Three-Dimensional Structure of a Cold-Core Arctic Eddy Interacting with the Chukchi Slope Current

A rapid, high-resolution shipboard survey, using a combination of lowered and expendable hydrographic measurements and vessel-mounted acoustic Doppler current profiler data, provided a unique three-dimensional view of an Arctic anti-cyclonic cold-core eddy. The eddy was situated 50-km seaward of the Chukchi Sea shelfbreak over the 1,000 m isobath, embedded in the offshore side of the Chukchi slope current. The eddy core, centered near 150-m depth, consisted of newly ventilated Pacific winter water which was high in nitrate and dissolved oxygen. Its fluorescence signal was due to phaeopigments rather than chlorophyll, indicating that photosynthesis was no longer active, consistent with an eddy age on the order of months. Subtracting out the slope current signal demonstrated that the eddy velocity field was symmetrical with a peak azimuthal speed of order 10 cm s(-1). Its Rossby number was similar to 0.4, consistent with the fact that the measured cyclogeostrophic velocity was dominated by the geostrophic component. Different scenarios are discussed regarding how the eddy became embedded in the slope current, and what the associated ramifications are with respect to eddy spin-down and ventilation of the Canada Basin halocline. Plain Language Summary A critical feature of the interior Arctic Ocean is the sharp vertical change in salinity between roughly 100-m to 200-m depth, known as the cold halocline. This shields the warm Atlantic-origin water below from mixing upward to the surface and melting the pack ice. The cold halocline is believed to be partially maintained by eddies of cold water emanating from the Chukchi Sea continental shelf. This paper presents measurements from a rapid, high-resolution shipboard survey of a cold-core Arctic eddy offshore of the shelf edge, providing a unique three-dimensional view of the feature. The eddy's core contained water near the freezing point with a high level of nitrate, but the biological activity had largely ceased because the eddy had descended below the part of the water column exposed to sunlight. The eddy was imbedded in the offshore edge of the westward-flowing Chukchi slope current. Different scenarios are discussed regarding how the eddy became embedded in the slope current, and what the associated ramifications are with respect to disintegration of the eddy and the manner in which the cold water feeds the halocline.