Role of coastally trapped waves of remote origin and local eddy-wind interaction in the formation of seasonal thermocline bulge in the Bay of Bengal

The Bay of Bengal is a unique tropical ocean basin because of seasonally reversing monsoon
winds; copious freshwater discharge from nearby continental rivers helps barrier layer formation. 
Again, due to its capacity to keep warm SST beyond 28^oC, the bay is prone to tropical cyclones
during the transition between two monsoons. Geographically, the basin is land bounded on three 
sides and connected to global oceans through its southern boundary. Vicinity to the equatorial 
The Indian Ocean facilitates the propagation of Kelvin waves through its rim as coastally trapped 
waves. Hence, local and remote forcings make the bay an active basin for brewing mesoscale 
features like eddies. Eddies play a vital role in the bay's upper ocean mesoscale dynamics (O[100s 
Km]). Surface intensified eddies are well studied, but very little is known about subsurface 
circulations. Limited literature reports active subsurface eddy fields in the basin. Observations by 
a RAMA buoy at 90^oE, 15^oN from 2007 to 2020 shows a thermocline bulge. For about a month, 
this peculiar subsurface feature is characterized by the doming (denting) of the seasonal 
thermocline's upper (lower) part. The bulge is a regular seasonal feature during the winter 
monsoon as denoted by time series analysis of D26 (depth of 26^oC isotherm) – D12 (depth of 
26^oC isotherm) from RAMA temperature. This research, using a suite of in-situ moored buoy 
observations, satellite altimetry, OSCAR surface current, near-surface ASCAT wind and HYCOM 
re-analysis data, suggests a possible mechanism for the formation of thermocline bulge. Usually, 
it isn't easy to detect a subsurface feature from the sea surface variables like SST or SLA. But 
eddy-wind interactions can lead to the local generation of lens-shaped features in the 
thermocline of a pre-existing surface-intensified anti-cyclonic eddy. Observations show the 
simultaneous development of a surface anti-cyclone off the Irrawaddy delta (hereafter referred 
to as ICAE) and upwelling-favourable winter monsoon winds in the background. The interaction 
of background wind stress with the IACE facilitates the formation of a bulge by doming the 
seasonal thermocline at the eddy core. The thermocline bulge starts its westward journey along 
with the parent eddy due to Rossby wave forcing in December and crosses the RAMA buoy in 
mid-January. Three factors are responsible for a bulged IACE off the Myanmar coast: 1. the arrival 
of coastal Kelvin waves due to intense remote equatorial forcing by Wrytki jets, 2. eddy 
separation from the coast, and 3. Ekman suction (or upwelling) at the centre of IACE due to local 
"eddy-wind" interaction during late fall to winter. The IACE that wraps a bulged thermocline in 
its core is an example of seasonal mode-water ACE or intra-thermocline eddy during the winter, 
typical in higher latitudes but only recently observed in tropical basins like the Bay of Bengal.