Intraseasonal variability (ISV) of the upper 500 m circulation in the southern Bay of Bengal (SBOB) is investigated by using mooring observations and analyzing model outputs. An enhanced ISV is found in the upper 500 m currents during summer 2016. In summer 2016, a boreal summer intraseasonal oscillation (BSISO) propagated northward in tropical Indian Ocean. The intraseasonal easterly anomaly associated with the BSISO forces the equatorial wave to propagate into SBOB, driving the ISV of the upper currents. However, the analysis shows that the enhancement of ISV may be more related to the oceanic instability. During summer 2016, the intensity of the southwest monsoon current (SMC) significantly increased with the position of the flow axis closer to the SBOB's eastern boundary. Energy analysis reveals that the SMC could strengthen the westward propagating Rossby wave through baroclinic and barotropic instability, which can lead to the enhanced ISV of the upper currents in SBOB.

Two extremely low surface chlorophyll concentration events in the southeast Arabian Sea (SEAS, 6oN-15oN, 72oE-77oE) during summers of 2015 and 2019 have been found since 1998. Although warm sea surface temperature (SST) and low nutrients are the direct cause for the anomalously low surface chlorophyll concentration, the physical processes leading to the warm SST anomalies during 2015 and 2019 summer are different. Satellite observations, model outputs and reanalysis data are used to explore the related mechanisms. In 2019, the combined effects of northward local wind anomaly due to extreme positive Indian ocean dipole (IOD) and westward-propagating downwelling Kelvin wave driven by the easterly anomaly in eastern Sri Lanka weaken the upwelling in the SEAS, leading to warm SST anomaly and suppressing the upward transport of the subsurface nutrients to the surface. A weaker positive IOD occurred in 2015, leading to stronger upwelling in the SEAS than during 2019. Yet, seawater in the SEAS experienced extreme warming (lowest SST exceeded 28.5oC) due to the development of super El Niño in 2015. The significant seawater warming can shoal mixed layer and prevent the nutrients in the subsurface from reaching surface, which is unfavorable for the chlorophyll growth. The thermal balance analysis suggests that the extreme warming in the SEAS was mainly related to more downward shortwave radiation and zonal advection.