ICES Journal of Marine Science, 59: 1133–1154. 2002
doi:10.1006/jmsc.2002.1284, available online at http://www.idealibrary.com 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. 2002. The East
Greenland Current and its contribution to the Denmark Strait overflow. – ICES
Journal of Marine Science, 59: 1133–1154.
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
(<0) part of the Arctic Ocean thermocline.
2002 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 2001.
B. Rudels and P. Eriksson: Finnish Institute of Marine Research, PL33, FIN-00931
Helsinki, Finland; tel: 00-358-9-61394428; fax: 00-358-9-61394494; e-mail:
[email protected]. E. Fahrbach and G. Budéus: Alfred-Wegener-Institut für Polar und
Meeresforschung, Postfach, 120161, D-27515 Bremerhaven, Germany. J. Meincke:
Institut für Meereskunde der Universität Hamburg, Troplowitzstraße 7, D-22529
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
1054–3139/02/061133+22 $35.00/0
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
2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.
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 -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<c27.97 the Arctic Ocean contribution, the
Arctic Atlantic Water (AAW), is colder and less saline.
In the density interval 27.97< and 0.5c30.44 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 -S diagram (stable
in temperature, stable in salinity stratification). The
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< and
0.5c30.44, but its temperature is below 0C. It is
distinguished by an almost constant negative slope in the
-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 -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 <34.915.
The water less dense than 27.70 and colder than 0C
comprises the Polar Surface Water (PSW). The surface
water in the same density range but with temperature
1136
B. Rudels et al.
above 0C 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
27.70 isopycnal and 0C (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 -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
0.5 =30.444 and the temperature maximum, the -S
curves show a mixture between upper Polar Deep Water
(uPDW) and AIW. The temperature maximum in 1998
was above 0C 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 -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
Polar Surface Water; PSW
Polar Surface Water warm; PSWw
Re-circulating Atlantic Water; RAW
Arctic Atlantic Water; AAW
Upper Polar Deep Water; uPDW
Arctic Intermediate Water; AIW
Canadian Basin Deep Water; CBDW
Eurasian Basin Deep Water; EBDW
Nordic Deep Water; NDW
Iceland Sea Arctic Intermediate Water;
IAIW
Polar Intermediate Water; PIW
Water mass boundaries
c27.70, <0.
c27.70, 0<.
(a) 27.70<c27.97, 2<.
(b) 27.97<, 0.5c30.444, 0<.
(a) 27.70<c27.97, <2, If <0
then S<34.676+0.232.
(b) 27.97<, 0.5c30.444, 0<.
27.97<, 0.5c30.444, <0.
Origin, remarks
Arctic Ocean.
Sea ice melting on warmer Atlantic Water
West Spitsbergen Current: (b) Slope in -S
diagram; positive.
Arctic Ocean: (b) Slope in -S diagram;
negative.
Arctic Ocean: Slope in -S diagram; negative,
almost constant with depth.
Greenland Sea: Slope in -S diagram;
27.97<, 0.5c30.444, <0.
through infinity, negative but increasing.
30.444<0.5, 1.5c35.142, 34.915<S. Canadian Basin, but also includes water from
the Eurasian Basin.
35.142<1.5, 2.5c39.738, 34.915<S. Eurasian Basin, lower boundary because of
the sill in Fram Strait.
Includes the Greenland, Iceland and
30.444<0.5, S<34.915.
Norwegian Seas deep waters (GSDW,
NSDW, ISDW).
(a) and (b) locally formed (c) advected from
(a) 27.70<, <0,
Greenland Sea and Arctic Ocean: Slope in
34.676+0.232<S.
-S diagram; negative and increasing.
(b) 27.70<, 0.5c30.444, <1,
34.676<S.
(c) 27.97<, 0.5c30.444, <0.
27.70<, <0, S<34.676+0.232. Approximate definition. Derives from the
colder parts of the Arctic Ocean thermocline.
The East Greenland Current and its contribution to the Denmark Strait overflow
Figure 2(a).
1137
1138
B. Rudels et al.
Figure 2(b).
