Equilibration of the Arctic halocline by eddies

The stratification of the Arctic's Beaufort Gyre (BG) comprises a strong, near-freezing surface halocline floating above an intermediate layer of (relatively) warm, salty water. The halocline suppresses communication between the warmth at depth and the cold surface and plays a major role in modulating sea-ice cover and Arctic climate.

The action of the wind over the BG pumps fresh water down from the sea surface resulting in density surfaces bowing down in to the interior. The resulting shallow lens of buoyant fluid will be susceptible to baroclinic instability. A plausible hypothesis is that the resulting lateral and upward eddy (bolus) fluxes can provide the balancing mass and salinity fluxes to arrest the deepening of the BG freshwater lens, enabling an equilibrium to be established.

Results from a laboratory experiment are described which can be interpreted as an analogue of the BG. A rotating tank of initially unstratified water is simultaneously warmed and mechanically pumped at the surface by means of a spinning perspex disc, to the bottom of which a heating pad is attached. A buoyant lens of warm water develops which deepens until lateral transfer of buoyancy by geostrophic eddies (spawned by the instability of the lens) offsets gain at the surface, arresting the deepening of the lens.

Theory used to interpret the laboratory experiment, and which is broadly supported by it, is applied to the BG. This yields plausible predictions for the depth and stratification of the halocline which is compared against observations from Ice Tethered Profilers deployed in the Arctic. We surmise that eddies may play a central role in the dynamics of the BG and in setting its stratification.