Oceanic Mixing over the Northern Arabian Sea in a Warming Scenario: Tug of War between Wind and Buoyancy Forces

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AbstractA consensus of the twenty-first-century climate change in the ocean is surface warming, stratification due to extreme freshening and subsequent weakening of mixing, overturning circulation, and biological production. Counterintuitively, certain parts of the tropical ocean may develop a resistance to changes in mixing, where the climate change impacts of atmosphere and ocean are complementary to each other. Under the poleward shift of monsoon low-level jet (LLJ) in the twenty-first century, a part of the northern Arabian Sea has a tendency to maintain the mixed layer depth intact. The process is studied using a set of high-resolution regional ocean model downscaling experiments for the present and future climate. It is found that the wind intensification caused by the shift in LLJ tends to counteract the stratification gained by surface ocean warming and maintains the mixing process in a warming scenario. The mixing energetics shed light on the way in which this is achieved. Intensified winds promote shear production and surface ocean warming demotes buoyancy production of turbulent kinetic energy (TKE), with a net effect of an increase in TKE. However, TKE appears to be dissipating quickly because of the presence of a larger number of small-scale eddies. This causes the mixing length and mixed layer depth to remain intact. Therefore, the interpretations of impacts of future climate change in ocean mixing should be viewed with caution, at least regionally, by focusing on the detailed changes of the governing mechanisms.