Surface chlorophyll blooms in the Southern Bay of Bengal during the extreme positive Indian Ocean dipole

The open ocean region of southern Bay of Bengal (BoB) (similar to 5 degrees N-10 degrees N; 87 degrees E-97 degrees E) is characterized as the region of low chlorophyll-a concentration (< 0.1 mg m(-3)) during winter (December-February). However, the analysis of satellite ocean colour data between 2003 and 2020 shows anomalous high surface chlorophyll concentration (similar to 0.35-0.5 mg m(-3)) in the southern BoB during the winters, followed by the extreme positive Indian Ocean Dipole (pIOD) events in 2006 (W06-2007) and 2019 (W19-2020). It is found that anomalous upwelling Rossby wave generated remotely through the anomalous easterly wind in the equatorial Indian Ocean (EIO) during October-November leads to a thin barrier layer (BL), shallow thermocline and nutricline in the southern BoB during W06-2007 and W19-2020. Thin BL, and shallow nutricline, provide a conducive environment for surface chlorophyll enhancement through much easier vertical transport of nutrient-rich water to the near-surface layer by local near-climatological wind-induced vertical mixing during W06-2007 and W19-2020. It is also found that surface chlorophyll bloom was absent in the southern BoB during the winters, followed by the weak pIOD event in 2012, 2015, and 2018. During the winters followed by the weak pIOD events, the westward propagating downwelling Rossby wave generated through the westerly wind, similar to the climatological state, in the EIO during October-November leads to a thick BL, deeper thermocline and nutricline in the southern BoB. Thick BL, and deeper nutricline, provide an unfavourable environment for the vertical transport of nutrient-rich water to the near-surface layers to generate surface chlorophyll bloom during the winters followed by the weak pIOD event.