CURRENTS AND TRANSPORTS OF THE MONSOON CURRENT SOUTH OF SRI-LANKA

The zonal monsoon circulation south of India/Sri Lanka is a crucial li nk for the exchange between the northeastern and the northwestern Indi an Ocean. The first direct measurements from moored stations and shipb oard profiling on the seasonal and shorter-period variability of this flow are presented here. Of the three moorings deployed from January 1 991 to February 1992 along 80 degrees 30'E between 4 degrees 11'N and 5 degrees 39'N, the outer two were equipped with upward looking acoust ic Doppler current profilers (ADCPs) at 260-m depth. The moored and sh ipboard ADCP measurements revealed a very shallow structure of the nea r-surface flow, which was mostly confined to the top 100 m and require d extrapolation of moored current shears toward the surface for transp ort calculations. During the winter monsoon, the westward flowing Nort heast Monsoon Current (NMC) carried a mean transport of about 12 Sv in early 1991 and 10 Sv in early 1992. During the summer monsoon, transp orts in the eastward Southwest Monsoon Current (SMC) were about 8 Sv f or the region north of 3 degrees 45'N, but the current might have exte nded further south, to 2 degrees N, which would increase the total SMC transport to about 15 Sv. The circulation during the summer was somet imes found to be more complicated, with the SMC occasionally being sep arated from the Sri Lankan coast by a band of westward flowing low-sal inity water originating in the Bay of Bengal. The annual-mean flow pas t Sri Lanka was weakly westward with a transport of only 2-3 Sv. Using seasonal-mean ship drift currents for surface values in the transport calculations yielded rather similar results to upward extrapolation o f the moored profiles. The observations are compared with output of re cent numerical models of the Indian Ocean circulation, which generally show the origin of the zonal flow past India/Sri Lanka to be at low l atitudes and driven by the large-scale tropical wind field. Superimpos ed on this zonal circulation is local communication along the coast be tween the Bay of Bengal and the Arabian Sea.