A Lagrangian Perspective of Coastally Trapped Kelvin Waves

The Bay of Bengal (BoB) annually experiences four phases of Coastally Trapped Kelvin Waves (CTKWs), namely, the first upwelling (January–March), first downwelling (April–June), second upwelling (July–September), and second downwelling (October–December). The upwelling and downwelling CTKWs are associated with propagating patches of negative and positive Sea-Surface Height Anomaly (SSHA) respectively. The second downwelling CTKW is the strongest and travel farthest . Given that the CTKWs are known to be confined near the coast, their behavior can also be effectively understood using the hyperbolic Lagrangian Coherent Structures (hLCSs) approach, wherein such structures act as barriers to material advection. The hLCSs are a set of material surfaces with pointwise maximal (minimal) repulsion and are referred to a repelling (attracting) hLCSs.  In this study, we have computed the repelling hLCSs from the climatological (1993–2020) geostrophic currents derived from Archiving, Validation and Interpretation of Oceanographic (AVISO) altimetry data. These hLCSs were found to show close correspondence with the climatological SSHA signatures of the different phases of the CTKWs in the BoB. An even better correspondence was seen between these repelling hLCSs and the gradient of climatological SSHA fields. Moreover, these repelling hLCS were found to be located almost at a distance of Rossby Radius of Deformation (Rd) away from the coast.  The hLCSs associated with the CTKWs have major implications on the advection patterns of passive tracer fields such as sea surface temperature, sea surface salinity, and chlorophyll-a. Since, these hLCSs act as material barriers, they were found to separate water masses with different thermodynamic and biological properties on either side of the hLCs. This can potentially result in distinct coastal circulations and biological productivity near the coast.