The East Greenland Current and its contribution to the Denmark Strait overflow

Transcription

1 ICES Journal of Marine Science, 59: doi: /jmsc , available online at on The East Greenland Current and its contribution to the Denmark Strait overflow Bert Rudels, Eberhard Fahrbach, Jens Meincke, Gereon Budéus, and Patrick Eriksson Rudels, B., Fahrbach, E., Meincke, J., Budéus, G., and Eriksson, P The East Greenland Current and its contribution to the Denmark Strait overflow. ICES Journal of Marine Science, 59: The East Greenland Current is the main conduit for the waters of the Arctic Ocean and the Nordic Seas to the North Atlantic. In addition to low salinity Polar Surface Water and sea ice, the East Greenland Current transports deep and intermediate waters exiting the Arctic Ocean and Atlantic Water re-circulating in the Fram Strait. These water masses are already in the Fram Strait and are dense enough to contribute to the Denmark Strait overflow and to the North Atlantic Deep Water. On its route along the Greenland slope the East Greenland Current exchanges waters with the Greenland and Iceland Seas and incorporates additional intermediate water masses. In 1998 RV Polarstern and RV Valdivia occupied hydrographic sections on the Greenland continental slope from the Fram Strait to south of the Denmark Strait, crossing the East Greenland Current at nine different locations. The Arctic Ocean waters and the re-circulating Atlantic Water could be followed to just north of Denmark Strait, where the East Greenland Current encounters the northward-flowing branch of the Irminger Current. There strong mixing occurs both within the East Greenland Current and between the waters of the two currents. No distinct contribution from the Iceland Sea was observed in the Denmark Strait but the temperature reduction of the warm core of the East Greenland Current just north of the strait could partly have been caused by mixing with the colder Iceland Sea Arctic Intermediate Water. The overflow plume south of the sill was stratified and covered by a low salinity lid. Less saline overflow water was also observed on the upper part of the slope. The less saline part of the overflow was identified as Polar Intermediate Water and its properties were similar to those of the thermocline present in the East Greenland Current already in the Fram Strait. It is thus conceivable that its source is the upper (Θ<0) part of the Arctic Ocean thermocline International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved. Keywords: Arctic Ocean, Denmark Strait overflow, East Greenland Current, Nordic Seas, T-S analysis, water masses. Received 21 June 2001; accepted 25 October B. Rudels and P. Eriksson: Finnish Institute of Marine Research, PL33, FIN Helsinki, Finland; tel: ; fax: ; rudels@fimr.f1. E. Fahrbach and G. Budéus: Alfred-Wegener-Institut für Polar und Meeresforschung, Postfach, , D Bremerhaven, Germany. J. Meincke: Institut für Meereskunde der Universität Hamburg, Troplowitzstraße 7, D Hamburg, Germany. Introduction The Arctic Mediterranean Sea comprises the Arctic Ocean and the Nordic Seas i.e. the Greenland, Iceland and Norwegian Seas respectively. The overflow of dense water from it across the Greenland Scotland Ridge into the North Atlantic is the main source of North Atlantic Deep Water. The largest contribution passes through the 640-m-deep Denmark Strait between Iceland and Greenland and the Denmark Strait Overflow Water (DSOW) becomes, because of weaker entrainment, the coldest and densest part of the Deep Northern/Western Boundary Current which ventilates the deep global ocean. Several areas in the Arctic Mediterranean Sea contribute to the Denmark Strait overflow and no agreement exists, so far, as to which source is the most prominent. In the early 1980s Swift et al. (1980) suggested that the /02/ $35.00/ International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

