Andaman sea circulation in a high-resolution OGCM

Abstract

The objective of this study is to understand the circulation of the Andaman Sea, which is unique by geography and oceanography, using a high-resolution Ocean General Circulation Model (ROMS) in tandem with altimeter and buoy observations. The north-easterly winds over the region reverse in May and switch back by November, so does the wind stress curl. Intense upwelling/downwelling along the north coast of Indonesia is a significant observation of this study, which is concomitant with the Ekman pumping (it pursues an annual cycle with its value being negative in summers and positive in winters) of the region. Further, there is seasonal reversal of currents (eastward in summers and westward in winters), which is a direct repercussion of Ekman drifts. The straits between the Andaman and Nicobar Islands, Sumatra, and Myanmar serve as the major inlets to this sea from the Bay of Bengal. The Indian Monsoon plays an indispensable role in regulating the flux of water through these straits, which is evident from mass transport studies as well as from spectral analysis of oceanographic parameters. It is found that during summer monsoon, there is heavy outflux of water from the basin through the Great Channel. The net inflow into this regime from the Bay of Bengal through these straits peaks in April and September, while it plummets during monsoon. Nevertheless, the influx from rivers and precipitation over the sea shoots up during monsoon. The mean sea level of the Andaman Sea goes up twice a year, each during summer and winter. This is accompanied by the persistence of cyclonic-anticyclonic vortices in the domain, as observed in the model and confirmed by observations. The performance of the model is satisfactory from the perspective of simulations of pertinent harmonics to which the basin resonates. The existence of low-frequency waves like the second baroclinic Rossby wave (of phase speed 0.017 m/s) of 240-day period and second baroclinic Kelvin wave (of phase speed 1.3 m/s) of 180-day period are some of the sterling observations of this study. The westward-propagating Rossby waves and trapping of Kelvin waves to the eastern coast of the basin throw light on the probable oceanic wave modes that determine the dynamics of the Andaman Sea.