We now present our results following the path of AW through the Fram
Strait, from the inflow in the WSC via the recirculation in the central Fram
Strait to the EGC. A particular emphasis is placed on the formation and
evolution of the EGC.
The AW inflow in the WSC
The most striking feature of section 79∘ N, as measured in summer 2016,
is the highly dynamic velocity field (Fig. c). This can also be
seen in daily averages from FESOM (Movie S1b in the Supplement) and in
the multi-year model average eddy kinetic energy at 79∘ N
(Fig. c), which is significant across the Fram Strait east of
5∘ W and highest over the Svalbard shelf slope. This agrees with
observations . The velocity field may be
comprised of eddies, which appear as strong velocity fluctuations paired
around domes in the temperature and density fields (Fig. ).
While the precise horizontal structure of these cannot be resolved here,
it matches that of the daily averages of the modelled velocity field
(Movie S1 in the Supplement). It is clear that the flow is not smooth,
i.e. unidirectional, in the WSC and EGC with near zero velocities as otherwise
seen in long-term mean sections e.g..
Separate from the eddies, we identify the northward velocities east of the
1000 m isobath on the Svalbard slope (Fig. c) as the WSC.
This location agrees with the location of the WSC core both in long-term
observations and FESOM output
(Fig. b). The velocities in the eddies are instantaneously
stronger than the WSC with peak velocities of -0.18 and 0.24 m s-1
(e.g. at 240 and 260 km in Fig. c). Whilst the
27.8 kg m-3 isopycnal (Fig. b) is almost flat in the
deep Fram Strait (west of 2.5∘ E), near the Svalbard slope it slopes
downward toward the east with 0.64 m km-1. The downward sloping of
isopycnals in the vicinity of the shelf break is a characteristic of
baroclinic boundary currents, such as the WSC and EGC. The isopycnal slope is
used to estimate the baroclinic velocity assuming a two-layer ocean as
described in Sect. . This conceptual estimate gives a
baroclinic velocity of 0.13 m s-1 in the WSC. Although only a rough
estimate, this value is close to the absolute geostrophic velocity in the WSC
of 0.11 m s-1 (Fig. c). We did not observe an offshore
branch of the WSC, which is consistent with long-term measurements where the
offshore branch is observed to be weakest or absent during summer months
. Additionally, the
presence of an offshore branch may be obscured by an eddy in our transect.
The water column in the WSC is temperature stratified with a temperature maximum at the surface,
while the minimum temperature is in the deep ocean (Fig. a, b).
The surface temperatures of over 9 ∘C on the west Spitsbergen slope are the
highest water temperatures in the WSC near 79∘ N published so far and
are likely due to the warming of the AW inflow to the Fram Strait .
The AW layer is over 500 m thick and is in contact with the atmosphere east of
5∘ E (Fig. a). Toward the west, the AW layer gets thinner
and the depth of the temperature maximum increases. Although water warmer than
2 ∘C is found in the upper 50 m west of 5∘ E, this water is
too fresh to fall into the AW definition (Fig. a, b).
Eight year July/August/September average FESOM realizations of the
sections crossing the east Greenland shelf break for (a) Potential temperature
(as in Fig. a), (b) velocity (as in Fig. c) and
(c) EKE.
The westward recirculation in the deep Fram Strait
The synoptic section in the central Fram Strait shows a south to north transition
along 0∘ EW. At the southernmost station (near 78∘ N) the water
has an almost uniform salinity with warm AW close to the surface (the water
in the upper tens of metres is too fresh to fall into the AW definition) and
colder water at depth (Fig. a), similar to the stations
sampled in the WSC along 79∘ N (Fig. a). With increasing
latitude, the observed AW layer gets thinner, colder, fresher and is located
deeper in the water column. This suggests that between the AW inflow at the
surface in the WSC and the subsurface AW layer in the northern part of the
central Fram Strait, AW subducts underneath colder and fresher PSW and
sea ice. This was also simulated in the eddy-resolving model study of the Fram
Strait by and it was hypothesized that baroclinic
instability may achieve this subduction. The subduction of AW under PSW is
also simulated in FESOM though this does not show a northward thinning of the
AW layer (Fig. b). In the observations, the Arctic Ocean
halocline, with cold, fresh PSW at the surface, is found in the upper
120 m of the water column north of 80∘ N below which Knee Water
(KW, the saltiest water close to the freezing point line) is found. The
properties of KW are indicative of the ice–ocean–atmosphere interaction in
the Arctic Ocean
signalling that we observe water modified in the Arctic Ocean north of
80∘ N. In addition to their maximum temperature (more or less than
2 ∘C) AW and AAW along 0∘ EW exhibit differences in oxygen
saturation. Since AAW has transited through the Arctic Ocean, its oxygen
saturation of typically ∼80 % is significantly lower
than the oxygen saturation of AW of typically ∼100 %.
