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

High density CTD and XBT sections were covered from 35 degrees to 56 degrees S along 45 degrees E and 57 degrees 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 similar to 35 degrees 30'S along 45 degrees E. Along 57 degrees 30'E, the signature of the Agulhas Return Front (ARF)+ Subtropical Front (STF) was identified with a rapid decrease in surface temperature between 43 degrees 30' and 45 degrees S and it is located with a southward shift compared to that at 45 degrees 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 degrees and 50 degrees S whereas Polar Front2 (PF2) was identified between 52 degrees and 54 degrees S along 45 degrees E. This study reveals a southward shift of the oceanic fronts (ARF+STF) from west to east, with a maximum southward displacement of > 2 degrees latitude at 57 degrees 30'E. A novel finding of this study is that along 45 degrees E, SAF1 merged with ARF and SSTF and SAF2 similar to 4 degrees latitude southwards from the merged fronts whereas along 57 degrees 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 degrees to 52 degrees 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 degrees E. Upper-ocean heat-content computation revealed a remarkable drop of 989 x 10(7) Jm(-2) at similar to 42 degrees S and 1405 x 10(7) Jm(-2) at similar to 44 degrees S along 45 degrees and 57 degrees 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. (c) 2006 Elsevier B.V. All rights reserved.