Effect of seasonal barrier layer on mixed-layer heat budget in the Bay of Bengal

Time series measurements (2010–2017) from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) moorings at 15°N, 90°E and 12°N, 90°E are used to investigate the effect of the seasonal barrier layer (BL) on the mixed-layer heat budget in the Bay of Bengal (BoB). The mixed-layer temperature tendency ( ∂T/∂t ) is primarily controlled by the net surface heat flux that remains in the mixed layer ( Q ′) from March to October, while both Q ′ and the vertical heat flux at the base of the mixed layer ( Q h ), estimated as the residual of the mixed-layer heat budget, dominate during winter (November–February). An inverse relation is observed between the BL thickness and the mixed-layer temperature (MLT). Based on the estimations at the moorings, it is suggested that when the BL thickness is ⩾25 m, it exerts a considerable influence on ∂T/∂t through the modulation of Q h (warming) in the BoB. The cooling associated with Q h is strongest when the BL thickness is ⩽10 m with the MLT exceeding 29°C, while the contribution from Q h remains nearly zero when the BL thickness varies between 10 m and 25 m. Temperature inversion is evident in the BoB during winter when the BL thickness remains ⩾25 m with an average MLT<28.5°C. Furthermore, Q h follows the seasonal cycle of the BL at these RAMA mooring locations, with r >0.72 at the 95% significance level.