2 1134 B. Rudels et al. main contribution was the Arctic Intermediate Water formed in winter in the central Iceland Sea. However, it was soon realized that the Denmark Strait Overflow Water (DSOW) incorporated water from more than one source and Smethie and Swift (1989) proposed that the densest part of the overflow, because of its greater age, did not originate in the Iceland Sea but derived from intermediate waters in the Greenland Sea. Aagaard et al. (1991) observed that some of the less dense deep waters exiting the Arctic Ocean through the Fram Strait remain in the East Greenland Current (EGC) and cross the Jan Mayen Fracture Zone into the western Iceland Sea, whilst Buch et al. (1996) reported that Arctic Ocean deep waters were occasionally present in Denmark Strait and could, at least intermittently, cross the sill into the North Atlantic. Strass et al. (1993) noted that Re-circulating Atlantic Water (RAW), constituting the eastern part of the East Greenland Current and comprising Atlantic Water of the West Spitsbergen Current that re-circulates in the Fram Strait, interacts with the colder water of the Greenland Sea to create a water mass similar to the Arctic Intermediate Water observed in the Iceland Sea. Mauritzen (1996a, b) proposed that water from the Atlantic layer of the Arctic Ocean, the Arctic Atlantic Water (AAW), together with the Re-circulating Atlantic Water (RAW), supplies most of the overflow water. This is a return to an early observation by Worthington (1970) that the density increase of the Atlantic Water entering the Arctic Mediterranean, and necessary to create the overflow water, primarily occurs in the Norwegian Sea. When the Atlantic waters of the different loops in the Arctic Ocean and the re-circulation in the Fram Strait eventually converge in the East Greenland Current, they are sufficiently dense to form overflow water. The overflow plume is often stratified, being capped by a less saline and less dense lid, most conspicuous in the upper part of its descent (Malmberg, 1972, 1978; Rudels et al., 1999a). The characteristics of this lid are similar to those of the Polar Intermediate Water (PIW), frequently present at the sill in the Denmark Strait (Malmberg, 1972). It has been suggested that the Polar Intermediate Water is formed on the Greenland continental shelf during winter, perhaps as far north as the Greenland Sea (Malmberg, 1972). The fact that the overflow plume retains this low salinity upper lid suggests that the entrainment of ambient water, especially into the lower, denser part of the plume is small. Furthermore, the changes in properties of the Denmark Strait Overflow Water observed south of the sill can, to a large extent, be explained by mixing between the different source waters within the plume without incorporation of ambient water masses (Müller, 1978). The alternative views on the origin of the DSOW have been based mainly on comparisons between the overflow characteristics and the properties of the water masses found in different parts of the Arctic Mediterranean. In many cases the observations have been made in different years. In 1998 the combined cruises of RV Polarstern (September October) and RV Valdivia (August) surveyed the East Greenland Current from the Fram Strait to well south of the Denmark Strait. In addition, the upstream conditions north of the Fram Strait were observed in 1997 by RV Polarstern (Figure 1). All the possible source waters mentioned above were encountered and the observations showed how the water masses evolve through external forcing and through mixing, both internally and with the neighbouring water masses in the Greenland and Iceland Seas, as they are transported southward in the East Greenland Current. To consider the 1998 observations a snapshot of the East Greenland Current we assume that the characteristics of the source waters in the Fram Strait remain approximately constant during the time it takes for the waters to flow along the Greenland slope into the Irminger Sea. Observations Data We discuss CTD observations only. On all cruises SeaBird SBE-911 CTD systems were used. On the Polarstern 98 cruise the sensors were calibrated at the SeaBird facilities in Seattle before and after the cruise, while on the Polarstern 97 and the Valdivia cruises the conductivity sensors were calibrated against water samples measured onboard on a Guildline 8400 salinometer. The Polarstern cruises are described in Stein and Fahl (1997) and Fahrbach (1999). The Polarstern sections are denoted Ps-I Ps-VII and the Valdivia sections V-1 V-4. The sections Ps-VI and Ps-VII repeated the previously taken sections V-1 and V-2. The source waters The different sources are easily distinguished on Θ-S diagrams, and Figure 2a shows the contributions from the Arctic Ocean and from the re-circulating part of the West Spitsbergen Current: the main currents are indicated on Figure 12. A simplified version of the water mass classification for the Fram Strait originally proposed by Friedrich et al. (1995) is used (see also Rudels et al., 1999b; and Table 1). The Re-circulating Atlantic Water (RAW) provides the warmest and most saline water. In the density range 27.70<σ θ the Arctic Ocean contribution, the Arctic Atlantic Water (AAW), is colder and less saline. In the density interval 27.97<σ θ and σ the characteristics of these two contributions overlap. The RAW is distinguished here by a positive slope (stable in temperature, unstable in salinity stratification), while the AAW has a negative slope in the Θ-S diagram (stable in temperature, stable in salinity stratification). The

3 The East Greenland Current and its contribution to the Denmark Strait overflow 1135 Figure 1. The Nordic Seas and the positions of the Polarstern (Ps) and Valdivia (V) sections taken in autumn 1998, and of the Polarstern stations from 1997 used in Figure 2a. warmest AAW, in fact, has a slight positive slope suggesting that this part derives from a return flow along the Nansen Gakkel Ridge (Rudels et al., 2000) of the anomalously warm Atlantic Water that recently entered the Arctic Ocean and was first reported by Quadfasel et al. (1991). The upper Polar Deep Water (updw) from the Arctic Ocean lies in the density range 27.97<σ θ and σ , but its temperature is below 0 C. It is distinguished by an almost constant negative slope in the Θ-S diagram. In the water column it is located between the AAW and an underlying intermediate salinity maximum that derives from the Canadian Basin Deep Water (CBDW). Below the salinity maximum lies the colder, denser European Basin Deep Water (EBDW) with its salinity maximum at the bottom. Two dense water masses from the south are identified. The Arctic Intermediate Water (AIW) occupies the same density range as the updw but is colder, less saline and in the Θ-S diagram its slope becomes vertical and then increases from negative infinity with increasing depth. The densest water mass of southern origin from the Nordic Seas is denoted Nordic Deep Water (NDW) and comprises deep waters from both the Greenland Sea and the Norwegian Sea present in the Fram Strait. Its density range corresponds to those of the CBDW and EBDW but its salinity is < The water less dense than and colder than 0 C comprises the Polar Surface Water (PSW). The surface water in the same density range but with temperature

