Observed interannual variability of the thermohaline structure in the south eastern Arabian Sea

Abstract

The XBT and sea surface salinity (SSS) measurements systematically carried out at near-fortnightly interval from May 2002 to August 2008 in the South Eastern Arabian Sea (SEAS) are utilised to describe the observed seasonal cycle and its interannual variability of the mixed layer depth (MLD), the near-surface thermal inversions, thermocline oscillations and the SSS. During the summer monsoon season (SMS) of 2005 (2002), the upwelling characterized by up-sloping of 25 degrees C isotherm is relatively weaker (stronger). In addition, the upwelling during 2005 persisted till November unlike in the other years. As a result of prolonged upwelling, the TMI sea surface temperature (SST) has shown cooling extending into the post-monsoon season. The governing mechanisms both in terms of local and remote forcings are examined to explain the observed anomalous upwelling during 2002 and 2005. During SMS and the following winter seasons of 2005, unusually heavy rainfall has occurred over the south-western Bay of Bengal (BoB) and SEAS compared to other years in the study. Despite transportation of large quantities of low salinity waters from the northern BoB into SEAS where relatively cooler near-surface thermal regime persisted due to the prolonged upwelling until November 2005, unusually fewer number of near-surface thermal inversions are noticed in the SEAS during the winter 2005–2006 compared to the other winters. In winter 2005–2006, the combination of prolonged upwelling and stronger surface wind field resulting in anomalous net surface heat loss caused weaker secondary warming of the near-surface waters in the SEAS. This led to a weaker horizontal SST gradient between the SEAS and the intruding BoB waters and hence a reduced number of thermal inversions compared to other winters despite the presence of stronger vertical haline stratification. The possible causative mechanisms are examined to explain the anomalous nature of the observed thermal inversions