The southern Bay of Bengal (BOB) cold pool (SCP) plays an important role in the regional climate fluctuation of the BOB. However, the interannual variability in the SCP is still unknown. Multisource satellite remote sensing data and assimilation have been applied to explore the interannual variability in the SCP and its relationship with El Niño–Southern Oscillation (ENSO) events for the period 1982–2020. The anomalous SST of the SCP in the summer following the peak phase (i.e., winter) of the ENSO was closely related to the ENSO events. El Niño (La Niña)-induced the warm (cold) anomaly of the SCP starting from May and persisted throughout August with a peak value appearing in June during the El Niño (La Niña) decaying years. In the El Niño decaying years, the southwest monsoon current (SMC) was weakened, forced locally by the weakening southwesterly wind and remotely by the easterly wind anomaly at the equator associated with El Niño. The El Niño-related weakening SMC and the associated less cold advection led to the warm anomaly of the SCP. In addition, El Niño-related atmospheric heating also made a comparable contribution to the evolution of the SCP’s SST. In the early stage (15 May to 10 June), its contribution to the warming of the SCP was much larger than that of the SMC, whereas from mid-June to August, it reversed to have a cooling effect and partially offset the advection heating induced by the SMC on the SCP. In the La Niña decaying years, similar oceanic and atmospheric processes operated but in an opposite way.

Strong salinity stratification induced by large freshwater fluxes in the northern Bay of Bengal (BOB) results in the formation of a quasi-permanent barrier layer (BL) that covers almost the entire BOB and leads to a unique temperature inversion within the thick BL in winter. In the presence of temperature inversions, the entrainment process at the bottom of the mixed layer (ML) induces warming effects in the ML, but little is known about this. In this paper, we quantify the contribution of the entrainment process to the ML temperature (MLT) in the northern BOB during the winter of 2013 using monthly and daily data from the Ocean General Circulation Model for the Earth Simulator version 2 (OFES2). It is found that the warming effect of the daily entrainment heat flux (EHF), which resolved the high-frequency variations, is 4 orders of magnitude larger than the monthly EHF for most of the wintertime. This significantly enhanced warming effect in daily data offsets up to 87% of the surface cooling induced by net heat flux during wintertime. A further analysis reveals that the larger daily EHF warming effect compared to its monthly counterpart is closely related to the deepened ML, the larger temperature difference within the ML and vertical velocity at the bottom of the ML.