Propagating waves and their influence on eddy field variability in the northeastern Arabian Sea and northern Bay of Bengal

Abstract

Theoretical modelling and observational studies confirmed the presence of planetary scale Kelvin and Rossby waves and their effect on circulation in the Bay of Bengal and the Arabian Sea. This study focuses on the role of propagating waves on eddy field variability. Analysis of geostrophic currents derived from 18 years of altimeter data revealed a large variability in the intensity and the number of eddies between boxes of the same area (8° × 10° above 12° N) in the northeastern Arabian Sea (NEAS) and the northern Bay of Bengal (NBoB). In the NEAS, eddy kinetic energy levels are low (50–250 cm2 s–2) when compared with NBoB (200–800 cm2 s–2) for a selected box of the same size. The number of eddies in the NEAS is 434, which is fewer than in the NBoB (452), for a span of 18 years in the representative boxes. The amplitudes and speeds of both cyclonic and anticyclonic eddies are very low in the NEAS compared with those in the NBoB. The estimated input wind energy is of same order in these regions. The lower eddy kinetic energy in the NEAS compared with the NBoB is attributed to the variability of coastally trapped Kelvin waves and radiated Rossby waves. Of two upwelling and two downwelling coastally trapped Kelvin waves on an annual cycle, only one downwelling Kelvin wave reached the west coast of India during December and January. Inter-annual variability of these waves also significantly connects to eddy kinetic energy variability.