Mixed Layer Temperature Budget in the Arabian Sea During Winter 2019 and Spring 2019: The Role of Diapycnal Heat Flux

Sea surface temperatures (SSTs) simulated by almost all models in Coupled Model Inter-comparison Project phase five are consistently colder (∼−1°C to −4°C) than the observation in the northern Arabian Sea (AS) during winter and spring. These biases significantly weaken the seasonal and extended summer monsoon prediction skills and climate change projection's reliability in this region. To understand the relative contribution of diapycnal heat flux (J_h) compared to other terms in the mixed layer (ML) temperature (MLT) budget during these two seasons, we use time series of vertical profiles of microstructure shear measurements collected in the northeastern AS (NEAS) at 18.4°N and 67.4°E during January 10–17, 2019 (W19) and May 7–21, 2019 (S19). It is found that the vertical processes term and net surface heat flux together determine the bulk of MLT tendency during S19 and W19, and the contribution of the horizontal advection and J_h is relatively smaller. The mean value of J_h (∼−2 W m⁻²) at the base of the ML shows a comparable magnitude during W19 and S19, and the median values of diapycnal diffusivity (K_ρ) at the ML base are not significantly different between W19 (∼3.1 × 10⁻⁶ m² s⁻¹) and S19 (∼5.2 × 10⁻⁶ m² s⁻¹). Besides, K_ρ and J_h values were estimated using different K_ρ-Richardson number (Ri) based interior ocean parameterization schemes at the ML base in the NEAS overestimated with respect to observation. The alleged role of overestimation of K_ρ and J_h in parameterization schemes on model simulation of cold SST bias in the AS is also discussed.