Frontal circulation and submesoscale variability during the formation of a Southern Ocean mesoscale eddy

Abstract

<jats:title>Abstract</jats:title><jats:p>Observations made in the Scotia Sea during the May 2015 Surface Mixed Layer Evolution at Submesoscales (SMILES) research cruise captured submesoscale, <jats:italic>O</jats:italic>(1–10) km, variability along the periphery of a mesoscale <jats:italic>O</jats:italic>(10–100) km meander precisely as it separated from the Antarctic Circumpolar Current (ACC) and formed a cyclonic eddy ~120 km in diameter. The meander developed in the Scotia Sea, an eddy-rich region east of the Drake Passage where the Subantarctic and Polar Fronts converge and modifications of Subantarctic Mode Water (SAMW) occur. In situ measurements reveal a rich submesoscale structure of temperature and salinity and a loss of frontal integrity along the newly formed southern sector of the eddy. A mathematical framework is developed to estimate vertical velocity from collocated drifter and horizontal water velocity time series, under certain simplifying assumptions appropriate for the current dataset. Upwelling (downwelling) rates of <jats:italic>O</jats:italic>(100) m day<jats:sup>−1</jats:sup> are found in the northern (southern) eddy sector. Favorable conditions for submesoscale instabilities are found in the mixed layer, particularly at the beginning of the survey in the vicinity of density fronts. Shallower mixed layer depths and increased stratification are observed later in the survey on the inner edge of the front. Evolution in temperature–salinity (<jats:italic>T</jats:italic>–<jats:italic>S</jats:italic>) space indicates modification of water mass properties in the upper 200 m over 2 days. Modifications along <jats:italic>σ</jats:italic><jats:sub><jats:italic>θ</jats:italic></jats:sub> = 27–27.2 kg m<jats:sup>−3</jats:sup> have climate-related implications for mode and intermediate water transformation in the Scotia Sea on finer spatiotemporal scales than observed previously.</jats:p>