Mesoscale variability along the east coast of India in spring as revealed from satellite data and OGCM simulations

Abstract

Since mesoscale features like meanders have great importance in nourishing the coastal fisheries, satellite data analyses and a numerical modeling study were carried out for the east coast of India during spring inter-monsoon time (March-May), when biological productivity is high. During this time, the East India Coastal Current (EICC) system appears as a northward flowing western boundary current of a seasonal subtropical gyre in the Bay of Bengal prior to the summer monsoon with a more intense upwelling in the coastal region. A relatively clear sky permits satellite remote sensing of Sea Surface Temperature (SST) and Chlorophyll-a (Chl-a), whose patterns were verified against geostrophic velocity in altimeter data: i.e., phytoplankton grows in colder and nutrient richer water bounded by the seaward meanders. Progression of meanders in the coastal current was revealed and compared with an eddy-resolving Ocean General Circulation Model (OGCM), which is capable of modeling wind-driven general circulation and each stage of the meander growth. The numerical solutions provided the following results, in reasonable agreement with the linear stability theory using a two-and-a-half layer quasi-geostrophic model. Baroclinic instability plays a key role for the meander growth and eddy generation, while meanders in the coastal current are initiated by isolated mesoscale rotations propagating westward. The baroclinically unstable meanders have a wavelength of 500∼700 km, grow in one month and propagate downstream of the coastal current at several kilometers per day. The instability is not strong enough for the meanders to detach an eddy from the western boundary current.