Evolution of the Arctic Ocean boundary current north of the Siberian shelves

Journal of Marine Systems 25 Ž2000. 77–99 www.elsevier.nlrlocaterjmarsys

Evolution of the Arctic Ocean boundary current north of the Siberian shelves Bert Rudels a,) , Robin D. Muench b, John Gunn b, Ursula Schauer c , Hans J. Friedrich d a

c

Finnish Institute of Marine Research, Lyypekinkuja 3A, P.O. Box 33, FIN-00931 Helsinki, Finland b Earth and Space Research, 1910 FairÕiew East, Suite 102, Seattle, WA 98102-3620, USA Alfred-Wegener-Institut fur ¨ Polar- und Meeresforschung, Postfach 120161, D-27515 BremerhaÕen, Germany d Institut fur ¨ Meereskunde der UniÕersitat ¨ Hamburg, Troplowitzstraße 7, D-22529 Hamburg, Germany Received 22 September 1998; accepted 10 January 2000

Abstract The Arctic Mediterranean Sea is the most important source for the North Atlantic Deep Water, and the Arctic Ocean, often neglected in this respect, may provide a significant amount of the overflow waters crossing the Greenland–Scotland Ridge. Warm water from the south enters the Arctic Ocean through two main passages, Fram Strait and the Barents Sea, and the inward flowing boundary current that overlies the Eurasian continental slope of the Arctic Ocean supplies heat to the Arctic Ocean and exerts a dominant influence over its internal temperature and salinity characteristics. Major transformations of the inflow occur in the Barents Sea and as the two inflow branches meet in the boundary current north of the Kara Sea their characteristics are different. Lateral mixing between the two branches dominates the further transformations of the Atlantic and intermediate layers occurring in the Eurasian Basin. Ice formation, brine rejection and dense water formation on the shelves and subsequent convection down the slope lead to transformation of the boundary current that crosses the Lomonosov Ridge, and determine the properties of the Canadian Basin water column. Changes in the inflow characteristics of the boundary current will gradually, but slowly, affect also the intermediate and deep-water characteristics of the water column in the interior of the Canadian Basin. In the Eurasian Basin the influences of the shelf processes and pure slope convection are smaller and the water mass characteristics are mostly determined by advection and mixing of the two inflows. Only in the deepest part of the water column does slope convection appear to dominate the water mass transformations. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Arctic Ocean; hydrography; water masses; continental shelf processes; convection

1. Introduction The meridional overturning circulation that ventilates the deeper layers of the world ocean is driven ) Corresponding author. Tel.: q358-9-6139-4428; fax: q358-9331025. E-mail address: [email protected] ŽB. Rudels..

by a combination of surface heat loss and subsequent convection of dense water over small areas at high latitudes, and upwelling occurring almost everywhere in the oceans ŽMunk and Wunsch, 1998.. This results in meridional advection of warm surface water and transport of oceanic sensible heat from low to high latitudes. Combined with different circulation gyres, it also leads to large zonal differences in

0924-7963r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 7 9 6 3 Ž 0 0 . 0 0 0 0 9 - 9

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climate between the eastern and western ocean margins. This difference is especially stark between northwestern Europe and northeastern North Amer-

ica because of the extreme northward penetration of warm surface water, partly derived from the North Atlantic subtropical gyre, which, after crossing the

Fig. 1. Ža. Map of Arctic Mediterranean Sea showing the positions of the 1995 Polarstern stations in the eastern and the 1991 Oden stations in the western Eurasian Basin. Two coordinate systems are used. The maps show the latitudes and longitudes of the familiar geographical system. The asymmetrical convergence of the meridians toward the pole and the slight distortion of the latitude circles arise from the fact that the maps are Mercator projections in a transformed spherical system. This second system was originally introduced for numerical models of the world ocean by putting both poles on land masses, thus avoiding metric singularities and allowing for cyclic lateral boundary conditions. The positive pole in this system is located at 308N; 1158E ŽChina.. The zero meridian is taken to pass through Greenwich, and the ‘‘longitude’’ is counted negative to the left. Our area of interest, the northern North Atlantic with the Arctic Mediterranean Sea, shows small distortions in this system in comparison to more commonplace presentations. The latitudes and longitudes of the transformed system are orthogonal straight lines and the distances relate in the customary way to the map coordinates Žlatitudes., given in degrees on the frame X of the maps. The topography has been developed from ETOPO 5 ŽHirtzler, 1985. with the full resolution of 5 . Isolines are shown for 1000, 2000, 3000 and 4000 m with lines of decreasing thickness. CB: Canada Basin, AR: Alpha Ridge, MR: Mendeleyev Ridge, EB: Eurasian Basin, FS: Fram Strait. Žb. Blow-up of the working area. The end stations of three sections A, B, C are numbered and the sections are connected with thin lines. CTD stations shown as profiles andror Q –S curves are indicated by stars. The position of the 1991 CTD stations 10, 13, 17, 20 and 26 are also shown. MB: Makarov Basin, AB: Amundsen Basin, LR: Lomonosov Ridge, NGR: Nansen–Gakkel Ridge, NB: Nansen Basin, SAT: St. Anna Trough, SZ: Severnaya Zemlya, FJL: Franz Josef Land, NZ: Novaya Zemlya, Sv: Svalbard.

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Fig. 1 Ž continued ..

