Eddy diffusivity in stratified ocean: a case study in Bay of Bengal.

The eddy diffusivity in the ocean pycnocline K N = γ ε / N 2 is analyzed based on field measurements of the turbulent kinetic energy (TKE) dissipation rate ε z and buoyancy frequency N z profiles and an assumed mixing efficiency of γ = 0.2 . The microstructure measurements were taken between 2013 and 2019 in various regions of the Bay of Bengal (BoB) as well as the central Gulf Stream (GS) and deep waters of the Southern California Bight (SCB). The space–time variability of K N z , t observed in the pycnocline of the southeastern BoB is likely related to internal-wave generated turbulence, identified by its following of MacKinnon–Gregg scaling (MacKinnon and Gregg in J Phys Oceanogr 33(7):1476–1492, 2003). The probability distribution function of diffusivity CDF K N could be well fitted by generalized extreme value distribution (GEVD) for all regions of BoB and other oceans. Mixing rates in the upper pycnocline of BoB during extended southwestern monsoon period tend to be larger in the southern parts of the Bay compared to central and northern parts. Statistics of K N in the GS and SCB waters appear to be similar to those in the southern BoB with a characteristic median value ~ 2 × 10 - 6 m²/s, suggesting relatively low intensity of vertical mixing therein compared to the canonical pycnocline diffusivity of K N ≈ 10 - 5 m²/s. Article Highlights: Internal wave instabilities appear to be a dominant mechanism of generating energetic mixing events in the Bay of Bengal (BoB). Mixing rates (diffusivities) in the upper pycnocline tends to be larger in the southern BoB than in the northern part of the Bay. Generalized extreme value distribution approximates well the probability functions of the diffusivity in all tested regions of BoB as well as in the other oceans. [ABSTRACT FROM AUTHOR]