The vertical structure of phytoplankton is key to understand the biological carbon pump in the Southern Ocean-Indian sector. This study systematically analyzed the seasonal and interannual variability in the vertical distribution of chlorophyll a （Chl a） and phytoplankton carbon biomass （C_phyto） and their environmental drivers in this region， using satellite observations， BGC-Argo floats， and reanalysis data （2015-2018）， with a focus on the 2015/2016 El Niño event. Results showed that Chl a was mainly distributed in the upper 0-100 m， with a pronounced subsurface chlorophyll maximum layer （SCML） at 10-100 m. Seasonal variability was significant： during austral summer （Dec-Feb）， sufficient light （PAR>100 μmol·m^-2·s^-1） supported peak Chl a and C_phyto in the upper 0-100 m [ρ（Chl a）>0.75 mg·m^-3， C_phyto>20 mg·m^-3）. In austral winter （Jun-Aug）， reduced light availability and a deeper mixed layer collectively led to the dissipation of the SCML and the lowest annual values of Chl a and C_phyto [ρ（Chl a）<0.5 mg·m^-3， C_phyto<10 mg·m^-3]. The deepening of the mixed layer extended the vertical distribution of Chl a to approximately 200 m， whereas the vertical structure of C_phyto showed no corresponding significant change， leading to a pronounced decoupling between Chl a and C_phyto in the 0-200 m. During the 2015/2016 El Niño， enhanced Ekman pumping shoaled the thermocline， increasing nutrient supply to the euphotic zone and intensifying and prolonging the summer phytoplankton bloom [ρ（Chl a）>1 mg·m^-3， lasting about 7 months]. This study provides scientific support for accurately assessing the Southern Ocean’s carbon sink potential and predicting the impact of extreme climate events on the phytoplankton-driven carbon pump.