The East Greenland Current and its contribution to the Denmark Strait overflow
For comparison we have kept the water mass boundaries introduced for the Fram Strait on the -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 -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 (< 1.5C), 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
1139
warmer Atlantic Water (PSWw). The Arctic Atlantic
Water (AAW) had maximum temperatures slightly
above 1C 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 0.5C and
0.5C 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 -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 -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 -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 75N (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) -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 =27.70 isopycnal separates the Polar
Surface Waters (PSW and PSWw) from the intermediate water masses and the 0.5 =30.444 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 0C isotherm. The RAW is, when less dense than =27.97, warmer than 2C and when denser than =27.97
it is distinguished from AAW by its positive slope (/) of the -S curves. In the same density range AAW has a negative slope (\)
in the -S diagram. The temperature maximum of the AAW is less dense than =27.97 and it has temperatures below 2C. The
uppermost part of the AAW, the thermocline, has temperatures below 0C 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 -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 34.915 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 0.5 =30.444 and 1.5 =35.142, the isopycnal present at the sill depth of the Lomonosov Ridge, is denoted
CBDW. The 2.5 =39.738 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) -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.
1140
B. Rudels et al.
Figure 3. Potential temperature, salinity and potential density distributions, and -S curves from stations 60, 66, 68, 69, 70, 71 on
section Ps-I in the Fram Strait. The lines and isopycnals in the -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
34.915 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 (c1C), 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 75N 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.
71N, 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 75N the intermediate water with temperatures above
0C constituted a 500 m thick layer centred at 300 m
The East Greenland Current and its contribution to the Denmark Strait overflow
1141
Figure 4. Potential temperature, salinity and potential density distributions, and -S curves from stations 132, 133, 134, 135,
136, 139, 141 on section Ps-II at 75N in the Greenland Sea. The lines and isopycnals in the -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 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 0C 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
1142
B. Rudels et al.
Figure 5. Potential temperature, salinity and potential density distributions, and -S curves from stations 218, 219, 220, 222, 223
on section Ps-III at 71N south of the Jan Mayen Fracture Zone. The lines and isopycnals in the -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 -S curves
from section Ps-III and the Greenland–Iceland section
(Ps-IV) suggests that the warm layer (>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 S34.3 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).
The East Greenland Current and its contribution to the Denmark Strait overflow
1143
Figure 6. Potential temperature, salinity and potential density distributions, and -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 -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 (]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 1C and salinities
between 34.75 and 34.85 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 -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).
–200
–400
–600
–800
–600
VEINS Ps V
R/V Polarstern
–1000
200
250
300
350 km
0
50
100
150
24
0
150
24
8
24
7
24
6
24
245
244
243
2
24
1
100
25
7
25
6
25
5
25
4
25
3
25
2
25
1
50
25
9
25
8
0
25
0
24
9
–1000
3
Potential temperature (°C)
–200
–400
–600
300
350 km
sigma 0
27.70
0
Pressure (dbar)
24
0
250
24
7
24
6
24
245
244
243
2
24
1
200
–400
–800
VEINS Ps V
R/V Polarstern
24
8
–200
25
0
24
9
0
Pressure (dbar)
0
25
7
25
6
25
5
25
4
25
3
25
2
25
1
25
9
25
8
24
0
24
8
24
7
24
6
24
245
244
243
2
24
1
25
0
24
9
25
7
25
6
25
5
25
4
25
3
25
2
25
1
B. Rudels et al.
25
9
25
8
1144
sigma 0
27.97
2
1
sigma 0.5
30.444
sigma 1.5
35.142
sigma 2.5
39.738
0
–1
–800
VEINS Ps V
R/V Polarstern
–2
34
34.2
34.4
34.6
34.8
35
Salinity
–1000
0
50
100
150
200
250
300
350 km
Figure 7. Potential temperature, salinity and potential density distributions, and -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 -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 (<0C), 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<<3C), 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
The East Greenland Current and its contribution to the Denmark Strait overflow
1145
Figure 8. Potential temperature, salinity and potential density distributions, and -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 -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
1146
B. Rudels et al.
Figure 9. Potential temperature, salinity and potential density distributions, and -S curves from the stations 273–278 on section
Ps-VII down the Greenland slope south of the Denmark Strait. The lines and isopycnals in the -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 -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
The East Greenland Current and its contribution to the Denmark Strait overflow
1147
Figure 10. Potential temperature, salinity and potential density distributions, and -S curves from the stations on section V-1
between Greenland and Iceland south of the Denmark Strait. The lines and isopycnals in the -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
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 27.75 and
27.85 but also between 27.85 and 27.95. 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 27.85–27.95 interval. The 27.75–
27.85 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
27.75–27.85, 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.
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.
