Hydrographic changes in the Labrador Sea, 1960–2005

Progress in Oceanography Progress in Oceanography 73 (2007) 242–276 www.elsevier.com/locate/pocean

Hydrographic changes in the Labrador Sea, 1960–2005 Igor Yashayaev

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Ocean Circulation Section, Ocean Sciences Division, Bedford Institute of Oceanography, Fisheries and Oceans Canada, 1 Challenger Drive, P.O. Box 1006, Dartmouth, NS, Canada B2Y 4A2 Received 23 March 2006; received in revised form 17 April 2007; accepted 17 April 2007 Available online 4 May 2007

Abstract The Labrador Sea has exhibited significant temperature and salinity variations over the past five decades. The whole basin was extremely warm and salty between the mid-1960s and early 1970s, and fresh and cold between the late 1980s and mid-1990s. The full column salinity change observed between these periods is equivalent to mixing a 6 m thick freshwater layer into the water column of the early 1970s. The freshening and cooling trends reversed in 1994 starting a new phase of heat and salt accumulation in the Labrador Sea sustained throughout the subsequent years. It took only a decade for the whole water column to lose most of its excessive freshwater, reinstate stratification and accumulate enough salt and heat to approach its record high salt and heat contents observed between the late 1960s and the early 1970s. If the recent tendencies persist, the basin’s storages of salt and heat will fairly soon, likely by 2008, exceed their historic highs. The main process responsible for the net cooling and freshening of the Labrador Sea between 1987 and 1994 was deep winter convection, which during this period progressively developed to its record depths. It was caused by the recurrence of severe winters during these years and in its turn produced the deepest, densest and most voluminous Labrador Sea Water (LSW1987–1994) ever observed. The estimated annual production of this water during the period of 1987–1994 is equivalent to the average volume flux of about 4.5 Sv with some individual annual rates exceeding 7.0 Sv. Once winter convection had lost its strength in the winter of 1994–1995, the deep LSW1987–1994 layer lost ‘‘communication’’ with the mixed layer above, consequently losing its volume, while gaining heat and salt from the intermediate waters outside the Labrador Sea. While the 1000–2000 m layer was steadily becoming warmer and saltier between 1994 and 2005, the upper 1000 m layer experienced another episode of cooling caused by an abrupt increase in the air-sea heat fluxes in the winter of 1999–2000. This change in the atmospheric forcing resulted in fairly intense convective mixing sufficient to produce a new prominent LSW class (LSW2000) penetrating deeper than 1300 m. This layer was steadily sinking or deepening over the years following its production and is presently overlain by even warmer and apparently less dense water mass, implying that LSW2000 is likely to follow the fate of its deeper precursor, LSW1987–1994. The increasing stratification of the intermediate layer implies intensification in the baroclinic component of the boundary currents around the mid-depth perimeter of the Labrador Sea. The near-bottom waters, originating from the Denmark Strait overflow, exhibit strong interannual variability featuring distinct short-term basin-scale events or pulses of anomalously cold and fresh water, separated by warm and salty overflow modifications. Regardless of their sign these anomalies pass through the abyss of the Labrador Sea, first appearing at the Greenland side and then, about a year later, at the Labrador side and in the central Labrador Basin. The Northeast Atlantic Deep Water (2500–3200 m), originating from the Iceland–Scotland Overflow Water, reached its historically freshest state in the 2000–2001 period and has been steadily becoming saltier since then. It is argued that

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0079-6611/$ - see front matter  2007 Published by Elsevier Ltd. doi:10.1016/j.pocean.2007.04.015

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LSW1987–1994 significantly contributed to the freshening, density decrease and volume loss experienced by this water mass between the late 1960s and the mid 1990s via the increased entrainment of freshening LSW, the hydrostatic adjustment to expanding LSW, or both.  2007 Published by Elsevier Ltd. Keywords: North Atlantic; Subpolar gyre; Climate change; Labrador Sea Water; Denmark Strait Overflow; Iceland–Scotland Overflow; Northeast Atlantic Deep Water; Water mass production

1. Introduction The Labrador Sea is the coldest and freshest basin of the subpolar North Atlantic. Two fresh and cold inflows arriving from the Arctic Ocean to the North Atlantic by way of the Canadian Arctic Archipelago and the East Greenland shelf pass around the Labrador Sea margins, forming boundary currents in the Labrador Sea (Fig. 1). These boundary currents, known as the Labrador and West Greenland Currents, are strongly baroclinic with rapid transition between the less-saline waters over the shelves and the more-saline waters over the deep basins. Formed in the northern and western outskirts of the sea, the Labrador Current also presents the main pathway for the equatorward export of upper cold and fresh waters from the Labrador Sea, strongly affecting the hydrography and ecosystems of the shelf-slope areas downstream. Distinct patches of warmer and saltier water are found near the Greenland and Labrador continental slopes, offshore into the Labrador Sea. These patches are associated with an inflow of Subpolar Mode Water (McCartney and Talley, 1982), originating from a branch of the North Atlantic Current. In fact, the Labrador Sea is the final destination for these warm and salty waters of the North Atlantic Current system. Cooling and freshening along the passage around the subpolar gyre reduces the stratification of the Atlantic waters, but there are still substantial temperature and salinity contrasts in the upper layer of the Labrador Sea. These lead to intense vertical and horizontal mixing and exchange of heat and freshwater (Straneo, 2006) and contribute to the formation of the specific water mass of the Labrador Sea that is referred to in this paper repeatedly. The counterclockwise flow of warm and salty water around the rim of the Labrador Sea has several naming conventions, the most common of which is the Irminger Current, reflecting the fact that this water arrives from the Irminger Sea. In the Labrador Sea the core of the Irminger Current (Fig. 1) is centered at around 500 m depth (Fig. 4). The Irminger Current and its derived eddies contribute to the overall heat, salt and freshwater budget of the basin and, by setting the flux of heat and salt toward the center of the Labrador Sea, influence the development of winter convection. In winter, cold Arctic air outbreaks from Labrador result in intense atmosphere–ocean heat exchanges. The extreme winter heat losses combined with the subpolar gyre circulation mean that the Labrador Sea creates the densest winter mixed layers in the North Atlantic excluding the Nordic Seas and Baffin Bay. Episodically this deep convective mixing produces a characteristic intermediate water mass, the Labrador Sea Water (LSW), and at times modifies water properties to depths exceeding 2000 m (Lazier, 1980; Clarke and Gascard, 1983; Gascard and Clarke, 1983; Lazier et al., 2002; Yashayaev et al., 2003, 2007). There is another view of the LSW formation, suggesting that a substantial part of this water originates outside the Labrador Sea. In particular, Pickart et al. (2003) rejected the assumption that the sole source of LSW is the Labrador Sea arguing that LSW is massively formed in the southwestern Irminger Sea. This idea was in part based on the fact that tracer observations by Sy et al. (1997) showed very high advective speed and quite short travel times, on the order of 6 months, for LSW spreading to the Irminger Sea. This is at odds with a more recent advective–diffusive model study by Straneo et al. (2003) showing that LSW arrives in the central Irminger basin two years after its formation. Recently this story took a new turn. Yashayaev et al. (2007) published observational evidence that the LSW transit times are at least twice as long as those found by Sy et al. (1997). This recent finding reinstates the dominant source of LSW in the Labrador Sea, indicating no evidence of its simultaneous formation in the Irminger Sea, and confirms that its eastward spreading is, indeed, advective–diffusive. The Labrador Sea is a principal contributor to the lower limb of the Atlantic meridional overturning circulation (MOC). The latter, in its turn, excites and regulates the great ocean conveyor belt (Haupt and Seidov, 2007). In addition, the deeper North Atlantic water masses originating from the deep dense overflows from the

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Fig. 1. Map of the Labrador Sea showing its major topographic features and currents. The yellow line labeled AR7W indicates a standard hydrographic section occupied annually between 1990 and 2005. The circles indicate locations of oceanographic stations occupied by the Bedford Institute of Oceanography in the Labrador Sea in the spring of 2005 (May 18–June 4). Bravo was an Ocean Weather Ship, which regularly collected oceanographic data between 1964 and 1974.

Nordic Seas crossing the Greenland–Scotland Ridge actively mix with the Labrador Sea-formed intermediate waters (Dickson et al., 2002). The vigor of the deep-ocean flow is believed to be largely controlled by the volume and properties of the latter rather than the overflow itself (Boessenkool et al., accepted for publication). The Labrador Sea with its intense air–sea interaction, mixing and circulation can therefore influence the strength of the global ocean conveyor belt. Strong large-scale temperature and salinity anomalies develop intermittently at and near the surface of the Labrador Sea. Following their formation, these anomalies move out of the Labrador Sea, enter the subpolar

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circulation gyre and spread east-to-northeast across the North Atlantic and south-to-southwest along the western boundary of the North Atlantic. In some cases these anomalies cross the Subpolar Front and influence the Gulf Stream and the North Atlantic Current (Yashayaev, 2000). The present article documents the major developments in the hydrographic structure of the Labrador basin over the past five decades, focusing on the most recent period starting in the mid-1980s when precise oceanographic observations have been routinely collected across the Labrador Sea. These full water column changes observed in the Labrador Sea are also analyzed in the context of the Subarctic climate dynamics and change. 2. Oceanographic monitoring of the Labrador Sea Over the past 17 years (1990–2006), the Ocean Sciences Division of the Bedford Institute of Oceanography (BIO) has conducted a series of annual occupations of a hydrographic section across the Labrador Sea known in the World Ocean Circulation Experiment (WOCE) and Climate Variability and Predictability (CLIVAR) terminology as AR7W (Fig. 1). Two more surveys covering the western part of the Labrador Sea close to AR7W were conducted by BIO in 1987 and 1988. These observations and several more extensive multi-section surveys combined with the US Coast Guard’s Ocean Weather Ship (OWS) Bravo time series (1964–1974) document prominent interannual and decadal changes through the entire water column of this key region (Figs. 21, 4 and 12). The annual AR7W surveys measure the distribution of seawater temperature, salinity as well as chemical and biological parameters across the Labrador Sea. The primary goal of these annual hydrographic surveys is to observe the interannual changes in the properties of the intermediate and deep waters of the North Atlantic, particularly the changes in the properties and volumes of Labrador Sea Water (LSW). In addition to physical parameters, contemporary studies of deep-water renewal widely use transient anthropogenic tracers. Chlorofluorocarbons (CFCs), the most popular of these tracers, are taken up by the ocean via air–sea gas exchange. They intermittently enter the deep layers via intense winter convection and are exported out of the region by major ocean currents. Since their atmospheric history is well documented, CFCs are effectively used to document production, evolution, relative age and spreading pathways of newly formed waters. Since 1991, CFCs have been measured at all water sampling depths as a part of the BIO-led monitoring of the Labrador Sea (Azetsu-Scott et al., 2003). 2.1. A look at vertically integrated changes The interannual changes in the properties of LSW can be captured by examining the average potential temperature2 and salinity of the upper 2000 m layer3 over the central part of the Labrador Sea basin (Fig. 2). This is a layer of complex dynamics, exhibiting considerable variability in water properties and stratification (Figs. 3–5). The cooling and freshening of this layer over the 1980s and early 1990s led to a version of LSW in 1994 that was the coldest, freshest, densest, deepest and most voluminous since the 1960s and indeed in the entire historical record back to the 1930s. The 45-year record clearly shows three periods of Labrador Sea warming. The first and second periods lasted from 1962 to 1971 and from 1977 to 1983. The first warming period was preceded by a fairly significant renewal of LSW that occurred in the late 1950s – early 1960s. At the end of this warming period, in 1970–1971, the Labrador Sea reached its warmest and saltiest state ever observed. The last warming started in 1994. It was seen through 2006 when the 150–2000 m average temperature and salinity returned to the levels typical to the mid-1960s. If these temperature and salinity increases persist, the freshwater and heat contents of the Labrador basin are soon to exceed record high values. 1

Fig. 2 only goes back to 1960; the preceding observations were generally coarser and with larger errors. Potential temperature, h, and potential density, rh, are defined as the temperature and density of a parcel of water after it has been raised adiabatically to the sea surface. Since h and rh were generally used in this study, any reference to temperature and density in the text will imply their potential values (unless other is specified). 3 To avoid the influence of unresolved seasonal variability, the upper 150 m measurements were not used in this analysis. Means were calculated for 150–2000 m only. 2

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Fig. 2. A predominance of high values of the North Atlantic Oscillation (NAO) index (upper plot, note that the NAO axis is inverted) is reflected in periods of cooling and freshening of the Labrador Sea associated with sustained renewal of the intermediate waters reaching 2000 m and deeper. The potential temperature and salinity values are 150–2000 m means in the central Labrador Sea. The central Labrador Sea was confined by the 3250 m isobath. The freshwater gain (FW, note that the FW axis is inverted) was calculated over the full water depth. The steric height (lower plot, red line, 1960–2004) represents the water column thickness; it was derived from all available temperature and salinity measurements in the central Labrador Sea. The observed sea level anomalies (brown line, 3-months low-pass filtered anomalies relative to the record mean, 1992–2004) were calculated from satellite altimeter data acquired by the Topex/Poseidon and Jason missions. The period of AR7W occupation, 1990–2005, is highlighted.

