Abstract
Abstract. The existence of a seasonally varying undercurrent along 8° N off the east coast of Sri Lanka is inferred from shipboard hydrography, Argo floats, glider measurements, and two ocean general circulation model simulations. Together, they reveal an undercurrent below 100–200 m flowing in the opposite direction to the surface current, which is most pronounced during boreal spring and summer and switches direction between these two seasons. The volume transport of the undercurrent (200–1000 m layer) can be more than 10 Sv in either direction, exceeding the transport of 1–6 Sv carried by the surface current (0–200 m layer). The undercurrent transports relatively fresher water southward during spring, while it advects more saline water northward along the east coast of Sri Lanka during summer. Although the undercurrent is potentially a pathway of salt exchange between the Arabian Sea and the Bay of Bengal, the observations and the ocean general circulation models suggest that the salinity contrast between seasons and between the boundary current and interior is less than 0.09 in the subsurface layer, suggesting a small salt transport by the undercurrent of less than 4 % of the salinity deficit in the Bay of Bengal.
Highlights
Knowledge of the circulation in the southern Bay of Bengal (BoB) is crucial to understanding the contrasting salinity distributions of the Arabian Sea (AS) and the BoB since it controls the amount of water and salt exchanged between the two ocean basins
The Sri Lanka Dome (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)
To gain a better understanding of the subsurface circulation, we examine the Parallel Ocean Program (POP) velocity fields at 729 m (Movie S1 in the Supplement), which encompasses the depth location of the undercurrent core in boreal spring and summer (Figs. 2, 3; Tables 1, 2)
Summary
Knowledge of the circulation in the southern Bay of Bengal (BoB) is crucial to understanding the contrasting salinity distributions of the Arabian Sea (AS) and the BoB since it controls the amount of water and salt exchanged between the two ocean basins. The surface current to the east of Sri Lanka is influenced by local alongshore winds, remote winds in the vicinity of the northern and eastern boundaries of the BoB, equatorial waves, and interior Ekman pumping (Yu et al, 1991; Shetye et al, 1993; McCreary et al, 1996; Shankar et al, 1996; Vinayachandran and Yamagata, 1997). The SLM suggests the presence of an undercurrent along the Sri Lankan east coast (centered at 8◦ N) that reverses its direction twice a year, with model speeds ranging from 6 cm s−1 equatorward during boreal spring to 8 cm s−1 poleward during summer This undercurrent has not been observed before and will be the focus of this study. Knowledge of the vertical structure, variability, and associated dynamics of the boundary current will contribute to a better understanding of mass and salt exchanges in the northern Indian Ocean
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