North Atlantic, follows the eastern boundary northward into the Arctic Mediterranean Sea ŽFig. 1a.. This northward flow is believed, at least partly, to be a response to the density increase and the deep convection occurring in the Nordic Seas and especially in the Greenland Sea. The surface water crossing the Greenland–Scotland Ridge becomes transformed into dense, cold water which re-crosses the ridge, supplies the North Atlantic Deep Water, and eventually renews the deep waters of the world ocean. Sites of deep convection are few and localised, and in contrast to the Nordic Seas the most extensive part of the Arctic Mediterranean, the Arctic Ocean, has often been neglected in this aspect. The large

freshwater input, from the Siberian rivers and carried by the Pacific inflow through the Bering Strait, creates a strong stratification that limits convection in the central Arctic Ocean to the upper 50–100 m and leads to a permanent ice cover and reduced heat loss. Water transformations taking place in the Arctic Ocean would then counteract those occurring in the Nordic Seas, creating less dense rather than denser water. An excessive outflow of low salinity surface water and ice from the Arctic Ocean through Fram Strait could also increase the stability of the water column and affect convection in the Greenland Sea. One hypothesis ŽAagaard and Carmack, 1989. is that if the freshwater content in the surface water of the Greenland Sea increases, the convection would be-

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come shallower. The water supplied to the overflow and to the North Atlantic Deep Water would be less dense and its volume reduced. The meridional thermohaline circulation would weaken, with inscrutable effects upon the climate of northwestern Europe. However, almost 100 years ago Nansen Ž1906. suggested that ice formation and brine rejection over the shallow Arctic Ocean shelves could create water dense enough to sink down the continental slope and renew the deep waters of the Arctic Ocean. This idea was revived in the mid-80s by Aagaard et al. Ž1985., who realised that the deep water in the Canadian Basin was too saline to derive, untransformed, from the Greenland Sea. The salinity increase could be explained by convecting, saline boundary plumes originating from the shelves, and Aagaard et al. Ž1985. proposed an internal circulation scheme for the deep waters of the Arctic Mediterranean. This circulation consisted of two sources, the Arctic Ocean shelves and the Greenland Sea, both eventually supplying the Norwegian Sea Deep Water. This scheme treated the deep circulation in the Arctic Mediterranean as separate from the global thermohaline circulation south of the Greenland–Scotland Ridge. The possibility that the Arctic Ocean also acts as a supplier of especially the Denmark Strait Overflow Water has been discussed by Mauritzen Ž1996a,b., and Anderson et al. Ž1999. consider the role of dense water formation in the Arctic Ocean, both as a direct contributor to the overflow and as a forcing agent, displacing less dense intermediate water of the Nordic Seas upward, causing it to cross the ridge. These recent studies rekindle the interest in how the intermediate and deep waters of the Arctic Ocean are transformed and renewed. Although much has been written about the importance of ice formation and brine rejection on the shelves, and about the sinking of dense boundary plumes down the continental slope, little observational support has been found in the Arctic Ocean. Much of the current ideas are based upon a few observations such as those of the Storfjorden outflow ŽQuadfasel et al., 1988; Schauer, 1995. and modelling work based thereon ŽJungclaus et al., 1995; Backhaus et al., 1997., and upon conjectures made from observations of Q –S characteristics and tracer concentrations of the water masses in the Arctic Ocean deep basins ŽAagaard et al., 1985; Rudels, 1986; Rudels et al., 1994; Smethie

et al., 1988; Schlosser et al., 1994; Jones et al., 1995.. The eastern Eurasian Basin from the eastern Kara Sea to the East Siberian Sea is an area, where many of the processes determining the water mass properties of the Arctic Ocean are expected to operate. It is downstream of the confluence area of the two inflow branches from the Norwegian Sea, the Fram Strait branch and the Barents Sea branch ŽRudels et al., 1994.. Much of the transformation of the Barents Sea branch water occurs in the Barents Sea, before it is injected into the Arctic Ocean water column in the St. Anna Trough. The subsequent mixing between the two branches determines much of the characteristics of the intermediate waters, especially in the Eurasian Basin. The salinity on the Kara Sea shelf is high, and the presence of Severnaya Zemlya favours the creation of lee polynyas, leading to large ice formation and strong brine rejection. The islands are located at the shelf break, which allows created dense water to reach the continental slope without much dilution, thus making it a site favourable for deep, penetrative slope convection. A splitting of the boundary current into loops re-circulating in the Eurasian Basin and a remaining boundary current crossing the Lomonosov Ridge takes place in the eastern Eurasian Basin. Observations from the Laptev Sea and the East Siberian Sea continental slopes make it possible to determine modification of the boundary current as it flows along the slope and to assess its characteristics as it enters the Makarov Basin. The ARKXI cruise in August–September 1995 with RV Polarstern was dedicated to multi-disciplinary work in the eastern Eurasian Basin ŽRachor, 1997., including a small oceanography programme mostly involving CTD observations. This work presents these observations and uses them to assess changes in water properties along the boundary current following the continental slope north of Siberia. The emphasis is on the intermediate and deep-water formation and on possible effects of the recently reported higher temperatures in the Atlantic Layer of the boundary current ŽQuadfasel et al., 1991; Carmack et al., 1995; Grotefendt et al., 1998. on the water columns in the interior of the basins. After briefly presenting the data in Section 2, Section 3 considers the confluence and mixing, in the bound-

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ary current, between the two inflows, and the splitting of the boundary current north of the Laptev Sea. Section 4 re-examines the assumptions of the box model of slope convection used by Rudels et al. Ž1994., and discusses possible effects of the recently observed property changes in the boundary current on the characteristics of the Canadian Basin water column. Section 5 examines the sources for the deep water in the Eurasian Basin. A short summary is given in Section 6. Note that Canadian Basin and Eurasian Basin refer to the deep parts of the Arctic Ocean separated by the Lomonosov Ridge and will be used when distinctions across the Lomonosov Ridge are to be emphasised. The Canadian Basin comprises the Makarov and the Canada basins as well as the Alpha and Medeleyev Ridges. The Eurasian Basin comprises the Amundsen and Nansen basins and the Nansen–Gakkel Ridge Žsee Fig. 1a and b..