4 1136 B. Rudels et al. above 0 C mostly derives from sea ice melting on Atlantic Water and is denoted Polar Surface Water warm (PSWw). The Polar Intermediate Water (PIW) is not included in this water mass classification but PIW essentially corresponds to the part of the thermocline in the Arctic Ocean water column that lies between the isopycnal and 0 C (see Table 1 and Figure 2a). The waters masses formed in the Greenland and Iceland Seas, which interact with the East Greenland Current waters on their route towards Denmark Strait, are shown in Figure 2b. The AIW in the Greenland Sea is cold, has low salinity and its slope in the Θ-S diagram, going through negative infinity and then increasing with depth, is clearly different from that of the upper Polar Deep Water (updw). The Greenland Sea Deep Water (GSDW) falls in the Nordic Deep Water (NDW) range but its intermediate temperature maximum and deeper lying salinity maximum line up isopycnally with the CBDW and EBDW respectively, showing the communication between the deep waters of the Arctic Ocean and the Greenland Sea (Meincke et al., 1997). The cold, less-saline bottom water indicates input from local deep convection in the Greenland Sea. In the Iceland Sea the bottom water has the same density as the CBDW and the temperature maximum in the Greenland Sea. Above this layer, between σ 0.5 = and the temperature maximum, the Θ-S curves show a mixture between upper Polar Deep Water (updw) and AIW. The temperature maximum in 1998 was above 0 C but colder and less saline than the RAW core in the Fram Strait. Above the temperature maximum a less saline temperature minimum was observed. This minimum was located in a part of the Θ-S diagram not occupied by any water mass present in the Fram Strait or in the Arctic Ocean (Figure 2a), and only sporadically by waters found in the Greenland Sea. According to the classification introduced by Swift and Aagaard (1981) and expounded by Carmack (1990), the part comprising the temperature minimum and the range with increasing temperature and salinity with depth is the upper Arctic Intermediate Water (UAIW), while the water mass at and below the temperature maximum is the lower Arctic Intermediate Water (LAIW). Some confusion about the water masses exists and occasionally the Re-circulating Atlantic Water (RAW) is denoted LAIW. Here we combine the two Iceland Sea intermediate water masses into Iceland Sea Arctic Intermediate Water (IAIW). It should be kept in mind that only the layers down to and including the temperature maximum is formed in the Iceland Sea, while the denser IAIW is derived from a mixing between AIW and updw. Table 1. Water masses discussed in the text. The definitions largely follow Friedrich et al. (1995) and Rudels et al. (1999b) but with some simplifications. The original definitions were made for the Fram Strait and boundaries for the Iceland Sea Arctic Intermediate Water (IAIW) and the Polar Intermediate Water (PIW) have been added. These definitions partly overlap those for the Arctic Atlantic Water (AAW) and upper Polar Deep Water (updw). To compare with a more conventional water mass classification see Carmack (1990). Water mass Water mass boundaries Origin, remarks Polar Surface Water; PSW σ θ 27.70, Θ<0. Arctic Ocean. Polar Surface Water warm; PSWw σ θ 27.70, 0<Θ. Sea ice melting on warmer Atlantic Water Re-circulating Atlantic Water; RAW (a) 27.70<σ θ 27.97, 2<Θ. (b) 27.97<σ θ, σ , 0<Θ. West Spitsbergen Current: (b) Slope in Θ-S diagram; positive. Arctic Atlantic Water; AAW (a) 27.70<σ θ 27.97, Θ<2, If Θ<0 then S< Θ. Arctic Ocean: (b) Slope in Θ-S diagram; negative. (b) 27.97<σ θ, σ , 0<Θ. Upper Polar Deep Water; updw 27.97<σ θ, σ , Θ<0. Arctic Ocean: Slope in Θ-S diagram; negative, almost constant with depth. Arctic Intermediate Water; AIW 27.97<σ θ, σ , Θ<0. Greenland Sea: Slope in Θ-S diagram; through infinity, negative but increasing. Canadian Basin Deep Water; CBDW <σ 0.5, σ , <S. Canadian Basin, but also includes water from Eurasian Basin Deep Water; EBDW the Eurasian Basin <σ 1.5, σ , <S. Eurasian Basin, lower boundary because of the sill in Fram Strait. Nordic Deep Water; NDW <σ 0.5, S< Includes the Greenland, Iceland and Norwegian Seas deep waters (GSDW, NSDW, ISDW). Iceland Sea Arctic Intermediate Water; IAIW (a) 27.70<σ θ, Θ<0, Θ<S. (b) 27.70<σ θ, σ , Θ<1, <S. (c) 27.97<σ θ, σ , Θ<0. (a) and (b) locally formed (c) advected from Greenland Sea and Arctic Ocean: Slope in Θ-S diagram; negative and increasing. Polar Intermediate Water; PIW 27.70<σ θ, Θ<0, S< Θ. Approximate definition. Derives from the colder parts of the Arctic Ocean thermocline.

5 The East Greenland Current and its contribution to the Denmark Strait overflow 1137 Figure 2(a).

6 1138 B. Rudels et al. Figure 2(b).

