Water mass properties and fluxes in the Rockall Trough, 1975–1998

Deep-Sea Research I 47 (2000) 1303}1332

Water mass properties and #uxes in the Rockall Trough, 1975}1998 N. Penny Holliday *, Raymond T. Pollard , Jane F. Read , Harry Leach
George Deacon Division for Ocean Processes, Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK
Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, UK Received 5 April 1999; received in revised form 8 September 1999; accepted 4 October 1999

Abstract A time series of a standard hydrographic section in the northern Rockall Trough spanning 23 yr is examined for changes in water mass properties and transport levels. The Rockall Trough is situated west of the British Isles and separated from the Iceland Basin by the Hatton and Rockall Banks and from the Nordic Seas by the shallow (500 m) Wyville}Thompson ridge. It is one pathway by which warm North Atlantic upper water reaches the Norwegian Sea and is converted into cold dense over#ow water as part of the thermohaline overturning in the northern North Atlantic and Nordic Seas. The upper water column is characterised by poleward moving Eastern North Atlantic Water (ENAW), which is warmer and saltier than the subpolar mode waters of the Iceland Basin, which also contribute to the Nordic Sea in#ow. Below 1200 m the deep Labrador Sea Water (LSW) is trapped by the shallowing topography to the north, which prevents through #ow but allows recirculation within the basin. The Rockall Trough experiences a strong seasonal signal in temperature and salinity with deep convective winter mixing to typically 600 m or more and the formation of a warm fresh summer surface layer. The time series reveals interannual changes in salinity of $0.05 in the ENAW and $0.04 in the LSW. The deep water freshening events are of a magnitude greater than that expected from changes in source characteristics of the LSW, and are shown to represent periodic pulses of newer LSW into a recirculating reservior. The mean poleward transport of ENAW is 3.7 Sv above 1200 dbar (of which 3.0 Sv is carried by the shelf edge current) but shows a high-level interannual variability, ranging from 0 to 8 Sv over the 23 yr period. The shelf edge current is shown to have a changing thermohaline structure and a baroclinic transport that varies from 0 to 8 Sv. The interannual signal in the total transport dominates the

* Corresponding author. Fax: #44-1703-596204. E-mail address: [email protected] (N. Penny Holliday). 0967-0637/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 9 9 ) 0 0 1 0 9 - 0

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observations, and no evidence is found of a seasonal signal.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Rockall Trough; Interannual variability; Salinity anomaly; Geostrophic transport; WOCE; Time series

1. Introduction The north eastern North Atlantic has a complex topography that divides the ocean into several basins separated by ridges and banks. Immediately west of the British Isles is a basin separated from the Iceland Basin to the west by the Rockall and Hatton Banks, and separated from the Faroe Bank Channel and the Faroe}Shetland Channel to the north by the Wyville}Thompson Ridge. This basin is the Rockall Trough, and from May 1975 to January 1996, David Ellett and colleagues at Dunsta!nage Marine Laboratory (DML) in Oban, UK, occupied the Anton Dohrn Section several times per year, mainly with the RRS Challenger. From October 1996 to May 1998, the Southampton Oceanography Centre (SOC) completed a further three sections as part of the UK WOCE programme. The location of the section and the topography of the region are shown in Fig. 1. The section runs from the Scottish continental shelf at Barra Head, over the Anton Dohrn seamount (least depth approximately 550 m) to Rockall (a small island at the shallowest point of the Rockall Plateau). The Rockall Trough shallows northwards, and so through-#ow is restricted to waters of certain depths. The entrance to the Rockall Trough west of the Porcupine Bank is between 3000 and 4000 m deep, the deepest part of the section in Fig. 1 extends to over 2300 m, but the ridges at the northern end of the basin are 1200 m (between George Bligh and Lousy Banks) and 500}600 m (Wyville}Thompson Ridge). The Rockall Trough is a small area of the subpolar North Atlantic but nonetheless supplies warm and saline water to the Nordic Seas, which eventually become transformed into cold fresh southward #owing deep water. The hydrography of the Rockall Trough was reviewed by Ellett et al. (1986), henceforth referred to as EEB86. In summary, there is a general northward drift of warm saline upper waters with anticyclonic recirculation and enhanced #ow in the shelf edge current at the continental shelf break, a cyclonic recirculation of deep fresher water which is trapped by the topography, and occasional evidence of cold over#ow water coming southwards across the Wyville}Thompson Ridge (see also Arhan et al., 1994). Estimates of total transport range from 0 to 4 Sv (1 Sv"10 m s\) (e.g. Otto and van Aken, 1996; Schmitz and McCartney, 1993). The circulation in this region is complex in mesoscale detail, with many eddies and anticyclonic recirculation features around banks and the seamounts. The upper waters lie within the temperature}salinity range of Eastern North Atlantic Central Water (ENAW) (Harvey, 1982) and are substantially more saline than the subpolar mode waters of the Iceland Basin and the subpolar gyre. A poleward #owing shelf edge current exists all along the European continental shelf break bringing warm and saline upper water with ENAW characteristics through the

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Fig. 1. Location of the Anton Dohrn seamount section (circles), and the topography of the northeastern North Atlantic. Station &F' and &M' are referred to in the text and are indicated by "lled circles. Stars indicate synoptic sections occupied in May 1998. Contours are 200, 1000, 2000, 3000 and 4000 m.

Rockall Trough (Huthnance, 1986; Hill and Mitchelson-Jacobs, 1993). The main branch of the North Atlantic Current (NAC) carrying fresher Western North Atlantic Central Water (WNAW) is generally considered to turn northwards to the west of Hatton Bank (e.g. Schmitz and McCartney, 1993) and does not enter the Rockall Trough from the south. The deep waters show a salinity and potential vorticity minimum usually associated with Labrador Sea Water (LSW); Talley and McCartney (1982) show distribution maps of LSW with a branch reaching northwards into the Rockall Trough having made its way to the eastern margin of the North Atlantic south of the Iceland Basin. Any water below 1200 m is trapped by the topography and must recirculate; EEB86 suggested that the LSW circulates cyclonically around the basin. The Wyville} Thompson over#ow water, which forms a small component of the Iceland}Scotland Over#ow Water (ISOW) (Mauritzen, 1996), is thought to be limited to the western side of the Rockall Trough and associated with the Feni Ridge, a sedimentary feature running the length of the eastern Rockall Trough (Ellett and Roberts, 1973). Schmitz and McCartney (1993) suggest that some Lower Deep Water (LDW) reaches the southern part of the Rockall Trough, revealed through a signature of high silicate levels that re#ect the diluted in#uence of Antarctic Bottom Water.