The Labrador Sea reached its extreme cold and fresh state in the 1990s (Fig. 2). The episode of freshening and cooling between the mid-1980s and the mid-1990s combined with the noted loss of freshwater and gain of heat over subsequent years form a cycle in the history of water mass transformation. This process is largely controlled by production, thickness and properties of convectively-formed waters and is characteristic for the Labrador Sea and through the inter-basin exchanges (Johnson et al., 2005; Yashayaev et al., 2007) for the subpolar North Atlantic in general. Stretching the research domain over all deep subpolar basins (Fig. 4), one can come to a conclusion that the observed cyclic changes in the regional freshwater and heat content (Fig. 2) are caused by shifting hydrographic balances, which in turn are largely controlled by the interaction or ‘‘interplay’’ between LSW and the other intermediate waters of the North Atlantic. These intermediate waters surrounding LSW are typically warmer and saltier than LSW (Fig. 4). The ‘‘life cycle’’ of LSW, this most renowned water mass of the subpolar North Atlantic, including its production, transformation and losses caused by export and mixing, is the central theme in this paper.

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Fig. 3. Vertical section plots of potential temperature (upper row, C) and salinity (lower row) for the 1994 (left column) and 2005 (right column) occupations of AR7W. The section runs from the Labrador coast (left side) to the west coast of Greenland (right side). The 1994 survey (left column) shows a pool of newly-formed extremely cold and fresh Labrador Sea Water (LSW) extending to 2400 m. LSW is formed in the Labrador Sea through deep convection caused by high heat losses during severe winters (Lazier, 1980; Lazier et al., 2002; Yashayaev et al., 2003). In 1994, its core can be best identified by a temperature–salinity or density class occupying the largest volume on the AR7W section, while in 2005, the remnants of this water mass can still be detected by local salinity minima in the potential density range between 27.77 and 27.80 kg m3 (black dotted lines). Two other isopycnic levels shown in this figure, 27.68 and 27.74 kg m3 (black dashed lines), are discussed at the end of Methods and definitions C. NEADW indicates the Northeast Atlantic Deep Water, typically seen as a broad salinity maximum centered at 2700–2900 m. DSOW denotes the characteristic bottom water mass of the Labrador Sea – the deep-basin coldest Denmark Strait Overflow Water.

3. Water masses and full-depth stratification in the Labrador Sea captured by the 1994 and 2005 hydrographic sections All the principal water masses of the subpolar North Atlantic are characterized by their unique combinations of physical and chemical properties, which can clearly be seen in vertical sections of temperature and salinity across the Labrador Sea. Fig. 3 illustrates how the properties and volumes of these water masses changed from the extremely cold and fresh phase of 1994 to the present generally warmer and saltier state. In 1994, LSW with potential temperatures (h) < 2.8 C and salinities (S) < 34.84 filled the entire central part of the Labrador Sea basin within the depth range of 500–2400 m, (Fig. 3, left column, LSW). Remnants of this water mass can still be detected eleven years after its formation, in 2005 (Fig. 3, right column, LSW). Its present signature is the slightly increased spreading of the isotherms between 2.9 and 3.1 C (Fig. 3, upper right panel) and the broad volume of water with salinities between 34.88 and 34.90, featuring a weak local salinity minimum (Fig. 3, lower right panel) in the depth range between 1500 and 2400 m.

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Fig. 4. (Upper and middle rows) Maps of the subpolar North Atlantic showing salinity (left) and thickness (in m, right) of two r2 layers individually confining the LSW formations of the late 1960s (upper row) and the mid-1990s (middle row). The corresponding LSW-specific r2 ranges are 36.86–36.88 for the late 1960s and 36.92–36.94 for the mid-1990s. The late 1960s include the years 1964–1972; the mid-1990s are represented by 1995–1997. (Lower row) Changes in salinity (left) and thickness (in m, right) of the LSW-characteristic r2 layer between the late 1960s and the mid-1990s. The ridges of high LSW thicknesses and changes reveal three major LSW pathways, which are indicated by the yellow arrow-headed lines.

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Fig. 5. Potential temperature (upper), salinity (middle) and thickness of Dr2 = 0.01 kg m3 layers (lower, m) in the central Labrador Sea, 1987–2005 (the region was defined by the 3250 m isobath and the 150 km distance range from the AR7W line, Fig. 1). The r2 (potential density anomaly referenced to 2000 dbar) isolines (dashed contours) indicate that the LSW produced between 1990 and 1994 was the densest on the record. A thick weakly stratified layer (high r2 layer thickness, weak density gradients, low vertical stability) reappears in the Labrador Sea in 2000 as a new LSW class, LSW2000. This class continued to develop and got deeper over the subsequent years.

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While abundant (e.g., 1994, Fig. 3, left column), the deep LSW can be identified by the temperature–salinity or density class with the largest volume (Yashayaev et al., 2003, 2007). The same water mass, but substantially modified and weakened over the years, was unable to form a distinct volumetric peak in 2005, however, its remnants could still be identified by the aforementioned local salinity minimum. The density of the deep LSW core had also changed between 1994 and 2005. The salinity minimum identifying the deep aged LSW core in 2005 is denser and closer to the deeper boundary of the density range indicated by the black dotted lines in Fig. 3 than the deep temperature minimum (<2.7 C) seen between 800 and 2400 m in 1994. As shown later in this paper, a newly formed LSW core can be unmistakably identified by a relative temperature minimum within several years following its formation. This statement is well supported by the 1994 section. Within the depth range between 200 and 2000 m, saltier water is found over and sometimes near both the Greenland and Labrador slopes (Fig. 3). Shallower than 1500 m, this water is warmer than the LSW. It originates from the warm salty inflow from the Irminger Sea, the Irminger Current. Remnants of this current can be traced around the Labrador Sea, all the way to its western side. The temperature and salinity signals associated with this flow are stronger on the Greenland side and weaker on the Labrador side. There are several reasons for this along-path thinning, cooling and freshening of the Irminger Current. The AR7W sections often show a southward flow of the warm and saline water located near the northward flowing Irminger Current. The change in the slope of the isopycnic levels in Fig. 3 indicates the reversal in geostrophic flow. Warmcore eddies are often seen in the eastern part of the section, as an example see the patch of warmer and saltier water at 750 km in 2005 (Fig. 3, right column). The Irminger Current recirculation and eddy production and diffusion lessen the net amount of warm and salty water passing around the Labrador Sea. Upon reaching the north-eastern slope, the flow bifurcates or splits into two branches: one of the branches continues northward and passes through Davies Strait into Baffin Bay, while the other turns westward rounding the deep Labrador basin (Fig. 1). The loss in the circum-basin transport caused by this bifurcation has not yet been quantified, but hydrographic measurements collected at various locations of the Irminger Current suggest that the major part of the flow does not leave the Labrador basin, showing only moderately small changes in width, heat and salt contents after passing the bifurcation site. In addition to its diminishing transport, the Irminger Current is influenced and altered by the vertical and lateral mixing with the fresher and colder waters. This mixing appears to be more enhanced over the northwest slope of the Labrador Sea, where the merging Arctic outflows spread offshore and then mix down as winter convection develops. In 2005, the central basin was warmer, saltier and more uniformly stratified over the upper 2000 m (Fig. 3, right column) than it was in 1994. Following the temporal evolution of the water masses by the means of h–S analysis shows that there are, in fact, two LSW masses in the 2005 sections. The deeper one shows its characteristic salinity minimum centered at 2000 m. The shallower one with h < 3.4 C and S < 34.86 is found in the depth range between 500 and 1500 m. This is a water mass that was first formed by winter convection in 2000 (Yashayaev et al., 2003, 2007) and has been subsequently modified through mixing and weak convection during, at least, some of the subsequent winters. Full-depth hydrographic sections across the Labrador Sea (e.g., Fig. 3) also reveal the Northeast Atlantic Deep Water (NEADW) and the Denmark Strait Overflow Water (DSOW), which occupy the deep and abyssal reservoirs beneath LSW. Interannual changes observed in these two water masses during the past two decades will be discussed later in the paper. 4. Changes in the Labrador Sea water between the 1960s and the 1990s The changes observed in the Labrador Sea and in its characteristic water mass over the past five decades are significant but are still not fully accounted for and understood. There are several factors controlling and influencing winter convection in the Labrador Sea. The atmospheric forcing (Lazier et al., 2002; Lu et al., 2007) needs to be considered not only for the season of cooling, but over the whole annual cycle. Air–sea heat and moisture (freshwater) fluxes over the warm part of the year increase stratification and build buoyancy in the upper layer (Straneo, 2006) that will work against convection in the following winter. Changes in the contributions of the Arctic inflows and continental run-off (Peterson et al., 2006), both including ice, and North Atlantic currents to the freshwater and salt budgets of the Labrador Sea also play a significant role in the development of winter convection.

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The LSW and the entire central Labrador Sea became warmer and saltier over the 1960s into the early 1970s. This was due to the mild winters of the late 1960s – early 1970s, causing the LSW at the intermediate depths to become isolated from the upper mixed layer. It stopped mixing with fresher shallower waters and also stopped exporting its excessive heat upwards. This LSW was steadily becoming warmer and more saline due to mixing with the surrounding intermediate waters from outside the Labrador Sea. A somewhat similar sustained change in the deep LSW layers is reported in the present study for the years from 1994 to 2005. This situation ended with the severe winters in 1972 and 1973, which produced mixed layers deeper than 1500 m (Lazier, 1980) leading to the creation of a new LSW vintage or class (the latter of these terms is introduced in Methods and definitions C). This vintage of LSW was colder and less saline than those formed in the mid and late 1960s. It continued to develop over several subsequent years, becoming anomalously cold and fresh during the severe winters of 1974 and 1976. However, the cooling and freshening of the mid-1970s was short-lived; following the year 1976 both temperature and salinity of the deep LSW increased again as horizontal mixing replaced vertical mixing as the dominant forcing agent. Between 1980 and 1985, the LSW was warmer and saltier than in the mid-1970s, but not as saline as it was between 1963 and 1971. These earlier LSW developments, transformations, dissipations and, ultimately, losses are reflected in the time series of depth averaged or integrated values in Fig. 2. The warm and salty state of the mid-1960s–early 1970s and the cold and fresh state of the early-mid-1990s are distinguishable not only in the Labrador Sea (Fig. 2; Yashayaev et al., 2003, 2007), but over the entire subpolar domain of the North Atlantic Ocean (Dickson et al., 2002; Yashayaev et al., 2003). These two hydrographically contrasting states, defined by their characteristic overall environmental conditions and circulations at all levels, are of special interest for the Arctic and North Atlantic ocean climate and ecosystem studies. The transition between these large-scale oceanic extremes and associated vertical and horizontal rearrangements in all major subpolar basins are insightful for comprehending the sources of these shifts, their dynamics and environmental impacts. Finally, a better understanding of the past and present states is valuable for improving the ocean climate predictions. The North Atlantic was well surveyed in 1964–1972 (hereafter the late 1960s) and in 1995–1997 as part of WOCE (hereafter the mid-1990s). The first period coincides with the record salty, warm and thin LSW, while the second, shorter and therefore better focused time interval brings up a three-year snap shot of the subpolar hydrography taken in the wake of the most voluminous LSW production ever recorded. The 1964–1972 and 1995–1997 hydrographic data sets were compiled from the National Oceanographic Data Center (NODC), WOCE, Variability of Exchanges in the Northern Seas (VEINS) and institutional archives of temperature and salinity measurements. All temperature, salinity and density profiles were quality controlled and vertically interpolated every 5 m. These data sets were used to construct the LSW salinity and thickness distributions (Fig. 4), the NEADW salinity and depth distributions (Fig. 12 discussed in Section 8) and volumetric inventories (Front Page discussed in the introductory paper of this special issue) representing these two hydrographic surveys of the subpolar North Atlantic. LSW, clearly distinguishable in both the late 1960s and the mid-1990s, was individually defined for the two hydrographic surveys. The potential density (r2, referenced to 2000 dbar) ranges defining the associated LSW layers were taken as 36.86 < r2 < 36.88 for the late 1960s and 36.92 < r2 < 36.94 for the mid-1990s. This choice was based on the corresponding volumetric r2 and h–S (potential temperature–salinity) census discussed in the issue’s introductory paper and shown on the Front Page. The main LSW reservoir can be easily identified in the examined hydrographic compilations by extremely low salinity and high thickness values (Fig. 4). The LSW salinities decreased notably throughout the whole region, while the corresponding layer thicknesses and the spatial salinity and thickness contrasts increased between the 1960s and the 1990s. At the same time, the main reservoir of this water retained its characteristic shape featuring a broad well-established ‘‘communication’’ between the Labrador and Irminger basins, an exiting pathway to the subtropics and a transit pathway to the eastern subpolar basins. Even though the main LSW reservoir forms a rather distinguishable spatial pattern, one can notice few local irregularities and discontinuities in its shape and margins. For example, in the case of the mid-1990s, the 1000 m LSW thickness contour forms two distinct LSW pools and even hints to another somewhat smaller pool in the central Irminger Sea. However, we argue that these and other local irregularities in the LSW thickness distribution are mostly caused by known methodological and observational (sampling) shortfalls rather