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
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
The East Greenland Current and its contribution to the Denmark Strait overflow
27.5
1149
IC
27.6
PIW
27.7
27.8
27.9
AAW
RAW
28.0
28.02
28.04
28.06
28.08
DSOW
AAW + RAW
+ uPDW
IAIW
uPDW
IAIW
RAW
AAW
uPDW
Bottom
density
AIW + uPDW
CBDW
AIW
CBDW
GSDW
EBDW
28.10
Arctic
Ocean
Fram GFZ
Strait
Input of
water masses
(density range)
Greenland
Sea
Mainly
isopycnal
mixing
JMFZ
Iceland
Sea
Mainly
diapycnal
mixing
Denmark Irminger
Strait
Sea
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 =27.9 and =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 0.9 10 4 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 (c27.70) 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
1150
B. Rudels et al.
PIW/AAW
82°
AW
uPDW/CBDW
EBDW
N
PSW
RAW
78°
(RAW)
WSC
G
C
AIW
E
74°
(EBDW, CBDW)
RAW?
70°
IAIW
AW?
(CBDW)
PIW
66°
IC
62°
LSW
NEADW
58°
40°W
30°
20°
10°
0°
10°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
The East Greenland Current and its contribution to the Denmark Strait overflow
contributions, were found at the same or at somewhat
shallower depths in the East Greenland Current. However, the -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?
1151
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
50109 m2, 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 3–4.5 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
1152
B. Rudels et al.
Figure 13. Potential temperature, salinity and potential density profiles, and -S curves from the stations 220 on the Greenland
Slope (red) and 216 in the central Iceland Sea (green) on Ps-III, 231 and 232 on the Greenland slope (blue) and 236 and 237 on
the Iceland slope (violet) on Ps-IV, and 243, 244, 245 and 246 at the sill in the Denmark Strait (black). The data are cut at
=27.85. The properties at 244–246 are present at the Greenland slope but could also be created by mixing the East Greenland
Current water masses between 300 and 900 m. In the Iceland Sea the characteristics most similar to those at the sill are found at
250 m. The -S properties of the deepest part of 236, 237 and 243 are seen at the same depth or somewhat deeper in the central
Iceland Sea. At the Greenland slope they are present between 900 and 1200 m.
The East Greenland Current and its contribution to the Denmark Strait overflow
Iceland Sea and become transformed into Iceland Sea
Arctic Intermediate Water (IAIW) before it continues
southward to the Denmark Strait. Such deflection could
occur at or slightly north of 6930N, the northwestern
end of section Ps-IV, where the Greenland shelf turns
abruptly towards west and where the warm core separates from the slope (Figure 6). To estimate if the heat
loss in the Iceland Sea is large enough to accomplish the
transformation into IAIW the same numbers are used as
in the earlier hypothesis. About 1 Sv RAW of 2C could
be cooled to 0–0.5C by the given heat loss. The other
water masses have temperatures close to those found in
the Iceland Sea and would not require much additional
heat loss. Assuming a renewal rate of 1.5 Sv and if the
thickness in the intermediate layers is assumed to be
600 m, then the residence time for the IAIW would be of
the order of half a year to a year. Such rapid renewal
is not very different from a direct flow of the East
Greenland Current to the Denmark Strait and should
lead, conceivably, to larger variability in the IAIW than
that observed.
Conclusions
It is proposed that in 1998 the East Greenland Current
supplied most of the Denmark Strait Overflow Water
and that the rest derived from the Iceland Sea and was
mixed into it north of and in the Denmark Strait. The
densest part of the Denmark Strait Overflow Water
(DSOW) was formed initially by isopycnal mixing in the
East Greenland Current between Re-circulating Atlantic
Water (RAW) and Arctic Atlantic Water (AAW) and
between upper Polar Deep Water (uPDW) and Arctic
Intermediate Water (AIW) respectively. These two
mixing products were then gradually homogenised by
diapycnal mixing at the sill in the Denmark Strait and in
the overflow plume as it descended south of the sill.
The Polar Intermediate Water (PIW) contributed to
the less dense, upper part of the plume and was also seen
as a separate upper core on the Greenland slope. The
Polar Intermediate Water characteristics suggest that it
is well ventilated and that it originates from the thermocline at 150–250 m depth in the Arctic Ocean rather than
from the Greenland shelf. North of the sill it is located in
the upper part of the water column and so does not have
to pass through the deepest part of the Denmark Strait
but could flow south over the shelf further to the west
and sink down the slope south of the sill.
Acknowledgements
We wish to thank Norbert Verch for technical assistance, and Bogi Hansen, Harald Loeng and Detlef Quadfasel for their comments. Economic support has been
received from the European Commission MAST III
1153
Programme VEINS through contract MAS3-CT960070.
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