Corresponding author: Rebecca Woodgate (email@example.com)
Transports and Pathways
Cooling of the Atlantic Layer by outflow from the Barents Sea
Halocline formation - advective or convective?
|Between summer 1995 and 1996, three moorings
and LM3) carrying current meters and
temperature-salinity recorders were deployed at the junction of the Lomonosov
Ridge and the Eurasian continent.
These data are the first-ever year-long
records in the Arctic Ocean Boundary Current (AOBC) within the Eurasian
Basin. The AOBC is a circumpolar current that transports water, tracers,
contaminants, heat and salt cyclonically (anti-clockwise) around the Arctic
These figures are stickplots of 6 hour filtered velocity at all measured depths, at each mooring. The magnitude of each stick gives the magnitude of the current, the direction of the stick gives the heading of the current. Click on each figure to get better resolution.
The mean flow in the boundary current is
cyclonic (i.e. anticlockwise around the Arctic). It is aligned along
the depth contours. In the annual mean, the flows are weak (1-5 cm/s).
The strongest mean flow is found at LM1, where the current is almost twice
that at LM2 and LM3. At all sites, the current has a similar structure
(equivalent barotropic) in the vertical. About half of the first vertical
EOF (which explains over 70% of the variance) can be attributed to the barotropic
mode (which is essentially depth independent).
Thus, vorticity constraints suggest the flow will follow the depth contours. In addition, the large barotropic componend means that transport estimates based on geostrophic calculations with a level of no motion will be seriously in error.
The tidal signal in the records is small (always less than 3 cm/s, generally less than 1 cm/s). Common to all 3 sites is the absence of a seasonal cycle in either speed, direction, temperature or salinity.
Using information on the width of the current from CTD sections, we estimate the transport of the current at 5±1 Sv at LM1 and 3±1 Sv at LM2 and LM3. Thus, the boundary current is split in two by the ridge, half continuing on into the Canadian Basin, and half being diverted north along the Lomonosov Ridge. The core of the boundary current is found over water depths of 3000 to 500m at LM1; 3000m to the top of the ridge (c.1500m) at LM2, and 2500 to 500m at LM3. This is consistent with the part of the current over deeper water being diverted northwards by the ridge, whilst the part of the current over shallower water (i.e. nearer the coast) continues on eastward to the Canadian Basin.
ATLANTIC WATER PATHWAY
DEEP WATER PATHWAY
|A cooling and freshening of the Atlantic
layer is observed at all three moorings, both in the timeseries data and
in CTD casts taken before and after the deployment (see left).
In CTD measurements from 1995 (see right), the cooling is seen to propagate around the shelf of the Eurasian Basin. The advection speed of the signal (obtained from the 1995 CTD measurements and the arrival of the cooling at the three mooring sites) agrees with the measured mean velocities at the moorings, i.e. 5 cm/s before the Lomonosov Ridge, and 1 cm/s and 3 cm/s at LM2 and LM3 respectively.
The T-S change is consistent with mixing
with Barents shelf water (see left).
We attribute the changes in the Atlantic layer to changes (in temperature and salinity and/or volume) in the outflow from the Barents Sea the previous winter, possibly caused by an observed increased flow of ice from the Arctic Ocean into the Barents Sea. The change in water properties also strengthens the cold halocline layer and increases the stability of the upper ocean. This suggests a feedback in which ice exported from the Arctic Ocean into the Barents Sea promotes ice growth elsewhere in the Arctic Ocean.
In the Eurasian Basin, the cold halocline layer, in which salinity increases with depth while temperature remains almost constant, lies at about 100m. On a T-S plot, this lower halocline water (LHW) is found as a sharp bend at salinities between 34.2 and 34.4 PSU and temperatures of -1°C or colder.
This water can be formed in two ways (cf.
Steele and Boyd, 1998)
- by advection of cold, salty waters from the shelves, which interleave in the water column above the warmer Atlantic layer and below the cold, but fresher mixed layer (Aagaard et al., 1981)
- by winter convection north of the Barents Sea, the convective water subsequently being covered by low salinity shelf water north of the Laptev Sea. (Rudels et al., 1996).
|This issue may, in fact, be
resolved by the different thermal signatures these processes leave in the
- If the LHW is formed by convection, then the water at the top of the thermocline will be near the freezing point, since it is the salt rejection on freezing that drives the convection. - In contrast, an advective source for this LHW does not necessarily result in a halocline at the freezing point.
The distinction is well illustrated
by a line of CTD casts taken in 1993 near 120 E, stretching from the shelf
into the deep Eurasian Basin. (see right)
Thus we conclude that shelf processes form the halocline in the boundary current, whilst in the deeper water, convection can play a role, although we cannot ascertain if the convection has occurred locally.
The table gives a catalogue of the eddies
found in the timeseries. There are predominantly, two different types:
- cold, low salinity eddies confined to the surface layer.
- warm, salty eddies extending over 1000m of the water column.
In both cases the majority of the eddies are anticyclonic.
COLD, LOW SALINITY, SHALLOW EDDIES
e.g.,\ at LM3 in January, at LM2 in May and June
The shallowness of the eddies and the properties of the core, viz., frequently near the freezing point and often comparatively homogeneous, suggest the formation mechanism modelled by Chapman (1999), which creates cold core eddies in the mixed layer and upper pycnocline via instabilities of the front between the dense waters formed under a wind-forced
coastal polynya and the surrounding shelf waters. (Note that although the water in the polynya is more saline than the surrounding shelf waters, the polynya water is fresher than waters of the same density over the slope.)
WARM, SALTY, DEEP EDDIES
(Table by R.Muench and J.Gunn, ESR)
K., and R.A. Woodgate, Some thoughts on the freezing and melting of sea
ice and their effects on the Ocean, in press in Ocean Modelling, 2001.
|Woodgate, R.A., K.Aagaard, R.D.Muench, J.Gunn, G.Bjork, B.Rudels, A.T.Roach, U.Schauer, 'Effects of barents Sea outflow on the upper waters of the eastern Arctic Ocean.', poster, 1999.|
We gratefully acknowledge financial support for this work from the Office of Naval Research (ONR), High Latitude Dynamics program.
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