Characteristics and Seasonality of eddies in the Eastern Arabian Sea
Midhila Varna V*, Jithin Abraham K, Dr Francis P. A
Indian National Centre for Ocean Information Services (INCOIS), Hyderabad, Telangana, India Pincode: 500090 Mobile: 9747811024 E-mail: firstname.lastname@example.org
Variability of ocean currents occur at various Spatio-temporal scales, mesoscale eddies (smaller than the size of gyres, with horizontal dimensions of around 30 km to 150 km, vary at the time scale of a few weeks) are one of the main sources of such variability, which are driven by the instabilities in the ocean circulation, especially in the vicinity of a boundary current system. These mesoscale eddies are ubiquitous in nature and they play an important role in the dynamics of ocean systems ranging from tracer exchange; to altering fluxes to having biogeochemical implications. The characteristics of eddies in the Arabian Sea with added significance to the Eastern Arabian sea have been studied using an eddy tracking algorithm (PyEddy tracker) developed by Mason et al., (2014) for a 26 year-long period (1993-2018). It is deciphered that most of the eddies propagating westward formed in the Eastern Arabian Sea are formed in Winter. It is also observed that the number of cyclonic eddies is higher than that of anticyclonic eddies in the Eastern Arabian Sea. However anticyclonic eddies show larger average amplitude and radius as compared to their cyclonic counterparts. It is also observed that more eddies are formed during Spring and Winter compared to Summer, during which the Arabian Sea is forced with strong monsoon winds. Our analysis suggests that the eddies in the Eastern Arabian Sea have a longer lifetime and larger amplitude during the Spring, however, though the horizontal dimension of the eddies is more during the Summer Monsoon season. Previous studies suggested that the forcing from the equatorial Indian Ocean via downwelling Kelvin waves, which propagates all through the coastal waveguide of the Bay of Bengal and turns around Sri Lanka to propagate further along the west coast of India influence the formation of the mesoscale eddies in the Eastern Arabian Sea (Rao et al., 2014). Our analysis based on the baroclinic instability calculations of the mean flow in the region using data from simulations from a state-of-the-art regional ocean circulation model (NEMO GLORYSV4) confirms this mechanism. Eddy heat flux and salt flux computed also discern the observed seasonality of winter dominance in eddies in the Eastern Arabian Sea. The observed interannual variability in the eddies has a high correlation with the DMI index which confirms the existence of dependence between interannual wind anomaly in the eastern equatorial Indian Ocean and eddy frequency in the Eastern Arabian Sea.
Mason, E., Pascual, A., & McWilliams, J. C. (2014).
A new sea surface height based code for oceanic mesoscale eddy tracking. Journal of atmospheric and oceanic technology. 31(5) 1181-1188
Raghunadha Rao, A., Nageswara Rao, G., & Hareesh Kumar, P. V. (2014).
Propagating waves and their influence on eddy field variability in the northeastern Arabian Sea and the northern Bay of Bengal. International Journal of Remote Sensing, 35(14), 5467-5478.