Low-Frequency Currents from Deep Moorings in the Southern Bay of Bengal

Abstract

igh-resolution currents and hydrographic fields were measured at six deep-water moorings in the southern Bay of Bengal (BoB) by the Naval Research Laboratory as part of an international effort focused on the dynamics of the Indian Ocean. Currents, temperature, and salinity were sampled over the upper 500 m for 20 months between December 2013 and August 2015. One of the major goals is to understand the space?time scales of the currents and physical processes that contribute to the exchange of water between the BoB and the Arabian Sea. The observations captured Southwest and Northeast Monsoon Currents, seasonally varying large eddies including a cyclonic eddy, the Sri Lanka dome (SLD), and an anticyclonic eddy southeast of the SLD. The observations further showed intraseasonal oscillations with periods of 30?70 days, near-inertial currents, and tides. Monthly averaged velocities commonly exceeded 50 cm s?1 near the surface, and extreme velocities exceeded 150 cm s?1 during the southwest monsoon. Tides were small and dominated by the M2 component with velocities of about 3 cm s?1. The average transport into the BoB over the measurement period was 2 Sv (1 Sv ≡ 106 m3 s?1) but likely exceeded 15 Sv during summer of 2014. This study suggests the water exchange away from coastal boundaries, in the interior of the BoB, may be largely influenced by the location and strength of the two eddies that modify the path of the Southwest Monsoon Current. In addition, there is a pathway below 200 m for transport of water into the BoB throughout the year.