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A feature of the upper ocean of the Rockall Trough is the unusually deep convective winter mixing that occurs to 600 m in typical winters, and may penetrate to 1000 m in severe winters (Meincke, 1986). Wade et al. (1997) demonstrate that the presence of SAIW provides stability to the upper and intermediate waters preventing deep winter mixing in parts of the eastern North Atlantic. The absence of an SAIW salinity minimum in the Rockall Trough means a weak density gradient allows deep convection to occur, resulting in high winter heat loss to the atmosphere helping to moderating the North European climate (Ellett, 1993a). Thus the Rockall Trough experiences a strong yet predictable seasonal cycle in temperature and salinity (summarised in EEB86). The North Atlantic is well known to exhibit temporal (seasonal to decadal) variability of water mass properties and circulation features, with successions of salinity anomalies in the upper water (Belkin et al., 1998) and changes in convective activity related to atmospheric patterns such as the North Atlantic Oscillation (NAO) propagating around the basin (Dickson et al., 1996). These review papers pull together many decades of observations to argue that some part of the variability can be viewed as climatic signals related to atmospheric patterns. These large-scale patterns explain some phases of variability, but there remain unanswered questions regarding the changes observed in the Rockall Trough. This paper analyses the details of the 23 yr hydrographic time series of the Anton Dohrn Section. We investigate all depths of the water column to assess the levels of variability in water masses properties, and changes in circulation and #uxes. The unusual time series with high seasonal and interannual resolution can provide insight into the levels of variability of properties and #uxes experienced in the eastern margin of the North Atlantic. We extend previous analysis of the time series data with a more intimate look at the di!erent water masses present in the regions and with an assessment of the variations in poleward transport of the upper ocean.

2. The Rockall Trough hydrographic time series Since 1975, the concerted e!orts of David Ellett and colleagues at DML have resulted in the accumulation of an unusual hydrographic time series with several sections of temperature and salinity in almost every year to 1996, achieving fairly even temporal coverage during that time (Table 1). The time series has been added to during 1996}1998 by the Southampton Oceanography Centre, though with reduced temporal and spatial sampling. The Anton Dohrn section consists of 22 full-depth, standard location stations which were repeated during each occupation (Fig. 1). During the 23 yr period to date the section has been visited 63 times, and a total of 38 full occupations achieved. The spread over the seasons is uneven because of the di$culties of surveying the region in poor weather: 4 in winter, 13 is spring, 14 in summer and 7 in autumn. In this paper the seasons are de"ned as follows: winter is December}January}February, spring is March}April}May, summer is June}July}August, and autumn is September}October}November. The severe weather often encountered in the Rockall Trough has led particularly to a small

1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

C12496

D123398

(CD9295)

LF189

C681 (C782)

C576 C677 C678

A

DI18089

C10193 C11094

(C480)

C477

C375

M

C1487

C7591

C278

F

C987

C185

C281

J

C10393 C11294

(C2588)

C485

C783

C779 C780

C775 C876

M

C6790

C3088

(C284)

C987 (5579)

J

C12095

(C8191)

C1081

C1177

C1075

J

C11494

LF189

C885

C1183

(C1275) (C1276) C1377 C11B78

A

D123097

C9792 C10593

C7190

C11D78 C1379

S

D122396

C1581 C1582

C1679

(C1576)

O

C11694

CD4489

C2287

C1084

C1478

C1475

N

D

Table 1 The Rockall Trough time series: occupations by season and years. Sections highlighted in bold text are complete occupations; sections in parentheses are not used in this analysis because of data quality problems or unavailability. The pre"x of each cruise signi"es the vessel with which the section was surveyed; `Ca indicates RRS Challenger, `DIa indicates RRS Discovery, `CDa indicates RRS Charles Darwin, `LFa indicates R/V Lough Foyle, and `Sa indicates RRS Shackleton N. Penny Holliday et al. / Deep-Sea Research I 47 (2000) 1303}1332 1307

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number of winter occupations. The DML data (pressure, temperature and salinity) were logged from a Bissett Berman STD from 1975 to 1978, a Bissett Berman 9040 CTD from 1978 to 1992 and a SeaBird CTD from 1991 to 1996 and were obtained by the authors from the British Oceanographic Data Centre where most are publicly available. Temperature, salinity and conductivity sensors were calibrated with reversing thermometers and bottle salinities obtained from Nansen bottles; pressure corrections were obtained from an acoustic pinger measuring depth o! the sea#oor. The SOC data (pressure, temperature, salinity and oxygen) were collected using Neil Brown Mk. III CTDs and calibrated with laboratory tests and bottle salinity and oxygen samples. Some cruises experienced di$culties with instrumentation resulting in noisy data; these sections were not used in the analysis (Table 1).

3. Analysis methods 3.1. Changes in water mass properties To consider the long-term changes in a time series of observations it is useful to develop a baseline from which relative changes can be calculated. This is particularly important in the Rockall Trough when variations on time-scales longer than one year are considered because of the very pronounced seasonal signal in temperature and the deep winter mixing. We have constructed a seasonal climatology shown in Fig. 2a}c. Pro"les at each standard station were averaged into 10 dbar pressure bins to form a seasonal temperature}salinity relationship. The winter means were calculated from only 4}7 occupations with none in December (Table 1), so are less representative of long-term conditions than the other seasonal means. The four seasonal climatologies have themselves been averaged to form a mean climatology with equal weight given to each season (Fig. 3). The main features of the section as described in EEB86 are clearly visible, including the shelf edge current indicated by high salinity and isolines sloping down toward the east, the deep permanent thermocline and pycnocline (800}1400 dbar), the general shoaling of isolines to the west indicating the general northward drift of the warm and saline upper ocean, and the cool, fresh, well-strati"ed deep water. All these features are more carefully considered throughout this paper, the value of the mean is in having a baseline condition from which to evaluate temporal changes. In order to determine the true interannual change in the Rockall Trough we must di!erentiate the e!ects of change in structure due to mesoscale variability from actual changes in water mass characteristics. Bindo! and McDougall (1994) show that the thermohaline characteristics at any pressure level of a section may change for two reasons: "rstly because of vertical motion of density surfaces (heave) caused by changes in rate of renewal of water masses or by dynamical changes, and secondly because of density compensating changes in temperature and salinity which re#ect altered conditions in ventilation of the water mass. The counterintuitive conclusion is that the warming of mixed layer water (with no change in salinity) will lead to a cooling and freshening of the subducted water mass along a constant density surface,

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Fig. 2. The climatology of the Anton Dohrn Section: seasonal means of (a) temperature (contour interval 0.5), (b) salinity (contour interval 0.025) and (c) potential density (p , contour interval 0.05). Winter is F December}January}February, spring is March}April}May, summer is June}July}August, autumn is September}October}November.