2. Observations and data quality Eighty-seven CTD stations were obtained during the ARK-XI expedition, many of these with multiple casts. Six crossings of the continental slope were made: The East Siberian Sea section, the Eastern, Central and Western Laptev Sea sections, the Severnaya Zemlya section and the Kara Sea section. Several stations were taken in the interior of the Eurasian Basin and one section across the Lomonosov Ridge was also occupied. The station positions are shown in Fig. 1a. The area north and northeast of Severnaya Zemlya is almost always covered by a thick, multiyear ice cover ŽTimokhov, personal communication.. This ice cover was absent in 1995, offering the rare opportunity to occupy the Severnaya Zemlya section ŽB. and perhaps also the Kara Sea section ŽA.. In the present work, we shall mainly discuss these two sections and a combined, long section running from the western Laptev Sea over the Lomonosov Ridge to the East Siberian Sea ŽC. ŽFig. 1b.. The hydrographic observations were made using a Sea-Bird SBE-911 plus CTD system and a 24 bottle Sea-Bird Carousel rosette sampler. Apart from a broken thermistor at station 33, no instrumental problems arose. Salinity samples for calibration were taken at each station, but mostly only from the

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deepest bottle. Because of the different depths of the stations, this gave samples from the entire observed pressure range. The salinity samples were analysed onboard on a Guildline Autosal 8400 and the CTD was found to perform well. Before the change of thermistor, the off-set in salinity Ž S btl –Sctd . was 0.003 and after the change 0.008. No temporal and pressure-dependent trends were detected. The accuracy in salinity is estimated to 0.003. 3. The confluence of the inflows and the splitting of the boundary current The two westernmost sections showed a cold, low salinity water column over the slope reaching deeper than 1000 m ŽFig. 2.. The temperature below 500 m was constant around y0.68C, but the salinity increased toward the bottom, indicating an isothermal, but salinity stratified, water column at the slope. The cold, low salinity water was separated by a 20-km Žsection A. to 40-km Žsection B. wide frontal zone from a warm, high salinity core located at about 300 m depth and 75 km into the basin. These temperature and salinity distributions are consistent with the hypothesis that the boundary current at the Eurasian continental slope is supplied by warm, saline water from Fram Strait and by a colder, less saline inflow passing over the Barents and Kara Seas ŽRudels et al., 1994; Schauer et al., 1997.. The temperature of the Fram Strait branch was high, almost 38. This is close to what was observed in 1993 by Schauer et al. Ž1997. northwest of Franz Josef Land. The Barents Sea branch characteristics resemble those reported by Loeng et al. Ž1993. from a section between Franz Josef Land and Novaya Zemlya. On section C, the Barents Sea branch water could be identified as salinity minima at three positions, on the Laptev Sea slope, at the Lomonosov Ridge, where it was most prominent on the Amundsen Basin side, and on the continental slope north of the East Siberian Sea ŽFig. 3.. This salinity distribution suggests that the boundary current splits at the Lomonosov Ridge with one part following the ridge and the other entering the Makarov Basin along the continental slope. The temperature of the Atlantic Layer was, except at some smaller patches, only slightly above 2.08C with the highest values close to the Nansen–Gakkel Ridge.

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Fig. 2. Potential temperature and salinity distributions on section A Žleft., and section B Žright..

Large variability was observed on stations where multiple casts were taken, as well as between closely spaced stations. Fig. 4 shows different casts from stations 25, 29, 40 and 80 north of Severnaya Zemlya and north of the Laptev Sea. The ship was drifting between the casts, and the two casts on stations 25 and 29 are shown as separate stations on section ŽB..

The drift of the ship on stations 40 and 80 was considerably less, 4 and 2 km, respectively. The maximum temperatures varied by more than half a degree, suggesting a narrow, perhaps meandering, front changing position with time, or that parcels of warm Fram Strait branch water were transferred across the front into the Barents Sea branch and vice

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Fig. 3. Potential temperature and salinity distributions on section C.

versa. The front between the two branches was characterised by sharp inversions in temperature and salinity, indicating the presence of interleaving ŽFig. 5b, d, f.. Such interactions would enhance the mixing between the two branches and accelerate the formation of a distinct water column in the boundary current. The difference in Atlantic Layer properties between section B and section C is more clearly re-

vealed by comparing potential temperature and salinity profiles and Q –S curves from the warmest stations on the two sections ŽFig. 5a, c, e.. The lower temperature and salinity in the Atlantic Layer on section C and the difference between stations 77 and 75 suggest two possibilities. Either that the warmest, most saline water of the Fram Strait branch did not cross the Nansen–Gakkel Ridge into the Amundsen Basin but returned within the Nansen Basin toward

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Fig. 4. Q –S diagrams from stations 25 Ža., 29 Žb., 80 Žc. and 40 Žd. where multiple casts were made.

Fram Strait, or it has become cooled by lateral mixing with Barents Sea branch water before reaching section C. Considering the short distance between the sections and the fact that the Barents Sea branch was still distinct, albeit warmer and more saline, the first scenario appears more likely. The vertical scale was larger and the property contrasts smaller in the layering observed in the Atlantic Layer on stations 75 and 77 than on the stations north of Severnaya Zemlya. The cooler and less saline end member for interleaving was here located on the basin side of the Fram Strait branch Ži.e. Fig. 5, station 47.. This is consistent with a separation of mainly Fram Strait branch, but also some Barents Sea branch, water from the continental

slope into the Nansen Basin before the boundary current crossed the Nansen–Gakkel Ridge into the Amundsen Basin. The salinity minimum below the Atlantic Layer on station 47 ŽFig. 5. agrees with a presence of Barents Sea branch water in the Amundsen Basin and a second splitting of the boundary current at the Lomonosov Ridge Žsee above, Fig. 2.. The Amundsen Basin contained water from both branches, and temperatures above 18C at the Makarov Basin slope indicate that not only Barents Sea branch water but also water from Fram Strait penetrated eastward across the Lomonosov Ridge. Schauer et al. Ž1997. found that the boundary current was broader north of the Laptev Sea than north of the Barents Sea and attributed this to insta-

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Fig. 5. Left: Profiles of potential temperature Ža. and salinity Žc. and Q –S curves Že. for the warmest stations on section B Ž27. and on section C Ž77 and 75.. Right: Profiles of potential temperature Žb. and salinity Žd. and Q –S curves Žf. for stations 25, 27 and 29 on section B.

