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
Authors: Kalita Bijit Kumar , Vinayachandran Pn 
Published: February 2023

The Bay of Bengal is a unique tropical ocean basin because of seasonally reversing monsoonwinds; copious freshwater discharge from nearby continental rivers helps barrier layer formation.&#160;Again, due to its capacity to keep warm SST beyond 28oC, the bay is prone to tropical cyclonesduring the transition between two monsoons. Geographically, the basin is land bounded on three&#160;sides and connected to global oceans through its southern boundary. Vicinity to the equatorial&#160;The Indian Ocean facilitates the propagation of Kelvin waves through its rim as coastally trapped&#160;waves. Hence, local and remote forcings make the bay an active basin for brewing mesoscale&#160;features like eddies. Eddies play a vital role in the bay's upper ocean mesoscale dynamics (O[100s&#160;Km]). Surface intensified eddies are well studied, but very little is known about subsurface&#160;circulations. Limited literature reports active subsurface eddy fields in the basin. Observations by&#160;a RAMA buoy at 90oE, 15oN from 2007 to 2020 shows a thermocline bulge. For about a month,&#160;this peculiar subsurface feature is characterized by the doming (denting) of the seasonal&#160;thermocline's upper (lower) part. The bulge is a regular seasonal feature during the winter&#160;monsoon as denoted by time series analysis of D26 (depth of 26oC isotherm) &#8211; D12 (depth of&#160;26oC isotherm) from RAMA temperature. This research, using a suite of in-situ moored buoy&#160;observations, satellite altimetry, OSCAR surface current, near-surface ASCAT wind and HYCOM&#160;re-analysis data, suggests a possible mechanism for the formation of thermocline bulge. Usually,&#160;it isn't easy to detect a subsurface feature from the sea surface variables like SST or SLA. But&#160;eddy-wind interactions can lead to the local generation of lens-shaped features in the&#160;thermocline of a pre-existing surface-intensified anti-cyclonic eddy. Observations show the&#160;simultaneous development of a surface anti-cyclone off the Irrawaddy delta (hereafter referred&#160;to as ICAE) and upwelling-favourable winter monsoon winds in the background. The interaction&#160;of background wind stress with the IACE facilitates the formation of a bulge by doming the&#160;seasonal thermocline at the eddy core. The thermocline bulge starts its westward journey along&#160;with the parent eddy due to Rossby wave forcing in December and crosses the RAMA buoy in&#160;mid-January. Three factors are responsible for a bulged IACE off the Myanmar coast: 1. the arrival&#160;of coastal Kelvin waves due to intense remote equatorial forcing by Wrytki jets, 2. eddy&#160;separation from the coast, and 3. Ekman suction (or upwelling) at the centre of IACE due to local&#160;"eddy-wind" interaction during late fall to winter. The IACE that wraps a bulged thermocline in&#160;its core is an example of seasonal mode-water ACE or intra-thermocline eddy during the winter,&#160;typical in higher latitudes but only recently observed in tropical basins like the Bay of Bengal.