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Fig. 2. (continued).

though freshening of the mixed layer by increased precipitation over evaporation will also lead to freshening of the subducted water mass. They compare the implication of observing changes on isobaric surfaces and constant density surfaces, and note that variability due to eddies is reduced on density surfaces, particularly in the upper thermocline. Thus we have chosen to calculate salinity anomalies from a standard potential temperature}salinity relationship (the seasonal mean) along isopycnals. This enables us to distinguish interannual changes from mesoscale and seasonal variability. We apply this method to a single station in order to gain an overview on depth dependency of temporal changes, and later analyse the individual water masses across the whole section. 3.2. Estimation of geostrophic transport Ellett and Martin (1973) quanti"ed the volume #ux through the Rockall Trough by selecting 1800 m as a reference level of no motion. At 0}500 m (which could #ow northwards over the Wyville}Thompson Ridge) they found a range of #4.7 to !0.6 Sv with mean of #2.7 Sv (1 Sv"10 m s\, positive is northwards). At 500}1200 m (which could #ow northwestwards through channels between banks) the mean was #1.0 Sv, range #3.2 to !2.7 Sv. Between 1200 and 1800 m (water with no northern exit) the mean was #0.2 Sv, range #0.7 to !0.3 Sv. The estimated transports depend on the problem of determining a level at which the velocity is

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Fig. 3. The climatology of the Anton Dohrn Section: annual means of (a) temperature, (b) salinity and (c) potential density (p ) with mean #ow directions resulting from level of no motion at 1200 dbar. F

assumed to be zero (or some known value) to reference the shear pro"le from the thermal wind equations. We repeat this analysis with the time series of sections (and direct current measurements) to reduce the error caused by the uncertainty of the reference level. The climatological mean temperature, salinity and potential density sections shown in Fig. 3 indicate that towards the base of the permanent thermocline/pycnocline (around 1200 dbar) the isobars are nearly horizontal across the section, implying close to zero baroclinic #ow. From the temperature}salinity relationship shown in Fig. 4 it can be seen that the permanent pynocline lies between the cool fresh Labrador Sea Water (LSW) and the warmer saltier Eastern North Atlantic Central Water (ENAW). If we assume that the LSW #ows independently from the ENAW, then it is possible, though not certain, that there may be a level of no motion between them. The topographic constraints mean that water above 1200 m can #ow through the Rockall Trough whereas water below 1200 m must recirculate and conserve its volume #ux (assuming little vertical mixing). An appropriate reference level will give close to zero net transport in the recirculating deep water. Using only complete and

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Fig. 4. Temperature}salinity from the Anton Dohrn Section in October 1996. Water masses are labelled: Eastern North Atlantic Central Water (ENAW), Labrador Sea Water (LSW), and Lower Deep Water (LDW). Solid curves are potential density surfaces (p ). F

full-depth sections we have calculated the net transport below 1200 dbar for a variety of reference levels (Fig. 5). The result is that 1200 dbar gives the net transport closest to zero with the smallest variability. The levels that give a net transport below the reference level of less than$0.25 Sv result in a reasonable level of up or downwelling (maximum of $90 m per year). Levels shallower than 1000 dbar and deeper than 1300 dbar, require an unrealistic vertical motion of isopycnals ($200 m or more per year) to support the possible net volume #uxes in the closed basin. A small source of error in this assumption though is the possibility of the presence of southward #owing ISOW at 1600 dbar in some of the sections. A UK cruise to the Rockall Trough included two synoptic sections south of the Anton Dohrn Section (RRS Discovery cruise 233, May 1998, D. Smythe-Wright and H. Bryden). The synoptic sections (DI23398-B and DI23398-C in Fig. 1) give an opportunity to empirically determine a reference level by assuming that the mean meridional transport hardly di!ered between the sections; again the aim is to choose a reference level that minimises the noise created by the choice of reference level. Conservation of mass means the transport de"cit between the sections will be closest to zero for the optimal reference level. The transport through each section was determined for a series of isobaric reference levels and the di!erence between the sections calculated. The de"cit is close to zero for the reference level of 1200 dbar,

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Fig. 5. Net transport across the Anton Dohrn Section below 1200 dbar as a function of depth of reference level of no motion. Diamonds and the solid curve are the mean values from 23 complete and full-depth sections; the bars represent$1 standard deviation. Positive transport is northwards through the Rockall Trough.

Table 2 Mean and standard deviations of "ltered current meter records at stations F and M (tidal components removed). `va is the northward component and `ua is the eastward component North Mean l (cm/s)

Std. Dev. l (cm/s)

Station M (east of the Anton Dohrn seamount) 150 0.2 15.8 600 0.4 13.1 1100 0.3 7.8 1800 0.1 4.3

!3.1 !3.0 !1.1 0.4

14.7 12.1 7.4 4.7

Station F (west of the Anton Dohrn seamount) 100 1.1 17.8 500 1.3 12.2 1000 0.3 8.0 1750 !0.10 7.1

0.2 0.3 1.1 !0.4

17.3 12.8 7.9 7.1

Nominal depth (m)

East Mean u (cm/s)

Std. Dev. u (cm/s)

rising to over 1.0 Sv at 1100 dbar, over 0.5 Sv at 1300 dbar, and reaching a maximum di!erence of 2.0 Sv at 1000 dbar. Current meter moorings were deployed at stations F and M periodically from 1978 to 1984 (Fig. 1). While they show a large variability in direction and size of de-tided currents, their means lend weight to the argument for the mid-depth reference level and the EEB86 mean circulation scheme. Table 2 lists the mean de-tided current

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obtained over the full time series of the current meter arrays. The upper currents (1000 m and above) have a mean northward #ow west of the Anton Dohrn Seamount, and mean southward #ow to the east (anticyclonic). Both moorings then exhibit a mid-depth reversal in currents, leading to a mean southward #ow in the deep water west of the Seamount and northward #ow to the east (cyclonic). The result of the analysis of the direct current measurements in the Rockall Trough and our empirical approach to determining the reference level of no motion is that 1200 dbar is the optimum level to use. The results of the volume transports calculated in this way are described in Section 5 and discussed in Section 7.

4. Long-term changes in water mass properties The strong upper ocean seasonal cycle is well de"ned by the climatology (Fig. 2), providing a clear visualisation of the cycle which needs to be taken into account when studying interannual to decadal changes. The mixing in the winter (DJF) is best illustrated by the near-homogeneous temperature and density down to 500}800 m; the salinity mean is rather noisy as a result of the low number of sections. Convection occurs over the winter period removing the seasonal thermocline and releasing heat accumulated in the summer to the atmosphere, and mixing heat and saline water down to around 700 m. Despite the noise in the winter salinity section it is clear that the salinity maximum of the ENAW ('35.40) which is present at 100}400 dbar in the other seasons extends to 600 dbar in the winter. Throughout this process the warmest and most saline waters remains associated with the shelf edge current, but extend away from the continental shelf break as far out as 11}123W. The coldest and freshest surface water in the winter is present over the Rockall Bank and Scottish continental shelf. The shoaling of isopycnals to the west is present throughout the winter and the other seasons maintaining the northward baroclinic transport. With the onset of warmer weather in spring (MAM), the surface layers gain heat, the seasonal thermocline begins to develop and the upper layer regains strati"cation, capping the winter mixed layer. During summer (JJA) the shallow seasonal thermocline is more pronounced with additional surface heating, and a surface layer of fresh water accumulates. The seasonal thermocline/pycnocline is restricted to the top 100 m, but heating which began in the spring continues through the summer causing the 9.53C isotherm to descend to 200 dbar. In the autumn months (SON) the downward transfer of the heat has increased still further (9.53C isotherm at 300 dbar), resulting in a less sharp thermocline, though the pycnocline remains restricted to the upper 100 dbar as it is dominated by the presence of the freshwater layer. The cycle is completed with the re-establishment of winter convection and the mixing away of the seasonal thermocline and pycnocline. With the seasonal cycle removed from the data we will "rst consider changes over the full depth of water column to gain a broad picture of the vertical structure to the variability. The deepest station of the section (M, Fig. 1) provides the fullest information as it samples all the main water types. The salinity anomaly is shown in Fig. 6a; the pattern retains a level of stochastic noise, but it is clear that the water column is divided into upper (above 1000}1200 dbar) and deep layers, and that the two layers

