ICES mar. Sei. Symp., 195: 40-51. 1992
The Central Bank vortex in the Barents Sea: watermass
transformation and circulation
D etlef Quadfasel, Bert Rudels, and Stefan Selchow
Quadfasel, D., Rudels, B., and Sclchow, S. 1992. The Central Bank vortex in the
Barents Sea: watermass transformation and circulation. - ICES mar. Sei. Symp 19540-51.
The formation and circulation of bottom water in the western Barents Sea are
examined. Freezing and induced salt ejection are identified as the mechanisms creating
the dense waters. The bottom water formed above the Central Bank creates a
stationary anticyclonic vortex which traps the water column above the bank and limits
lateral exchanges. Only under extreme conditions, as during the strong inflow of warm
Atlantic W ater in the early 1980s, will the water column be completely replaced, thus
inhibiting ice formation and reducing the bottom-watcr production. The situation
described lasted until the Atlantic inflow to the Barents Sea returned to normal and the
Central Bank vortex was re-established.
D etlef Quadfasel, Bert Rudels, and Stefan Selchow: Institut fü r M eereskunde der
Universität Hamburg, Troplowitzstrasse 7, 2000 Ham burg 54, Germany.
Introduction
The Barents Sea is perhaps the most important entrance
to the Arctic Ocean (Blindheim, 1989; Rudels, 1987).
Atlantic and Norwegian Coastal Current waters enter
between the Norwegian coast and Bear Island (Fig. 1)
and continue into the Arctic Ocean, passing primarily
between Novaya Zemlya and Frans Josef Land. A
substantial recirculation takes place in the western
Barents Sea, which returns part of the water to the
Norwegian Sea (B lin d h eim , 1989).
The Barents Sea is also a large shelf sea, where the
inflowing water is transformed by cooling, freezing, and
melting into low salinity, low density surface water and
saline, cold and dense bottom waters, which are injected
into the Arctic Ocean water column (Midttun, 1985;
Nansen, 1906). The water mass formation takes place
primarily in the shallow areas west of Novaya Zemlya
and around Frans Josef Land (Midttun, 1985). Dense
water is also formed in the central and western Barents
Sea: over the Central Bank; on the shallow area east of
Edgeöya and Hopen; and in Storfjorden (Anderson et
al., 1988; Midttun, 1985).
This work concentrates on dense water formation in
the central and western Barents Sea. The discussion is
based on CTD observations collected by R V “Lance” in
1986 on cruises arranged by the Norwegian Polar R e­
search Institute (NP) and by the PRO M A R E
programme. Additional data from NP cruises with
40
“Lance” in 1983 and 1988 as well as tem perature sections
taken from the Russian vessels “Pachtussov” in 1901 and
“Rossiya” in 1990 have also been used. The station
positions and the locations of the sections are shown in
Figure 1. Because it is almost isolated from the rest
of the Barents Sea, Storfjorden is not considered.
Storfjorden has been described by Anderson et al.
(1988) and Quadfasel etal. (1988)
The formation of dense water
The areas of dense water production can be inferred
from the distribution of bottom densities (Fig. 2). Two
sources are discernible: the Central Bank and the
shallow area east of Edgeöya and Hopen.
Stations occupied in May 1986 east of H open show an
almost homogeneous water column with temperatures
at freezing point (Fig. 3). The slight freshening at the
surface is due to spring ice melt. The cold temperatures
indicate that the convection is driven by freezing and salt
ejection. Saline water is accumulated on the banks, and
the salinity increases throughout the winter. Cold Arctic
W ater dominates east of Edgeöya and only the surface
summer heating has to be removed before the ice forma­
tion can begin. The water does not reach the extremely
high salinities and densities found in Storfjorden
(Anderson et al., 1988). This could partly be due to
80° E
--
■VIÇJORIA
IS. \
N o rth e a s t,
Ba sin (
Novaya
Zemlya
Bank
Great
Bank
y j
)Svalbar<
Bank
/H O P E N
Hopen
■Qeep
/ßEÄR
*IS . /
S o uth east
Ba sin
Bear Is.
