SIDDIES Corridor: A Major East-West Pathway of Long-Lived Surface and Subsurface Eddies Crossing the Subtropical South Indian Ocean

Abstract

South Indian Ocean eddies (SIDDIES), originating from a high evaporation region in the eastern Indian Ocean, are investigated by tracking individual eddies from satellite data and co-located Argo floats. A subsurface-eddy identification method, based on its steric dynamic height anomaly, is devised to assign Argo profiles to surface eddies (surfSIDDIES) or subsurface eddies (subSIDDIES). These westward-propagating, long-lived features (>3 months) prevail over a preferential latitudinal band, forming a permanent structure linking the eastern to the western Indian Ocean, that we call the 'SIDDIES Corridor’. Key features have been revealed in the mean thermohaline vertical structure of these eddies. Anticyclonic SIDDIES are characterized by positive subsurface salinity anomalies, with subSIDDIES not exhibiting negative surface anomalies, as opposed to surfSIDDIES. Cyclonic subSIDDIES also occur, but their related salinity anomalies are weaker. SubSIDDIES exhibit two cores of different temperature polarities in their surface and subsurface levels. Cyclonic subSIDDIES have their cores at around 150-200 m depth along the 25.4-25.8 kg m−3 potential density layer with anticyclonic subSIDDIES having their cores at 250-300 m along the 26-26.4 kg m−3 density layer. The SIDDIES corridor acts as a zonal pathway for both eddy-types to advect water masses and biogeochemical properties across the basin. This study provides a new insight on heat/salt fluxes, showing that 58% (32%) of the total heat eddy-flux is ascribed to cyclonic (anticyclonic) subSIDDIES, respectively, in the eastern South Indian Ocean. Anticyclonic subSIDDIES have also been found to be the sole high-saline water eddy-conveyor toward the western Indian Ocean.