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than by the reoccurrence of specific processes and local circulations. This statement is supported by the following facts: (1) Baroclinic mesoscale eddies can change the LSW layer thickness, without changing its temperature and salinity much. (2) The LSW thickness distributions (Fig. 4, right column) robustly define the large-scale inventories of this water mass, however significant interannual variations might have also aliased in with the analyzed observations. Pickart et al. (2003) based their LSW map on the years 1989–1997 coinciding with the period of exceptionally large changes in the LSW thickness and properties (Fig. 5). This raises doubts concerning the observational evidence of deep convection in the Irminger Sea used by (Pickart et al., 2003). Specific local extrema found by these authors could be ‘‘shaped’’ by different stages of progressive LSW development and spatial irregularities in the body of this water mass. Note that the mid-1990s LSW thickness map in Fig. 4 (right middle) is based on a shorter time window that comprises only the first three years of post-production evolution of the deep and dense LSW. (3) Horizontal interpolation (gridding) of locally sparse data can also ‘‘isolate’’ an occasional cluster of observations resulting in a closed contour line surrounding this cluster. The mentioned spot to the south–southwest of Greenland in the mid-1990s LSW thickness map (Fig. 4, right middle) shows an exemplarily case of such discontinuity in certain contour lines caused by interpolation over a domain that lacks observations. A split in the ridge of high thickness (>1000 m) coincides with the spot with no observations. This drop in the LSW thickness matching the bifocal pattern in the deep potential vorticity minimum (Pickart et al., 2003) cannot be justified on the basis of the two observational compilations presented here. Excluding the mentioned data-free area with artificially thinned LSW and assuming that the distributions of interest can be aliased by smaller scale variations and trends, we conclude that the LSW thickness and property distributions for the two episodes show a continuous subpolar reservoir of LSW, fed from the same source or, possibly, sources. Once a new water mass is produced, it advectively diffuses over this reservoir (Straneo et al., 2003) and mixes with other waters as it moves away from its source. The ridges or sleeves of high LSW thicknesses and thickness changes (Fig. 4, right column) reveal major spreading routes of LSW, indicated in the maps by the yellow arrow-headed lines. Since the LSW layer is generally thinner, saltier and warmer in the Irminger Sea than in the Labrador Sea (Fig. 4), we still recognize that the Labrador Sea is the main formation region for LSW. The map showing differences between the LSW salinities of the 1960s and 1990s (Fig. 4, lower left panel) reveal another region of substantial changes. This region is stretched along the perimeter of the main subpolar reservoir of LSW and is likely a result of its horizontal expansion following the most massive LSW production of the 1990s. As a result of such expansion the fronts separating LSW and other intermediate waters (warmer and saltier than LSW) shifted outward, which is likely to affect the mid-depth circulation. The reported long-term cyclic changes in the stratification of the Labrador Sea and the properties of LSW can be linked to regime changes in certain large-scale atmospheric patterns, the most common of which is the North Atlantic Oscillation (NAO) (Hurrell and Van Loon, 1997). NAO is the primary mode of climate variability that involves the winter atmospheric circulation over the northern hemisphere and principally over the North Atlantic. The NAO index is the normalized Azores-to-Iceland sea level pressure difference.4 A positive NAO means a stronger pressure difference over the central North Atlantic and hence stronger winter winds over this region. A predominance of negative NAO years from 1962 through to 1971 coincides with the period of little convective renewal of LSW (Fig. 2) during which this water was becoming warmer and more saline. The convective renewal of LSW from 1972 to 1976 and from 1988 to 1994 can in contrast be associated with periods of strong positive NAO values. However, the agreement between convection and the NAO is not perfect for a number of reasons:

4

Since the meteorological record at Lisbon is longer than at Azores, Lisbon is often used in the place of Azores.

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Firstly, the correlation between the NAO index and the net air–sea heat flux over the Labrador Sea (e.g., Lu et al., 2007) strongly depends on the cut-off frequency employed in high-pass filtering of the multidecadal series of these variables. The essentially high correlation between the unfiltered NAO and heat flux series is largely a result of the low frequency shift in the atmospheric conditions between the 1960s and the early 1990s. This correlation weakens in response to shortening of the filter’s cut-off window, implying less agreement between the NAO index and net heat flux on shorter time scales. Indeed, because the NAO index has little autocorrelation between months, a winter can have a positive NAO index but be preceded and followed by warm seasons or mild winters that inhibit convection. Secondly, the ocean, because of its thermal inertia, creates a ‘‘memory’’ of past winters through its stratification; it might take several severe winters to erode away the stratification built up over several years of mild winters, meaning that on short timescales, the ocean does not convect deep despite fairly strong forcing. On the other hand, intense convection in preceding years homogenizes the water column, making it easier to remix large volumes of water in subsequent years, thus letting a weaker forcing operate over the whole mixed layer. The interannual changes in the sources of saltier and fresher waters feeding the upper layers of the Labrador Sea can, to a certain degree, influence the development of convection and determine the properties of its product, the LSW. A study of the mechanisms controlling renewal of the intermediate waters must be based on model simulations for different configurations of boundary conditions (sources of heat and freshwater), followed by a comparison with the observed changes. The first results of such effort are discussed in a companion paper by Lu et al. (2007). Thirdly, a strong NAO does mean stronger westerlies over the central North Atlantic. This is usually associated with stronger winds in the Labrador Sea but not always. Significant variations in atmospheric fields over the North Atlantic (Yashayaev and Zveryaev, 2001) weaken the link between the NAO index and winter conditions in the Labrador Sea, including convection. 5. Production, export and transformation of the Labrador Sea Water, 1987–2005 The present analysis is focused on more recent developments and changes in the Labrador Sea and in order to bring full attention to these, Figs. 5–10 start in 1987. The Methods and definitions A and B sections at the end of the paper discuss some principles and procedures used in this analysis. The estimation of year or survey typical conditions employs robust averaging over r2 (potential density anomaly referenced to 2000 dbar) bins as briefly outlined in Methods and definitions A. The values shown in the time-depth coordinates (Fig. 5) were generated by a procedure based on this approach. Methods and definitions B discusses some ideas and methods of water mass analysis used to create the diagrams in Figs. 6 and 7. The LSW cores and classes documented by Figs. 5–8 and discussed in our present summation of the Labrador Sea changes are introduced and identified in Methods and definitions C. Despite its seeming simplicity, water mass identification is not a trivial task. Any misconceptions may lead to wrong conclusions on production, spreading and mixing of a specific water mass. In particular, in order to properly identify a certain LSW class (e.g., either LSW1987–1994 or LSW2000, see below) over a multiyear period one should not unconditionally rely on a predetermined criterion set by time-invariant ranges of certain parameter (e.g., density). Since strengthening and deepening convection remixes LSW, altering practically all of its parameters, the LSW-characteristic property ranges or other criteria chosen to define the core and margins of this water mass may change as well. In their turn, mixing, diluting, discharge, export of LSW and other processes responsible for the losses of LSW do also change the properties of LSW and, as a result, alter its core and boundaries. Indeed, the AR7W section plots (e.g., Fig. 3) and volumetric inventories (e.g., Figs. 5–8) demonstrate that a universal definition of a given LSW class (e.g., LSW1987–1994) meant to be used through its development and transformation history should not be based on a static range of a certain variable. On the contrary, any criteria that involves the LSW properties needs to be brought into agreement with the changes in such properties. The r2 and h–S volumetric approaches are used through our studies for routine identification of LSW and computation of its volume and key characteristics. The water mass definitions coming out of these techniques automatically adjusts to a specific LSW core, following its year-to-year dynamics and transformation and revealing spatiotemporal changes in this LSW. The r2 volumetric method was also employed by Yashayaev et al. (2007) for tracking LSW anomalies spreading across the subpolar North Atlantic. These authors also

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Fig. 6. (a) A r2-time plot showing average thickness (m) of Dr2 = 0.01 kg m3 layers in the Labrador Sea (r2 is potential density anomaly referenced to 2000 dbar). For the construction of this plot overlapping r2 layers set by 0.002 kg m3 have been used. The volumetric peaks reveal two temporally evolving LSW classes, LSW1987–1994 and LSW2000 (Yashayaev et al., 2007). (b) Potential temperature (h)–salinity (S) ‘‘volumetric’’ projections of the 1994, 2000 and 2004 AR7W hydrographic section (Fig. 1). Each projection (colored accordingly to its label) comprises average vertical thicknesses (m) of h–S layers defined by two-dimensional h–S intervals with Dh = 0.1 C and DS = 0.01, set by 1 Dh and 12DS. The local h–S thickness peaks reveal two characteristic LSW classes: LSW1987–1994 and LSW2000. The solid and dashed 2 contours are isolines of r2 (kg m3) defined as a function of h and S.

argued that using time-invariant density ranges as definitions of LSW (e.g., Kieke et al., 2006) can lead to erroneous water column partitioning and misrepresentation of specific LSW classes and other intermediate and upper water masses arriving or formed in the region. These concerns are illustrated and further discussed at the end of Methods and definitions C.

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Fig. 7. Potential temperature vs. salinity curves representing typical conditions in the Labrador Sea during 1987–1994 (upper panel) and 1994–2005 (lower panel). The years of AR7W occupations, arranged diagonally, correspond to the curves of matching color. LSW1987–1994 denotes the record coldest LSW progressively produced through the winters of 1987–1994, LSW2000 denotes the LSW produced in the winter of 1999–2000, NEADW and DSOW are the Northeast Atlantic Deep and Denmark Strait Overflow waters. The light-colored trajectories indicate the development (upper panel) and subsequent transformation (lower panel, lower trajectory) of the densest and most voluminous LSW class (LSW1987–1994), the evolutions of the LSW2000 class (lower panel, upper trajectory) and the development of the warmer and saltier layer separating these two LSW classes (lower panel, middle trajectory). The contour lines show potential density anomaly referenced to 2000 dbar (r2) as a function of salinity and potential temperature.

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Fig. 8. Average potential density (r2), salinity, potential temperature (h), thickness (shown in vertical bars), depth and CFCs (F11 and F12) of the LSW1987–1994 (labeled as LSW1990) and LSW2000 classes. The method of robust averaging described in Methods and definitions A have been adapted to construct the LSW series. Vertical bars in temperature and salinity series represent annual median absolute deviation.

The h–S curves presented in Fig. 7 indicate yearly typical conditions in the Labrador Sea during 1987–1994 (upper panel) and 1994–2005 (lower panel) and are based on all available measurements within a 50 km range of AR7W. The choice of these two time intervals reflects the 1987-to-1994 build-up and development, followed by the 1994-to-2005 decline and decay of the most remarkable intermediate water mass ever observed in the subpolar North Atlantic. The two characteristic LSW classes in the focus of this study are defined as LSW1987–1994 and LSW2000 (Figs. 5–7). Their complete development and transformation history for 1987–2005 is presented in Fig. 8

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Fig. 9. (Upper row) Potential density anomaly averaged over the 200–800 m (red-shaded; lower surface in left panel, upper surface in right panel) and 1200–2000 m (blue-shaded; upper surface in left panel, lower surface in right panel) layers of the Labrador Sea along the AR7W line (Fig. 1). Inversion of the Density axis (right panel) exposes some additional details of the coupled variability in the appointed layers hidden in the back of the original drawing (left panel). (Middle and lower rows) Potential temperature (left) and salinity (right) averaged over the 200–800 m (middle) and 1200–2000 m (lower) layers of the Labrador Sea along the AR7W line (Fig. 1). The Distance axis expresses the distance to the central point of the section line with its values increasing toward the Greenland coast (positive direction) and decreasing toward the Labrador coast (negative direction). OWS Bravo is near 100 km. The bottom depth to distance correspondence is shown in Fig. 11.

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Fig. 10. Potential density anomaly (upper panel), potential temperature (middle panel) and salinity (lower panel) averaged over the bottommost 100 m layer of the Labrador Sea along the AR7W line (Fig. 1). The Distance axis expresses the distance to the central point of the section line with its values increasing toward the Greenland coast (positive direction) and decreasing toward the Labrador coast (negative direction). OWS Bravo is near 100 km. The bottom depth to distance correspondence is shown in Fig. 11.

