On the variability of Arabian Sea mixing and its energetics

The mechanism of interannual variability of Arabian Sea (AS) mixing is studied by examining its energetics from 60 years of model output (1950-2009) generated using an ocean general circulation model. The model results are compared with ocean re-analysis data for the same period. Both model and re-analysis data show consistent patterns of interannual variability of AS mixing. It is observed that the dominant mode of interannual variability has a basin-wide structure with monotonic sign throughout the basin, albeit with a higher intensity in the central and southeast regions of the AS. The variability is primarily controlled by El Niño-Southern Oscillation with shallower (deeper) mixed layer during El Niño (La Niña). During a typical La-Niña year, the wind stress forcing, heat fluxes and evaporation minus precipitation together cause deepening of mixed layer depth (MLD) in AS. This MLD variability has the strongest signal in winter months (i.e. December to February; DJF). Further, the resolved microstructures of mixing energetics suggest that the dominant forcing of MLD variability changes with seasons and has a regional preference. The study identifies the regions in AS with the corresponding governing mechanisms of MLD variability. Buoyancy production of turbulent kinetic energy (TKE) governs the mixing and associated MLD variability in central AS during September to November (SON) and in northeast AS during DJF. Both buoyancy and mechanical production of TKE governs MLD variability in southwest to central AS from June to August and southeast AS in SON. Thus, the study highlights basin-wide structure of interannual variability of AS mixing along with its controlling mechanisms with strong regional dependency owing to the contrasting nature of forcing prevailing over various parts of the AS.