On an upwelling front, propagation of upwelling and vertical velocity in the eastern Arabian sea during monsoon, 1987

Abstract

A coastal upwelling front parallel to the coast and identifiable upto a depth of 75 m was observed between 12.5 and 16 degrees N along the eastern Arabian Sea in September, 1987 from closely spaced digital BT data. With a north-south slope in the thermocline, the front has a temperature gradient of about 6 degrees C over a distance of 30-50 km and appears to be closer to the coast on the northern side than the southern side. Information derived from data during July 1987 shows no such front upto 30-m depth at this location. An estimate based on the positions of isotherms indicates offshore movement of surface water at a rate of about 1-2 km/day from July to September. Vertical velocities computed in the frontal region using the conservation equation of temperature show an average value of -2.77 x 10 sup(-2) cm/sec which is indicative of upwelling. A comparison of offshore volume transport and a temperature anomaly near the coast did not show good correlation. These observations suggest that upwelling along this coast propagates from south to north in the form of a coastally trapped wave