7 The East Greenland Current and its contribution to the Denmark Strait overflow 1139 For comparison we have kept the water mass boundaries introduced for the Fram Strait on the Θ-S diagram for the Greenland and Iceland Seas. Some of the names given for the Fram Strait are no longer relevant, and some water mass names used for the Nordic Seas are shown in Figure 2b and their characteristics are given in Table 1. In summer the uppermost layer in the Greenland and Iceland Seas is dominated by seasonal heating and occasional ice melt. This layer, commonly called Arctic Surface Water (ASW), is not included in the present water mass classification. In the Θ-S diagrams shown and discussed below the Fram Strait water mass boundaries are retained to facilitate the detection of changes in the water masses but the names assigned to the different parts are not given. The Fram Strait (Ps-I) The East Greenland Current is often associated with the outflow of low salinity Polar Surface Water (PSW) and sea ice from the Arctic Ocean, which passes through the Fram Strait and continues southward along the Greenland shelf and slope. However, even before it is joined by the RAW, the main transport of the East Greenland Current consists of denser water masses from the Arctic Ocean (Rudels, 1987; Foldvik et al., 1988). In the Fram Strait in 1998 the PSW was confined to the Greenland shelf and slope and characterized by a temperature minimum (Θ< 1.5 C), having a salinity of 34.3 and located at or slightly deeper than 100 m. Its upper part was less saline and slightly warmer, indicating seasonal heating and ice melt (Figure 3). Some low salinity surface water was found further to the east but that is mainly the result of sea ice melting on top of warmer Atlantic Water (PSWw). The Arctic Atlantic Water (AAW) had maximum temperatures slightly above 1 C and was located over the Greenland slope as were the upper Polar Deep Water (updw) and the Arctic Ocean deep waters. The presence of AAW and updw was recognized by the spreading of the 0.5 C and 0.5 C isotherms, and the CBDW was present as a salinity maximum at 1800 m. The EBDW provided a deeper salinity maximum at about 2200 m not seen on the section which is cut at 2000 m but evident on the Θ-S diagram (Figure 3). The warmer and more saline RAW was found further to the east, although some detached lenses of RAW had penetrated closer to the slope. The difference in temperature between the two sources, the re-circulation and the outflow, can be clearly seen on the Θ-S diagram in Figure 3, where the temperature minimum of the PSW is easily identified also. The inversions observed in the deep waters suggest isopycnal mixing and that the characteristics of the Arctic Ocean deep water were diluted by incorporating re-circulating NDW. The Θ-S diagrams are included to document the evolution of the water masses as they flow south in the East Greenland Current. The station numbers are given for completeness rather than for the identification of different stations in the diagrams. Greenland Sea at 75 N (Ps-II) Further to the south, in the Greenland Sea, the maximum temperatures of the Re-circulating Atlantic Water (RAW) had decreased while the temperatures at the slope were higher. This suggests that RAW had penetrated to the slope and mixed with the Arctic Atlantic Figure 2. (a) Θ-S diagram showing the East Greenland Current source water masses present in the Fram Strait. The water mass classification is a simplified version of that introduced by Friedrich et al. (1995). The σ θ =27.70 isopycnal separates the Polar Surface Waters (PSW and PSWw) from the intermediate water masses and the σ 0.5 = separates the intermediate and deep waters. In the intermediate range the less dense Re-circulating Atlantic Water (RAW) from the Nordic Seas and Arctic Atlantic Water (AAW) from the Arctic Ocean are separated from the denser upper Polar Deep Water (updw) and Arctic Intermediate Water (AIW) by the 0 C isotherm. The RAW is, when less dense than σ θ =27.97, warmer than 2 C and when denser than σ θ =27.97 it is distinguished from AAW by its positive slope (/) of the Θ-S curves. In the same density range AAW has a negative slope (\) in the Θ-S diagram. The temperature maximum of the AAW is less dense than σ θ =27.97 and it has temperatures below 2 C. The uppermost part of the AAW, the thermocline, has temperatures below 0 C and exhibits similar characteristics to the Polar Intermediate Water (PIW) observed in the Nordic Seas. The updw from the Arctic Ocean and the AIW from the Nordic Seas are separated by the more distinct negative (\) slope of the updw Θ-S curves. The AIW is also cooler, less saline and denser than the updw. In the deep water range the European Basin Deep Water (EBDW) is separated from the Nordic Deep Water (NDW) from the Nordic Seas by the isohaline. The EBDW has an intermediate salinity maximum derived from the Canadian Basin. Although this Canadian Basin Deep Water (CBDW) is much diluted by EBDW occupying the same density range, the part of the EBDW lying between σ 0.5 = and σ 1.5 =35.142, the isopycnal present at the sill depth of the Lomonosov Ridge, is denoted CBDW. The σ 2.5 = corresponds to the density at sill depth in the Fram Strait. The station numbers are given in the Figure and their positions are shown on Figure 1. The stations in the West Spitsbergen Current and the Return Atlantic Current are shown red, those in the Arctic Ocean outflow blue. (b) Θ-S diagram showing the source water masses in the Greenland Sea and the Iceland Sea. The isopycnals separating the upper, intermediate and deep water masses are the same as in Figure 2a. The traditional water masses, AIW, Greenland Sea Deep Water (GSDW) and the upper and Lower Arctic Intermediate Water (UAIW) and (LAIW) are indicated. We only distinguish between the AIW from the Greenland Sea and Iceland Sea Arctic Intermediate Water (IAIW), which is comprised of both the UAIW and the LAIW. The deep waters in both basins occupy the NDW range introduced for the Fram Strait. The station numbers are given in the figure and their positions are indicated on Figure 1. Stations from the Greenland Sea are shown black and from the Iceland Sea green. For further water mass definitions see Table 1.

8 1140 B. Rudels et al. Figure 3. Potential temperature, salinity and potential density distributions, and Θ-S curves from stations 60, 66, 68, 69, 70, 71 on section Ps-I in the Fram Strait. The lines and isopycnals in the Θ-S diagram are the same as in Figure 2. Water (AAW). In the deeper layers the higher salinity at the slope indicated a continued presence of Arctic Ocean deep waters, but the salinity was reduced to less than here making them formally Nordic Deep Water (NDW). Polar Surface Water (PSW) was still confined to the shelf and slope regions and separated by a sharp front from the low salinity upper waters of the Greenland Sea, which were considerably more saline and warmed by summer heating (Figure 4). Cores or lenses of colder (Θ 1 C), less saline RAW were observed further to the east, suggesting that it was mixed with the low salinity upper waters of the Greenland Sea. There were also indications not shown that a part of the RAW had separated from the East Greenland Current at the Greenland Fracture Zone and entered the Boreas Basin. This loss could, combined with the mixing with the AAW, contribute to the lower temperatures of the RAW at 75 N as compared to the Fram Strait. The Arctic Intermediate Water (AIW) closer to the central Greenland Sea was located directly beneath the surface layer and no conspicuous, shallow temperature maximum was present. The AIW was dense enough to interact, not with the RAW, but with the updw and the CBDW in the East Greenland Current. The lower salinities and temperatures of these waters as compared to those in the Fram Strait indicated that they had become diluted by, presumably isopycnal, mixing with AIW. 71 N, the Jan Mayen Fracture Zone (Ps-III) The warm, saline core of the Re-circulating Atlantic Water (RAW) remained at the slope also south of the Jan Mayen Fracture Zone, confining the Polar Surface Water (PSW) and its low temperature core to above the shelf and slope (Figure 5). Compared with section Ps-II at 75 N the intermediate water with temperatures above 0 C constituted a 500 m thick layer centred at 300 m