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bilities in the St. Anna Trough inflow as it enters the basin. An additional cause for the instabilities and broadening of the boundary current after the confluence of the two branches could be a reversal of the directions of the pressure and density gradients between the slope and the basin water columns with depth. North of the Barents Sea the lower part of the boundary current was, because of the presence of plumes at the Barents Sea continental slope, denser than the water at corresponding depths in the interior of the basin. A deep, geostrophically balanced flow will be trapped at the slope ŽSchauer et al., 1997.. North of the Kara Sea and Severnaya Zemlya, this was no longer the case. The water column was less stratified at the slope than in the interior of the basin, as the upper part was denser; the deeper part less dense than basin water at the same depths ŽFig. 6.. The baroclinic velocity of an eastward flowing boundary current then decreases toward the bottom, and its deeper part can only move eastward, if the sea surface rises from the basin toward the shelf. Variations in sea level slope between shelf and basin, perhaps caused by meteorological forcing or excessive river discharge, could then change the direction of the pressure gradient in the deeper layers, arresting, and perhaps reversing, the flow in the lower part of the boundary current. Further to the east, the density contrasts between the slope and the interior diminished and north of the Laptev and the East Siberian seas the isopycnals were almost horizontal ŽFig. 6.. The section across the Lomonosov Ridge taken in 1995 was located much closer to the continental slope than the crossing made by IB Oden in 1991 Žfor positions of the Oden-91 stations, see Fig. 1a.. On the 1991 section, a front in potential temperature and salinity was observed in the Atlantic Layer. It became especially strong below 600 m where the warmer and more saline upper Polar Deep Water in the Makarov Basin and the Barents Sea branch water at the Amundsen Basin side suggested different circulation gyres ŽRudels et al., 1994.. On the 1995 Polarstern, crossing the temperature profiles at each side of the ridge was similar below the temperature maximum suggesting that both sides were within the domain of the boundary current. Salinity was lower in the Amundsen Basin from 400 m downwards, indicating a stronger presence of Barents Sea branch

water on this side of the ridge ŽFig. 7.. The warmer, more saline Canadian Basin Deep Water of the Makarov Basin was found first below 1600 m, implying that the sill depth of the Lomonosov Ridge close to the continental slope is about 1600 m and that the boundary current dominates the water mass properties at the slope down to this level. The deeper part of the Q –S curves at the Makarov Basin slope in 1995 ŽFig. 7. was similar to what was observed in the Makarov Basin close to the Mendeleyev Ridge during the AOS94 expedition ŽSwift et al., 1997.. This supports the conjecture made by Swift et al. Ž1997. that the deeper layers of the boundary current crossing the Lomonosov Ridge partly leaves the slope at the Medeleyev Ridge and enters the interior Makarov Basin. A small salinity maximum was observed between 1600 and 1650 m on the Amundsen Basin side of the ridge ŽFig. 7.. The maximum had the same density and similar properties as water at the same level in the Makarov Basin suggesting an injection of Canadian Basin Deep Water into the Amundsen Basin through a deeper gap in the Lomonosov Ridge. No such intrusions and no intermediate depth salinity maxima were observed close to the Lomonosov Ridge on Oden in 1991. However, an intermediate depth salinity maximum was present on almost all Oden stations in the interior of the Amundsen Basin Že.g. Fig. 8d, station 17.. This salinity maximum became more prominent toward Greenland, and it was assumed to originate from the boundary current returning from the Canadian Basin north of Greenland, carrying Canadian Basin Deep Water into the Eurasian Basin ŽJones et al., 1995.. However, intermittent, but probably small, deep exchanges between the basins are likely to occur along the entire Lomonosov Ridge. Because of its higher temperature, the intermediate depth water of the Makarov Basin is less compressible than intermediate depth water of the Amundsen Basin and referred to 1500 dB the Amundsen Basin water column was already at 1700 dB denser than the Makarov Basin Bottom Water ŽFig. 7d.. It must, if it enters the Amundsen Basin, remain at sill depth and will be recognised as an intermediate depth salinity maximum. Unusually high core temperatures in the Atlantic Layer have recently been reported by Quadfasel et al. Ž1991., Carmack et al. Ž1995., McLaughlin et al.

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Fig. 6. Density distributions on section A Župper left., section B Župper right. and section C Žbottom..

Ž1996., Grotefendt et al., Ž1998. and Morison et al. Ž1998.. High temperatures were observed in 1990 at the continental slope north of Franz Josef Land ŽQuadfasel et al., 1991. and in 1993 at the slope

north of the Laptev Sea ŽSchauer et al., 1997. but not in the interior of the Eurasian Basin in 1991 ŽAnderson et al., 1994.. In 1994, the AOS94 expedition practically re-occupied the Oden 1991 stations at the

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Fig. 7. Profiles of potential temperature Ža. and salinity Žb. and Q –S curves Žc. and blow-up Žd. from stations at the Lomonosov Ridge Ž51., in the Amundsen Basin Ž47., in the Makarov Basin Ž57. and at the East Siberian Sea slope Ž60..