Channel
Goose
Bank
vMurman
VRise
North
Kanin
Bank
Figure 1. (a) (above) The indicated sections are “Lance”
1988 (A ), “Lance” 1986 (B), “Pachtussov” 1901 (C), “ Rossiya”
1990 (D ), and “ Lance” 1983 (E). (b) (left) Station positions in
1986. Crosses represent the spring cruise, dots the summer
cruise. Numbered stations are discussed in the text. Stations
320 and 323 (open circles) were taken in 1983.
lower initial salinities, but is primarily due to strong
currents and rapid exchanges of water.
In the depressions further to the northeast the vertical
profiles show an increase in salinity with depth, which
indicates that the dense water has been supplied from
the surrounding shallower areas (Fig. 4). The tem pera­
ture, salinity, and density near the bottom show large
variations, suggesting also a second source area (Fig. 4).
This is most likely the Central Bank, where the
salinities and densities are higher, albeit not as high as
those in Storfjorden (Figs. 2 and 3). However, at no
station, available to us, is a homogeneous water column
to be seen. The profiles show low salinity surface water,
a warmer intermediate layer of Atlantic origin, and
bottom waters with somewhat lower salinities and tem41
20e
The removal of fresh water during ice formation in­
creases the salinity of the upper layer and the saline
water at the surface may attain such a large density
anomaly that it can sink through the intermediate layers
towards the bottom (Rudels, 1990). The intermediate
layer is displaced towards the surface, the freezing stops
and the ice begins to melt.
In winter 1986 ice formed over the Central Bank
between 10 and 18 February (Norwegian Meteorologi­
cal Institute ice charts). The ice cover expanded south­
ward 150 km in one week. During this period the winds
were m oderate, excluding the possibility of older ice
drifting over the Central Bank from the north (Fig. 5).
The bank remained ice covered up to 3 March, but by 10
March the ice edge was found 150 km to the north. A t
the beginning of March the winds were easterly and the
temperatures were extremely low ( —35°C) (Fig. 5). This
supports the view that the ice is removed by melting
caused by warm water being brought to the surface to
replace brine enriched, dense, convecting surface water.
The tem perature of the bottom water will, because of
entrained intermediate water be above freezing (Fig. 3).
A t some stations the differences are only a tenth of a
degree, showing that the entrained water must be cold.
Sites must exist where the intermediate layer is of small
vertical extent and/or has been cooled by successive
convection events. A section taken across the bank in
September 1988 shows localized domes of colder water,
which a re , however, not properly resolved by our station
grid, suggesting that the convection occurs on scalcs
smaller than the horizontal extension of the bank (Fig.
E 40°
»05
78
•05.
76°'
.13
-
.10
.05
2 8 .0 0
Figure 2. Distribution of bottom densities in 1986, based on all
available stations.
peratures close to, but clearly above freezing point (Fig.
3).
The cold tem perature of the bottom water indicates
that also here ice formation is driving the convection.
The fact that no station with remnants of a homogeneous
water column is found implies that the convection to the
bottom might be localized to shallow, unsampled areas.
A nother possibility is that the convection is penetrative.
27.4
34 .2
-2
27.6
34.4
278
3 4 .6
-1
STAT. 2 9
28.0
34.8
0
35 .0
1
28.2
352
2
6).
öe
S
0
27.4
34 .2
-2
276
34.4
-1
27.8
34.6
280
34.8
0
35.0
1
282
35.2
2
STAT. 56
Figure 3. Potential tem perature, salinity, and density profiles from east of Hopen (stn 56) and from the Central Bank (stn 29).
42
STAT. 61
STAT. 62
Figure 4. Tem perature and salinity profiles from stations (61) and (62) east of Edgeöya.
IC E
Circulation of the bottom water
As the water sinks off the Central Bank, it becomes
deflected by the E arth ’s rotation and an anticyclonic
vortex is created. The vortex remains almost stationary,
as a Taylor column, over the bank and it is perm anent
enough to divide the Atlantic inflow into two branches.
The southern Atlantic branch flows seaward of the
Norwegian Coastal Current towards the east, while the
other branch turns northward to split a second time. One
part follows the anticyclonal flow eastward north of the
Central Bank and the other part is deflected by the
Persey Current and recirculates in the Hopen Deep
(Figs. 6 and 7; Tantsiura, 1959; Midttun, 1989).
