On the dynamics of the Lakshadweep high and low in the southeastern Arabian Sea

Abstract

Hydrography and altimetry show that a "high" in surface topography forms off southwest India in January (midnortheast monsoon) and that a "low" forms during the southwest monsoon; the high and low, called Lakshadweep high and low after the island chain in the vicinity of where they form, propagate westward, extending across the southern Arabian Sea a few months after genesis. We investigate the dynamics of the high and low with an analytic model and with numerical simulations using a dynamical reduced-gravity model for the north Indian Ocean. We conclude that the high and low do not owe their existence to nonlinearity. They are a consequence of westward propagating Rossby waves radiated by Kelvin waves propagating poleward along the western margin of the Indian subcontinent. Most important for the annual cycle of the high and low are the annual and semiannual Kelvin waves off southwest India; these appear to be forced primarily by alongshore winds in the Bay of Bengal and by winds in the equatorial Indian Ocean. Other Kelvin waves, provided their period is greater than about 40 days, can contribute to the high and low; these Kelvin waves are expected to introduce intra-annual and interannual variability in the annual cycle. Below the critical period of 40 days, energy is trapped at the coast. The analysis provides a mechanism for the early onset of upwelling off southwest India, which is known to begin in February, well before the onset of the upwelling-favorable southwest monsoon winds.