Lomonosov Ridge, and a temperature increase was found in the Atlantic Layer ŽAagaard et al., 1996; Carmack et al., 1997; Swift et al., 1997.. In spite of the large distance between the stations taken by Oden in 1991 and by Polarstern in 1995 Žsee Fig. 1a and b., we shall compare the 1991 and the 1995 salinity and temperature profiles in the Nansen Basin and at the Nansen–Gakkel Ridge, in the Amundsen Basin and at the Lomonosov Ridge ŽFig. 8.. In 1995, the Atlantic and the intermediate depth waters were warmer. The differences in salinity were more ambiguous. At the Nansen–Gakkel Ridge, the salinity was higher in the Atlantic Layer in 1995 but equal to that observed in 1991 below

500 m. In the Amundsen Basin, the salinity was lower between 400 and 800 m in 1995, perhaps due to a larger presence of Barents Sea branch water, or to a change in the Barents Sea branch characteristics. The most drastic difference is seen at the Lomonosov Ridge, where the salinity in 1995 was lower than in 1991 below 300 m and significantly lower deeper than 400 m. Together with the increased temperature, this suggests a presence of less saline and warmer Barents Sea branch water than during earlier years. In the interior of the basins, we do not expect large, local water mass transformations to occur below the mixed layer, and observed changes in the

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Fig. 8. Profiles of potential temperature and salinity from selected stations taken in 1991 and in 1995 Ža. at the Nansen–Gakkel Ridge, Žb. in the Amundsen Basin, Žc. at the Lomonosov Ridge.

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water column are likely caused by advection. The increased temperature at the Lomonosov Ridge found in 1994 ŽSwift et al., 1997. then indicates a return flow from the boundary current toward Fram Strait along the Lomonosov Ridge. This agrees with CTD observations taken from submarines in recent years ŽSteele and Boyd, 1998.. The higher temperatures found in the eastern parts of the Amundsen Basin and the Nansen–Gakkel Ridge in 1995 then suggest that also these parts, including the Nansen–Gakkel Ridge, are ventilated by water separating from the boundary current north of the Laptev Sea ŽRudels et al., 1994. rather than by a direct flow from the west. However, here opinions diverge. Swift et al. Ž1997. have proposed that the Atlantic Layer in the Nansen Basin up to the Nansen–Gakkel Ridge is renewed by double-diffusively driven intrusions penetrating the basin water column from the boundary current, and not by a branching of the boundary current from the continental slope.

4. The intermediate layers of the Canadian Basin In the Canadian Basin, no strong, dense second inflow exists, corresponding to that at the St. Anna Trough. The boundary current at the slope is exposed to shelf-slope convection, leading to cooling and vertical redistribution of its water through entrainment into descending plumes. This transforms the water of the boundary current and creates the Canadian Basin intermediate and deep water. These waters re-cross the Lomonosov Ridge into the Eurasian Basin north of Greenland and have also been estimated to provide a substantial fraction of the overflow waters crossing the Greenland–Scotland Ridge into the North Atlantic ŽAnderson et al., 1999.. Because of this possible importance of the slope convection in the Canadian Basin we shall, in the light of the 1995 observations, re-examine two assumptions made by Rudels et al. Ž1994. and Anderson et al. Ž1999. in their description of the transformations of the Atlantic and intermediate depth waters in the Canadian Basin to judge, if that approach is at all valid. Ž1. Rudels et al. Ž1994. lacked observations at the continental slope in the Canadian Basin but assumed that the boundary current splits at the Lomonosov

Ridge and that Q –S characteristics of the boundary currents at the continental slope and along the Lomonosov Ridge were similar. Q –S characteristics observed at the Amundsen Basin side of the Lomonosov Ridge could then be substituted for those of the boundary current. Ž2. The slope plumes are entraining, diluting the original shelf water. And this dilution was assumed constant with depth and set so that the plumes incorporate twice their original volume for each 300 m of descent, 300 m being the applied layer thickness. It should be emphasised that this is not a dynamical model. The characteristics of the shelf input and the slope plumes are determined by assuming that the merging of the plumes and the boundary current reproduces the Makarov Basin Water columns at all levels. The characteristics of the boundary current, and of the Makarov Basin, are known, and the Makarov Basin properties are assumed to represent the entire Canadian Basin. In 1995, observations were made at the Makarov Basin slope as well as at the Lomonosov Ridge. The boundary current there was slightly colder in the Atlantic Layer but otherwise its characteristics were similar to those found at the Lomonosov Ridge ŽFig. 7.. Assumption 1 may thus be defended. To determine, if assumption 2 is realistic, we redo the calculation for the temperature changes of the plumes, as they sink down the slope using different dilution factors. Fig. 9a shows temperature profiles formed by plumes entering at different depth of the water column. Drawn are also the five layer temperature profiles of the Canadian Basin and of the boundary current. The dilution factor 2 is found to be the only one of the four Ž1, 1.5, 2 and 3. examined, which by mixing the plumes with the boundary current can, at all levels, reproduce the Canadian Basin water column. Note that this can only be done for the temperature, which is initially the same for all plumes regardless of how dense they are, and how deep they sink. Once the ratio of the contributions from the plumes and from the boundary current has been determined at each level, the initial salinity of the plumes, sinking to different levels, can be deduced. These calculations ignore along slope changes of the boundary current properties and assume that the entire Canadian Basin water column can be repre-

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Fig. 9. Ža. Potential temperature profiles from the boundary current Žopen circles. and the Canadian Basin Žopen squares. in 1991 and the temperature of the descending plume at different depths with dilution factors 1, 1.5, 2 and 3 for each 300 m layer Žcrosses.. Žb. Potential temperature profiles from the boundary current Žfilled circles. and the Canadian Basin Žopen squares. in 1995 and the temperature of the boundary plumes at different depths with dilution factors 1 and 2 Žcrosses.. Žc. The salinity profiles from boundary current Žopen circles. and the Canadian Basin Žopen squares. in 1991 and the salinity of descending plumes reaching different terminal depths using dilution factor 2 Žbroken lines and crosses.. The salinity of the different plumes at their terminal depths are connected with a solid line. Žd. The salinity profiles from the boundary current in 1995 Žfilled circles. and the Canadian Basin in 1991 Žopen squares. and the salinity of descending plumes reaching different terminal depths using entrainment rate 2 Žbroken lines and crosses.. The salinity at the different plumes at their terminal depths are connected with a solid line.

