Abstract
The Bay of Bengal, subjected to monsoonal forcing and tropical cyclones, displays a complex field of ocean eddies. On 5 December 2013 a sub-surface vortex or Intrathermocline Eddy (ITE) composed of water characteristic of the Andaman Sea was observed within the thermocline of the western Bay of Bengal. We propose that the ITE was the product of Tropical Cyclone Lehar interaction on 27 November 2013 with a westward propagating surface eddy from the eastern Bay of Bengal. While Lehar’s interaction with the ocean initially removes heat from the upper layers of the eddy, air-sea flux is limited as the deeper portions of the eddy was subducted into the stratified thermocline, inhibiting further interaction with the atmosphere. The ITE core from 30 to 150 m is thus isolated from local air-sea fluxes by strong stratification at the mixed layer base, and its periphery is stable to shear instability, suggestive of longevity and the ability to carry water far distances with minimal modification.
Highlights
The Bay of Bengal (BoB) embayment of the northeastern Indian Ocean, receives an abundance of freshwater by massive river outflow as well as local precipitation, resulting in a warm, low salinity, buoyant surface layer
A subsurface vortex fitting the characteristics of an Intrathermocline Eddy (ITE) was observed in the western (BoB) during a regional survey by an underway Conductivity Temperature Depth, which gathered data within the upper ~200 m from the Research Vessel (RV) Revelle in December 2013 (Fig. 2) as part of the Air-Sea Interactions Regional Initiative (ASIRI) in the northern Indian Ocean[12,13]
It is expected that the response of anticyclonic ocean eddy (AOE) would be opposite that that of the cyclonic ocean eddies (COE), as supported by the Parallel Ocean Program model using a spatially uniform, but rotating wind field[27]
Summary
An ITE, a class of subsurface vorticities[4], have been detected in numerous regions including marginal seas, eastern boundary currents, and within subtropical gyres[5,6,7,8]. The feature does not have the low salinity subsurface core, and is quite different than the ITE of Andaman Sea water observed by the RV Revelle. As the Rossby number is 0.2 the ITE would be marginally or quasi-geostrophic, none-the-less the ADCP values agree quite well with the geostrophic flow derived from the uCTD data, relative to 200 decibar (Fig. 3c), with the maximum geostrophic current depth, zero geostrophic shear, of ~0.25 to 0.30 m/sec within the 50–80 m depth interval. The BoB is quiescent below the mixed layer and is characterized by a complex vertical structure where both salinity and temperature contribute strongly to density (Fig. 4a) Due to these properties, and Figure 4. High values of Ri along the stratified edges of the ITE combined with the lack of fine structure at its upper and lower boundaries, suggest that shear-driven mixing is unlikely to be a significant factor in water mass evolution at the time of the ship transect
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