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(‘LSW1990’ in this figure indicates the LSW1987–1994 progressions). The values plotted in Fig. 8 are the robust means over LSW characteristic density ranges (layers) confined by r2|LSW  0.017 kg m3 and r2|LSW + 0.017 kg m3, where r2|LSW is r2 of the LSW core associated with a specific LSW class. The LSW core in its turn was identified from the corresponding annual h–S diagram (e.g., Fig. 6b). A solitary volumetric h–S peak (maximum), which was first observed in 1987 at r2 = 36.885 kg m3, reached its all-time high in 1994 at r2 = 36.940 kg m3 (Figs. 6a, 7 and 8), thus completing the most extraordinary development of LSW in the entire documented history of this water mass. This peak related to LSW1987–1994 has then substantially diminished over the subsequent years, becoming barely identifiable in the volumetric diagrams constructed for the early-mid 2000s (Fig. 6a and the year 2004 in Fig. 6b). On the other hand, the remnants of the transformed LSW1987–1994 still show a characteristic salinity minimum, helping with objective identification of this LSW class. In addition to complementing and extending the documented production and transformation history of the deep LSW of the 1990s (Lazier et al., 2002; Yashayaev et al., 2003), Figs. 5–8 also illustrate a newer LSW development or an LSW class, defined as LSW2000. This water was massively formed in 2000 and is found at shallower depths and in lower density ranges than the LSW1987–1994 class. Note that these two classes were named after the years over which they were progressing to their coldest, densest and most voluminous states. The two LSW classes have much in common. Both of their developments were preceded by freshening of the upper layer (Fig. 5), followed by the years of increased wintertime atmospheric forcing. Both waters are surrounded by saltier and often warmer layers (Figs. 5 and 7) and water columns (Fig. 4) and both now show steady increases in their temperature and salinity (Figs. 5–8). The following provides a detailed overview of the recent LSW developments and discusses some impacts of these developments on the stratification at the upper to intermediate depths in the Labrador and, consequently, entire subpolar North Atlantic basins. During the late 1980s, a freshwater anomaly appeared in the upper 500 m. Over the subsequent years it was mixed down through winter convection (Figs. 5 and 7). A series of strong winter convection events from 1987– 1988 to 1993–1994 produced a large homogeneous volume of exceptionally cold, fresh and dense LSW reaching 2400 m (LSW in Figs. 3 and 5, LSW1987–1994 in Figs. 6–8). The progressive development of this densest and most voluminous LSW class can be presented as the corresponding h–S-time trajectory shown as the lightcolored arrow-headed line behind the 1987–1994 annual h–S curves (Fig. 7, upper panel). This trajectory indicates that the density increase observed during this LSW development was mostly due to the cooling caused by excessive high heat losses during the severe winters of the early 1990s (Lazier et al., 2002), associated with the high-NAO phase. Between 1987 and 1994 the LSW1987–1994 was rapidly changing in its formation region, cooling by about 0.45 C, becoming 0.06 kg m3 denser and approximately doubling in volume (Figs. 5–8). Remarkably, the LSW1987–1994 salinity (Figs. 5, 7 and 8) was not changing monotonically during all these years. While convection was getting deeper between 1990 and 1993, the LSW1987–1994 class was steadily becoming saltier. This salinity increase is best illustrated in the h–S plot in Fig. 7 (upper panel), where in 1993 the well mixed layer of LSW can be identified by a narrow temperature and salinity minimum close to r2 = 36.940 kg m3. Despite further cooling of the whole mixed layer caused by winter convection in 1994, this layer did not seem to extend noticeably deeper in the spring of 1994 than the mixed layer of the previous year. At the same time, the average salinity of the deep mixed layer decreased by almost 0.01. This 1993-to-1994 salinity decrease formed a singleyear reversal in the LSW salinity increase maintained through the previous three years (Figs. 5, middle panel, 7, upper panel and 8). Such a short-lived LSW freshening was likely a result of convection to the same depth in both 1993 and 1994 and is discussed below. The increase in the LSW1987–1994 salinities between 1990 (even 1988) and 1993 was due to mixing with the saltier NEADW that is entrained into LSW every time convection deepens. This statement can be supported by the analyses presented in Fig. 5, showing how the lower margin of LSW and therefore of winter convection had progressively gotten deeper in each year between 1988 and 1993 (Fig. 5). The last occurrence of intense convection that took place in 1994 did not penetrate deeper than the convection of 1993 and thus did not bring up enough saline NEADW from below to overcompensate the freshening effect of convective entrainment of the upper-layer less-saline waters. The LSW1987–1994 salinity decrease further means that the amount of war-

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mer and saltier brought into the Labrador Sea by the Irminger Current was also insufficient to compensate the freshening resulting from the entrainment of less-saline waters. This single-year salinity decrease by almost 0.01 (Fig. 8) resulted in an instantaneous shift or disruption in nearly a record-long positive trend of the LSW1987–1994 salinity, thus forming the second minimum (the first low is found in the mid-to-late 1980s) in the bi-minimum LSW1987–1994 salinity record (Figs. 5, middle panel, 7, both panels and 8). This remarkably sustained increase in the LSW1987–1994 salinity was first maintained by the NEADW entrainment and after the cessation of LSW1987–1994 convective renewal following 1994 by the isopycnal mixing with saltier intermediate waters bordering LSW outside of the Labrador Sea. Such a switch in the dominant source of ‘‘salt’’ has altered the salinity increase rate (Fig. 8). Even though the LSW1987–1994 salinity increase was disrupted in 1994, the newly formed or remixed LSW found in the same year was significantly colder than its 1993 precursor, meaning that the continuous cooling of LSW1987–1994 that started in the mid or late 1980s was neither reversed nor even stopped or slowed down on its way to the LSW all-time-coldest state observed in 1994 (Fig. 8). In addition to the abovementioned details of the LSW1987–1994 development, one can note that the annual increases in salinity from 1988 to 1993 (Fig. 8) added to the density increases primarily triggered by the convective cooling of LSW1987–1994 (Fig. 7, upper panel). These matching tendencies in the temperature and salinity contributions explain the rapid increase in the LSW1987–1994 density (Fig. 6a) and the progressing buoyancy loss in the top 2000 m. The LSW vintage of 1994 was slightly more saline than those of 1987 and 1990 (Fig. 8). However, because the volume of this water mass increased between 1987 and 1994, mostly at the expense of more-saline waters (e.g., NEADW), the net freshwater storage of the Labrador Sea basin reached its record high level in 1994 (Fig. 2, the FW Gain reflects the net freshwater storage change). After the body of LSW achieved its greatest thickness and property anomalies in 1994, it began to thin out and weaken (Fig. 8). Since 1994 the deep reservoir of LSW1987–1994 has remained isolated from the winter mixed layer. Such isolation of the deep and dense version of LSW (LSW1987–1994) was due to a substantial decrease in the net annual heat loss from the Labrador Sea to the atmosphere after 1994 (Lazier et al., 2002). This weakening in the atmospheric forcing (linked to the low NAO, Fig. 2) resulted in less intense convective mixing, mostly limited to the shallower reservoirs which were becoming filled with the less-dense LSW (ultimately, LSW2000; Fig. 5). The mild winters allowed temperature and density stratification to be re-established above the thinning patch of LSW1987–1994, which evolved without much interaction with the layers above. Since its last convective renewal in 1994, the deep LSW core was steadily becoming warmer and saltier, bearing just slight changes in its density (Figs. 5–8). The volume of the LSW1987–1994 class declines as this water drains away from the Labrador Sea to other regions of the ocean (thickness in Figs. 5, lower panel, 6 and 8). Since 1994 the thickness of LSW1987–1994 in the density range of 36.92 < r2 < 36.95 kg m3 decreased from 1900 m to 250 m, a reduction by 87%. The average thickness of the LSW1987–1994 based on its more precise time-dependent LSW definition and unsmoothed measurements (Fig. 8) decreased from 1835 m in 1994 to 350 m in 2005 (81% reduction). Even after 11 years of isolation, this much of the LSW1987–1994 can be well identified in the h–S curves by the characteristic LSW salinity minimum (Fig. 7, lower panel, r2 = 36.935 kg m3). Between 1994 and 2005 the water at this salinity minimum became warmer and more saline by 0.34 C and 0.062, respectively. This increase in temperature and salinity in the LSW1987–1994 class (often referred to as the deep LSW) was likely due to the isopycnal mixing as this water exhibits a h–S minimum in the corresponding density and vertical layers in the whole North Atlantic. Even larger changes are seen in the layers above the deep LSW core. The combination of lower heat loss to the atmosphere and continued horizontal mixing of heat and salt into the LSW and shallower layers resulted in the entire upper 2000 m of the Labrador Sea becoming warmer, saltier and less dense. This change was not vertically uniform and equally persistent at all depths (Fig. 5). The annual changes between 1800 and 2300 m are smaller but steadier than those occurring at shallower depths. Although we do not dismiss the possibility that there might have been occasional short convective events penetrating deeper than 1500 m and reaching into the deep and dense LSW classes after 1994, there is no evidence that such events made any significant change in the overall properties and volume of this deep LSW created in 1994 and earlier. The fact that the annual changes observed in the deepest LSW after 1994 had nearly the same magnitude while this water was gradually thinning out is key for understanding the LSW export and transformation pro-

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cesses. The whole body of the deep LSW loses its mass by draining away or leaking from its main subpolar reservoir (Fig. 4) to other regions of the Atlantic Ocean. The loss of LSW is being replaced by warmer, saltier and less dense waters mostly arriving from the boundary currents and building up above the deep LSW core. Despite its losses, a significant part of this deep and dense LSW is still recirculating within the Labrador and Irminger seas and possibly elsewhere steadily becoming warmer and saltier. Along its extensive outer boundary, this deep recirculation places LSW into contact with warmer and saltier waters of the same density, allowing their mixing and exchange, and, consequently, maintaining the steady transfer of heat and salt into the LSW core within the Labrador Sea. The steadiness of the net annual fluxes results in the noted quasi-constant temperature and salinity increases between consecutive AR7W occupations in the deep LSW core (Fig. 5, 2000 m). This change can be explained by advective–diffusive process similar to that explored by Straneo et al. (2003), but acting in both ways. The general increase in temperature and salinity of the upper 2000 m since 1994 (Fig. 2) was disrupted by a cooling and freshening event that occurred in 2000, the most significant winter convection since 1994. A freshwater anomaly that appeared in the upper 500 m in the previous years was carried down through winter convection in 2000 (salinities <34.82, Fig. 5, lower panel) and some of the following years, similar to the convection-driven redistribution of freshwater seen over the late 1980s – early 1990s. The 2000 convection reached 1600 m and was extensive enough to produce a distinct LSW class that is still seen in the Labrador and Irminger seas (designated LSW2000 in Figs. 3, 5 and 7 lower panel). While the LSW1987–1994 has substantially declined and diminished over the past decade, the volume of the less-dense LSW classes, dominated by the LSW2000, has increased. Since 2000, the LSW2000 class has been thicker than its deeper and gradually decaying companion. With regard to its appearance, the LSW2000 class occupies a fairly warm, low-salinity and low-density range (Fig. 8) of the historic temperatures, salinities and densities of LSW (Yashayaev et al., 2003). Even though this LSW is warmer, less dense and shallower than its precursor formed in 1990–1994 (Figs. 5– 8), these two LSW classes have similar rates of annual change and comparable transit times to the Irminger and Iceland basins (Yashayaev et al., 2007). It is not clear if the most recent LSW class was massively renewed between 2001 and present, whether in the Labrador Sea, in the Irminger Sea, or in both. Winter convection in the years 2001–2005 failed to increase the LSW2000 density. In fact, the density of the mixed layer in our spring surveys decreased from r2 = 36.87 kg m3 in 2000 to r2 = 36.85 kg m3 in 2005 (Figs. 7, lower panel, and 8). Similar to what we saw in the denser and deeper LSW1987–1994 after its renewal had stopped, we now see in the layer occupied by the LSW2000 – it is becoming warmer and saltier. The light-gray trajectory shown in Fig. 7 (lower panel) follows the LSW2000 transformation history. Yashayaev et al. (2007), who reconstructed the LSW2000 signals between Labrador and Iceland basins, suggest that this change can result from mixing with warmer and saltier waters from outside the Labrador Sea. However, there are indications that winter convection after 2000 has also brought warmer and saltier waters into the LSW2000 layer. In 2001–2003 LSW2000 became thicker and showed irregular year-to-year changes in salinity, CFCs (Fig. 8) and oxygen, suggesting its, at least partial, renewal after 2000. On the other hand, the salinity increases observed in 2004 and 2005 in the upper 500 m are insufficient to compensate the buoyancy gain in this layer caused by its recent warming (Fig. 5). This increases the chances of LSW2000 repeating the fate of its denser predecessor – becoming replaced by less-dense LSW classes. There is another reason that adds to such an expectation. The aforementioned sustained weakening and discharge of the deeper LSW1987–1994 produced a niche above it. The lower part of this niche is becoming filled by the Icelandic Slope Water – a narrow temperature and salinity maximum just above the deep LSW (Fig. 3, 2005), while the top part is now being taken by the LSW2000, leading to apparent deepening of LSW2000. This vertical rearrangement of the intermediate water masses in the Labrador Sea shows up in Figs. 5 and 8 as an evident subduction or sinking of the LSW2000 core during the 2000–2004 period. On the other side, the ‘‘sinking’’ LSW2000 is progressively withdrawing from its upper compartments, thus vacating more and more room for even shallower, warmer and less dense waters. Being regularly mixed by weak convection these new warm low-density waters are presently building up, expanding vertically over the past several years, and thus increasing the density stratification in the top layers of the Labrador Sea (>3.4 C in Figs. 3 and 5). This makes it more difficult for winter renewal of the LSW2000 to take place, consequently leading to the vertical isolation of LSW2000 that will eventually close another page in the LSW development history.

