The monsoon currents in the north Indian Ocean

Abstract

The monsoon currents are the seasonally reversing, open-ocean currents that flow between the Arabian Sea and the Bay of Bengal, the two wings of the north Indian Ocean. The Summer Monsoon Current (SMC) flows eastward during the summer monsoon (May-September) and the Winter Monsoon Current (WMC) flows westward during the winter monsoon (November-February). We assemble data on ship drifts, winds and Ekman drift, and geostrophic currents derived from altimetry and hydrography to describe the observed climatological seasonal cycle of the monsoon currents. We then use an Oceanic General Circulation Model (OGCM) to simulate these currents and estimate their transports, and a 1.5-layer reduced-gravity model to investigate the processes that force them. The monsoon currents extend over the entire basin, from the Somali coast to the eastern Bay of Bengal. They do not, however, come into being, or decay, over this entire region at a given time. Different parts of the currents form at different times, and it is only in their mature phase that the currents exist as trans-basin flows. The westward WMC first forms south of Sri Lanka in November and is initially fed by the equatorward East India Coastal Current (EICC); the westward WMC in the southern bay appears later. In its mature phase during December-March, the WMC flows westward across the southern bay; it divides into two branches in the Arabian Sea, one branch flowing westward, the other turning around the Lakshadweep high (a sea-level high off southwest India) to flow into the poleward West India Coastal Current (WICC). The WMC is primarily a geostrophic current, with Ekman drift modulating it. The eastward SMC first appears in the southern bay during May. In its mature phase, which peaks with the summer monsoon in July, the SMC in the Arabian Sea is a continuation of the Somali Current and the coastal current off Oman. It flows eastward and southeastward across the Arabian Sea and around the Lakshadweep low (a sea-level low off southwest India), eastward south of Sri Lanka, and into the Bay of Bengal. The strong winds during the summer monsoon ensure that Ekman drift dominates at the surface, leading to a more complex vertical structure in the SMC than in the WMC. In the depth-averaged flow over 50 m, the mature phase of the SMC extends from May to September. The numerical experiments show that the dynamics of the north Indian Ocean on seasonal time scales is explicable by linear wave theory. Circulation at any point is decided by both local forcing and remote forcing, whose signals are carried by equatorial and coastal waves. Superimposed on the currents associated with these waves is the local Ekman drift. The geostrophic component of the monsoon currents is forced by several processes. In the Bay of Bengal, the monsoon currents are forced by Ekman pumping and by the winds in the equatorial Indian Ocean. In the eastern Arabian Sea, the major forcing mechanisms are the winds along the east and west coasts of India and Sri Lanka; these processes link the parts of the SMC in the Arabian Sea and the bay during the summer monsoon, and of the WMC early during winter. Ekman pumping in the central Arabian Sea and off the Somali coast force the monsoon currents in the central and western Arabian Sea, with the Rossby waves radiated from the Indian west coast also playing a role. Therefore, the monsoon currents consist of several parts, each of which is forced by one or more processes, which act in concert to produce the continuous currents seen flowing across the breadth of the north Indian Ocean.