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Fig. 6. Salinity anomaly at station M east of the Anton Dohrn seamount. Anomalies are from the 1975}1998 seasonal climatologies and calculated on isopycnals. Shaded areas are positive anomalies, the heavy black line represents zero anomaly and the thin black lines are at 0.02 intervals.

show anomalies with di!erent amplitudes and di!erent phases. Fig. 6b shows that the correlation of the time series at 2000 dbar becomes statistically zero at the 95% level with series below 1000 dbar. The cross-correlation function shown is for a time lag of zero years; in fact the correlation is signi"cant for lags of $1 yr down to 500 dbar and !1 to 2 yr from 500 to 800 dbar. In all cases the series were smoothed to yearly intervals to calculate the correlation coe$cients. The mid-depth decorrelation occurs towards the base of permanent thermocline, between the ENAW and the LSW. It is the approximate depth of the northwestern sills which limits the depth of water that can #ow through the Rockall Trough, and it is also the depth to which extreme winter mixing may penetrate (though deeper than average winter convection). In both depth bands there are periods of 5}10 yr of relatively high or relatively low salinity, with the amplitude of the anomalies being greater in the ENAW. The greatest changes in salinity of the section are experienced by ENAW between 300 and 600 dbar, though the anomaly retains a coherent pattern to depths of 100 dbar (deeper than average winter mixing). In order to study the details of changes in ENAW we have chosen to look at the salinity anomaly on the 27.36 potential density surface. This isopycnal was selected as one that is present in all sections and captures the core of ENAW (Fig. 4). In fact a similar pattern is observed on lighter and denser surfaces above the permanent pycnocline. The ENAW anomalies were obtained by subtracting the seasonal climatological temperature}salinity relationship from a mean pro"le representing the whole Anton Dohrn Section. Data from sections with less than six stations, and stations on the continental shelf (not ENAW) were excluded. The evolution of the salinity anomaly over time is shown in Fig. 7a. The range in anomaly is 0.1, with a period of very low salinity in the late 1970s followed by a rise to a maximum in 1983}1985 (equating to a temperature anomaly change of !0.25}0.303C). The salinity then proceeds to decrease to a further minimum in 1990,

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Fig. 7. Time evolution of water masses of the Rockall Trough. (a) ENAW salinity anomaly from the 1975}1995 mean (sigma}theta"27.360, seasonal cycle removed). Solid curve and "lled diamonds are the salinity anomaly; open diamonds represent the approximate depth of the isopycnal at each occupation; (b) the deep water: LSW `corea as indicated by a minimum in potential vorticity. Solid curve and "lled diamonds are LSW salinity; dashed curve and open circles are the bottom water salinity; "lled squares are the pressure of the LSW PV minimum.

when there is a sudden rise of nearly 0.08 to a peak in 1992. By the mid-1990s the anomaly has reverted to around average for the 23 yr period. In Fig. 6 the Labrador Sea Water (LSW) below 1000}1200 m undergoes a pattern of salinity variability rather di!erent from that in the upper water. Despite the fact that changes in LSW (which undergoes small changes in density over time) is not especially well represented by this initial approach, clear periods of relatively constant salinity lasting 5}10 yr can be seen separated by periods of freshening, and over the whole

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time series the deep water is becoming progressively fresher. The pattern of change is notably di!erent from that in the upper ocean, and the amplitude of the anomaly from the mean is a factor of two smaller in the deeper water. This suggests that there are di!erent mechanisms causing the variations in the deep water compared to the upper ocean. A more robust description of the LSW may be obtained by "nding the `corea of the LSW. The core can be de"ned by its characteristic minimum in potential vorticity (PV) which is a result of its formation by deep convective winter mixing. The PV minimum persists throughout the LSW distribution and has been used to identify spreading pathways in the North Atlantic (Talley and McCartney, 1982). It is known that the density and depth of LSW change on interannual time-scales, but while the absolute value of the PV minimum may vary at source and by mixing during advection, the presence of the PV minimum persists. By looking at the properties of the LSW at the PV minimum rather than at a selected depth or density, we capture changes that a!ect the entire water mass at this location rather than focusing on changes as a result of vertical displacement of isopycnals or the depth of the core itself. Fig. 7b shows the depth of the PV minimum and the salinity of the LSW core varying with time. During the time period sampled the core of the LSW underwent two major freshening events (reduction of salinity by 0.02 during 1983}1985 and 0.04 during 1990). The freshening events separate periods of fairly stable, though slightly increasing salinity of LSW and are associated with freshening bottom water. Over the entire time series (23 yr) the LSW core has freshened by 0.04. Each freshening event renews the LSW salinity minimum, and between events the minimum is gradually eroded by the higher salinity waters above and below. The over#ow water that periodically #ows southwards over the Wyville}Thompson Ridge is characteristically cold and fresh (around 13C and 34.91}34.95 in the Faroe}Shetland Channel according to Turrell et al., 1999). Ellett and Roberts (1973) implied from sedimentary features that the path of the over#ow through the Rockall Trough is limited to the western side and associated with the Feni Ridge. Swift (1984) demonstrated that immediately south of the Wyville}Thompson ridge the bottom water retained just 10% of the Norwegian Sea over#ow water properties during an extensive survey in 1973. This would suggest that any in#uence of the over#ow temperature and salinity properties may be very small as far south as the Anton Dohrn section. In fact throughout the time series it is rarely possible to distinguish any evidence of over#ow water; on occasion one or two stations adjacent to the Rockall Bank may show a temperature}salinity relationship without the LSW salinity minimum seen east of the seamount (for example Fig. 4, October 1996). It is possible that this is the in#uence of the over#ow water but the e!ect is limited and intermittent. The western side of Rockall Trough is thus not a major pathway for over#ow water to reach the rest of the North Atlantic.

