Abstract

Eddies are circulating water bodies and is a dominant phenomenon at all latitude bands of the world ocean. The eddy consist a vorticity dominated inner region, the core, surrounded by a strain-dominated region, the circulation cell. In the northern hemisphere, eddies with clockwise circulation are called anti-cyclonic (warm water and high sea level at its core) and those with counter clockwise circulation are called cyclonic (cold water and low sea level at its core). An anti-cyclonic eddy is characterized by convergence at the center and divergence at its periphery and vice versa for a cyclonic eddy. The generating mechanisms of eddies include baroclinic instability, interaction of ocean currents with bottom topography and the local wind stress curl. Eddies can move from their source regions; in general they move westward with southward tendency for anticyclones and northward tendency in the case of cyclones (Cushman-Roisin et al., 1990).

Eddies play a key role in moving energy and matter through the ocean and impact mixing processes in the surface layer of the ocean. They also cause significant variability in thermohaline field and to the transfer of temperature and salinity of one region to another. Eddies can also affect watermass properties far from their generation point (McDonald, 1999). The horizontal and vertical variation in the thermohaline field caused by eddies may lead to the formation of oceanic fronts (Hareesh Kumar et al., 2013). The anomalous temperature and salinity structure associated with eddies cause sound speed variation in all direction; hence influence the long-range sound propagation (Akulichev et al., 2011). The transport of heat by eddies are crucial for balancing the global ocean heat budget (Colas et al., 2013). Because of their ability to transport heat, salt, carbon, and nutrients as they propagate in the ocean, they play a significant role in the global budgets of these tracers (Nencioli et al., 2008). From a biological point of view the divergence associated with the cold core eddies bring nutrients to the surface and support phytoplankton blooms hence biological productivity (Nuncio and Prasanna Kumar, 2013). Thus, a better understanding of eddies are important at regional and global scales at several discipline of Oceanography.

The North Indian Ocean (NIO), one of the most dynamic regions among the world oceans, is also famous for the occurrence of meso-scale eddies. Despite the importance of eddies, our understanding of eddy activity in the NIO remains incomplete.