sented by the water column observed in the Makarov Basin close to the Lomonosov Ridge. The AOS94 expedition has shown that part of the boundary

current enters the interior of the Makarov Basin along the Mendeleyev Ridge, retaining much of the characteristics it had crossing the Lomonosov Ridge

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ŽSwift et al., 1997.. The main part of the Makarov Basin has different water mass properties, and Swift et al. Ž1997. proposed that the Makarov Basin is also fed by a loop that detaches from the boundary current at the North American side and flows along the Lomonosov Ridge into the basin. This part of the boundary current has been exposed to slope convection much longer and has become more transformed. The merging of the contributions from the ‘‘young’’ and the ‘‘old’’ boundary current would make the water mass properties in the Makarov Basin more representative of the transformation of the boundary current occurring at the slope in the entire Canadian Basin than it initially appears. Because of the advective changes in properties of the boundary current, the Q –S characteristics observed at the Lomonosov Ridge and over the Makarov Basin slope in 1995 were different from those found at Amundsen Basin side of the ridge in 1991 ŽFig. 8.. If an entrainment rate of 2 again is used, it is not possible to merge the plumes and the boundary current to reproduce the Canadian Basin water column with the boundary current properties observed in 1995. A 50% lower entrainment rate Ž1. would fit the first layer, and in the deeper layers the difference in temperature between the boundary current and the Canadian Basin is small, and almost no input from the shelves is required to change the temperature of the boundary current into that of the Canadian Basin water column ŽFig. 9b.. However, the salinity profiles in the boundary current and in the interior of the Canadian Basin are very different and larger contributions from the shelves are needed to increase the salinity of the boundary current to that of the interior of the Canadian Basin ŽFig. 9c and d.. Merging between sinking plumes and the boundary current then cannot reproduce both the temperature and the salinity distribution in the Canadian Basin water column irrespective of the applied entrainment rate. The boundary current and the interior of the Canadian Basin are out of balance. As the interior water column is renewed by water leaving the continental slope, the advected variations of the boundary current properties will, modified by slope convection, change the characteristics of the interior water column. To describe a possible evolution of the water mass properties in the Canadian Basin, we apply the input from the shelves deduced

by Rudels et al. Ž1994. and compute the resulting plume characteristics, using the same dilution factor Ž2.. The plumes are then allowed to merge with the 1995 boundary current to create a new interior water column ŽFig. 10.. The water to the Canadian Basin water column from above, carried by the plumes, comprises about 1r4 of the volume in the upper three layers and it increases to 1r3 of the volume in layer 5. The rest is supplied by the boundary current. The plume input into a layer also includes water entrained from shallower layers, and the part originating from above 200 m decreases from 25% in layer 1 to 3% in layer 5. The changes in the boundary current will make the Canadian Basin water column warmer and less saline ŽFig. 10.. Changes are expected to appear first in the upper, more rapidly renewed layers, and at prominent bathymetric features, where the boundary current communicates with the basin interior. Slope convection gradually alters the properties and redistributes water of the boundary current downward. The transport in the deeper layers increases and the transport in the upper layers decreases, and changes will eventually be seen in the deeper layers.

Fig. 10. Schematics showing Q – S curves from the boundary current in 1991 Žopen circles. and in 1995 Žfilled circles. and from the Canadian Basin in 1991 Žopen squares. and the plume characteristics in 1991 Žthin crosses. and in 1995 Žcrosses. and the projected change of the Makarov Basin water column Žfilled squares..

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About 3 Sv enter the Arctic Ocean from the Norwegian Sea ŽAagaard and Carmack, 1989., and if 1r3 of this inflow reaches the Canadian Basin around 100 years would be needed to renew the Canadian Basin water column between 200 and 1700 m. During such long time spans, we expect several changes in the properties of the inflowing waters, which might counteract the present trend. Perhaps a balance never is, or was, established. 5. Formation of Eurasian Basin Deep and Bottom Water As the Barents Sea branch water enters the Arctic Ocean in the St. Anna Trough, it is less saline but denser than the water at the same levels in the interior of the basin ŽHanzlick and Aagaard, 1980; Schauer et al., 1997.. The inflow displaces the Fram Strait branch from the slope towards the interior of the basin Žcompare Figs. 2 and 3., but it is constrained to the continental slope by the earth’s rotation, and its volume flux can only be accommodated by a downward displacement of the deeper isopycnals at the slope ŽSchauer et al., 1997.. The lower part of the Barents Sea branch water, thus penetrates to depths where it becomes less dense than the water column at the same depth in the interior of the basin ŽFig. 6.. Water denser than what was found at the Eurasian continental slope in 1993 and 1995 has been observed in the eastern Barents Sea ŽNansen, 1906; Midttun, 1985.. Mostly, the temperature has been above, but close to, freezing and the salinity has varied from slightly below to slightly above that of the Atlantic Water entering the Barents Sea ŽPfirman et al., 1994.. The highest salinities and the lowest temperatures have been observed on the shallow shelf west of Novaya Zemlya, but entrainment could change the characteristics as the water sinks into the deeper depressions and continues toward St. Anna Trough ŽNansen, 1906.. The bottom layer of the St. Anna outflow may thus occasionally be denser than the Barents Sea branch water observed at the slope in 1995. The lower part of the main St. Anna inflow reaches as deep as 1000 m as it enters the Nansen Basin ŽFig. 2.. A denser bottom layer would then separate from the main, eastward flowing, part and