9 The East Greenland Current and its contribution to the Denmark Strait overflow 1141 Figure 4. Potential temperature, salinity and potential density distributions, and Θ-S curves from stations 132, 133, 134, 135, 136, 139, 141 on section Ps-II at 75 N in the Greenland Sea. The lines and isopycnals in the Θ-S diagram are the same as in Figure 2. depth here that extended across the Iceland Sea. The temperatures and salinities of the intermediate depth water in the central basin were lower than those of the RAW and it superficially resembled Arctic Atlantic Water (AAW). The overlying temperature minimum was, however, warmer and more saline than the temperature minimum of the PSW in the East Greenland Current (Figure 5). As mentioned earlier, the term IAIW is used for the water mass comprising both the temperature minimum and the temperature maximum and extending down to the σ 0.5 =30.44 isopycnal separating the intermediate from the deep waters (Figures 2 and 5). Part of the East Greenland Current separates from the slope at the Jan Mayen Fracture Zone and enters the Greenland Sea, flowing in the Jan Mayen Current toward Jan Mayen and the Mohn Ridge. According to Bourke et al. (1992) this flow mainly involves the upper, less saline Polar Surface Water (PSW), while the deeper lying RAW only appears to make a short incursion into the Greenland Sea and then returns to and crosses the Jan Mayen Fracture Zone in the East Greenland Current. The eastward extension of water with temperatures above 0 C could be caused by such branching, if the most eastward part of the RAW, cooled and freshened by mixing with the less saline upper waters in the Greenland Sea, crosses the Jan Mayen Fracture Zone outside the East Greenland Current and directly supplies intermediate water to the central Iceland Sea. The colder RAW core mentioned in section 2.4 could provide such input. In the deeper layers the difference between basin and slope was less marked in the Iceland Sea than in the Greenland Sea. The characteristics were between those of the updw and the CBDW from the Arctic Ocean and the AIW from the Greenland Sea, indicating that at least

10 1142 B. Rudels et al. Figure 5. Potential temperature, salinity and potential density distributions, and Θ-S curves from stations 218, 219, 220, 222, 223 on section Ps-III at 71 N south of the Jan Mayen Fracture Zone. The lines and isopycnals in the Θ-S diagram are the same as in Figure 2. the Arctic Ocean intermediate and deep waters, albeit diluted, were crossing the Jan Mayen Fracture Zone as a part of the East Greenland Current (Figure 5). Greenland Iceland section (Ps-IV) As the East Greenland Current approached Denmark Strait, the warm core became separated from the slope and moved towards the centre of the channel. It also became colder and less saline, possibly due to mixing with Iceland Sea Arctic Intermediate Water (IAIW). Nevertheless, a comparison between the Θ-S curves from section Ps-III and the Greenland Iceland section (Ps-IV) suggests that the warm layer (Θ>0) on Ps-IV was supplied by water from the East Greenland Current rather than by IAIW. The low-salinity, upper waters had similar characteristics to the Polar Surface Water (PSW) in the East Greenland Current further to the north and the cold, less-saline, upper part of the IAIW, observed on section Ps-III, could no longer be identified. In particular, the temperature minimum at S 34.3 was present, although it had become slightly warmer (Figure 6). This implies that the front separating the Polar Surface Water (PSW) at the shelf and slope from the upper layers in the interior of the basins had disappeared and that the PSW had penetrated into the southern part of the Iceland Sea. Because of this the core of Re-circulating Atlantic Water (RAW) was suppressed towards greater depths. Close to Iceland warm, saline Atlantic Water of the Irminger Current was also observed. At the bottom the densest water was found at shallower levels on the Iceland slope than on the Greenland side, suggesting a re-circulation toward the east of water too dense, and lying too deep, to cross the sill in the Denmark Strait (Figure 6).

11 The East Greenland Current and its contribution to the Denmark Strait overflow 1143 Figure 6. Potential temperature, salinity and potential density distributions, and Θ-S curves from stations (229), 230, 231, 232, 234, 236, 237 on section Ps-IV between Greenland and Iceland. The lines and isopycnals in the Θ-S diagram are the same as in Figure 2. The thinner lines indicate the stations close to Iceland. The Denmark Strait section (Ps-V). When Polarstern took section Ps-V at the sill in the Denmark Strait, Atlantic Water from the Irminger Current was present in the eastern half of the deep channel down to 400 m (Figure 7). Further to the west Re-circulating Atlantic Water (RAW) could no longer be clearly identified. Its warmest and most saline part had disappeared but a distinct, colder temperature maximum was observed at one station in the deepest part of the channel (σ θ 27.96, see Figure 7). The RAW temperature maximum and the denser part of the overlying thermocline appeared displaced westward onto the shelf, where bottom temperatures above 1 C and salinities between and were observed. The lower temperatures and salinities indicated diapycnal mixing between RAW and the waters of the thermocline. The density was still mostly above 27.80, high enough to contribute to the overflow water. The shapes of the Θ-S curves on the Denmark Strait section (Ps-V) were more horizontal and flatter than further to the north, as if the water column had become partly homogenized by diapycnal mixing, reducing the temperature and salinity maxima of the intermediate RAW layer. The convergence of the waters at the sill thus caused a significant weakening of the RAW characteristics. This is surprising, considering how dominant the RAW had been in the East Greenland Current further to the north and the comparatively large area of the sections it occupied there. At the deepest part of the sill denser water was present. It had similar properties to the deeper layers of the East Greenland Current from Ps-II onwards, implying that updw as well as AIW contribute to the densest overflow water (Figure 7).