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Both section plots and time series of vertical profiles and h–S curves (Figs. 3, 5 and 7) show two other characteristic features, which in some way are related to the two aforementioned LSW formations. These are the thin but distinct temperature maxima underlying LSW1987–1994 and LSW2000, hereafter, the sub-LSW1987–1994 and sub-LSW2000 maxima. In 1994, the sub-LSW1987–1994 maximum was located at about 2500 m (Fig. 3). Between the mid-1980s and the early 2000s, this feature was typically 0.02–0.03 kg m3 (r2) denser than the deep LSW core (Fig. 7). The origin and fate of the sub-LSW1987–1994 maximum will be discussed with the NEADW changes. The sub-LSW2000 salinity and temperature maxima are associated with the relatively saline and warm intermediate layer separating the LSW1987–1994 and LSW2000 in the Labrador (Figs. 3, 5 and 7) and Irminger (Yashayaev et al., 2007) basins. This layer is formed by the core of saltier and warmer water that can be traced back to the Icelandic Slope Water (ISW) seen near the Reykjanes Ridge. ISW is formed through a direct linear mixture of the original Iceland–Scotland Overflow Water with the overlying Atlantic thermocline water near the Faroes, without interacting much with LSW in its formation region (van Aken and de Boer, 1995). That ISW then follows the slopes of Iceland and the Reykjanes Ridge until it enters the Irminger Sea through the Charlie–Gibbs Fracture zone (Dickson et al., 2002). From the western slope of the Reykjanes Ridge the ISW intrudes into the centre of the Irminger gyre, forming a relatively thin, salty and warm layer, now prominently seen between the LSW1987–1994 and LSW2000 cores (Yashayaev et al., 2007). Finally, this water mass arrives in the Labrador Sea where it partially replaces the underlying LSW. The ISW-characteristic salinity maximum is typically 140–200 m deeper in the Labrador Sea than its temperature companion (Figs. 3 and 5). While the mid-depth temperature maximum appeared in our annual surveys only in 2000, the saltier water started to form a weak characteristic feature at about 800 m in 1995. This happened almost immediately after winter convection had lost its strength and was not able to compensate for the losses in the discharging LSW1987–1994. In 1995 this new saltier feature (r2 = 36.92 kg m3, Fig. 7, lower panel) was less dense than the deep LSW1987–1994 by only 0.01–0.02 kg m3. However, this water soon became less dense, arriving at r2 = 36.90 kg m3. As time progressed, it developed into a prominent h–S maximum also characterized by a volumetric peak, forming a unique water mass derived from the warmer and saltier ISW (the light-pink arrow-headed line in Fig. 7, lower panel, indicates the trajectory through its evolution). The core of this water (r2 = 36.90 kg m3) is steadily becoming warmer, saltier and deeper, tending to replace the LSW1987–1994, which in its turn has substantially drained away and strongly diminished in its size and contrasts with the surrounding waters. While the sinking of the warm and salty ISW-derived layer stopped in 2004, it still continues to become warmer and saltier. 6. Changes in the intermediate layers across the Labrador Sea Repeat occupations of a particular hydrographic section can be used for studying spatiotemporal variability of an observed property in a variety of ways. We have already discussed temporal changes in some integral characteristics of the entire water column of the Labrador basin or a large portion of such (Figs. 2 and 8). Averaging a survey collection of hydrographic measurements over individual layers defined by relatively narrow depth (pressure), r2 (Figs. 5 and 6a), h–S (Figs. 6b and 7) or other property ranges adds a vertical dimension to the water column development and transformation history. Figs. 9 and 10 introduce a spatial dimension to the temporally evolving hydrographic syntheses. Distance vs. time mapping of layer mean or another characteristic parameter provides an effective way for examining the temporal evolution of an analyzed layer across the Labrador Basin and also for determining the spreading pathways of major signals either arriving from other basins or being formed locally. In particular, Fig. 9 demonstrates the role of the subpolar gyre circulation in the regional transports of heat and salt and thus in the three-dimensional restratification of the Labrador Sea. This figure shows how the average density (upper panels) of the upper (200–800 m) and lower (1200–2000 m) intermediate layers and the corresponding temperature and salinity contributions (middle and lower panels) vary over the entire length of the AR7W section from 1987 to 2005. The values used to construct Fig. 9 were produced by averaging all measurements from individual hydrographic stations fitting into one of two depth (pressure) ranges. The first range, 200– 800 m, represents the upper intermediate layer featuring a quite diverse water mass composition. In particular,

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this layer comprises all shallow LSW classes (e.g., LSW2000) and at times the top segments of the deep LSW classes (e.g., LSW1987–1994 during the 1987–1995 period). The second range, 1200–2000 m, represents the lower intermediate layer containing all deep LSW classes (e.g., LSW1987–1994) and other waters that eventually feed the void left by the draining LSW. The bathymetric depths of all hydrographic stations used in these compilations are shown in Fig. 11. The distances used in this figure match the distances of Fig. 9, so it can be used to relate the contents of Fig. 9 to the key topographic features of the Labrador basin. Projecting the average densities of the upper and lower intermediate layers on the same coordinates (Fig. 9, upper panels) is an effective way to compare and link their changes in both time and distance coordinates. Even though the density plots shown in Fig. 9 (upper panels) map identical values, the inversion of the Density axis (upper right panel) reveals some interesting features hidden in the back of the other (upright) drawing. Responding to the formation of the deep and dense LSW, the lower intermediate layer (blue-tone shaded) of the interior basin reached its highest density in 1993–1995, at the time when its body was mostly occupied by the dense LSW. During this period, the density difference between the interior and the margins of the lower intermediate layer was much smaller than during the years preceding the production of the densest LSW as well as during the following years, when the upper part of this layer was becoming reoccupied by less dense waters. Indeed, in the weak convection years the lower intermediate layer is colder and denser at the boundaries than in the interior of the Labrador Sea. This changed, when the extreme convection of 1993 and 1994 filled the deep reservoir of the central basin with the waters almost as dense as the boundary waters at the same depth, practically erasing the spatial gradients, in turn strongly affecting the layer’s baroclinic currents. In contrast to the fact that the boundaries of the lower intermediate layer are typically denser and more stable in time than its interior, the average density of the upper intermediate layer decreases and shows larger temporal variations in the direction of the basin’s edges. During the high convection years the upper intermediate layer became denser; in the central part its density approached the levels characteristic to the deep intermediate layer of the same years. As soon as winter convection had lost its strength in the mid-1990s, the upper intermediate layer started to become less dense. This density decrease continued until the revived convection of 1999–2000 made the whole layer denser than it was in the two previous years. Very similar to the lower intermediate layer of the years 1993–1995, the upper intermediate layer showed much weaker spatial contrasts in 2000 than it did in the years of weaker convection, meaning a smaller contribution of this layer to the adjacent baroclinic boundary currents in the year 2000. The changes observed in layer-specific densities can be understood if considered as a joint product of the spatial and vertical exchanges and redistributions of heat, fresh-

Fig. 11. Seafloor depths at the AR7W hydrographic stations occupied between 1987 and 2005. The black dots show the depths of individual stations corresponding to the distances used in the Distance axes of Figs. 9 and 10.

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water and salt in the Labrador Sea. The contributing variations of temperature and salinity averaged over the examined layers are discussed below. The upper intermediate layer features two major LSW events since 1987, while the lower intermediate layer shows just one LSW development. Through the early 1990s, both layers had similar temperatures and salinities, because the entire depth range was filled with the developing 1994 class of LSW (LSW1987–1994). After 1994, the upper intermediate layer (200–800 m; Fig. 9, middle panels) showed continued freshening (except near the Greenland coast) as winter convection renewed the upper LSW layers; while the lower intermediate layer (1200–2000 m; Fig. 9, lower panels) exhibited the decay of the LSW1987–1994 through advection and mixing. Since 1994, the deep LSW and the whole lower intermediate layer have been steadily becoming warmer and saltier over the entire basin, whereas the asymmetric and irregular warming of the upper intermediate layer was interrupted by the cooling set by fairly intense winter convection of 1999–2000. In 2000, the western part of the upper intermediate layer was the freshest in 15 years. The May 2004 survey showed that the upper 1500 m layer of the Labrador Sea was the warmest since the first annual occupation of the AR7W hydrographic section in 1990. The most rapid warming of this layer occurred between 2003 and 2004. This warming was not simply a consequence of the warming in the surface and LSW layers. In 2004, a large volume of warm and salty water appeared over the continental slopes on the Greenland and Labrador ends of the section. This water is thought to have come from the Irminger Sea, carried north and west by the Irminger Sea branch of the North Atlantic Current. The upper intermediate layer (Fig. 9, middle panels) shows a rapid increase in temperature and salinity over the whole eastern part of the Labrador Sea between 2003 and 2004. This warm and salty water from the Irminger Sea, seen along the eastern and (in some years) western rims of the Labrador Sea, spread out to the center of the Labrador Basin in 2004, filling the whole eastern part of the basin between 100 and 800 m. As a result, temperature and salinity at 700 m in the eastern part of the Labrador Sea increased by 0.6 C and 0.05 in one year. The 2005 spring occupation did not show a further increase in the volume of these Irminger Sea waters, but the upward trends in both temperature and salinity in the upper 2000 m over most of the AR7W line did persist (Figs. 5–9). The Labrador Sea is approaching the conditions last seen in the late 1960s. If these trends continue, the Labrador Sea temperatures could soon become the warmest ever recorded. Even if the mean water temperature is still lower than the record high (Fig. 2), the sea level (estimated as the steric height, discussed below) in the central region of the Labrador Sea has already approached the record high levels observed between 1969 and 1972. 7. Changes in the deep waters of the Labrador Sea The North Atlantic Deep Water is a hydrographic entity that is commonly used in large-scale analyses and generalizations to identify a vast bulk of the Atlantic water column containing a collection of intermediate, deep and bottom waters formed within the Arctic and Subarctic domains. However, in the case of regional North Atlantic studies this term is rather obscure and we often operate with more specific definitions of water masses, which typify or distinguish the waters by their formation regions, transformation and evolution histories. Both vertical sections of salinity (Fig. 3) and plots showing the depth-dependent temporal evolution of salinity (Fig. 5) and density-dependent h–S curves (Fig. 7) exhibit a relatively salty water mass below the LSW. This is the Northeast Atlantic Deep Water (NEADW). NEADW lies beneath LSW, typically between 2300 and 3300 m; its core can be easily identified as the salinity maximum below 1500 m (Figs. 3 and 5). In the h–S curves shown in Fig. 7, this characteristic salinity maximum is clearly visible in the r2 range confined by 36.98 and 37.05 kg m3. This water mass originates from the dense overflow entering the North Atlantic through the deep trenches in the Iceland–Faeroe–Scotland Ridge, which is commonly known as the Iceland–Scotland Overflow Water (ISOW). ISOW undergoes complex transformation. It acquires its high-saline signature through vigorous mixing with upper warm and salty waters (e.g., the Subpolar Mode Water) in the northern Iceland Basin. Then this overflow entrains fresher LSW while it is descending into the basin’s deep reservoir (Dickson et al., 2002). After ISOW crosses the deep trenches or fractures in the Mid-Atlantic Ridge to enter the Irminger Sea, it mixes with fresher LSW, deep and bottom waters, causing an increase in its volume and a reduction in salinity. The version of this water mass found in the Labrador

