Currents and transports of the Monsoon Current south of Sri Lanka

The zonal monsoon circulation south of India/Sri Lanka is a crucial link for the exchange between the northeastern and the northwestern Indian Ocean. The first direct measurements from moored stations and shipboard profiling on the seasonal and shorter‐period variability of this flow are presented here. Of the three moorings deployed from January 1991 to February 1992 along 80°30′E between 4°11′N and 5°39′N, the outer two were equipped with upward looking acoustic Doppler current profilers (ADCPs) at 260‐m depth. The moored and shipboard ADCP measurements revealed a very shallow structure of the near‐surface flow, which was mostly confined to the top 100 m and required extrapolation of moored current shears toward the surface for transport calculations. During the winter monsoon, the westward flowing Northeast Monsoon Current (NMC) carried a mean transport of about 12 Sv in early 1991 and 10 Sv in early 1992. During the summer monsoon, transports in the eastward Southwest Monsoon Current (SMC) were about 8 Sv for the region north of 3°45′N, but the current might have extended further south, to 2°N, which would increase the total SMC transport to about 15 Sv. The circulation during the summer was sometimes found to be more complicated, with the SMC occasionally being separated from the Sri Lankan coast by a band of westward flowing low‐salinity water originating in the Bay of Bengal. The annual‐mean flow past 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 of the moored profiles. The observations are compared with output of recent numerical models of the Indian Ocean circulation, which generally show the origin of the zonal flow past India/Sri Lanka to be at low latitudes and driven by the large‐scale tropical wind field. Superimposed on this zonal circulation is local communication along the coast between the Bay of Bengal and the Arabian Sea.