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sink as an entraining boundary plume down the slope. Because it starts this deep, no warm Atlantic Water but only Eurasian Basin Deep Water will be entrained. The depth such dense bottom layer would reach depends upon its initial density. The inflow at the St. Anna Trough is not the only conceivable source for the Eurasian Basin Deep Water. North of Severnaya Zemlya intrusions of water colder than the St. Anna inflow were observed intruding into the Atlantic core. These intrusions were not present on the Kara Sea section further west ŽFig. 11. and suggest the presence of cold, shelf-derived waters originating further to the east, perhaps around Severnaya Zemlya. These waters were apparently dense enough to sink into and merge with the boundary current at the level of the Atlantic Layer. The northeastern Kara Sea and the area around Severnaya Zemlya are ideal for the formation of dense, brine-enriched water. The islands cause lee polynyas to form. The initial shelf salinity is high, the water depth is small, and the distance to the continental slope short. Plumes descending from Severnaya Zemlya would, however, start to entrain much higher up on the slope than the plumes separating from the St. Anna inflow, and to reach large depths, their initial salinity must be high. In contrast to the Canadian Basin, the cold water column of the Barents Sea branch here dominates at the continental slope, and the ambient bottom temperatures are always considerably below 08C ŽFig. 2.. Boundary plumes sinking down the slope do not pass through, and entrain, any water from the Atlantic Layer, and the temperature increase will be less than in the Canadian Basin. The Barents Sea inflow causes the isohalines and isopycnals to be depressed relative to the interior, but the deeper stations on section B showed near bottom salinities and densities that were higher than those at the same depth in the interior of the basin ŽFig. 5.. This increase could be caused by dense, saline bottom water originating either from the St. Anna Trough or the eastern Kara Sea shelf and around Severnaya Zemlya. To see how the characteristics of plumes sinking from the shelf break off Severnaya Zemlya, and from 1000 m at the St. Anna Trough, might evolve, we assume that the bottom water properties on the slope are those observed on section B, and use the same

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Fig. 11. Q –S diagram showing stations 89, 92, 93 on the Kara Sea section and intruding colder water observed on stations 31 and 29 at the slope north of Severnaya Zemlya.

dilution factor as for the Canadian Basin. As the Severnya Zemlya plumes cross the shelf break Ž100 m., their initial temperature would be at the freezing point. The initial salinities needed for the plumes to sink into the deeper layers have been estimated by trial and error Žrequiring that it should be large enough for the plumes to reach 3000 m.. Because they start high up on the slope, the temperature increase, due to entrainment, is large, and the characteristics of the plumes will approach those of the Eurasian Basin Deep Water from higher temperatures, warming the deep waters, as they merge with their surroundings. The initial properties of the plumes from the St. Anna Trough have also been chosen by trial and error. If their initial temperature is lower than that of the Eurasian Basin Deep Water, when they separates from the main inflow, entrainment will cause their properties to approach the Q –S curve of the Eurasian Basin Deep Water from lower temperatures, cooling the deeper layers. The two

inflows could thus combine to produce the deep Eurasian Basin Q –S curves ŽFig. 12.. To get a smaller temperature increase for the Severnaya Zemlya plumes, the bottom temperatures at the slope must be lower than those observed in summer 1995. This could be the situation in winter, when also the bottom salinities may be higher. The initial shelf salinities could then be lower than those shown in Fig. 12. A smaller, or a depth varying, entrainment rate may also reduce the required initial salinities, but to justify such assumptions the use of a dynamical plume model, rather than the simple property matching employed here, is necessary. This has not been attempted. The deepest layers on the slope stations north of Severnaya Zemlya shallower than 2400 m showed that the temperature decreased, while the salinity increased. On deeper stations also, a temperature increase was detected at the bottom ŽFig. 13.. This is what to expect when entraining plumes pass through

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Fig. 12. Profiles of potential temperature Ža. and salinity Žb. showing schematically the characteristics of the bottom water at the continental slope north of Severnaya Zemlya Žfilled circles. and the evolution of the potential temperature and the salinity of plumes sinking from the shelf at Severnaya Zemlya Žcrosses ands open squares., and from the bottom of the St. Anna Trough Žfilled diamonds.. Žc. The evolution of Q –S characteristics of the sinking plumes and the initial Q –S ranges required for the plumes to reach deeper than 3000 m. Žd. Blow-up of the Q –S characteristics of the plumes as they approach their terminal depths.

intermediate warmer water ŽQuadfasel et al., 1988.. The bottom layer was also thin as was that observed by Quadfasel et al. Ž1988. in Fram Strait suggesting that bottom stress may be strong enough to explain such large entrainment, and to break the balance between the Coriolis and the buoyancy accelerations, allowing the plumes to sink.

The Q –S characteristics in the bottom layer were different from those in the interior of the basin, which also is consistent with the presence of deep sinking plumes ŽFig. 13.. The density was higher than at the same level in the interior but not high enough for the water to reach the bottom of the basins. Station 47 from the Amundsen Basin showed

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Fig. 13. Profiles of potential temperature Ža., salinity Žb., and potential density Žc. and Q –S diagram Žd. from the deeper parts of stations at the slope north of the eastern Kara Sea Ž89. and north of Severnaya Zemlya Ž29, 229, 25, 26. and from the interior of the Nansen Basin Ž27. and the Amundsen Basin Ž47..