5. Transport in the upper ocean Using 1200 dbar (or the sea #oor for stations less than 1200 m deep) as the reference level of no motion we can proceed to calculate the mean circulation and the temporal

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variability over the period of the time series. Consider "rst the annual climatological mean density structure shown in Fig. 3, which represents the mean conditions in the northern Rockall Trough with all seasonal signals and mesoscale variability removed. By calculating the geostrophic velocities and transports from the mean section we can derive a circulation scheme that re#ects the average conditions (Fig. 3c). Below the reference level the deep water travels northward to the east of the Anton Dohrn seamount, and southwards to the west, i.e. with a cyclonic sense and recirculating approximately 0.5 Sv of LSW. The shelf edge current is associated with the high salinity ('35.40) concentrated over the continental shelf break at the east of the section. It can be seen extending to 700 dbar and contains a transport of 2.8 Sv northwards. Between the shelf edge current and the Anton Dohrn seamount is a southward recirculation of !2.5 Sv of ENAW associated with isopycnals that slope gently down to the west at 10}10.53W. The main northward current of ENAW west of the seamount is located between 11.5 and 133W in the climatology and transports 4.1 Sv poleward. A small southward #ow of cool, fresh ENAW (!0.6 Sv) can be seen on the east #ank of the Rockall Bank, evident in the density section as isopycnals sloping downwards to the west at the depth of 500}700 m. The resulting mean net northward transport of ENAW is 3.8 Sv above 1200 dbar through the Rockall Trough during the period 1975}1998. The mean conditions disguise a large range of total net transport through the Rockall Trough. Considering only the complete sections that represent the same total area we can calculate the transport above 1200 dbar from 1975 to 1998 (Fig. 8a). It can immediately be seen that there is a wide scatter in total transport of the upper ocean, varying from near zero to 8 Sv, and reaching the most energetic state in 1980, 1989}1990 and 1998 and relatively lower energy in the mid-1970s, mid-1980s and mid-1990s. The total transport is a product of the four circulation features described above, so Fig. 8 includes a breakdown into those component features and Table 3 summarises the mean values. What emerges from these "gures is a sense that the variability of the total transport is a balance of the variations in all the component currents, rather than being dominated by one. The following paragraphs consider the variations of the individual features. The warm and saline shelf edge current has previously been described (from short-term current meter measurements) as a rather steady and consistent northward transport of about 1.2}2.2 Sv (Booth and Ellett, 1983; Huthnance, 1986; Huthnance and Gould, 1989). The shelf edge current is concentrated in a core over the continental shelf break, and its in#uence spreads to typically 700 dbar as indicated by its high salinity signature (Hill and Mitchelson-Jacob, 1993). The latter chose to use 1000 dbar (or the sea-#oor) as a reference level of no motion for a series of CTD sections across the shelf edge current, and found that the resulting transport was close to the 1.2}2.2 Sv from current meters, arguing that this was thus a reasonable approach. Extending that argument to the Anton Dohrn Sections results in the transport values seen in Fig. 8b, which are not so consistent with the short-term current meter results. The range of total poleward transport of the shelf edge current is remarkably high (0}8 Sv) with a mean of 3.0 Sv. The highest transports are reached in 1980 and 1994, though these peaks are short-lived since the transport returned to the mean value in

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Fig. 8. Transport across the Anton Dohrn Section above (and referenced to) 1200 dbar; positive values represent northward transport. (a) Total volume #ux across the whole section, (b) shelf edge current, (c) ENAW west of the seamount ("lled circles) and east of the seamount (open circles), (d) cool fresh ENAW against the Rockall Bank.

Table 3 Mean and standard deviation of geostrophic currents across the Anton Dohrn section above and referenced to 1200 dbar. Positive values are northward transport; climatological net transport also shown for comparison Current

Mean (Sv)

Shelf edge current ENAW west of the seamount ENAW east of the seamount ENAW against Rockall Bank Net transport across the section

3.0 4.3 !2.6 !0.7 3.7

Climatology

Std. Dev. 2.1 2.7 2.1 2.1 2.4

3.8

1981 (and 1996). There is no correlation between the level of transport and the overall ENAW salinity, but there are clear changes in the thermohaline structure of the current which result in the range of baroclinic transports. The high transport periods are associated with a pronounced saline core of the current on the shelf break, with tight,

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steeply sloping isohalines that reach down to pressures of 800}1000 dbar. It is the combination of a steep density gradient with a strong saline core (relative to the ambient waters) and the current being o! the continental shelf, extending deeper than the mean position, that results in high transport; the strength of the transport is not correlated with interannual changes in salinity. This is demonstrated in Fig. 9, where the high transport in C11294 (May 1994) results from a core extending o! the shelf break to depths of nearly 1000 m, compared to the very low transport during C11b78 (August 1978), where there is no visible salinity core and the isopycnals slope very gently into the shelf edge. Two other examples illustrate the structure at times of average transport, with either a deep current but lack of a strong core (C1487, April 1987) or a strong core but shallow current (DI22396, October 1996). The examples used here are taken from various times of the year, and the e!ect of seasonality on the transport is discussed below. The interior of the Rockall Trough (away from the shelf edge current) consists entirely of ENAW characterised by its high salinity ('35.30) circulating through an anticyclonic gyre but with a net northward #ow of 1.7 Sv. The #ow of ENAW can be separated into the northward current west of the seamount (Fig. 8c), the southward #ow east of the seamount (Fig. 8c), and some southward #ow on the eastern #ank of the Rockall Bank (Fig. 8d). The highest transports and highest salinities occur west

Fig. 9. Variations in the structure of the shelf edge current as indicated by vertical salinity sections. Salinities above 35.35 are shaded grey, contour intervals are 0.025. (a) High transport in C11294 (May 1994) resulting from a strong core extending o! the shelf break and reaching depths of nearly 1000 m. (b) Low transport during C11b78 (August 1978) with no distinguishable core and a shallow current indicated by gently sloping isohalines. (c) Average transport during C1487 in April 1987 with a deep current but a lack of strong salinity core. (d) Average transport during DI22396 (October 1996) due to a shallow current with a strong core over the shelf break.

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of the seamount in a current that forms a front between cooler, fresher less strati"ed ENAW and a saline core of ENAW. The transport is predominantly northwards (4.1 Sv) except on sections c780 (spring 1980) and c7591 (winter 1991), when it appears to be absent. The range in transport of the current is high (0}8.7 Sv) as the structure of the front varies between being tight with steep and deep-reaching isopycnals with a large cross-front density di!erence, and being wide, gentle sloping isopycnals. Fig. 10 illustrates how the northward current west of the seamount can alter in structure independently of the overall salinity of the ENAW; during the saline C485 (May 1985) and the fresher C1075 (July 1975) the front is deep and steep with greater transport than during LF289 (August 1989) and the rather fresher C1177 (July 1977) when the front is shallow and #ow is weaker. Taking a broad view over time, the greatest transports occur in 1989}1990 and 1998 with a stable spread of values for the remainder of the time series between 2 and 6 Sv. However, as observed in the shelf edge current, there is occasionally a rapid shift in the nature of the transport, such as the drop from a high in September 1990 to zero in February 1991, followed by a rise to nearly 5 Sv in June 1991. The rapid change at that time is re#ected in the total transport across the section (Fig. 8a). On the #anks of the Rockall Bank, at depths of between 400 and 900 dbar there is frequently a parcel of cooler fresher ENAW that is distinguishable from the rest of the ENAW mainly because it is signi"cantly fresher, but also because it has a lower strati"cation. It has salinity of 35.20}35.30, potential temperature of 8.0}8.73C. Examples of this water can be seen in Fig. 10, particularly in section LF289 (August 1989),

Fig. 10. Variations in the structure of the poleward western Rockall Trough current as indicated by vertical salinity sections. Salinities above 35.35 are shaded grey, contour intervals are 0.025. (a) C485 (May 1985) and (b) C1075 (July 1975) have high transport due to a deep strong front; (c) LF289 (August 1989) and (d) C1177 (July 1977) have weaker #ow due to a shallow front with gently sloping isohalines.