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Sea is typically about 3 C warmer than the cold and dense overflow entering the Iceland Basin. Since the properties of ISOW are strongly altered along its spreading pathway, while its thickness tends to increase, it is appropriate to use some other name for the fresher and less dense ISOW-derived water found in the Irminger and Labrador basin. Following Yashayaev et al. (2003) and related large scale studies (Dickson et al., 2002) this water is referred through the present paper as NEADW. Between the 1960s and the late 1990s, the deep layers of the entire subpolar North Atlantic experienced widespread and sustained freshening (Dickson et al., 2002). The associated declines in both salinity and temperature of NEADW are well seen in the h–S curves from 1987 to 2000 (Fig. 7, 36.98 < r2 < 37.05 kg m3). This decrease in salinity is especially clear in Fig. 5 (lower panel), which shows the salinity at 2800 m decreasing from above 34.925 in 1987 to near 34.900 in 2001. The temperature decrease over the record is also evident in Fig. 5 (lower panel) by the slight rise in the isotherms over the record. We estimate the 1987-to-2001 decreases of temperature and salinity in the core of this water mass to be 0.15 C from 2.80 C to 2.65 C and 0.025 from 34.925 to 34.900, respectively. As already mentioned, these changes are the best resolved segments of the persistent declines of NEADW salinity (Dickson et al., 2002) and temperature5 between the end of the 1960s and the beginning of the 2000s. The cause of these trends is the long term freshening of the original overflow combined with the freshening and cooling of the Subpolar Mode and Labrador Sea Waters both mixing into the overflow. The continuing Labrador Sea observations indicate that this NEADW freshening tendency has recently changed. In 2000, NEADW reached its all-time freshest state (Figs. 5 and 7) and since then it has been becoming saltier. Although the individual annual increases of NEADW salinity were small and about equal (Fig. 5), they have been highly persistent, resulting in a significant salinity increase over the last 5 years. Since NEADW arrives from the eastern North Atlantic by the way of the Irminger Sea, the change seen in the Labrador Sea suggests that NEADW was also becoming saltier in the rest of the subpolar North Atlantic. Although NEADW salinity has recently started to recover, the rate of salinity increase is still greater for the remnant of the deep LSW. This change is obvious in the h–S curves in Fig. 7 in which the difference between the LSW salinity minimum and the NEADW salinity maximum has been reduced from about 0.105 in 1987 to 0.020 in 2005. The rapidly decreasing salinity difference of these water masses will inevitably lead to their blending into a single h–S class. The warming of the deep LSW and mixing at the upper boundary of NEADW resulted in the disappearance of the temperature maximum between the cores of these water masses, clearly seen at about 2400 m inside the 2.9 C isotherm between 1990 and 1994 (Fig. 5). This maximum first appeared at 1900–2400 m in the mid1980s as winter convection increased in strength producing the deep LSW that was colder than the underlying NEADW. This layer narrowed, deepened and became colder as convection developed to 2400 m (Fig. 5). As the contrast between the deep LSW and NEADW decreases and the sharp interface separating these water masses diminishes, this maximum also fades away. The deep temperature maximum greatly weakened, diminished or even vanished over most of the section by the 2002 occupation of AR7W. As already mentioned, the disappearance of the temperature maximum can be attributed to the warming of the deep LSW inverting the thermal contrast at the upper boundary of NEADW and mixing between these water masses. On the other hand, the multidecadal freshening of NEADW was also accompanied by its cooling (2500–3200 m, Fig. 5, upper panel), including the upper part of this water mass where the temperature maximum was formerly seen. This cooling tendency made the last pentad the record coldest for NEADW. The relatively cold, dense and fresh water mass below NEADW is the Denmark Strait Overflow Water (DSOW). DSOW enters the Irminger Sea from the Greenland Sea as a narrow, cold and dense flow at the sills of the Denmark Strait. DSOW is the densest water found in the Irminger Sea, Labrador Sea and Newfoundland Basins. It is seen on vertical sections as a thin layer of relatively cold and fresh water found near the bottom of these basins (Fig. 3). This bottom water mass found in the Labrador Sea is always denser than r2 = 37.10 kg m3 (Fig. 7), but the density of its near-bottom core significantly fluctuates in time over the range confined by r2 = 37.145 kg m3 (1993) and r2 = 37.180 kg m3 (1996, 19-year high). Therefore, the core of DSOW is better defined by a range of elevations from the seafloor rather than by ranges of density or other

5

The temperature changes were not discussed by Dickson et al. (2002); however they are well correlated with those in salinity and are equally important in deep ocean studies.

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seawater properties. In this study, the core of DSOW was simply defined as a water layer within a 100 m range off the seafloor. Fig. 10 exhibits how the values of density (upper panel), temperature (middle panels) and salinity averaged over the DSOW core defined as a 100 m thick near bottom layer vary along the AR7W line from 1990 to 2005. Before averaging over this layer the hydrographic measurements were interpolated to the same vertical grid with 2 m spacing between the grid points. The bathymetric depths matching the distances in Fig. 10 are shown in Fig. 11. The latter can be used to link the contents of Fig. 10 with the key topographic features of the Labrador basin. Similar to NEADW, DSOW also exhibits multidecadal cooling and freshening. Between 1965 and 2005 the DSOW in the Labrador Sea cooled and freshened by 0.4 C and 0.05. In addition to the long-term tendencies the near-bottom layer of the DSOW-derived water shows large interannual variations of temperature, salinity and density. These variations have shorter time scales than those in NEADW and even in LSW and can be easily seen in Figs. 5, 7 and 10. The annual h–S curves for the last 11 years (Fig. 7, lower panel) show that the magnitude of changes in the deep waters (r2 > 37.10 kg m3) increases with density and is the largest in the densest nearbottom part of the curves. Recall that these h–S curves (Fig. 7) are based on the entire AR7W annual compilations, so the changes corresponding to the h–S shifts that are seen in the near-bottom core of DSOW (r2 > 37.14 kg m3) become even larger if original CTD profiles are used to compute the near-bottom values. Indeed, the annual compilations (Figs. 5 and 7) moderate the changes introducing year-long basin-wide averaging. The individual profiles reflect passages of specific events at different locations along AR7W, discussed further in this paragraph. Fig. 10 is based on the near-bottom data from original hydrographic profiles and shows a high coherence of the near bottom changes across the Labrador Sea caused by anomalously cold and fresh events (also seen in Fig. 5, 3300–3600 m and in Fig. 7, r2 > 37.14 kg m3). At the same there is a distinct and mostly systematic lag in arriving of the observed DSOW signals that predictably increases toward the Labrador side of AR7W, which is well illustrated by Fig. 10. The mentioned cold, fresh and also reach in dissolved oxygen and CFCs (to be published separately) events arriving from the Irminger Sea, in most cases first show on the eastern side of the Labrador Sea (Fig. 10, large distance values) and exit about a year later from the southwestern part of the basin (negative distances). The most recent cooling and freshening of the DSOW layer was observed in the Labrador Sea in 2005. Although both salinity and temperature signals are fairly strong and distinct, the salinity variations have higher spatial regularity than the corresponding temperature variations; their magnitude also increases at the entrance point on the eastern side of AR7W. Note the earlier appearance of high and low salinities (Fig. 10, lower panel) in the high end range of the distances, followed by arrival of these signals in the other parts of AR7W with a delay, generally increasing to the west. In some cases the signals reach the western side of AR7W before the central basin. This is likely due to faster advection of anomalies with the deep western boundary current along the margins of the Labrador Sea. The passage of such signals through the deep layers of the Labrador Sea combined with oceanographic records from their upstream locations provides a good indicator of the operational state of the abyssal branch of the Atlantic meridional overturning circulation (MOC). 8. Concluding points 8.1. Inventory of hydrographic extremes: two past, one imminent The observational results presented in this paper help to understand how the recent water mass developments and changes recorded along the AR7W repeat hydrography section contributed to the current state of thermohaline oceanographic conditions in the Labrador Sea. These developments have added another full cycle to the longer-term observational record. At the same time, their magnitude, rates and impacts on the subpolar hydrography and ecosystem are rather unique. The warmer and saltier conditions recorded through the mid-1960s – early 1970s and the fresher and colder conditions recorded through the late 1980s – mid-1990s define two extreme states of both LSW and the entire water column in the history of reliable hydrographic observations in the Labrador Sea that began in the 1930s. In fact, the changes in LSW are the most notable in the whole subpolar water column. The longer-term hydrographic implications of the discussed LSW development of 1987–1994 are exceptional: the LSW progressively formed during these years was colder, fresher, denser and deeper than in any previous deep measurements in the Labrador Sea; from 1970 to 1994 (almost a 25-year period), the LSW

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became 0.08 fresher, 0.9 C colder and 0.08 kg m3 denser, and its thickness more than doubled. This signal is a part of what Dickson et al. (2002) called ‘‘the largest change in the modern oceanographic record’’. For most parts, the Labrador Sea is now rapidly becoming saltier and warmer: the net change of its salinity over the past decade is equivalent to a removal or an extraction of a layer of fresh water that is 2 m thick (Fig. 2), while over the same period of time the upper 2000 m layer have warmed up by 0.7 C. These recent tendencies contrast with the previous freshening of the whole Labrador Sea water column, equivalent to adding an extra 2.5 m layer of fresh water (Fig. 2, 1985–1994), accompanied by 0.7 C cooling of the upper 2000 m layer. The major agent of these changes, the Labrador Sea Water (LSW), is not just found in the Labrador Sea – it is the principal intermediate water mass of the subpolar gyre of the North Atlantic and spreads southward into the sub-tropical gyre and beyond within the deep western boundary current (Talley and McCartney, 1982; Yashayaev et al., 2004, 2007). These changes have already been seen or will soon to be seen in fairly distant regions of the Atlantic Ocean. 8.2. Restratification of the Labrador Sea water column and possible implications for the ocean circulation Within the subpolar gyre, LSW can be easily identified as a layer of low salinity and stability below 500 m. During the milder years most of this cold, fresh and dense LSW was mixed into the boundary currents and drained away from the region while the remaining portion became warmer and saltier as the warmer and saltier intermediate waters bordering the sea were mixed toward the centre. The loss of the LSW led to a restratification and complete restructuring of the upper 2000 m layer across the sea. This was marked by uncompensating changes in temperature and salinity changing the gradients in the layer-averaged density (Fig. 9) and, therefore, mid-depth circulation. The year-to-year changes in spatial and vertical density gradients discussed with Fig. 9 imply that the upper, intermediate and deep water currents of the Labrador Sea including, for example, its major cyclonic circulation and prominent anticyclonic recirculation gyres experienced significant changes over the past two decades. In particular, the boundary currents seemed to be more uniform showing a weak vertical geostrophic shear in the years of strong convection homogenizing the upper 2000 m column. The other informative dynamic and kinematic index is potential vorticity, presented here by two its proxies – inverse vertical density gradient (Fig. 5, lower panel) and thickness of r2 layers (Figs. 4, right, 6–8). Taken as they are or integrated over a characteristic layer or even a full water column, these two closely related measures of vertical stability and stratification can be used to construct new valuable indexes for the monitoring and diagnosis of the general ocean circulation, water mass renewal, transports and inter-gyre exchange. These stratification indexes will also complement those based on the sea surface height measurements (e.g., Fig. 2; Hakkinen and Rhines, 2004). The observed basin-wide changes in the water mass properties and stratification caused quite notable variations in the Labrador Sea steric height. A similar signal can be seen in the observed sea level (Fig. 2), computed from the sea level anomalies extracted from the globally gridded sea level maps (1/3 · 1/3). These maps (downloaded from http://www.jason.oceanobs.com/html/donnees/) are based on the observations available from the Topex/Poseidon and Jason satellite missions since 1992. The sea level height anomalies used to construct the time series shown in Fig. 2 are referenced to a seven-year mean. Temporal variations of the steric height and sea level agree well and reflect most of the major changes seen in the Labrador Sea hydrography. The intense cooling of the early 1990s resulted in a 10-cm drop of steric height in the central Labrador Sea, and the subsequent restratification over the past decade raised the steric height to its earlier levels. The warming observed between 1994 and 2005 led to the sea level rise. In 2005, the sea level in the central Labrador Sea was about 7–8 cm higher than in 1994, approaching the record highest level observed between the late 1960s and the early 1970s. 8.3. Impact of LSW on the deeper layers The steady freshening of the NEADW came to its end in 2000–2001 and since then this water is steadily becoming saltier. Remarkably, the reversal in salinity trend happened in this water about 10–12 years later than the deep LSW class (LSW1987–1994) stopped becoming fresher and started accumulating salt (Fig. 5). The LSW transit time to the Iceland Basin is about 5 years (Yashayaev et al., 2007). It probably takes between