Left column: (a) Potential temperature, (c) absolute geostrophic
velocity and (e) salinity as in Fig. but for the section at
0∘ EW. The southernmost station was sampled last, 24 days after its
northern neighbour. The next two stations were sampled 13 days after their
northern neighbour. All remaining stations were occupied within 4 days. Right
column: eight year July/August/September average FESOM realizations of
(b) potential temperature, (d) velocity and (f) EKE as in Fig.
but for the section at 0∘ EW. Positive velocities are eastward,
negative velocities are westward.
AW is present somewhere in the water column at all stations along 0∘ EW
except for the northernmost station at 80.8∘ N
(Fig. a). This implies that we sampled either the northern
rim of the recirculation as it was at the time of our measurements or that
we sampled a passing AAW filament. We cannot decide which of the two
explanations is true since no measurements farther north than 80.8∘ N
were taken during the cruise. Examining the mean temperature in FESOM at
0∘ EW (Fig. b) shows average temperatures above
2 ∘C at 80.8∘ N in the central Fram Strait. This suggests that
the northern rim of the recirculation in the model lies northward of this.
Alternatively, the presence of warm water at this latitude in the model may
be related to the presence of the Yermak branch flowing into the Arctic Ocean
close to 0∘ EW. However, this does not agree with the modelled average
velocities in the AW layer (Figs. d and c, d),
which are southeastward north of ∼80∘ N. AAW eddies with
a temperature maximum below 2 ∘C are seen in the daily averages
of the model run for 2009 (Movie S2 in the Supplement). Hence the model
does not allow us to judge which of the two possible explanations is more
likely. The synoptic observations made here do, however, show that the
recirculation in the Fram Strait can reach as far north as 80.7∘ N. A
repeat synoptic survey along 0∘ EW, with a higher resolution than in
the present study, extending beyond 81∘ N, supported by a mooring
array, could provide a more definite picture of the northern limit of the
Fram Strait recirculation and its meridional and temporal structure. This
could then be used to verify numerical models. In the central Fram Strait
along 0∘ EW (Fig. c), the coarse resolution section
depicts an absolute geostrophic velocity field which switches between broad
sectors of weak eastward (∼78∘ N and ∼79.5∘ N) and westward
velocity (∼78.5 to ∼79∘ N and around 80 to 80.5∘ N); Velocities reach
±0.12 m s-1. The velocity field appears mostly barotropic but
the station spacing of ∼40 km is not able to resolve
the flow structure. We expect the velocity field, at least in the vicinity of
79∘ N, to be similar to the velocity field shown in Fig. c
at 79∘ N and 0∘ EW. This is supported by the modelled EKE
(Fig. f), which is highest close to 79∘ N and the daily
velocity field (Movie S2 in the Supplement), which shows much narrower
velocity structures. Further, the section at 0∘ EW is less synoptic
than the other sections presented in this study due to large time gaps
between some stations (see caption of Fig. ). Note that the
water-mass properties are not affected by the coarse temporal and spatial
resolution. Previous studies have reported eastward transport north of
79∘30′ N at 0∘ EW variously
related to the Molloy Hole e.g.. This is seen
in the observations though not in the model average. The model study by
described two branches of westward recirculation
through the Fram Strait, at 78.5∘ N and at 80∘ N. This agrees well
with our synoptic section at 0∘ EW, both with the velocity field and,
more conclusively, with the location of two salinity fronts
(Fig. b, c). FESOM also shows two recirculation branches,
which merge at 0∘ EW (Fig. c, d). Long-term averages of
model output suggest that the mean current through 0∘ EW is
southwestward and Fig. d in this
study and daily
averages of the velocity field from FESOM show eddies advected southwestward
(Movie S2 in the Supplement).
Potential temperature–salinity diagrams for three sections crossing
the east Greenland shelf break (WT1, 79.6∘ N and NT1). Individual casts
are colour coded depending on their distance to the east Greenland shelf break
(positive = offshore). Please note that the x axis changes scale at 33. The
solid black line shows the water-mass boundary between AW and AAW (see Table ).