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and a fresher variety in section C1177 (July 1977). It di!ers from the main body of ENAW because it is apparently travelling predominantly southwards (Fig. 8d). Since the core of the parcel of water is close to the slope of the Rockall Bank, it is not clear that referencing the geostrophic shear to the sea#oor is appropriate. However, Dooley (1984, discussed in Dickson et al., 1986) demonstrated that the circulation around Rockall Bank was indeed anticyclonic, and current meter observations from the northern edge of the Rockall Bank indicate eastward #ow of up to 25 cm/s during 1982}1983. This water type is also seen west and north of the Rockall Bank in the Vivaldi 1996 data, and the corresponding LADCP pro"les indicate currents travelling northwards at 57.53N 13.63W and westwards at 58.53N 13.83W (between Rockall and George Bligh Bank) with velocities of 10}20 cm/s. The total transport of the cool fresh ENAW is small, but again there is relatively high variability (mean of !0.75 Sv with a standard deviation of 1.4 Sv). The Anton Dohrn Section crosses only one side of the anticyclonic recirculation of the cool fresh ENAW, so the net transport across the whole section is a slight underestimate of the poleward transport of ENAW. It is possible to further sub-divide the upper 1200 dbar of water to consider the layers whose pathways into the Nordic Seas are constrained by topography. Only water above 500 dbar can exit the Rockall Trough across the Wyville}Thompson Ridge, the remaining #ow being northwestwards into the Iceland Basin north of the Rockall and Hatton Banks. From the Iceland Basin, only water above 500 dbar may traverse the Iceland}Faroe Ridge, and though the Faroe}Bank Channel is 800 m deep, only water above 300}500 dbar #ows northwards (Saunders, 1990). Thus of the ENAW #owing through the Rockall Trough, all water above 500 dbar could potentially reach the Nordic Seas. The average transport above 500 bdar is 2.5 Sv (standard deviation 1.6), with the shelf edge current making up the largest component (mean 1.9 Sv, standard deviation 1.3). The remaining 1.6 Sv must recirculate back into the Iceland Basin. A prominent feature of the hydrography of the Rockall Trough is the seasonal signal in upper ocean temperature and mixed layer depth as described in Section 4. Dickson et al. (1986) demonstrated from overlapping current meter records from 1977 to 1994 that there is also a seasonality in the eddy kinetic energy of currents of the Rockall Trough related to variations in windstress. Fig. 11 illustrates all the occupations sorted by month and shows that there is no signi"cant observed seasonal signal in the geostrophic transports. The spread of transports in each season over the time series is more informative than an occupation in each season of a single year, because in one year a change of transports between seasons does not necessarily mean the variability is due to processes directly linked to each season every year. The advantage of the time series is that it indicates the range of conditions over the 23 yr. The smoothed seasonal climatology shows small variations between seasons both in total transport above 1200 dbar (3.0 Sv in spring and summer, 3.6 Sv in autumn and 4.3 Sv in winter) and in the strength of most of the component currents. The winter transport is based on a low number of sections, so while it suggests increased transport, further observations are required for con"rmation. The climatology does hint at a seasonality in the shelf edge current, with transports ranging from a spring maximum of 2.1 Sv to a winter minimum of 0.7 Sv, with summer transport of 0.9 Sv and autumn 1.4 Sv.

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Fig. 11. Volume #uxes above (and reference to) 1200 dbar against month of the year; positive values represent northward transport. (a) Total volume #ux across the whole section, (b) shelf edge current, (c) ENAW west of seamount ("lled circles) and east of the seamount (open circles), (d) ENAW against the Rockall Bank. Interannual variability dominates the signal and masks any seasonality of transport.

This is in direct contrast to the "ndings of Dickson et al. (1986), who observed an autumn}winter maximum in the strength of the shelf edge current. The conclusion is that the interannual signal dominates the observed variability of baroclinic transports, and that if a seasonal signal does exist, this time series does not have su$cient sampling to resolve it.

6. Salt and temperature 6uxes in the upper ocean With the established circulation scheme it is possible to estimate the salt and temperature #uxes (with non-zero volume #ux) from the baroclinic transport (after Bacon and Fofono!, 1996). First consider the mean scenario from the climatology (Fig. 3). The mean volume #ux is 3.8 Sv poleward above 1200 dbar, and using the climatological salinity and temperature this amounts to a mean northward salt #ux of 123.8;10 kg/s and a mean northward temperature #ux of 0.13 PW. But as demonstrated already both the transport and the salinity (and temperature) vary from year to year and over decadal time-scales, so it is logical to assume that the salt and

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Fig. 12. Salt #ux (square symbols and dashed curve) and temperature #ux (triangle symbols and solid curve) as a function of time.

temperature #uxes will vary also. Fig. 12 illustrates the variation in salt and temperature #uxes and shows that substantial changes have occurred over the length of the time series. The periods 1980, 1989}1990 and 1998 all show very high #ux levels (reaching around 0.25 PW and 200}250;10 kg/s salt #ux), and the lowest #uxes occurred during 1976, 1978, 1991 and 1997}1997, when they were less than 0.08 PW and 50;10 kg/s. The pattern of #ux levels from years to year is dominated by the transport described in Section 5 and illustrated in Fig. 8a rather than changes in the salinity of the ENAW described in Section 4 and illustrated in Fig. 7a. The periods of greater salt and temperature #uxes correspond with periods of high transport, and low #uxes result from low transport. Unsurprisingly the salt #ux was enhanced during the very salty years of 1991 and 1998, though the peak of a long period of saline ENAW in the early 1980s did not result in high salt #ux because the transport during that time was low.

7. Discussion 7.1. Origins of salinity changes in Rockall Trough ENAW Water mass properties in any one location are the result of the properties of the water advected into the region and subsequent local modi"cation. Here we discuss the possible origins of the observed changes in the Rockall Trough ENAW, and conclude that the main controls are not the characteristics of the source waters, but modi"cation through air}sea exchange and possibly mixing with the fresher WNAW and SAIW. The atmospheric interaction is a local response and not directly related to changes in the NAO.

