The monsoon currents in the north Indian Ocean

The north Indian Ocean is distinguished by the presence of seasonally reversing currents that flow between the Bay of Bengal and the Arabian Sea. These currents are located between the equator and approximately 10 N. 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), March– April and October being months of transition between these well-defined current systems. We assemble data on ship drifts, winds and Ekman drift, geostrophic currents derive from TOPEX/Poseidon sea-level anomalies, and hydrography to define the climatological currents in observations. An Oceanic General Circulation Model (OGCM) is used to simulate the climatology of these currents and estimate transports, and numerical experiments with a simpler model are used to investigate the processes that force these currents. The ship drifts show that the monsoon currents extend over the entire basin, from the Somali coast in the west to the Andaman Sea in the east. 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 the 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. The WMC divides into two branches in the Arabian Sea, one branch continuing to flow westward, and the other turning around the Lakshadweep high off southwest India to flow into the poleward West India Coastal Current (WICC). The SMC in the Arabian Sea is a continuation of the Somali Current and the coastal current off Oman. Preprint submitted to Elsevier Preprint 10 January 2001 It flows eastward and southeastward across the Arabian Sea and around the Lakshadweep low off southwest India. It continues as the eastward SMC south of Sri Lanka. In the Bay of Bengal, the SMC branches, one branch turning into the Bay of Bengal and the other flowing eastward. Ekman drift driven by the monsoon winds overwhelms the geostrophic flow at the surface in the western Arabian Sea. During the summer monsoon, Ekman drift dominates over most of the Arabian Sea; it is only in the eastern Arabian Sea, in the eddies off Somalia, and in the Bay of Bengal that the geostrophic current makes a significant contribution. During the winter monsoon, geostrophy dominates, and Ekman drift modulates the geostrophic current. The Ekman drift shows much less spatial structure than the geostrophic current. Signatures of westward propagation of sea-level anomalies are evident in the altimeter data in the regime of the monsoon currents. The OGCM simulations show that Ekman drift dominates in a shallow surface layer (about 20 m deep), but geostrophy dominates below this. The WMC is primarily a geostrophic current, with Ekman drift modulating it. 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. At the surface, the SMC in the Arabian Sea flows eastward and southeastward, feeding into the eastward SMC south of Sri Lanka. This flow branches east of Sri Lanka, one branch flowing into the bay, the other continuing to flow eastward. The geostrophic component of the SMC is a continuation of the Somali Current. A part of the recirculation around the eddies off Somalia merges with the flow to the west of the Lakshadweep low off southwest India to form a curving SMC that flows into the eastward SMC south of Sri Lanka. The net transport due to the shallow monsoon currents is due to both Ekman drift and geostrophic flow. The WMC (SMC) transports 7 Sv ( 6 Sv) westward (eastward) in the top 100 m between 3-6 N at 80.5 E (south of Sri Lanka) during the winter (summer) monsoon. Numerical experiments with a 12 -layer reduced-gravity model show that the dynamics of the north Indian Ocean on seasonal time scales is explicable by linear wave theory. The equatorial Rossby wave, the equatorial Kelvin wave, and the coastal Kelvin wave merge the Arabian Sea, the Bay of Bengal, and the equatorial Indian Ocean into a single dynamical entity, the north Indian Ocean, which must be modelled as a whole even to simulate circulation in its parts. Circulation at any point is decided by both local forcing and remote forcing, whose signals are carried by the 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