Fluctuations in the mixed layer,vertical thermal cross-section and its interannual variability in the Arabian sea

Abstract

In 2002-2004 as part of the Arabian Sea Monsoon Experiment (ARMEX), cruises were conducted in the Cochin-Lakshadweep area to collect temperature data up to depths of about 800 meters deploying XBT probes. The variation of ocean temperature in the vertical, for May 2002 shows the mixed layer is having a depth of about 30m from the sea surface. In the coastal zone at depths of 60m and below the temperatures are about 8°C colder than the corresponding open ocean temperature. In active monsoon season the mixed layer depth has decreased by about 5m from its depth during premonsoon. But the mixed layer temperature decreased by about 3°C. The thermocline has also gone up by about 50m from the May position. In the coastal area the mixed layer depth is only about 10m and mixed layer temperature about 1°C colder than the open sea temperature. Thus it is seen that from May to August there is strong upwelling both in the coastal zone and in the open sea. From active to post monsoon, the mixed layer temperature has warmed through 1°C and mixed layer depth has deepened from 25m to about 40m. The thermo cline has moved downwards through greater than 30m showing strong downwelling. The vertical thermal cross sections in winter show that the mixed layer temperature has not changed from that of postmonsoon(October to november). But mixed layer depth has further deepened by about 30- 40m, showing the continuation of the down welling process. In the coastal zone some of the winter profiles have shown an inversion in temperature. In pre-monsoon(April to May) 2003 mixed layer is slightly colder than in 2002 and mixed layer depth is about 10m less. The thermocline is higher in 2003. This preliminary examination therefore show strong interannual variability between 2002 and 2003.