Variability of water properties, heat and salt fluxes in the Arabian Sea, between the onset and wane of the 1995 southwest monsoon

Abstract

Abstract We investigate the variability of the circulation, water masses, heat and salt fluxes in the Arabian Sea over the course of the southwest monsoon. Two zonal sections taken along 8°30′N in 1995 as part of the Indian Ocean WOCE hydrographic program are used. The first was occupied in early June at the onset of the southwest monsoon winds, the second in late September, at the wane of the monsoon. The September section was found to be generally warmer (+0.32°C) and saltier (+0.04) than in June, despite a 50 mm drop in mean sea level. Therefore, the common assumption that an increase in sea-surface height follows an increase in heat content (the hydrostatic response) does not hold. Instead, we conclude that the heat content increases due to the advection of Arabian Sea Surface Water and Red Sea Water onto the section from the north, and the drop in sea level is due to a loss of mass, rather than heat, from the water column. There are large uncertainties involved in diagnosing the heat-flux divergence across the Arabian Sea, because the seasonal variability of the water masses and circulation in the basin mean that our data are not representative of a steady state. We treat each section separately and find an oceanic heat export of −0.72 PW in June and −0.19 PW in September, implying a basin cooling rate of about −0.36 PW in June and a slight heating of 0.12 PW in September. In June the mass and heat balances are dominated by the Ekman transport and the Somali Current, with very flat density surfaces resulting in a small interior geostrophic transport. By September the Ekman transport has reduced, and it is primarily the interior transport that balances a strong Somali Current. There are two main overturning cells in June and September: A shallow one of approximate magnitude 15 Sv in June and 0 Sv in September, which reaches depths of no more than 500 m and is driven by Ekman divergence at the surface; and a deep cell of magnitude 1 Sv representing a weak inflow and subsequent upwelling of Circumpolar Deep water. The deep cell implies a basin-averaged upwelling velocity of 3.2×10−5 cm s−1 through 2200 m.