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The ENAW has its origins in the relatively quiet intergyre Biscay region between the diverging boundary currents of the subpolar gyre (NAC) and the subtropical gyre (Azores Current). Deep winter mixing gives rise to a mode water which is advected slowly northwards in the eastern boundary current (Pollard et al., 1996). The Biscay upper waters are thought to be fed by a small branch of the NAC that has separated from the main NAC #owing into the Iceland Basin (Pingree, 1993; Pollard et al., 1996). The characteristic high salinity of the ENAW in its formation region is produced by air}sea interaction and mixing with high-salinity surface Mediterranean out#ow. It is subjected to interannual changes in its temperature}salinity characteristics related to changes in wind stress and hence levels of winter mixing and cooling, and so there can be no single `de"nitiona of ENAW temperature and salinity (Perez et al., 1995). They particularly noted high salinities in the early 1970s, and from 1991 to 1993, and low-salinity periods during the 1980s (reproduced in Fig. 13, values kindly provided by F.F. Perez), and demonstrated the latitudinal variations of ENAW as it is subjected to further modi"cation as it spreads northwards. The variations in the properties of the ENAW observed in the northern part of the source region show statistically signi"cant negative correlation with the Rockall Trough anomalies (with a time lag of 1}2 yr, !0.6 for n"20). This correlation should be viewed with caution; both time series exhibit a low-frequency variability with a periodicity of 10}15 yr, which may not be adequately de"ned by the time series of 25 yr. Whether the negative correlation is physically meaningful or not, it is clear that the characteristics of ENAW in the Bay of Biscay are not the source of the salinity variations observed in the northern Rockall Trough. The Rockall Trough upper waters may be in#uenced by the occasional input of an additional water mass; the cooler fresher WNAW. However, there is evidence that WNAW does not usually enter the Rockall Trough (EEB86). Most recently, Pollard et al. (1996) showed clearly that in 1991 all the WNAW was #owing west of Hatton Bank and that only ENAW was entering the Trough. Read and Ellett (1991) observed that any cool fresh Sub-Arctic Intermediate Water (SAIW, found below WNAW) that

Fig. 13. Time series of ENAW salinity anomaly at the Anton Dohrn Section (solid line and "lled diamonds), ENAW salinity at 423N 103W (solid line and "lled triangles), NAO winter index (dashed line) and NAO winter index integrated since 1971 (dash}dot line). Shaded periods are the Great Salinity Anomaly events de"ned by Dickson et al. (1988) in 1975 and Belkin et al. (1998) in 1985.

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reaches the eastern margin is mixed away in the entrance of the Rockall Trough and does not reach the Anton Dohrn Section. However, they suggested that this process may sometimes lead to a freshening of the thermocline water. Periods of low salinity in the Rockall Trough may be a re#ection of greater in#uence of WNAW and SAIW. The properties of the North Atlantic subpolar gyre water masses are a!ected by the propagation of climatic signals. Belkin et al. (1998) discuss the progression of signals in surface salinity around the North Atlantic. They suggest that the low-salinity period from 1975 to 1980 in the Rockall Trough is the signature of the `Great Salinity Anomalya of 1968}1982 (GSA '70s) "rst described by Dickson et al. (1988) and that the decrease in salinity seen from 1985 to 1990 is a subsequent Great Salinity Anomaly (GSA '80s) (shaded periods on Fig. 13). There is indeed a distinct salinity minimum in the late 1970s, but while there is a drop in salinity during 1985 there is no minimum that can be reliably associated with the GSA'80s in the time frame suggested. In fact the GSAs described are predominantly a feature in the WNAW, not the ENAW, which has its source to the south, rather than within the subpolar gyre proper. It is possible that the salinity anomalies in the Rockall Trough are not connected to the GSAs which are dominant to the west and north. The estimated residence time of ENAW in the Rockall Trough is one to three years because of the recirculation and relatively low mean current speeds. The deep winter mixing means that almost the entire ENAW layer is subjected to annual exchange with the atmosphere. It is therefore likely that changes in local atmospheric conditions will have the signi"cant impact on the character of the ENAW. The range of salinity is small compared to that observed in other regions such as the Iceland Basin (Bersch, 1999) and could be due to changes in precipitation. The dominant mode of atmospheric variability in the North Atlantic is the North Atlantic Oscillation (NAO). The NAO is associated with a change in the location and strength of westerlies across the Atlantic and is represented by an index resulting from winter pressure di!erences between Iceland and the Azores (Hurrell, 1995). In many regions observed variability of temperature and salinity can be directly correlated with changes in the winter NAO index. However, in the Rockall Trough the NAO index does not correlate with the anomalies observed; cross-correlation coe$cients for time lags of anything up to 5 yr are statistically zero at the 95% signi"cance limit. Fig. 13 shows the observed ENAW salinity anomaly overlain by the winter NAO index and the integrated index since 1971. The index used here is the winter mean (December}March) of normalised sea level pressure di!erence between Iceland and the Azores, obtained from the Climate Research Unit at the University of East Anglia. This "nding is in agreement with the general spatial nature of the North Atlantic sea surface temperature correlation with the NAO which generally displays a dipole nature with the zero correlation line running between the subpolar and subtropical gyres, and through the Rockall Trough (e.g. Rodwell et al., 1999). 7.2. Poleward transport and recirculation of ENAW Existing literature contains a small number of calculations of mean transport through the Rockall Trough, mostly through geostrophic estimates of baroclinic #ow

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by assuming zero velocity at a selected reference pressure or density level. Often the Rockall Trough is referred to as an oceanic `back-watera, which implies a stagnant pool of water with zero volume #ux (e.g. Otto and van Aken, 1996), and near-zero #ux was found by Bacon (1997) in an inverse model of the transports between Greenland and Ireland in 1991. EEB86 limited their transport estimates to three hydrographic sections taken prior to 1973 and calculated a mean northward through-#ow of 3.7 Sv. Meincke (1986) estimate 2 Sv from a summer cruise in 1984, and Schmitz and McCartney (1993) estimate 4 Sv of subpolar mode water #owing through the Rockall Trough as part of their circulation scheme for the North Atlantic. Recent estimates are of northward transport of 3 Sv, both above the potential density surface 27.70 from data during 1990}1993 (van Aken and Becker, 1996) and above 1400 m from a hydrographic section in 1996 (Pollard et al., 1998). With the evidence presented in Section 3.2 we argue that the spread of transport values calculated using a level of no motion at 1200 dbar is a true representation of the variability, and not an artefact of the choice of reference level. The mean net transport over the whole section is close to previous estimates at 3.7 Sv, indicating the Rockall Trough is not generally an oceanic backwater with zero through#ow. The range of values is greater than previously noted, and this will have implications for conditions downstream of the region. Fig. 14 shows the winter and integrated-winter NAO indices plotted alongside the net transport above 1200 dbar across the Anton Dohrn Section. There is no signi"cant correlation between the winter NAO index and the net transport above 1200 dbar across the Anton Dohrn Section smoothed to yearly intervals with time lags of up to 4 yr. The observations provide an insight into the smaller scale quasi-permanent features of the Rockall Trough which are related to the complex topography. From the single Anton Dohrn Section it is not clear whether the anticyclonic recirculation observed west and east of the seamount is limited to the region around the seamount, or whether it is a larger, basin-wide feature. The two additional synoptic sections occupied during DI23398 provide evidence that recirculation is also present in the

Fig. 14. Time series of net transport across the Anton Dohrn Section above (and referenced to) 1200 dbar ("lled circles), NAO winter index (dashed line) and NAO winter integrated since 1971 (dash}dot line). Shaded periods are freshening events in the Rockall Trough LSW.