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5 and 8 years for the NEADW signals to travel from the Iceland Basin to the Labrador Sea (Boessenkool et al., accepted for publication). Now, if LSW is a principal contributor to NEADW, than a reversing salinity trend in the former will have a response in the salinity of the latter. Consequently, a 10–12 year delay in the noted trend reversals can be reasonably linked to the next three-step process: (1) Transit of LSW from the Labrador Sea to the Iceland Basin ! (2) Entrainment of LSW by the Iceland–Scotland Overflow Water (ISOW) and subsequent transformation of ISOW into fresher NEADW ! (3) Transit of NEADW to the Labrador Sea. This hypothesis is now being tested by analyzing the ISOW–NEADW salinities, depths and thicknesses compiled along the entire subpolar pathway of ISOW–NEADW for the past two decades. DSOW shows temperature, salinity and density variations that are larger than the observed variations in NEADW. The DSOW-characteristic signals are strong short-term freshening and cooling events spreading through the near-bottom reservoir of the Labrador Sea from east to west. There were four such cold and fresh DSOW events over the past 17 years. Deep convection does not just create salinity, temperature and density anomalies at the intermediate depths (Fig. 9) through the formation of LSW; it also carries climatologically and biologically active substances such as greenhouse gases and nutrients out of the seasonally active surface and subsurface layers (Fig. 8). Because the LSW is found throughout the subpolar gyre (Fig. 4), it is also mixed with the various Arctic-derived components, DSOW and NEADW, when these waters enter the North Atlantic through the Greenland–Iceland– Faeroe–Scotland Ridge. These dense waters from the Nordic seas enter the North Atlantic at the depths of less than 1000 m and form boundary currents as they descend the continental slope to their equilibrium depths. There is a great deal of mixing associated with these overflows, and one of the water masses that the overflows are mixed with is the LSW. The regions with a thick LSW layer, indicated by the arrow-headed export pathways in Fig. 4, probably show higher mixing rates of LSW and the deeper waters. Through this mixing and, ultimately, by entering the North Atlantic Deep Water via LSW, DSOW and NEADW, freshwater, greenhouse gases, nutrients and other substances descent into the abyssal oceans, which serve as the deep reservoirs for such substances. This, in fact, presents an effective pathway for the named substances from the surface layers to the deep ocean where such can be isolated from the atmosphere for centuries. This pathway requires winter convection that occurs to depths greater than 1000 m in the Labrador Sea. However, it would continue to function for some years following the cessation of the convective renewal. Indeed, by compiling and examining the most complete hydrographic archive of annual occupations of the AR7 section crossing all subpolar basins Yashayaev et al. (2004, 2007) concluded that it took about 2 and 5 years for the newly formed LSW1987–1994 to arrive in the northern Irminger and Iceland basins, respectively. The maps presented in Fig. 12 add another dimension to the discussion sparked earlier in this chapter on the role of LSW in formation, development and control of the deeper components of the North Atlantic Deep Water, specifically NEADW. The sequence of NEADW transformations is quite complex. This water intensively freshens and gains thickness on its entry into the Irminger Sea. In the northern and western parts of the Labrador Sea it experiences even more freshening, but in the Newfoundland Basin it becomes saltier again. Such a reversal in the along-path salinity tendency for the inshore NEADW suggests that a significant fraction of NEADW advects from the Charlie–Gibbs Fracture zone or the Irminger Sea directly to the Newfoundland Basin without entering the Labrador Sea. In the Newfoundland Basin this saltier version of NEADW mixes with the fresher inshore modifications of NEADW arriving from the Labrador Sea. What facilitates such NEADW remixing in the Newfoundland Basin is its characteristic anticyclonic circulation gyre, known as the Mann Eddy. The Mann Eddy spins NEADW and also DSOW below forming a local recirculation of these waters. The increased depth, density and volume of LSW led to considerable changes in the mixing rates, properties and overall appearance of NEADW. This water mass became substantially fresher between the late 1960s and the mid-1990s (Fig. 12c), showing extreme salinity decreases in the northern Irminger Sea and along the boundaries of the Labrador basin, where in addition to LSW, NEADW vigorously entrains DSOW. In the Labrador and Irminger basins NEADW notably thinned between the two periods (Fig. 12d). The main agent for such a volume loss was LSW that between the late 1960 and the mid-1990s substantially expanded vertically (Fig. 4). If we assume that the entrainment rates did not change over the years (in fact, because of the LSW expansion these rates probably increased, implying even greater freshening), than a thinner volume of NEADW would experience larger freshening. Indeed, the NEADW freshening in the Labrador Sea was about there times larger than that in the Iceland Basin. Finally, the average depth of

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Fig. 12. (a and b) Maps of the subpolar North Atlantic showing salinity on the r2 = 37.00 kg m3 surface representing the deep salinity maximum associated with the Northeast Atlantic Deep Water (NEADW) in the late 1960s, 1964–1972 (a), and in the mid-1990s, 1995– 1997 (b). (c and d) Changes in salinity (c) and pressure (d) of the NEADW-characteristic (r2 = 37.00 kg m3) layer between the late 1960s and the mid-1990s.

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r2 = 37.00 kg m3 increased in the western basins by about 260 m, suggesting that the density at the pressure levels occupied by this water decreased. 8.4. The LSW production rates: two approaches – two results The impact of deep convection of the late 1980s – mid-1990s has spread beyond the intermediate depths and far away from its source (Fig. 4). This raises two questions: How much LSW was actually produced during the years of strong convection? and What role did this bulk of LSW play in maintaining or altering the strength and pathways of the Atlantic MOC? While the second question is up for a more extensive study outside the scope of the present paper, the first can be attempted here. The LSW volume production rate is the most unknown and also challenging characteristic of the best explored water mass of the North Atlantic. Indeed, according to Haine et al. (submitted), who compiled most of the published estimates of such rates, these vary from 2 to 12.7 Sv (106 m3/s). We see four obvious reasons for such a broad scatter in the rates: (1) different time interval, seasons and locations; (2) different temporal and spatial averaging; (3) different methodological, instrumental and computational approaches; and (4) different criteria used to identify LSW. In particular, the closing part of Methods and definitions C discusses sensitivity of LSW characteristics to a definition selection. Both of the following estimates arrive from volumetric techniques, but are built on different initial assumption. The first approach looks in two large-scale hydrographic surveys presented in Fig. 4 and assumes that the observed LSW volume change is a result of a constant increase in the LSW production over 25 years passed between these surveys. The second approach operates with much better temporally resolved hydrographic surveys, but lacks the spatial broadness of the first method. Because the North Atlantic was well surveyed in 1995–1997 as part of WOCE (coincidentally and fortunately following the years of the most voluminous LSW production), and back in 1964–1972 (these years happen to feature the record low LSW volumes), the net LSW volume increase caused by excessive LSW production before the mid-1990s and related seawater property changes can be properly estimated (Fig. 4). The difference in the LSW thickness between these two surveys (Fig. 4, right column) was integrated over the examined domain to estimate the total LSW accumulation in the region. Then assuming that convective ventilation of the intermediate depth providing this net increase took place over the entire 25-year period (1970–1995), we can estimate the mean annual LSW production rate over this period, which is about 2 Sv (106 m3/s). However, as shown earlier in the paper, LSW production is not a continuously sustained steady process, but rather episodic by its nature, meaning that a much larger LSW volume can be produced in a year with intense winter convection than the volume equivalent to the 2 Sv production rate. Furthermore, there are reasons to believe that such an estimate is considerably lower than the true mean rate of LSW production. The underestimation in the LSW production rate is due to several reasons. We just mention here that a portion of newly formed LSW taken by the equatorward currents rapidly drains out from the subpolar North Atlantic, while the LSW staying in the subpolar region contributes to other water masses via mixing and entrainment disseminating spatially and over a broader water column. A more realistic estimate of the LSW production rates can be derived from annual r2 and h–S volumetric censuses based on systematic hydrographic surveys of the regions, which collectively capture most of visible LSW and therefore can be used to compute annual LSW production rates. Such regions jointly form a reservoir receiving and storing most of an excessive LSW volume within two years after its formation. Observational evidence for this (Yashayaev et al., 2007) is in surprisingly good agreement with a model study (Straneo et al., 2003). The LSW thickness maps (Fig. 4) also support the idea that the deep Labrador and Irminger basin jointly form a good proxy for the principal reservoir of LSW. It takes notably more time for a newly produced surplus of LSW to drain from its subpolar reservoir (Fig. 8; Lazier et al., 2002; Yashayaev et al., 2007), implying that the exporting function of the major LSW reservoir is not as strongly controlled by LSW production as by the volume of LSW itself. This, in fact, offers a key to the estimation of export and consequently production rates of some prominent LSW classes. Such rates for the LSW1987–1994 class were obtained as follows. Annual values of the LSW1987–1994 basin-survey mean thickness for the Labrador (Figs. 6 and 8) and Irminger Seas were used to calculate year-to-year changes in the total volume of LSW1987–1994 stored in each basin. These mean LSW thicknesses were actually estimated for the repeat hydrography sections crossing

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the Labrador and Irminger Seas, AR7W and AR7E, respectively. The total volumes of LSW could be adequately estimated only for the full-basin hydrographic surveys, which are rare. However, it is reasonable to assume that these two measures of LSW mightiness are tightly correlated and to employ a simple linear model to translate the LSW section-mean thickness to basin-wide volumes values. The LSW thickness-to-volume conversion coefficients, linear slope and offset, were computed separately for the Labrador and Irminger basins using four reference thickness-volume pairs for each basin. These reference values were based on basin-scale hydrographic surveys, two are presented in Fig. 4. A compilation of the annual LSW1987–1994 volume changes in the LSW reservoir basins ‘‘measured’’ as a volume flux is shown in Fig. 13. This diagram facilitates the following discussion on the LSW export, accumulation and production rates. As the next step, the annual volumetric censuses for the years 1994–2005 showing no visible renewal of LSW1987–1994 (Figs. 5–8) and, therefore, representing an almost ideal case of its draining reservoir, were used to build a relationship between the year-to-year volume losses (negative annual changes) and annual volumes of LSW1987–1994. Assuming that a significant thinning or volume loss of dense LSW in a year with weak convective renewal is mostly a result of draining or exporting this water out of the basin of study, one evidently concludes that such a volume loss is a direct measure of the LSW export rate. If convection did happen to add to denser LSW, which could not have been much, because its layer was systematically thinning since 1994, then the actual export rates were somewhat higher than based on the volume loss. This reasoning allowed us to link the LSW1987–1994 export rate (derived from annual volume loss) with the volume of LSW1987–1994 available in the intermediate reservoirs of the Labrador and Irminger seas. Assuming that the found relationship between the LSW volume and export rates is applicable to the years preceding 1994–1995, one can reconstruct all annual LSW losses to its export or export rates during the years of active convective renewal of this water just by using the LSW volume series (Fig. 8), thus defining one of the unknowns needed in the estimation of production rates in a strong convection year. As the next step, the annual LSW1987–1994 volume increases (positive annual changes) were computed for 1987–1994 (Fig. 13). These were the years of strong or even intensifying convection, leading to progressive deepening of the LSW1987–1994 class and resulting in accumulation of this water (Figs. 5–8) in the Labrador and Irminger Seas. In the context of the present discussion this means that LSW was produced in larger quantities than could be exported or removed from the region in any other way between two consecutive convection

Fig. 13. The annual volume changes of LSW1987–1994 converted to volume flux (1 Sv = 106 m3/s). The estimates for the Labrador and Irminger basins are stacked together to show the corresponding changes in the main subpolar reservoir of LSW. The positive rates imply LSW accumulation, while the negative indicate its loss due to export, mixing, etc.

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seasons. The annual LSW1987–1994 accumulation rates decreased over the first five years of our annual surveys in the Labrador Sea from 2.25 to 0.75 Sv. However, this seemingly accumulation rate drop in the Labrador Sea was practically fully compensated by a greater accumulation in the Irminger Sea seen in the last two years of this eventful pentad, jointly with the Labrador Sea maintaining 2 Sv. In addition, as already mentioned, the increased LSW volume in 1992–1994 also meant a high export (5 Sv), implying that the LSW production was still at its top levels until 1994. Finally, the LSW1987–1994 production rate for each year with progressing or deepening convection was simply calculated as a sum of the accumulation and export rates estimated for a given year. When LSW did not show any annual change, loss or gain in its volumes, the LSW export and production were assumed to compensate one another and the production rate was assumed the same as the export rate. To remind, the export rates were reconstructed for the years 1987–1994 by using the volume-export ratio from the post-deep-convection years. This simple approach shows that for the years 1987–1994 with strong deep convection the average annual LSW production rate was 4.5 Sv. At the same time, the individual rates for 1992–1994 could possibly be on the order of 7.0 Sv or even higher. Impressively, this is about a third to a half of what is commonly accepted as the average rate of the North Atlantic overturning circulation (13–16 Sv, IPCC 4th assessment). In reality, the LSW production rates were probably even higher than those obtained by summation of the two considered components: a significant part of LSW might escape its designated reservoir or be taken by diapycnic mixing and entrainment during and shortly after convection thus being left out of our LSW inventories, which were mostly based on the spring and summer occupations of AR7W. This contemplation lifts up three tasks for prospective collaborations on the North Atlantic circulation and climate change studies. The first task would be to determine the large scale and full depth impacts of the rapid increase and subsequent decline in the most massive LSW production of the 1990s. The second task would address climatic significance and impacts of shallower but fairly voluminous LSW (e.g., LSW2000) and other intermediate water mass production. The last task would be devoted to finding the most optimal and efficient ways and working protocols for collaborative data acquisition, consolidation and analysis improving our collective abilities to identify individual water masses and detect their impacts on the hydrography and circulation of the North Atlantic Ocean, and, ultimately, on the global ocean climate. The 17-year series of annual occupations of the Labrador Sea repeat hydrography section AR7W, first undertaken as a contribution to WOCE and then continued as part of the CLIVAR and Global Ocean Observing System (GOOS) projects, is key to observing the interannual variability of the North Atlantic within and near the high latitude source regions of the MOC. The value of this program to the understanding of ocean climate lies in the long-term sustained nature of the sampling because the variability is significant over years, decades and even longer. Lately, the importance of the Labrador Sea monitoring was recaptured by the internationally led and highly collaborative Arctic–Subarctic Ocean Flux (ASOF) research initiative. ASOF, aiming ‘‘to measure and model the variability of fluxes between the Arctic Ocean and the Atlantic Ocean with the view to implementing a longer-term system of critical measurements needed to understand the high-latitude ocean’s steering role in decadal climate variability’’ (http://asof.npolar.no/), considered the Labrador Sea as a key receiving basin transferring the signal arriving from the rapidly changing Arctic to the rest of the Atlantic Ocean. The full water column changes observed in the Labrador Sea report on the entire system of water mass development and transformation, ocean dynamics and climate change spanning over the subpolar North Atlantic. Thorough description and systematization of these signals combined with analogous studies for other basins form a solid observational base for ‘‘tuning’’ and verification of a variety of ocean and climate models. The themes broad in the paper will hopefully be revisited in the near future and a further advance will be made in observing and understanding the changing Labrador Sea, including its strong ties with the circulation and climate of the Atlantic Ocean. 9. Methods and definitions A: Construction of time series Temporal evolution of a whole water column can be effectively visualized by producing an average or characteristic vertical profile of a given seawater property for each year or hydrographic survey and mapping a succession of such profiles in the time-depth coordinates.