12 B. Rudels et al Pressure (dbar) VEINS Ps V R/V Polarstern 800 VEINS Ps V R/V Polarstern km km sigma Pressure (dbar) Potential temperature ( C) sigma sigma sigma sigma VEINS Ps V R/V Polarstern km Salinity Figure 7. Potential temperature, salinity and potential density distributions, and Θ-S curves from stations 243, 244, 245, 246, 248 and 253 on section Ps-V in the Denmark Strait. The lines and isopycnals in the Θ-S diagram are the same as in Figure 2. The Atlantic Water of the Irminger Current occupied the same density range as the Polar Intermediate Water (PIW), as represented by the colder (<0 C), less dense part of thermocline in the East Greenland Current. Above the deepest part of the channel a core of PIW could be identified at 200 m, just west of the Irminger Current. An isolated lens of Atlantic Water from the Irminger Current was found at mid depth (150 m) over the Greenland shelf. It could either be water separated from the northward flow, joining the southward moving East Greenland Current to the west, or be part of another branch of the Irminger Current related to the circulation at the Dohrn Bank (Jónsson, pers. comm.) that had penetrated northward on the Greenland shelf. Since no correspondingly warm water was found in the western part of the sections further to the north such northward flow must be limited and likely intermittent. The first possibility is thus more plausible. On the Greenland shelf the PIW appeared to mix with Irminger Current water to create a warmer (1<Θ<3 C), and slightly more saline (S>34.5), water mass, lacking the Arctic characteristics of the PIW (Figure 7). South of the sill (Ps-VI, Ps-VII, V-1, V-2, V-3 V-4) South of the sill the overflow plume was recognised as a cold, low salinity bottom layer. On the Polarstern section Ps-VI the highest densities were found on the slope and not in the deepest part of the section

13 The East Greenland Current and its contribution to the Denmark Strait overflow 1145 Figure 8. Potential temperature, salinity and potential density distributions, and Θ-S curves from the stations 268, 269, 270, 271 on section Ps-VI down the Greenland slope south of Denmark Strait. The lines and isopycnals in the Θ-S diagram are the same as in Figure 2. The arrow indicates stations 268 and 269 on the slope. (Figure 8). The salinity and temperature were higher in the deeper part and this especially the higher temperatures could indicate either entrainment of ambient water into the overflow plume or the presence of Northeast Atlantic Deep Water (NEADW). On section Ps-VII further downstream the coldest and densest overflow water was present over the entire lower part of the slope. The salinity was higher and the temperature lower than on Ps-VI section, suggesting intermittent overflow of the densest components (Cooper, 1955; Mann, 1969) (Figure 9). On the northernmost Valdivia section (V-1), which was occupied earlier and coincided with Ps-VI but extended across the Irminger Sea onto the Iceland slope, a core of low salinity overflow water was observed on the slope at 1500 m (Figure 10). This core, less saline than the low-salinity lid observed on section Ps-VI, had similar properties to the Polar Intermediate Water

14 1146 B. Rudels et al. Figure 9. Potential temperature, salinity and potential density distributions, and Θ-S curves from the stations on section Ps-VII down the Greenland slope south of the Denmark Strait. The lines and isopycnals in the Θ-S diagram are the same as in Figure 2. (PIW). At the sill on section Ps-V water with PIW characteristics was only found in the deep part of the channel, and if this situation also held at the time section V-1 was taken, it would be the only source of PIW to supply the observed low-salinity overflow water. The fact that Polar Intermediate Water was found at great depth so close to the sill and with much of its Θ-S characteristics retained supports the idea that it had passed through the deepest part of the strait. A few deep troughs, cutting into the Greenland slope, exist

15 The East Greenland Current and its contribution to the Denmark Strait overflow 1147 Figure 10. Potential temperature, salinity and potential density distributions, and Θ-S curves from the stations on section V-1 between Greenland and Iceland south of the Denmark Strait. The lines and isopycnals in the Θ-S diagram are the same as in Figure 2. south of the sill, and via these less dense the East Greenland Current water that passes south over the shelf west of the deep channel may sink into the deep Irminger Sea. Such input could, conceivably, also have provided the low-salinity overflow water observed on V-1. On the Denmark Strait section (Ps-V) the bottom water on the Greenland shelf was warmer and more saline than the Polar Intermediate Water but dense enough to sink down the slope, should it cross the shelf break south of the sill. On section Ps-VII (Figure 9) the bottom water on the upper part of the slope was colder

