[A-OS11] Ocean Mixing Frontiers

[AOS11-P09] Double diffusion in the Arabian Sea during winter and spring

*Ashin Kuriakose^1,2, Girishkumar M S -¹, Jofia Joseph -¹, Eric D'Asaro -³, Sureshkumar N -¹, Sherin V R -¹, Murali B -¹, Thangaprakash V P -¹, E Pattabhi Ram Rao¹, S. S. C. Shenoi (1.Indian National Centre for Ocean Information Services, 2.School of Ocean Science and Technology, Kerala University of Fisheries and Ocean Studies, 3.Applied Physics Laboratory and School of Oceanography, University of Washington, Seattle, Washington, USA)

Keywords:Double Diffusion in the Arabian Sea, Heat flux due to Double diffusion

Microstructure measurements from two cruises during winter and spring 2019 documented the importance of double diffusion processes for small-scale mixing in the upper 400 m of the open ocean region of the eastern Arabian Sea (EAS). The data indicated that shear-driven mixing rates are weak, contributing diapycnal diffusivity (K_ρ) of not more than 5.2x10^-6 m² s^-1 in the EAS. Instead, signatures of double diffusion were strong, with the stratification favorable for salt fingers in 70% of the region and favorable for diffusive convection in 2-3% of the region. Well-defined thermohaline staircases were present in all the profiles in these regions that occupied 20% of the water column. Stratification favorable for strong diffusive convection occurred in ~ 45% of data in the barrier layer region of the south EAS (SEAS). The mean dissipation ratio (γ_χε) in the salt fingering (0.8) and diffusive convection (19-39) regimes were much higher than the expected turbulence mixing efficiency (γ_Rf=0.2), again suggesting the importance of double-diffusive processes here. Validation of different parameterizations of double diffusion with the measurements of vertical heat diffusivity (K_T) found that the Radko and Smith (2012) salt fingering scheme and the Kelley (1984) diffusive convection scheme showed the best agreement with observations. The downward heat flux associated with salt fingering in the thermocline region of the EAS of approximately -1.5 Wm^-2 and upward heat flux due to diffusive convection in the barrier layer region of the SEAS of ~10 Wm^-2 combine to cool the thermocline.