Fronts, water masses and heat content variability in the Western Indian sector of the Southern Ocean during austral summer 2004

Abstract

Two meridional CTD and XBT sections were covered along 45°E and 57°30’E to investigate the morphology of main fronts in the southwest Indian Ocean as a part of Indian pilot expedition to the Southern Ocean on board ORV Sagar Kanya. The frontal systems distinguish the different regimes of cold Antarctic waters from the warmer and saltier waters of the subtropical regime. Along 57°30’E, the signature of the Subtropical Front (STF) was a rapid decrease in surface temperature from 17°C to 10.6°C between 43°50’S and 45°40’S and it was identified as a broad frontal region with a southward shift compared to that at 45°E. The Subantarctic Front (SAF) and Polar Front (PF) were also located with remarkable features noted in the literature. PF was identified between 48° and 52°S along 45°E, where the surface temperature reduced from 5.5 to 2.7°C and where the northern limit of sub-surface temperature minimum layer (<2°C) is extended. This study reveals a southward shift of the oceanic fronts (STF and SAF) from west to east, with a maximum southward displacement of ~4° latitude for STF. The thermocline region was absent south of PF. An enhancement in the mixed layer thickness from 42° to 52°S occurred in association with the strengthening of the wind forcing. The presents of major watermasses like Subtropical Surface Water, Subantarctic Surface Water, Mode Water, Antarctic Intermediate Water, Circumpolar Deep Water and Antarctic Bottom Water were identified along the 45°E meridional section. Upper-ocean heat-content computation revealed a remarkable drop of 989 ×107 J/m2 (~42°S) and 1405 × 107 J/m2 (~44°S) along 45° and 57°30’E, respectively. This sudden drop in heat content in turn affects the meridional heat transfer which is crucial for the studies related to global climatic variability.