The evolution of the EGC from the northern Fram Strait to the Greenland Sea
In the synoptic section roughly perpendicular to the east Greenland
shelf break at ∼80.3∘ N (Section WT1), AW is only found
in the central Fram Strait near 0∘ EW, some 130 km east of the
Greenland shelf break (Fig. a). This is closer to the Svalbard
shelf break than the Greenland shelf break. The deep θ maxima sampled
west of 0∘ EW at WT1 have temperatures around 1 ∘C, well
below the temperature of AW, and salinities between 34.8 and 34.9
(Fig. a). This agrees with deep θ maxima from stations
sampled between 82–83∘ N and 10–5∘ W in 2004
, which, together with the transport measured there
, indicates that the AAW sampled at WT1 may be
advected from the northwest along the east Greenland shelf break. Thus, the
Arctic Ocean outflow of AAW sampled at 80.3∘ N is uninfluenced by
directly recirculating AW west of 0∘ EW. Salinity (Fig. b)
increases strongly in the halocline over the upper 150 m. The density
field (thin contour lines in Fig. b) closely follows the
salinity field. At the mouth of the Westwind Trough the temperature of the deep
θ maximum is ∼0.8 ∘C. Outside of the
trough, two regions of southward flow were sampled (Fig. c). The
local velocity maximum between 0 and 20 km offshore of the shelf break
with relatively weak core velocities of -0.09 m s-1 is at a
cross-shelf-break distance where the shelf break EGC is found farther south.
The broad southward flow between 5∘ W and 0∘ EW (30 and
120 km), identified as the Arctic Ocean outflow, is also visible in
the modelled velocity field (Fig. b). Both bands of southward
flow are highly barotropic and modelled EKE is negligible at WT1
(Fig. c). In the Arctic Ocean outflow, at ∼80.3∘ N
(section WT1), the slope of the 27.8 kg m-3 isopycnal
between 0∘ EW and the shelf break (Fig. b) is very weak
(0.25 m km-1, corresponding to a baroclinic velocity of only
0.05 m s-1). In this respect the southward flow at WT1 is different
from the well-defined baroclinic boundary current structure of the EGC
farther south commonly described in the literature. Likewise, the 2001–2009
FESOM model mean shows weak isopycnal slopes (Fig. ). Thus we
hypothesize that the southward flow at WT1 may not be a boundary current tied
to the shelf break. In the 8-year model average, the AW reaches much closer
to the shelf break at 80.3∘ N than in the synoptic section
(Fig. a) and actually reaches the shelf break during
20 % of the year, though it does not propagate into the Westwind Trough.
In the FESOM configuration used here , runoff is taken
from the interannual dataset of , which does not take
into account subglacial and submarine melting of the Greenland ice sheet.
This, however, may be crucial to represent the northeast Greenland shelf
circulation correctly. A different freshwater input from Greenland would
likely have effects both on the circulation in the troughs and on
water-mass transport and transformation in the southward flow along the
shelf break. It may thus impact the distance from the shelf break at which AW
is found in the model. From comparison with the sparse observations available
(this study, a synoptic section in , and the
climatology in ) we are inclined to trust the density
and velocity field in FESOM in the northern Fram Strait, but are more cautious
about the distribution of AW. Thus, correctly modelled currents may advect
the wrong water mass in the model, specifically AW may be simulated too far
in the west.
Just 50 km farther to the south, at 79.6∘ N, AW is found merely 30 km
offshore of the shelf break in a core between 150 and 450 m of depth (Fig. a).
The 27.8 kg m-3 isopycnal has a downward slope of 0.5 m km-1 toward
the shelf break (this corresponds to a baroclinic velocity of 0.1 m s-1),
which has a greater similarity to the EGC structure farther south
than the WT1 section. The offshore divergence of the isopycnals may be
caused by AW intruding below, into and/or above the AAW layer at depth.
The spreading apart of the isopycnals in the ambient AAW by intruding AW
is likely a generic process
(i.e. not just present in this synoptic section), taking place whenever AW
meets AAW at depth with a distinct and strong horizontal gradient in
stratification. Intruding AW at depth has lower stratification consistent
with the strong atmospheric cooling experienced relatively recently by the AW
in the Nordic Seas boundary current loop. Some interleaving is present in the
CTD profiles at the transition between AW and AAW 30 km from the
shelf break (orange profile in Fig. b). Largely barotropic
southward velocities (∼0.16 m s-1,
Fig. c) are found just offshore of the shelf break. While the
isopycnal slope at 79.6∘ N in the synoptic section and the 8-year
model average (Fig. ) are similar to the familiar boundary
current structure of the EGC farther south, the core of the modelled
southward velocities lies farther from the shelf break than in our synoptic
section. The modelled daily average velocities (Movie S1 in the Supplement) suggest that the main cause of
high southward velocities near the
shelf break are eddies passing through 79.6∘ N. The mean modelled EKE
and velocity (Fig. b, c) show higher values at the same
distance from the shelf break supporting this interpretation. This means that
our observation may either have resolved the southward flowing rim of an
eddy or we sampled 79.6∘ N at a time when the EGC was a boundary
current and close to the shelf break. The latter is supported by the fact that
in the model the southward flow at 79.6∘ N lies closer to the
shelf break in summer than in winter (Fig. ). Conversely, the
upward sloping isopycnals seen below 200 m suggest the presence of an
AW eddy in the synoptic section.
