THE ARCTIC OCEAN BOUNDARY CURRENT

Mooring Overview
Currents
Transports and Pathways
Cooling of the Atlantic Layer by outflow from the Barents Sea
Halocline formation - advective or convective?
Eddies
Publications

MOORING OVERVIEW (back to top)

Between summer 1995 and 1996, three moorings (LM1, LM2 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 Ocean.

CURRENTS (back to top)

LM1

LM2

LM3

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 component 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.
 

TRANSPORTS AND PATHWAYS (back to top)

TRANSPORTS 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 The warm waters (temperatures greater than or equal to 1.4 degC) of the Atlantic layer are also found on the Canadian Basin side of the ridge south of 86.5 degrees N, but not north of this latitude. This suggests that the Atlantic layer crosses the ridge at various latitudes south of 86.5 degrees N and flows southward along the Canadian Basin side of the ridge. DEEP WATER PATHWAY Temperature and salinity records indicate a small (0.02 Sv), episodic flow of Canadian Basin deep water into the Eurasian Basin at c.1700 m, providing a possible source for an anomalous eddy observed in the Amundsen Basin in 1996. There is also a similar flow of Eurasian Basin deep water into the Canadian Basin.  Both flows probably pass through a gap in the Lomonosov Ridge at 80.4 degrees N.

COOLING OF THE ATLANTIC LAYER BY OUTFLOW FROM THE BARENTS SEA(back to top) 

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.

HALOCLINE FORMATION - ADVECTIVE OR CONVECTIVE?(back to top)

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 degC 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 water column.  - 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) - The deeper casts (blue) (in water depths greater than 2500~m) show a comparatively fresh halocline with temperatures near the freezing point. This may have a convective origin.  - At stations nearer to the shelf (red) (those shallower than 1200~m), the cold halocline layer is warmer and saltier, and is most probably of shelf origin. - Intermediate depth (green) stations show some features of both haloclines. 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.

EDDIES(back to top)

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 modeled 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 e.g.,\at LM2 in October The large depth range suggests the eddies are formed from instabilities of a front with a similar vertical extent.  The most obvious such front is that formed between the Fram Strait and Barents Sea Branch waters in or near the St.Anna Trough.  This front will be sharpest (and hence most unstable) in its formation region.  Downstream, horizontal mixing between the FSBW and the BSBW erode the front, leaving no clear horizontal distinction between the two branches. If this is indeed the formation area, the eddies will be c.16 months old when they reach the moorings.  In this context, note that Manley and Hunkins (1985) suggest lifetimes in excess of 13.5 months for eddies in the Beaufort Sea.   (Table by R.Muench and J.Gunn, ESR)

PUBLICATIONS (back to top)

Aagaard, 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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