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

Abstract

High-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.

An undercurrent off the east coast of Sri Lanka

“…During boreal fall, the surface current is northward. Similarly, northward surface flow was also observed in mooring measurements in the southern BoB in late July of 2015 at 85.5 • E (Wijesekera et al, 2016b). During the winter, the southward-flowing EICC extends from the Indian east coast to the southern tip of Sri Lanka (Schott et al, 1994;Wijesekera et al, 2015), resulting in strong southward flow at 8 • N (Fig.…”

“…An alternative interbasin-exchange pathway is through the interior of the BoB, particularly during the southwest and northeast monsoon (Vinayachandran et al, 1999;Wijesekera et al, 2015Wijesekera et al, , 2016b. Observations indicate that the eastward-flowing southwest monsoon current (SMC) has a role in the injection of saline water, originating in the AS, into the southern central BoB during early summer.…”

“…The SLD is driven by Rossby waves radiating from the eastern boundary and intensified Ekman pumping inside the BoB (Vinayachandran et al, 1999;Shankar et al, 2002;de Vos et al, 2014). The SLD propagates westward (Wijesekera et al, 2016b) toward the east coast of Sri Lanka, resulting in a southward coastal surface flow during early summer. In October, the prevailing wind in the BoB starts reversing direction and blows south-westward.…”

“…The subsurface circulation in this region is also potentially important, as high-salinity water from the AS can be subducted beneath the fresher surface water originating from river runoff and precipitation in the northern BoB (Rao and Sivakumar, 2003;Sengupta et al, 2006;Vinayachandran et al, 2013;Gordon et al, 2016;Jensen et al, 2016); this is evident in observations and model studies both during the northeast (Wijesekera et al, 2015) and southwest monsoons (June-August) (Wijesekera et al, 2016b). Little is known about the subsurface structure of the boundary current along the east coast of Sri Lanka.…”

Interactive comment on “Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal” by Venugopal Thushara et al.

“…The southern BoB, characterised by strong currents, intense mixing and upwelling, is one of the most dynamically active regions of the northern Indian Ocean (Murty et al, 1992;Schott et al, 1994;McCreary et al, 1996;Vinayachandran and Yamagata, 1998;Vinayachandran et al, 1999;Shankar et al, 2002;Lee et al, 2016;Wijesekera et al, 2016b). Unlike the northern BoB, salinity stratification is relatively weak in the south, resulting in a deeper mixed layer.…”