Mixed Layer heat budget in the North Eastern Arabian Sea during winter and spring and relative contribution of diapycnal heat flux

Jofia Joseph1, Ashin Kuriakose2, Girishkumar M S3, N Sureshkumar4, Sherin V Raju5, Suprit Kumar6, V P Thangaprakash4, Shivaprasad. S7, E Pattabhi Ram Rao6 and S. S. C. Shenoi6, (1)Indian National Center for Ocean Information Services, Hyderbad, India, (2)INCOIS, India, (3)Scientist, Indian National Centre for Ocean Information Services (INCOIS), Hyderabad, India, (4)Indian National Centre for Ocean Information Services (INCOIS), Hyderabad, India, (5)National Institute of Oceanography, Dona Paula, India, (6)Indian National Center for Ocean Information Services, Hyderabad, India, (7)Indian National Centre for Ocean Information Services, Hyderabad, India
The Sea Surface Temperature (SST) variability in the Arabian Sea (AS) has a significant role to determine the Indian Summer Monsoon Rainfall (ISMR) variability in various spatio-temporal timescales. Earlier studies have shown that the cold bias in the AS SST, which develops in winter and persists through spring and summer, leads to significant dry bias in ISMR in the coupled model simulation. Though several observation campaigns were carried out in the Arabian Sea, systematic measurements of fine scale fast evolving turbulent mixing processes using vertical microstructure profiler (VMP) were non-existent in the Arabian Sea to the best of our knowledge. Considering this point, Indian National Centre for Ocean Information Services (INCOIS) conducted 3 scientific cruises during 2018-2019 to document upper ocean turbulent mixing characteristics during winter cooling (12 January 2018 – 12 February 2018, 4 January-5 February 2019) and spring warming (1-20 May 2019) period. One of the major highlights of these cruises are 16-day time series measurements using VMP in every 3-hour interval in the north eastern Arabian Sea (18.5 °N and 67.5 °E) during winter and spring. In addition, near surface meteorological parameters, 10-km underway CTD transects either side of time series stations and radiometer observations provide an unprecedented opportunity to document all the terms in the mixed layer heat budget, such as, net surface heat flux, horizontal advection, penetrative shortwave radiation, entrainment and diapycnal heat flux. The mixed layer heat budget analysis shows the dominant role of net surface heat flux to determine SST during winter and spring. The diapycnal heat flux at the base of the Mixed layer is obtained around -1.5W/m2 in winter and -6.0 W/m2 in spring and the corresponding turbulent eddy diffusivities are 1.5*10-5 and 1.2*10-4 respectively. In addition, we also compared the in-situ observation of turbulence parameters with one dimensional mixed layer models (K-Profile Parameterization (KPP), Mellor–Yamada (MY) TKE model, and K-ε model).