Another 80 km farther to the south, at 79∘ N, AW is found at
∼200 m depth at the east Greenland shelf break though no
AW is found on the east Greenland shelf (Fig. a). The
27.9 kg m-3 isopycnal undulates strongly, following the temperature
field. Whilst the isopycnals <27.8 kg m-3 are almost flat above
100 m depth in the deep Fram Strait (between 2.5∘ W and
2.5∘ E) they deepen toward the west. The downward sloping isopycnals
(a slope of 0.75 m km-1 toward the shelf break for the
27.8 kg m-3 isopycnal corresponding to a baroclinic velocity of
0.15 m s-1) are located at a distance from the shelf break at which
the shelf break EGC is found in mooring observations
e.g. and our model average
(Fig. b), and coincide with southward absolute geostrophic
velocities (Fig. c). Thus this section shows the familiar
structure of the EGC as a baroclinic boundary current.
At 79∘ N there are two cores of southward velocities
(Fig. c). We identify the core just offshore of the shelf break
centred around 5∘ W (20 km) and reaching -0.15 m s-1
as the shelf break EGC. The modelled average temperature and velocity field
are naturally smoother than the synoptic section but show the same general
structure with AW subducting westward below PSW (Fig. a). The
EKE at 79∘ N is much higher than at the sections sampled to the north
and south of this and has a peak where the EGC is found. This high
variability can also be seen in the daily averages of the velocity fields
(Movie S1 in the Supplement).
At the mouth of the Norske Trough (76.6∘ N, i.e. another 270 km
farther to the south along the shelf break), AW is found in a broad core
between 100 and 350 m depth at and offshore of the shelf break
(Fig. a). Inside the trough, a thin layer of AW is found
between 200 and 250 m, i.e. above 320 m, which is the depth of
the shallowest sill between the shelf break and the inner shelf near the NEGIS
glaciers . The model also shows an AW layer within
the Norske Trough, both in the 8-year average (Fig. a) and in
the daily averages for 2009 (Movie S1 in the Supplement). Thus, AW is
able to propagate through the Norske Trough to the termini of the NEGIS glaciers.
However, the modelled AW layer is thicker inside the Norske Trough than in the
observations and thins eastward. Since this also does not agree with the
temperature observations in the Norske Trough reported in
, we again conclude that the model transports too much
AW too far eastward. The temperature of the synoptic deep θ maximum
decreases from east to west and its depth increases (Fig. c).
Observed salinities (Fig. b) are lowest at the surface and on
the shelf. The density field largely follows the salinity field and
isopycnals deepen toward the west (Fig. b). The
27.8 kg m-3 isopycnal has a downward slope of 1.66 m km-1
toward the west which corresponds to a baroclinic velocity of
0.33 m s-1. Absolute geostrophic velocities on the shelf are
northeastward, whereas the shelf break EGC flows southwestward on the slope,
both in the observations and in the model, with high velocities
(0.15–0.3 m s-1) throughout the water column
(Fig. c). The core of the measured flow is located around
7∘ W (at 20 km) and reaches -0.26 m s-1. The EGC has a
width of approximately 40 km and the observations show some surface
intensification in its western half, whereas the eastern part is more
barotropic. The location and width of the shelf break EGC at NT1 agree well
with section 10 from , which is located
∼30 km to the north of section NT1.
Properties of the θ maximum in the shelf break EGC from north
to south: (a) potential temperature, salinity and oxygen saturation;
(b) depth and potential density; and (c) AW fraction as a function of AW + AAW for
individual stations (blue, horizontally offset for clarity) and for the
average at each section (black). Error bars show the ±1 standard
deviation. Transport, velocity and width of the shelf break EGC in the Fram Strait
as defined in are shown in (d). Southward transport
is negative. Water-mass definitions are as in Table . Downstream
distance (in km) is 0 km at WT1 and follows the east Greenland
shelf break southward. Values for sections 9 and 10 of
are taken from their paper.