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Fig. 5 shows a progression of annually averaged vertical profiles constructed for the central region of the Labrador Sea. The central Labrador Sea was defined for this task by the bottom depth exceeding 3300 and by the horizontal distance range from the AR7W line not exceeding 150 km (Fig. 1). Each characteristic vertical profile used in the compilation was formed by robust averaging of temperature, salinity, pressure (depth) and Dr2 = 0.01 kg m3 layer thickness (r2 is potential density anomaly referenced to 2000 dbar). This averaging was performed individually for each calendar year with available observations and over each r2 bin (layer), predefined by Dr2 = 0.005 kg m3. If a certain r2 bin did not have enough data points to produce reliable estimates, it was incrementally expanded in size until it contained a sufficient number of observations or its range exceeded 0.020 kg m3. In order to better identify a year or survey typical state and also minimize the contributions of data outliers, eddies and other small scale irregularities to this synthetic state, the technique of robust averaging used in this study involved two statistical weights that needed to be applied to each value found in a studied bin. The first weight or so-called biweight ‘‘measured’’ the ‘‘closeness’’ of individual values to their group median, helping to suppress various effects of uncharacteristic signals (even those of large magnitude) and therefore increase statistic robustness of the method’s outcome. The biweights constructed in a variety of ways are widely used in experimental data processing (Wilcox, 1997). Their values are most commonly defined via an inverse function of absolute deviation from a median over a group of samples selected for averaging. This function yields zero if individual absolute deviation is greater than group median absolute deviation multiplied by a certain coefficient regulating the method’s sensitivity to uncharacteristic samples, ultimately allowing to fully suppress the contribution of such samples to the result. The other weights account for uneven vertical spacing of observations and are generally proportional to separation between measurements. Finally, the estimates produced for r2 bins were projected to a depth/pressure grid with 5 m/dbar step, forming a uniform time-depth array that permits accurate mapping. 10. Methods and definitions B: Methods of water mass analysis Volumetric (statistical) oceanographic analysis or census of a selected domain, basin or reservoir remains the most effective and accurate tool for estimation of the relative contribution of a specific water mass (type, class, etc.) to the analysed volume. The volumetric approach also provides a comprehensive and scrupulous picture of temporal and spatial water mass development and transformation in both regional and ocean-wide water columns. This is achieved by mapping cumulative volumes of individual layers defined by preset (typically uniform) intervals of one (e.g., density, Fig. 6a) or several (e.g., temperature and salinity, Fig. 6b) seawater properties. Since the volume of a given layer is a simple product of its average thickness and the studied region’s area, the term ‘volumetric analysis’ is also used when dealing with layer thickness (Fig. 6). The AR7W section well represents all hydrographic constituencies of the Labrador Sea (Fig. 1) and ‘‘senses’’ changes in such. This notion supported by several full-basin surveys allows the layer thickness-to-volume conversion for each AR7W occupation. Note that the relative contribution of a certain layer to the total basin’s volume can be approximated by the corresponding ratio of the average thickness of this layer to the average bottom depth. A convectively formed water mass, e.g., LSW, can be reliably identified and monitored by the density layer volumetric method, which essence can be expressed by plotting basin-mean thickness (in m) of individual density layers (Dr2 = 0.01 kg m3) in density–time coordinates. Such a diagram for the Labrador Sea is presented in Fig. 6a. It was constructed by averaging layer thicknesses from individual hydrographic stations (in a single year or survey) weighted by the distance or area represented by these stations. A variety of temperature–salinity (T–S or, in this paper, h–S) projections of hydrographic data can also be used for identification of water masses and for more thorough (than the density volumetric method) examination of their ‘‘life cycle’’. Each of these cycles may include the phases of water mass production, development, transformation, advection, spreading, mixing and decay. The latter can be a result of water mass diluting, discharge or both. In addition to this, T–S (hereafter, h–S) analysis and, particularly, its volumetric editions are the most effective means of studying spreading and transformation of water masses, freshwater and heat, and detecting passages of watermass signals from one basin to another. The layers examined by a volumetric h–S analysis are commonly defined by two-dimensional h–S intervals, which in this study span over 0.1 C · 0.01 (Dh · DS) and are set by 12Dh and 12DS in the corresponding directions. This approach was applied

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to the Labrador Sea hydrographic data collection to produce annual volumetric h–S projections. Average vertical thicknesses (in m) of the 0.1 C · 0.01 h–S layers for three annual Labrador Sea surveys are shown in Fig. 6b. The h–S curves shown in Fig. 7 represent yearly typical hydrographic conditions in the Labrador Sea for the years 1987–2005. The curves are organized in two panels to reflect the two principal phases in the LSW1987–1994 history: the progressive development of this water mass (1987–1994), followed by its rapid decline and subsequent decay (1994–2005). First, all available measurements within a 50 km range of AR7W were used to construct annual volumetric h–S diagrams (0.1 C · 0.01 h–S intervals set by 0.05 C and 0.005). Each annual h–S curve comprises h–S points which in their turn are created by weighted averaging of h–S values over individual r2 intervals. The r2 intervals (representing r2 bins) used in this averaging were set by 0.005 kg m3 and had a span of 0.010 kg m3. In the case of insufficient data coverage the span of a given r2 interval was set to increase incrementally by 0.002 kg m3 at a time until this interval contained a required number of valid h–S classes or exceeded 0.020 kg m3. As its input values, this procedure of h–S averaging uses an annual volumetric h–S census, which, in its turn, is comprised of average thickness of h–S layers (defined by Dh · DS = 0.1 C · 0.01 intervals offset by 12Dh and 12DS). Each of the h–S intervals and its mid-point temperature and salinity values were assigned a weight defined through a certain function of the corresponding layer thickness. A good choice for this function, determining the thickness-to-weight conversion method, is a second degree polynomial function, achieving the least noise in the resulting h–S curves (Fig. 7) and, at the same time, providing a good match between these curves and the volumetric h–S ridges (Fig. 6b). 11. Methods and definitions C: Identification of LSW cores and classes All distinguishable isolated LSW formations can be consequently identified in the time series of vertical profiles (Fig. 5), in the corresponding density (r2, Fig. 6a) and potential temperature–salinity (h–S, Fig. 6b) volumetric censuses, and also in the compilation of h–S curves (Fig. 7) where each curve was constructed from an annual volumetric h–S projection (the procedure is described in Methods and definitions B). The named methods applied to each basin-survey reveal all principal water masses residing in the Labrador Sea; however, the r2 and h–S volumetric approaches remain the most efficient tools for identification and examination of a convectivelyformed water mass, namely, LSW. Even though the identifications of LSW by r2 and h–S volumetric methods are nearly identical, these two methods complement one another. While the first method (Fig. 6a) jointly with the time series of r2 layer thickness profiles (Fig. 5, lower panel) progressively documents build-up, development and transformation of mode density (r2) layers, the second method (h–S analysis, Fig. 6b) precisely identifies specific LSW formations distinguishable in each AR7W occupation. A volumetric peak with its h–S–r2 coordinates, area and also integrated and mean heights identifies a specific LSW formation, while the peak’s maximum or (if this maximum is vague) its central point represents the core of the examined water mass. Each prominent LSW class has been individually identified at each station of a basin-survey by a Dr2 = ±0.017 kg m3 range (for some tasks h–S or combined h–S and r2 ranges were used) centered at its volumetric core that was previously detected in a full-column volumetric census of this basin-survey. A compilation of such censuses for the Labrador Sea is shown in Fig. 6a; examples of the LSW core r2 are 36.916|1990 and 36.940|1993, (the subscript indicates a year of survey). These individual density ranges collectively form a historic density range of LSW (Fig. 6a). All annual volumetric peaks like those shown in both Figs. 6a and b (the volumetric h–S analysis was conducted for all years with hydrographic surveys) can be grouped into two continuous progressions, each reflecting year-to-year development and transformation of a given LSW core. This grouping introduces an LSW class – a sequence of LSW types with a common development history. The annual hydrographic monitoring of the Labrador Sea, allowed us to build a complete history of formation, development, evolution and decay of two well-defined LSW classes observed during the past two decades. The first class, LSW1987–1994 (the subscript indicates a time interval over which this LSW had likely been formed), is associated with the most extraordinary documented LSW production (Figs. 5–8). Record voluminous in 1994, it has strongly diminished over the past twelve years, recently becoming barely identifiable in the volumetric diagrams. The other LSW class, LSW2000 (Figs. 5–8), was formed in 2000. It is found at shallower depths and lower densities than LSW1987–1994. These two classes are named here by the years when they have achieved their coldest,

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densest (note temporal r2 changes in each class) and most voluminous states. When a certain LSW class loses its volumetric prominence (LSW1987–1994 of the recent years) some supporting criteria can be used to validate and refine its volumetric definitions. Among these are salinity (Fig. 3, lower row), temperature (Fig. 3, upper row) and potential vorticity minima (Fig. 5, lower panel), oxygen maximum (Clarke and Coote, 1988) and the anthropogenic tracers (Azetsu-Scott et al., 2003). Even when the thinning remnants of highly modified LSW1987–1994 form weaker volumetric maxima, the mentioned relative (local) extrema are in good agreement with the volumetric definitions endorsing the volumetric identification of the LSW1987–1994 at different stages of its transformation. To compute basin-survey typical LSW values (e.g., Fig. 8), the analyzed measurements were weight-averaged over LSW density ranges determined previously for each basin-survey. This computation involved two types of weights. The first weight reflects the depth-distance area represented by a measurement brought into averaging, while the second weigh (biweight) measures how ‘‘close’’ is the point’s h–S index to the h–S index of the corresponding LSW core. 11.1. Can a time-invariant density range be used to identify LSW? Yashayaev et al. (2007) raised a concern about identification of LSW by Kieke et al. (2006). Here we examine how the use of ‘‘classical’’ and ‘‘upper’’ LSW, defined by potential density ranges of 27.74–27.80 and 27.68– 27.74 kg m3 (Kieke et al., 2006), could misrepresent or ‘‘derail’’ both recent and historic LSW formations. Firstly, if combined, these two layers (see the 27.68–27.80 kg m3 layer in Fig. 3) fill almost the entire top 2000–2200 m reservoir of the Labrador Sea with ‘‘LSW’’ leaving no room for other known upper and intermediate water masses, arriving from outside the Labrador Sea to replace draining LSW. This, for example, is at odds with the observed receiving of the Icelandic Slope Water (ISW) by the Labrador Sea (Yashayaev et al., 2007). ISW can be clearly seen between LSW and LSW2000 in the 2005 section shown in Fig. 3. Secondly, LSW does change its density in time (Figs. 5–8), occasionally ‘‘crossing’’ the 27.74 kg m3 boundary. In fact, upper LSW, defined by Kieke et al. (2006) as 27.68–27.74 kg m3, missed a significant part of LSW2000 that we recognize as a unique LSW class formed over the years 2000–2002 (Fig. 3). Instead the portion of our LSW2000 that escaped their upper LSW was included into their classical LSW. Thirdly, the density range defining classical LSW is excessively broad, resulting in this water to remain as thick as 1000 m even a decade after its renewal stopped, at odds with the recent hydrographic section (Fig. 3) and other analyses (e.g., Fig. 5). This excessively high thickness of classical LSW is likely to result in systematic overestimation of the LSW formation rates. Indeed, the ‘‘combined’’ (classical + upper) LSW thickness shows little change over 55 years (Kieke et al., 2006) and the implications of this would be that LSW formation rates are rather invariant and that LSW production does not have strong interannual variability, which are largely inconsistent with the atmospheric data (e.g., the NAO series) and also at odds with our observations and interpretation. Fourthly, this ‘‘combined’’ LSW also includes fresher and salty upper waters, note 27.68 kg m3 in Fig. 3, characterized by larger spatial and temporal variations than LSW, threatening to ‘‘contaminate’’ results. Finally, since the LSW development of the 1990s was historically unprecedented it is probably inappropriate to label this unique formation as ‘‘classical’’. Acknowledgements Allyn Clarke, Dan Wright, John Loder, Brian Petrie (BIO), Jean-Claude Therriault, Laure Devine (Institut Maurice Lamontagne, Que., Canada) and Detlef Quadfasel (University of Hamburg, Germany) provided valuable comments and helpful suggestions during the work on the manuscript. The AR7E (A1E) hydrographic data used to calculate annual changes of the LSW volume in the Irminger Sea shown in Fig. 13 are courtesy of Manfred Bersch (University of Hamburg, Germany) and Hendrik van Aken (Royal Netherlands Institute for Sea Research). The author also thanks Robert Gershey (BDR Research) and Kumiko Azetsu-Scott (BIO) who kindly provided the Chlorofluorocarbons (CFCs) measurements used to construct the F11 and F12 series for the LSW inventory presented in Fig. 8. The sea level anomalies from the Topex/Poseidon and Jason satellite missions were downloaded from the website hosted by Aviso, Observing the Ocean from Space in France (http://www.jason.oceanobs.com/html/ donnees/produits/msla_uk.html).

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