16 1148 B. Rudels et al. and less saline than the Atlantic Water of the Irminger Current, which could indicate drainage from the shelf south of the sill. The temperatures and salinities were, however, considerably higher than those found at the bottom of the shelf on section Ps-V, implying stronger entrainment and mixing with ambient water than that occurring for the denser main overflow. The densities of these waters were also too low for them to contribute to the Denmark Strait Overflow Water (DSOW). The vertical distances between the isopycnals increased south of the sill, especially between and but also between and The larger thickness of the denser interval, which lies inside the overflow plume, can be explained by mixing within the plume, since the 28.0 isopycnal, present at the sill was not found further south. The densest water is then transferred into the interval. The density interval comprises some Denmark Strait Overflow Water (DSOW) but mostly water masses originating from outside the overflow plume, especially in the less dense part of the range. The increase in distance between these isopycnals is mainly due to the presence of Labrador Sea Water (LSW) and Northeast Atlantic Deep Water (NEADW), which lie in the density range , the same as that of the Polar Intermediate Water (PIW). However, entrainment of these ambient waters would increase the thickness of the plume and also widen the distance between isopycnals within the plume. Spall and Price (1998) proposed a different explanation for the larger distances between the isopycnals south of the sill. They think that to conserve potential vorticity the plume is stretched vertically as it sinks down the slope. They also suggest that the eddy activity observed in the East Greenland Current south of the sill (Bruce, 1995) is due to an increase in cyclonic relative vorticity caused by the stretching of the overflow water column, especially its intermediate part. Synopsis The detailed consideration of the individual sections across the East Greenland shelf and slope that has been made suggests the summary of the currents and water masses contributing to and forming the East Greenland Current as presented in Figure 11. The water masses are shown on a density axis as they move and evolve along the Greenland slope from north of the Fram Strait to south of the Denmark Strait. Along this path the East Greenland Current progressively loses its densest components as waters lying too deep to cross the sills of the Fram Strait, the Jan Mayen Fracture Zone and the Denmark Strait are deflected into the interior of the adjacent basins. These losses affect the EBDW in the Fram Strait, the GSDW, the EBDW and the denser part of the CBDW at the Jan Mayen Fracture Zone, and the CBDW and some of the AIW and updw at the Denmark Strait. However inputs from the re-circulating part of the West Spitsbergen Current (RAW) and the mixing with the intermediate waters of the Greenland Sea (AIW) and the Iceland Sea (IAIW) also occur at various points. While the mixing along the Greenland continental slope between the waters of the gyre and of the East Greenland Current is mainly isopycnal, that in the Denmark Strait is strongly diapycnal. The East Greenland Current appears, Figure 11 suggests, to be the main component of the Denmark Strait overflow, at least during the time of this survey. Figure 12 puts Figure 11 into a geographical context. It shows the sources of the East Greenland Current and its interactions with the waters of the Nordic Seas and the various contributions to the Denmark Strait Overflow. The different sources of the Denmark Strait overflow water It is suggested here that the main source of the overflow water is the East Greenland Current and not the IAIW of the Iceland Sea as proposed by Swift et al. (1980) and Swift and Aagaard (1981). However, Jónsson (1999) has recently advocated that the Iceland Sea is the primary source of the DSOW using current measurements from a section located roughly between our sections Ps-IV and Ps-V sections (Figure 1) as his evidence. The most persistent southward flow in his data was found over the Iceland slope. The flow was almost constant with depth down to 800 m and Jónsson concluded that the main contribution to the overflow must have originated in the Iceland Sea in accordance with Swift et al. (1980). This would imply the existence of a narrow jet that is not found in the water mass distributions. However, the fact that the strongest overflow occurs closer to Iceland does not, by itself, confirm that the Iceland Sea is the principal source. A qualitative interpretation of the flow field in the Denmark Strait The denser East Greenland Current water masses, lying below sill depth in the Denmark Strait, feel the shallowing of the bottom as the sill is approached. To conserve potential vorticity they turn and follow the bottom contours eastward but as the waters too dense to cross the sill reach the Iceland continental slope they have to return northward. The area available for northward flow is narrow and to conserve mass the velocity must increase. However, as the dense waters are forced onto the Iceland continental slope the densest isopycnals rise toward the slope. This creates a density structure favourable to a rapid, northward-flowing, geostrophic deep

17 The East Greenland Current and its contribution to the Denmark Strait overflow IC 27.6 PIW AAW updw CBDW RAW AAW + RAW + updw AIW IAIW AIW + updw CBDW IAIW RAW AAW updw DSOW Bottom density EBDW GSDW Arctic Ocean Fram Strait GFZ Greenland Sea JMFZ Iceland Sea Denmark Strait Irminger Sea Input of water masses (density range) Mainly isopycnal mixing Mainly diapycnal mixing Water mass too dense to cross sill Mainly isopycnal mixing Mixture Figure 11. The different water massesas they flow along the Greenland slope shown on a density axis. The East Greenland Current loses its densest components progressively as the waters lying too deep to cross the different sills in the Nordic Seas are deflected into the interior of the basins. This loss is compensated by an input of and mixing with less dense gyre waters, the AIW and IAIW, The mixing between the gyre waters and the East Greenland Current is mainly isopycnal, while the mixing taking place at the sill in the Denmark Strait is strongly diapycnal. The water mass acronyms are the same as defined in the text and in Table 1, GFZ (Greenland Fracture Zone), JMFZ (Jan Mayen Fracture Zone). boundary current. The eastern boundary of the channel is short and opens to the central Iceland Sea and ultimately to the Norwegian Sea. The less dense intermediate waters are pushed upward but become trapped at the Iceland slope between the deep waters too dense to cross the sill and waters of the northward-flowing branch of the Irminger Current. The distances between the isopycnals, especially between σ θ =27.9 and σ θ =28.0, then decrease and, to conserve potential vorticity, the water in this density range acquires negative relative vorticity. The density sections shown in Figures 6 and 7 indicate that the distance between these isopycnals at the Iceland slope is about a third of the corresponding distance close to the Greenland slope. Assuming that the relative positive vorticity at the Greenland slope is given by a velocity change of 0.2 ms 1 over 20 km the negative vorticity required to conserve the potential vorticity in the layer becomes s 1 which corresponds to a velocity increase of 1.8 ms 1 over 20 km towards the Iceland slope. The water at this level becomes deflected southward and crosses the sill at the deepest part of the channel. The Irminger Current frequently occupies more than half of the cross-section in the deepest part of the strait (Figure 7). Its presence forces the shallower part of the East Greenland Current, which is not obstructed by the topography, to veer westward onto the Greenland shelf. The warm core comprising the RAW and AAW then separates from the denser, eastward-moving waters below and the East Greenland Current water column splits, removing the barotropic character of the flow. Some of the Irminger Current water entering the strait penetrates onto the Greenland shelf and returns southward with the East Greenland current. The density range of the Irminger Current is much narrower than that of the East Greenland Current and the intruding water splits the East Greenland Current into a low density, low salinity upper part (σ θ 27.70) that carries the main freshwater flux from the Arctic Ocean and a denser part that slides beneath the re-circulating branch of the Irminger Current and supplies the overflow plume

