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Eddies in the Beaufort Gyre

A. Plueddemann
Associate Scientist, WHOI

Supported by: National Science Foundation Office of Polar Programs

This project used observations of velocity in the western Arctic pycnocline (25-300~m depth) made with Acoustic Doppler Current Profilers (ADCPs) to investigate the distribution and properties of subsurface eddies. The ADCPs were deployed on autonomous drifters called, Ice Ocean Environmental Buoys (IOEBs), that were frozen into the pack ice (Figure 1).

Figure 1. Ice-Ocean Environmental Buoy [Larger image]
Data were available from three IOEB deployments within the Beaufort Gyre between 1992 and 1998. Ninety five probable eddy encounters were identified during the four IOEB deployments, which included 44 months of buoy drift. Physical properties were determined for 62 encounters in a series of steps, the most important of which was estimating the eddy center. The typical eddy encounter rate was 1 per 100 km of drift (Figure 2). The highest encounter rate was in the Canada Basin, but a significant number of eddies were also found to the west of the basin, over the Chukchi Plateau.

Figure 2. IOEB drift tracks and eddy locations [Larger image]
The number of eddies with reliable physical property estimates was sufficient to generate meaningful statistics of eddy size, strength, and vertical structure (Figure 3). The majority of center depths were between 90 and 160~m and the mean vertical extent was 130~m. Thus, eddies were found predominantly within the cold halocline. Maximum rotation speeds were typically 20-35 cm/s, with some greater than 40 cm/s. Typical radii were 3-8 km. Faster rotation speeds were associated with larger vertical extent and larger radius. The sense of rotation was predominantly anticyclonic.

Dynamical properties were determined for 22 eddies. Tangential velocity increased nearly linearly within the eddy core (i.e. out to the radius of maximum velocity). Velocity decay in the eddy periphery was rapid, with values approaching background velocities within 2.5 times the radius of maximum velocity. Relative vorticity was maximum in the eddy core, with values nearly equal and opposite to the local planetary vorticity. Strain was largest outside the radius of maximum velocity.  These results are consistent with isolated eddy cores in approximate solid body rotation.

Figure 3. Physical properties of Arctic eddies [Larger image]

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