Keywords:Climate change, Southern Ocean, Phytoplankton
Long-term monitoring of phytoplankton biomass is valuable to detect climate change impact on surface ecosystems in the Southern Ocean. Chlorophyll a (chl-a) observations by Japanese Antarctic Research Expedition (JARE) started in the 1965/1966 during austral summer and continued for over 50 years in the Indian sector of the Southern Ocean. We analyzed long-term chl-a dataset along 110° E in December and January to investigate inter-annual and decadal variations of phytoplankton biomass. In the region between 40°-60°S, chl-a values exceeded 0.5 mg m−3 were detected after the 1990s more frequently than before the 1980s. The similar long-term trend was also found in vertically integrated chl-a values. There was an increasing trend in the ten-year moving average of the mean surface chl-a value in the waters between 45°-55°S over the past 50 years. Moreover, this increasing trend in chl-a was correlated with the Southern Annular Mode (SAM) index positively. The positive correlation between trends of chl-a and the SAM index could be associated with enhanced westerly winds, which can lead to the supply of cold, iron-rich waters by upwelling (e.g., Lovenduski and Gruber, 2005). Recent observation along 110° E by Umitaka-maru (TUMSAT) and satellite remote sensing dataset revealed that inter-annual variation of surface chl-a in seasonal sea ice zone in January, although relationships among chl-a, sea ice, and climate index such as the SAM are still obscure.