The Central Bank supplies, laterally, the bottom
water of the Hopen D eep, where the local thermal
convection is limited to the upper 200 m. The salinity
and temperature distributions (Figs. 6 and 7) indicate
that the bottom water follows the rim of the H open
D eep and exits into the Norwegian Sea south of Björnöya (Blindheim, 1989). During its transit, the bottom
water mixes with incoming Atlantic W ater and becomes
warmer and saltier, as can be seen by comparing TS
curves from the Central Bank with those from the inflow
and outflow areas in the Bear Island Channel (Fig. 8).
Not all dense water formed over the Central Bank
returns to the Norwegian Sea. A part follows the anti­
cyclonic flow into the eastern Barents Sea and may
W IN D
N
î
0
A IR T E M P E R A T U R E
10i
-
10 -
-
20-
-3 0 -4 0 J
T
1.
1----------------------- 1--------------------- 1-----------------------T-
10.
20.
FEBRUARY
1.
10.
-------------------1--------------------------1
20.
MARCH
31
Figure 5. The wind vectors and the air temperatures measured
at Hopen Radio Station in February and March 1986, taken
from the Germ an W eather Service. The time when the Central
Bank was covered by ice is indicated.
43
db ar
100
-0.5
-0 .5 '
-0.5
"•1.5
200
•
O (°c)
S E C T IO N
A
300
N
20
3 4 .C
.5 0
.9 4
d bar
.8 0
100
35.00
.9 8
.9 8
S E C T IO N
A
300
Figure 6. Vertical sections of tem perature (a) and salinity (b) over the Central Bank to the G reat Bank. “ Lance” , September 1988.
Section A in Figure 1.
continue to the Arctic Ocean (Fig. 6). Some intermedi­
ate water also crosses the sill between Edgeöya and the
G reat Bank to form bottom water of the northern
Barents Sea (Fig. 2 and also Pfirman (1984)).
The water from the western source adds, as its density
level, to the outflow into the Norwegian Sea (Blindheim, 1989). Most of this water is colder, less dense, and
44
found higher up on the slope of the Svalbard Bank (Figs.
6 and 7).
Permanence of the Central Bank vortex
The cold, dense water above the Central Bank is a
persistent feature. Figure 9 shows two tem perature
100
90
85
ø(°c)
SECTIO N B
35.00
35.00
35.00
35.00
3&00
SECTION B
(b)
Figure 7. Vertical sections of tem perature (a) and salinity (b) from the Norwegian coast over the Central Bank to the Svalbard
Bank. "Lance” , July 1986. Section B in Figure 1.
sections: the first, taken from RV “Pachtussov” in July
1901, the second from A I “Rossiya” in August 1990. On
both sections the dome of cold water is present. It
appears that the water leaking out at the bottom is
replaced not through mid-level exchanges with Atlantic
Water but by low salinity Arctic W ater from across the
Polar Front. This maintains the stratification and per­
mits ice formation, haline convection, and reinforces the
vortex.
Rapid complete renewals of the Central Bank water
column are possible. In 1982 a strong inflow of warm
Atlantic W ater was recorded (Loeng, 1983; Midttun,
1989). Little or no ice formed over the bank during the
winter 1983 (Fig. 10 and Midttun, 1989) and in the
following summer neither a low salinity surface layer,
nor cold, dense bottom water was observed over the
bank (Fig. 11). The profiles and the TS curves observed
at the bank were similar to those found in the Hopen
Deep (Figs. 12 and 13). The inflow had replaced the
intermediate water on the bank.
The near absence of a low salinity surface layer in the
summer 1983 confirms that only limited ice formation
45
0 (° C )
and haline convection occurred before the intermediate
Atlantic W ater was brought to the surface. Its high
temperature and the weak stratification prevented
further ice formation and the deep water renewal was
limited to a small volume dominated by warm entrained
water (Figs. 11 and 13).
In the absence of ice production and haline convec­
tion, the anticyclonic vortex cannot be maintained. The
splitting of the Atlantic inflow is weakened and the
lateral, advective exchanges of the Central Bank water
column can continue to bring warm water onto the bank
and delay the re-establishment of the ice formation.