18 1150 B. Rudels et al. 82 N 78 PIW/AAW updw/cbdw EBDW PSW RAW AW (RAW) WSC 74 EGC AIW 70 RAW? (CBDW) IAIW (EBDW, CBDW) AW? 66 PIW IC W NEADW LSW E Figure 12. The East Greenland Current (EGC): its interaction with the waters of the Nordic Seas and the different contributions to the the Denmark Strait overflow. IC (Irminger Current), WSC (West Spitbergen Current), LSW (Labrador Sea Water) NEADW (Northeast Atlantic Deep Water), the other water masses as in the text and in Table 1. (Rudels et al., 1999a). Depending upon the density of the Irminger Current water this would make a varying volume of the low salinity, less dense Polar Intermediate Water (PIW) join the Denmark Strait Overflow Water (DSOW) together with the waters of the temperature maximum (the RAW and the AAW) and the colder deeper layers, giving the overflow plume its initial, stratified character. Consequently the upper boundary of the DSOW could reflect the density variations of the Irminger Current, which are determined by conditions in the Irminger Sea rather than what happens in the DSOW source areas. The characteristics of the Denmark Strait overflow water Waters with the density and the characteristics of the overflow water at the sill, excluding the densest

19 The East Greenland Current and its contribution to the Denmark Strait overflow 1151 contributions, were found at the same or at somewhat shallower depths in the East Greenland Current. However, the Θ-S properties at the sill could also be created by diapycnal mixing of the East Greenland Current water masses lying between 300 m and 900 m in the East Greenland Current. Similar characteristics are found at much shallower levels, or not at all, in the Iceland Sea (Figure 13). If the Iceland Sea contributes to the overflow in this density range, the Iceland Sea Intermediate Water (IAIW) has to join and become incorporated into the East Greenland Current north of the sill. The densest water at the sill had about the same density, potential temperature and salinity as the water found at the same depth in the Iceland Sea. On the Iceland slope similar characteristics were found at a shallower depth (400 m), while in the East Greenland Current similar characteristics were first observed deeper than 1000 m (Figure 13). We have argued that water with these characteristics is formed by the mixing of Arctic Intermediate Water (AIW) and Arctic Ocean deep waters (updw and CBDW) and that it crosses the Jan Mayen Fracture Zone as the deepest part of the East Greenland Current. The denser waters of the Iceland Sea are then supplied from water returning northwards, because it is too dense to cross the sill in the Denmark Strait, in the boundary current along the Iceland continental slope. We favour this interpretation but we have no better support to offer for it than what is presented above. There is also the question of how representative these observations really are. Station 216, for example, was chosen as being typical of the central Iceland Sea and yet it is, perhaps, located too far north. Then again, how representative were conditions in 1998? The Iceland Sea as source for the Denmark Strait overflow water The alternative is that the Iceland Sea is the main source for the Denmark Strait Overflow Water. The flow field is then the opposite. The water masses must leave the Greenland slope further to the north and penetrate into the central part of the Iceland Sea from north and west. It also implies that they are forced upward onto the Iceland slope from the central Iceland Sea and then flow south towards the strait. Furthermore, if the Iceland Sea provides most of the overflow water, i.e. the less dense as well as the densest, two questions arise: (1) What, then, are the sources that renew the intermediate and deep waters of the Iceland Sea? and (2) What happens to the bulk of the East Greenland Current once it reaches the south-western Iceland Sea? The first of these questions mainly concerns the intermediate water down to and including the temperature maximum, since no deep water is formed as such in the Iceland Sea but is advected from the north in the East Greenland Current or enters from the Norwegian Sea. In section 2.5 above it was suggested that the Iceland Sea Arctic Intermediate Water (IAIW) is renewed by an inflow of cooled and diluted RAW crossing the central part of the Jan Mayen Fracture Zone into the Iceland Sea. The inflow of Atlantic Water in the Irminger Current mainly stays close to the coast and becomes diluted by runoff from Iceland (Hansen and Østerhus, 2000). It is thus not a likely alternative source for the Iceland Sea Arctic Intermediate Water (IAIW). Another possibility is that IAIW is formed out of a westward flow of Atlantic Water as it separates from the Norwegian Atlantic Current south of Jan Mayen (Swift and Aagaard, 1981). This would imply a large, circa six degrees, reduction in temperature of the Atlantic Water that has newly entered the Arctic Mediterranean. The mean surface heat loss in the Iceland Sea has been estimated to be 70 Wm 2 (Malkus, 1962; Worthington, 1970; Mauritzen, 1996b). If this heat is provided by cooling Atlantic Water (AW) from the Norwegian Sea and the area of the northern Iceland Sea is taken to be m 2, then about 0.1 to 0.15 Sv of Atlantic Water is required. If this inflow renews a 300 m thick layer the ventilation time for the IAIW in the northern Iceland Sea becomes years. This is slow enough to mask the warm signature of the entering Atlantic Water. The production of IAIW would then be small. Production would be larger if the Atlantic Water becomes substantially cooled in the Norwegian Sea before it enters the northern Iceland Sea. Assuming that the effective cooling area in the Norwegian and Iceland Sea is ten times as large then 1 to 1.5 Sv is formed. Convection must allow less saline surface water to become mixed into the Atlantic Water to account for the lower salinity of the IAIW, which also would increase the production of IAIW. However, this intermediate water will not all enter the Iceland Sea and continue to the Denmark Strait. Some, maybe the largest part, crosses the Greenland Scotland Ridge east of Iceland (Hansen and Østerhus, 2000). The ventilation time would be about one year or less in this case. An alternative, which is also an answer to the second question, is that denser water masses of the East Greenland Current are the source waters of the Iceland Sea intermediate and deep waters. The 3 4 Sv of dense water transported by the East Greenland Current in the Fram Strait (Rudels, 1987; Foldvik et al., 1988) must, if no significant net loss to the Greenland Sea occurs, continue across the Jan Mayen Fracture Zone within the East Greenland Current in the main. If the East Greenland Current does not contribute to the Denmark Strait overflow this volume must enter the central