a significantly colder deep temperature minimum and a higher deep and bottom water salinity ŽFig. 13.. The bottom ‘‘plumes’’ at stations 26 and 27 had reached their terminal density level Žwith respect to station 47., but were still warmer and more saline than the water column at station 47. The ‘‘plumes’’ at the other stations were denser than at the corresponding depth on station 47 and would continue to sink down the slope. However, all deep slope ‘‘plumes’’ were warmer and more saline than their corresponding density levels in the interior of the basin indicating that changes in the characteristics of the Eurasian Basin Deep Water could be taking

place. The characteristics of station 47 then suggest the existence of periods with a colder and denser St. Anna inflow, and more saline and denser slope convection. In the depth range between 1500 and 2000 m, station 47 was both warmer and more saline, as were also stations 75 and 77, than the stations in the southern Nansen Basin Žsee Fig. 5.. This indicates the presence of Canadian Basin Deep Water, which, after it has crossed the Lomonosov Ridge north of Greenland, spreads eastward at this levels. This far east, it did not show up as an intermediate salinity maximum as further to the west Žcompare the 1991

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station 17 and the 1995 station 47 in Fig. 8d.. The lower salinity at this level closer to the Laptev Sea slope then indicates a gradual dilution of the Canadian Basin Deep Water with Eurasian Basin Deep Water as it penetrates eastward. There, nevertheless, exists a weak feedback loop whereby water from Canadian Basin, which to a significant extent reflects input from the Eurasian Basin slope current, can act to modify the Eurasian Basin Deep Water, and the same boundary current close to its source. If the interpretation of the high density bottom water at the slope as descending plumes is correct, it would be one of the first, perhaps the first, instanceŽs. where the slope plumes warmer and more saline than the ambient water, invoked to explain the Q –S characteristics of the Arctic Ocean deep waters, have been observed in the Arctic Ocean. So far, most observations have indicating colder, less saline and shallower inputs from the shelves, which would cool and freshen the water column ŽSchauer et al., 1997.. An exception is the Storfjorden outflow ŽQuadfasel et al., 1988.. However, this outflow does not enter directly into the Arctic Ocean. The source of the higher salinity, higher density layer at the bottom of the boundary current could be either one of the two source areas, the Barents Sea and Severnaya Zemlya, discussed above. The increased temperatures at the bottom at the deeper slope stations indicate that the waters may, at least partly, originate from the Severnaya Zemlya area. The fact that a temperature minimum is observed about 800 m above the bottom over the entire Eurasian Basin then suggests that the deepest Žbottom. layers are predominantly supplied by slope convection from Severnaya Zemlya, and that the Barents Sea branch provides most water to the temperature minimum layer. These two sources alone are sufficient to explain the Eurasian Basin Deep Water properties. There is one colder, less saline and one warmer, more saline source, which can supply water both for the salinity increase at the bottom and for the deep temperature minimum. An inflow of deep water from the Nordic Seas through Fram Strait is then not needed to provide the low salinity, cold end member. An inflow through Fram Strait would, because of the 2600-m deep sill, also not be dense enough to influence the deeper parts of the Eurasian Basin water column, where the salinity minimum is encountered.

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The difference between the Nansen Basin and the Amundsen Basin in the 1500–2000 m range could also be explained by an input of less saline Barents Sea branch water Žsee Fig. 5.. However, Arctic Intermediate Water and Norwegian Sea Deep Water do enter the Nansen Basin through Fram Strait ŽJones et al., 1995., and have also been identified in the tracer distributions as far east as the Laptev Sea slope ŽFrank et al., 1998.. The low salinity in the intermediate depth range, as seen in station 27 ŽFig. 5. could be due to a weak presence of these water masses.

6. Summary The Atlantic and intermediate layers in the Arctic Ocean are renewed advectively by the Fram Strait and the Barents Sea branch. The Q –S distribution within the Arctic Ocean shows that isopycnal mixing, as well as brine rejection and slope convection, are active and important in determining the characteristics of the intermediate and deep waters circulating within and exiting from the Arctic Ocean. The water transformations in the Arctic Ocean act as separators, changing the inflowing waters from the North Atlantic into both less dense Polar Surface Water and denser intermediate and deep waters. The most conspicuous change occurs in the Barents Sea, as the Atlantic Water passes from the Norwegian Sea to the Arctic Ocean. Cooling, freezing and melting lead to the formation of the embryo of the Polar Surface Water that absorbs the run-off from the Siberian rivers farther east, and to the formation of the denser fractions sinking down the St. Anna Trough. The Fram Strait branch, excluding the part being diluted into less dense upper water north of Svalbard, is deflected from the continental slope by the Barents Sea branch, and becomes modified by the isopycnal mixing between the two branches. The density of the part of the Fram Strait branch water, which re-circulates within the Eurasian Basin, then does not change, as it returns toward, and exits through, Fram Strait. It has just become colder and less saline than the Return Atlantic Current re-circulating in Fram Strait ŽBourke et al., 1988.. East of the St Anna Trough, the Barents Sea branch comprises most of

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boundary current and sinking boundary plumes entrain and vertically redistribute mainly cold Barents Sea branch water to deeper levels. The Barents Sea branch thus supplies most of the intermediate water and deep water of the Arctic Ocean, which ultimately exit through Fram Strait. The Arctic Ocean deep waters will, in periods of reduced deep convection in the Greenland Sea, renew, and eventually replace, the deep, and finally the bottom, water in the Greenland Sea. The Atlantic and intermediate layers of the Arctic Ocean are dense enough to supply the Denmark Strait Overflow Water and the North Atlantic Deep Water. Because of its large volume and long renewal time, the Arctic Ocean may be the most steady of the sources contributing to the overflow.

Acknowledgements Economic support for ŽBR. has been received from the Deutsche Forschungsgemeinschaft ŽDFGSFB-18, TP B3. and from the European Commission MAST III Programme VEINS, through contract MAS3-CT96-0070. R.D. Muench and J. Gunn have been supported in this programme by grants from the US National Science Foundation and the Office of Naval Research.

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