General Circulation and Principal Wave Modes in Andaman Sea from Observations

Objectives: This study intends to describe the Andaman Sea circulation and investigate the dominant modes of variability in the basin. Analysis/Observations: The domain experiences stronger South-westerly winds from May to September and relatively weaker North-easterlies from November to February. A strong negative Ekman pumping along the north coast of Indonesia is observed during Summer. The transport of water across the straits of Andaman and Nicobar Islands (ANI) is computed by simple mass balance and is found to be in phase with the monthly averaged Mean Sea Level Anomalies (MSLA) of Andaman Sea. There occurs high surface outflux through Great channel and Ten-Degree channel in Summer. In April and October, rate of transport through the straits is maximum. During the same months, meridional surface currents intensify along the eastern boundary of the basin and are associated with signatures of downwelling. Findings: Intense down welling occurs to the north coast of Indonesia during Summer, locally forced by south-westerlies. There occurs large influx of water into Andaman Sea between April and November through the straits between the islands. Equatorial Wyrtki jets remotely force Kelvin waves of downwelling nature in the basin during April and October. The circulation in Andaman Sea is characterised by gyres or vortices, which is the manifestation of Rossby waves of semi-annual mode. The basin has a characteristic 120-day mode of westward propagating long Rossby wave packets which reflect from the coast of ANI as eastward propagating short Rossby waves. Novelty: The first attempt to completely describe the dynamics of Andaman Sea circulation exclusively from observations.