This study investigates the impact of salinity stratification on the upper-ocean response to a category 5 tropical cyclone, Phailin, that crossed the northern Bay of Bengal (BOB) from 8 to 13 October 2013. A drastic increase of up to 5.0 psu in sea surface salinity (SSS) was observed after Phailin’s passage, whereas a weak drop of below 0.5°C was observed in sea surface temperature (SST). Rightward biases were apparent in surface current and SSS but not evident in SST. Phailin-induced SST variations can be divided into the warming and cooling stages, corresponding to the existence of the thick barrier layer (BL) and temperature inversion before and erosion after Phailin’s passage, respectively. During the warming stage, SST increased due to strong entrainment of warmer water from the BL, which overcame the cooling induced by surface heat fluxes and horizontal advection. During the cooling stage, the entrainment and upwelling dominated the SST decrease. The preexistence of the BL, which reduced entrainment cooling by ~1.09°C day⁻¹, significantly weakened the overall Phailin-induced SST cooling. The Hybrid Coordinate Ocean Model (HYCOM) experiments confirm the crucial roles of entrainment and upwelling in the Phailin-induced dramatic SSS increase and weak SST decrease. Analyses of upper-ocean stratification associated with 16 super TCs that occurred in the BOB during 1980–2015 show that intensifications of 13 TCs were associated with a thick isothermal layer, and 5 out of the 13 were associated with a thick BL. The calculation of TC intensity with and without considering subsurface temperature demonstrates the importance of large upper-ocean heat storage in TC growth.

Knowledge of mesoscale eddies in the Bay of Bengal (BOB) is key for further understanding the climate variability in this region and beyond, but little is known about the vertical structure of these eddies. In this study, the three−dimensional structure and transport characteristics of mesoscale eddies in the BOB were comprehensively investigated by the combined use of Argo profiles and satellite data. The composite analysis showed that eddy−induced ocean anomalies are mainly confined to the upper 300 m of the water. The spatial structure of eddy−induced thermohaline perturbations is characterized by a dominant dipole structure in the near surface layer, arising from horizontal advection of the background thermohaline gradient by eddy rotation, and a monopole structure in the subsurface layer, caused by eddy−induced vertical displacements of the isopycnal surfaces. In the eddy core, the maximum temperature anomalies induced by a cyclonic eddy (CE) and an anticyclonic eddy (AE) are about −1.2 °C and +1.2 °C, respectively. The anomalies are located at approximately 100 m. The corresponding salinity anomalies are located at approximately 50 m with a value of −0.1 psu (0.1 psu) for CE (AE). The eddy thermohaline structure has a seasonal character. A deeper temperature and salinity core occurs in both CE and AE in spring compared to that in other seasons, which is primarily caused by the relatively deep thermocline and halocline during that season. In addition, unique warming anomalies induced by CE are present in the mixed layer during winter due to the vertical advection of the BL (Barrier Layer) warmer water by eddies. The total meridional heat transport induced by the composite eddy is poleward (equatorward) south (north) to 10°N with a value of 0.01 PW (−0.013 PW), whereas the total meridional freshwater transport is equatorward with a value of 0.046 Sv over a one−year period. The volume of freshwater export out of the bay is approximately 35% of the annual net freshwater input from local precipitation and river discharge.

This study explored the characteristics of marine heatwaves (MHWs) and the related dynamics in Bay of Bengal (BOB) associated with El Niño events during 1982–2020. This showed that El Niño can induce more MHW days, which displays remarkable seasonality. The strong influences of El Niño were found in the southern BOB at its peak phase in winter and the following spring, characterized by an increase in MHW days by 106% in winter and 83% in spring. In the proceeding summer, its influences weakened and shifted to the central bay. The more MHW days during winter and spring are primarily caused by the enhanced solar radiation associated with El Niño and partly contributed by the thick barrier layer in winter. In contrast, besides atmospheric heating, the deepening thermocline in the central bay induced by equatorial and local winds also plays an important role in the increase in MHWs during summer.

The Arabian Sea (AS) hosts the world's thickest and most intense oxygen minimum zone (OMZ), and previous studies have documented a dramatic decline of dissolved oxygen (DO) in the northeastern AS in recent decades. In this study, using the recently released data from Biogeochemical‐Argo floats, we found a surprising trend of recovery in deoxygenation within the core region of the OMZ in the AS (ASOMZ) since 2013. The average DO concentration increased by approximately threefold, from ∼0.63 μM in 2013 to ∼1.68 μM in 2022, and the thickness of the ASOMZ decreased by 13%. We find that the weakening of Oman upwelling resulting from the weakening of the summer monsoon is the main driver of oxygenation in the ASOMZ. In addition, the reduction of primary production linked to warming‐driven stratification reinforces deoxygenation recovery at depth.

Multisource satellite remote sensing data have been used to analyze the strong upwelling event off the southern coast of Sri Lanka in 2013 and its relationship with Indian Ocean dipole (IOD) events. The upwelling area in 2013 is 5.7 times larger than that in a normal year and lasts from June to August, with the peaks of the cooling anomaly reaching −1.5°C and the positive chlorophyll a concentration anomaly exceeding 3.1 mg m⁻³. In 2013, the negative unseasonable IOD (IOD_JJA) event enhances the southwest monsoon, while the blocking of the monsoon wind by the island results in a stronger westerly/northwesterly wind stress off the southern coast of Sri Lanka and a weaker westerly/northwesterly wind stress over the eastern Sri Lanka waters. This causes stronger offshore transport and positive Ekman pumping off the southern coast, forming a strong upwelling event there. Further analysis indicates that the interannual variability of the upwelling, as represented by a newly constructed index based on satellite observations, is primarily caused by the variations of local wind associated with the IOD. The upwelling off the southern coast of Sri Lanka weakens (strengthens) in the positive (negative) IOD years. However, an analysis based on 21 IOD events during 1982–2019 demonstrates that the effects of the three types of IOD events, including IOD_JJA, prolonged IOD (IOD_LONG), and normal IOD (IOD_SON), on the upwelling are different. Compared to the IOD_SON events, the IOD_JJA and IOD_LONG events tend to have stronger influences due to their earlier developing phases.