The warm inflow persisted at a reduced rate in 1984—
1985 (Midttun, 1989). The ice edge merely touched the
Central Bank in 1984 and the bank was ice-free again in
1985 (Fig. 10). Not until 1986 did the ice cover reach its
long-term mean extent. The inflow of Atlantic W ater
was then less warm and the leaking of low salinity Arctic
W ater and ice across the Polar Front could compete with
BEAR IS L.C H .
HOPEN DEEP
-2-*
3 4 .8 5
1
1
3 4 .9 0
1
1
3 5 .0 0
1
35.10
1---S
Figure 8. T -S curves from station (95) in the Hopen Deep,
station (247) from the inflow area and station (253) from the
outflow are in the Bear Island Channel.
500
505
509
< -1 .5
100-
200
-
Q PC)
SECTION
300 -
s
N
205
200
195
dbar
100
' - 0.5
■
(- 0.5
•05
-
0.5
200
ØCC)
S E C T IO N
D
300
Figure 9. Tem perature section taken by (a) RV "Pachtussov" in July I90l and by (b) AI "Rossiya” in August 1990 across the
Central Bank. Sections C and D in Figure 1.
46
V i / 1/4/
Figure 10. Maximum extent of the ice cover in the Barents Sea 1979-1990 (adapted from Midttun, 1989)
47
dbar
S E C T IO N
E
34.00
S E C T IO N E
Figure 11 . Vertical sections of tem perature (a) and salinity (b) from the Svalbard Bank over the Central Bank to the G reat Bank
"Lance", August 1983. Section E in Figure 1.
48
27 .4
3 4 .2
2 7 .6
3 4 .4
27 .8
3 4 .6
2 8 .0
3 4 .8
3 5 .0
262
3 5 .2
274
S
3 4 .2
0
-2
276
3 4 .4
2 7 .8
280
262
—i---- 1----- 1---- 1---- 1
3 4 .6
3 4 .8
3 5 .0
3 5 .2
I--------i-------- 1-------- 1_____ I_____ ._____ I_____ I_____ I_____ 1_____ I
1--------1--------1---------!-------- 1-------- !-------- ■-------- 1--------1--------«--------1
0
2
4
6
dbar
100
100-
S ta t. 323
200-
S ta t. 89
300-
STAT. 8 9
STAT. 3 2 3
400
3 4 .6
3 4 .8
35 .0
S
Figure 12. T em perature and salinity profiles (a) and T -S curves (b) from station (323) in 1983 and station (89) in 1986 on the Central Bank.
3 5 .2
STAT. 9 4
STAT. 3 2 0
Figure 13. T em p e ratu re and salinity profiles (a) and T -S curves (b) from station (320) in 1983 and station (94) in 1986 in the Hopen Deep.
the advective transport of Atlantic W ater, reform the
vortex and recreate the long-term circulation pattern.
It should be kept in mind that we have no data from
1984 and 1985 or from the years before 1983. It is the
correlation between ice formation and convection found
above which makes us assume that the circulation and
hydrography of the western Barents Sea during the icefree years 1984-1985 resembled those of 1983, while the
situation in the period before was similar to that of 1986
and 1988.
The Central Bank remained ice-free for two to three
years. It is not clear if this represents the time needed to
reform the stratification or whether the time span was
determined by the persistence of the inflow. In view of
the rapid return of the bottom water production in 1986
and of the permanence of the vortex throughout the
years, it is likely that the long-term hydrographic state
can be established within one season, once the disturb­
ance is gone.
The circulation pattern in the central Barents Sea is
normally stable and controlled by the Central Bank
vortex. The situation which prevailed in the early 1980s
was not created locally but was caused by climatic
conditions outside the Barents Sea, confirming HellandHansen and Nansen's (1909) finding that the advection
from the N orth Atlantic determines the convection and
the climate state of the Barents Sea.
Acknowledgments
Most data used here have been collected by the Norwe­
gian Polar Research Institute in connection with its
climate programme and PR O M A R E . We thank the
captains and crews of RV “Lance" for their support at
sea. This work is part of a diploma thesis by Stefan
Selchow. Financial support for the study was granted by
Deutsche Forschungsgemeinschaft (SFB 318).
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51