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

Abstract

High density CTD and XBT sections were covered from 35° to 56°S along 45°E and 57°30'E to investigate the morphology of the main fronts in the southwest Indian Ocean, as a part of the Indian pilot expedition to the Southern Ocean on board ORV Sagar Kanya. Northern branch of the Subtropical Front (NSTF) was observed at ~35°30'S along 45°E. Along 57°30'E, the signature of the Agulhas Return Front (ARF) + Subtropical Front (STF) was identified with a rapid decrease in surface temperature between 43°30' and 45°S and it is located with a southward shift compared to that at 45°E. The Subantarctic Front (SAF) was distinguished as two fronts as northern SAF (SAF1) and southern SAF (SAF2) along both the meridional sections. Polar Front1 (PF1) was identified between 49° and 50°S whereas Polar Front2 (PF2) was identified between 52° and 54°S along 45°E. This study reveals a southward shift of the oceanic fronts (ARF + STF) from west to east, with a maximum southward displacement of > 2° latitude at 57°30'E. A novel finding of this study is that along 45°E, SAF1 merged with ARF and SSTF and SAF2 ~4° latitude southwards from the merged fronts whereas along 57°30'E, SAF1 was not identified as a merged front with ARF and STF as opposed to earlier studies [Belkin, I.M., Gordon, A.L., 1996. Southern Ocean fronts from the Greenwich Meridian to Tasmania. Journal of Geophysical Research 101, 3675-3696]. 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. Major water masses like Subtropical Surface Water, Subantarctic Surface Water, Mode Water, Antarctic Intermediate Water, Circumpolar Deep Water and Antarctic Bottom Water were identified along 45°E. Upper-ocean heat-content computation revealed a remarkable drop of 989×10⁷ J m⁻² at ~42°S and 1405×10⁷ J m⁻² at ~44°S along 45° and 57°30'E, respectively. We believe that this sudden drop in heat content affects the meridional heat transfer which is crucial to the regional climatic variability.