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 ($\partial T/\partial t $) is primarily controlled by the net surface heat flux that remains in the mixed layer ($ {Q}^{\text{'}} $) from March to October, while both $ {Q}^{\text{'}} $ and the vertical heat flux at the base of the mixed layer ($ {Q}_{h} $), estimated as the residual of the mixed-layer heat budget, dominate during winter (November–February). An inverse relation is observed between the BL thickness and the mixed-layer temperature ($ \mathrm{M}\mathrm{L}\mathrm{T} $). Based on the estimations at the moorings, it is suggested that when the BL thickness is ≥25 m, it exerts a considerable influence on $\partial T/\partial t$ 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 $ \mathrm{M}\mathrm{L}\mathrm{T} $ 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 $ \mathrm{M}\mathrm{L}\mathrm{T} $ <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.