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centre of the Rockall Trough. The schematic of Fig. 4 in EEB86 (upper layer circulation derived from a variety of sources including surfaces drifters) suggests that the circulation around the seamount is a separate feature to the circulation in the widest part of the Rockall Trough (centered on 553N). A recent release of surface drifters in winter and summer (Burrows et al., 1999) also revealed separate recirculation features centred on 55}563N and around the Anton Dohrn Seamount. It seems likely that there are a series of semi-permanent or transient recirculation features at various locations in the Rockall Trough, rather than a single large-scale recirculating of the upper water. The variability of the transport of shelf edge current is a surprising "nd in the light of existing knowledge. These results indicate that the structure of the shelf edge current and the baroclinic component of the transport varies widely on an interannual basis. Existing current meter records are short-term measurements and have not captured the interannual variability the hydrographic data have demonstrated. Huthnance (1986) proposes that the forcing of the shelf edge current is due to a poleward decline of dynamic height forcing an increasing poleward barotropic shelf edge current, the constant #ow being a balance between the barotropic #ow and friction. This implies that the cross-slope density gradients and therefore the baroclinic shears do not tell the whole story. So although it is certain that the shelf edge current is not as consistent as previously thought, any study of the long-term variability of the transport would need to include a measure of the barotropic #ow. The upper water in the Rockall Trough provides some portion of the warm and saline Atlantic Water that invades the Nordic Seas and eventually becomes converted into southward #owing cold and fresh deep water. Considering the changing properties and the varying amount of ENAW transported through the Rockall Trough, we have shown that the #ux of salt and temperature is greatly dependent on the volume #ux and has varied by a factor of 5 between 1975 and 1998. Existing studies of the budget of the Nordic and Arctic Seas begin with Atlantic water, which has some level of transport and salinity value (and hence salt or freshwater #ux) which di!ers between authors but remains constant in individual studies. However, we have shown that the Rockall input to the Atlantic in#ow into the Nordic Seas does change on interannual to decadal time-scales. The next step should be to determine how the larger part of the Nordic Sea in#ow (via the Iceland Basin) also varies in volume, salt and temperature #ux and whether these changes are in anyway related to those observed in the Rockall Trough. 7.3. Origins of freshening of Rockall Trough LSW The substantial changes in the Rockall Trough LSW have been noted in the literature already; the freshening event of 1990 was used to estimate transit times of 18}20 yr for the LSW to spread from its source to the northern Rockall Trough by arguing it was the manifestation of a drop in salinity in the Labrador Sea in 1971} 1972 (Ellett, 1993b). This conclusion was queried by Sy et al. (1997) when they uncovered transit times of as little as four to "ve years to the eastern margin. In order to understand the cause of the freshening events in the Rockall Trough we have

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clari"ed their timing and amplitude by analysing the properties in the core of the LSW and now discuss the nature of LSW circulation in this region. The LSW in the Rockall Trough occupies a horizontally enclosed basin, and water entering from the south must circulate around the basin and exit through the southern entrance. A small amount of this water may experience vertical mixing, but broadly speaking the net volume #ux of LSW is topographically constrained to zero in this region. However, the nature of the circulation of the LSW observed at the Anton Dohrn seamount section is rather unclear. Does it represent two sides of an open loop of LSW continually #owing cyclonically from the south, around the basin and exiting to the south, or is there a stagnant pool of LSW circulating in a closed loop occasionally supplemented by an in#ux of LSW from the south? In the "rst scenario changes observed in the LSW would represent changes in the properties of the water mass advected into the eastern margin from the source region, and in the second scenario they would represent changes in local circulation and a `#ushinga of the stagnant pool. We interpret the change to the LSW core shown in Fig. 7b evidence for the second scenario; a stagnant pool of LSW gradually mixing vertically with more saline water lying above and below, periodically #ushed by `newa, relatively unmodi"ed and therefore fresher LSW from the south. The size of freshening events is too large to be explained by changes in source properties of the LSW (as discussed by Dickson et al., 1996). Cunningham and Haine (1995) calculated that salinity anomaly amplitude from conditions in the source region would be changed by a factor of "ve by the time the water reaches the eastern margin. Typical salinity changes observed in the Labrador Sea are of the order 0.05 (e.g. Lazier, 1988), so the observed decreases in salinity in the Rockall Trough (0.02}0.04) are greater than one would expect from source changes. The sudden sharpening of the LSW core salinity minimum in theta}S space seen in the context of a well-mixed reservoir (compared to the LSW outside the Rockall Trough) suggests periodic input, and the LSW mass is seen to experience subsequent erosion of the salinity minimum after each freshening event due to mixing with the saltier waters lying above and below. In conclusion the freshening events in the deep water are caused by a local change in circulation rather than re#ecting changes in the source characteristics of the LSW. Thus estimating transit times of the LSW from its formation regions to the northern Rockall Trough based on those freshening events is misleading.

8. Summary The times series of temperature and salinity data in the northern Rockall Trough has monitored changes in the water mass properties and the #uxes across the section since 1975. The waters above and below approximately 1000}1200 dbar exhibit variations in salinity (and temperature) that are distinct from the other both in terms of the levels of salinity and the time-scales of the changes. The upper water (warm, saline Eastern North Atlantic Central Water) has experienced changes in salinity of $0.05 with high-salinity periods in the mid-1980s, 1991 and 1998. These patterns in

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salinity levels appear to be mainly unrelated to basin-scale and climatic events but are instead a result of local modi"cation through exchange with the atmosphere and possibly mixing with fresher WNAW and SAIW. The fresher deep water (Labrador Sea Water) has experienced two freshening events (early 1980s and 1990) which re#ect periodic in#ow of newer LSW into a recirculating reservoir, and are not manifestations of freshening events in the source region. The upper water (0}1200 dbar) contains much mesoscale recirculation but culminates in a mean #ow of 3.7 Sv northwards through the Rockall Trough. The Scottish continental shelf edge current carries over half of that transport, on average transporting 3.0 Sv northwards. The structure of the shelf edge current changes interannually both in depth to which it extends and the salinity gradient into the core, with consequential changes in cross-slope baroclinic transport (0}8 Sv). The total amount of transport above 1200 dbar through the Rockall Trough varies on an interannual basis from 0 to 8 Sv northwards, and that signal dominates the series, masking any seasonal signal in transport that may exist. No correlation has been found between the transport levels (with the low temporal resolution of the time series) and the NAO. The transport level rather than the changing water mass properties has a profound e!ect on the amount of temperature and salt #ux through the section. The mean salt #ux is 123.8;10 kgls but varies from 15 to 260;10 kgls, and the mean temperature #ux is 0.13 Pw but varies from 0.01 to 0.31 Pw. The high periods of transport and #uxes were in 1977}1981, 1989}1990 and 1998. The warm saline upper water of the Rockall Trough #ows northwards either directly over the Wyville}Thompson Ridge, or into the Iceland Basin to join North Atlantic Current waters which are also #owing north. These subpolar mode waters make their way into the Nordic Sea through the Faroe}Shetland Channel and enter the Arctic system where they are cooled and freshened, to return southwards back into the North Atlantic as over#ow water.

Acknowledgements The e!orts of Dave Ellett and many of his colleagues at DML in maintaining this unique time series in often di$cult circumstances are acknowledged by the authors. The British Oceanographic Data Centre supplied data from the DML cruises from 1975 to 1996. The NAO index data were obtained from the web pages of the Climate Research Unit at the University of East Anglia. Thanks are due to Colin Gri$ths who supplied the current meter data analysis. This is a UK contribution to the World Ocean Circulation Experiment.

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