Effect of Antarctic sea ice on chlorophyll concentration in the Southern Ocean

Abstract

Sea-ice extent is very sensitive to climate change and its minor variations can significantly affect the regional biota of the Southern Ocean (SO). Chlorophyll-a concentration (Chl-a) is a primary proxy for the understanding of phytoplankton distribution and primary productivity in the oceans. Therefore, analysing the relation between Chl-a and sea-ice extent could be significant in understanding the role of SO sea-ice extent on regional Chl-a variability. Local Chl-a variability in the SO was analysed in five major sectors, utilizing 39 years of remotely sensed sea ice and 15 years of Chl-a and Sea Surface Temperature (SST) datasets. Katabatic winds blowing offshore from the Antarctic coast were found to enhance the sea-ice extent during winter and diminish sea-ice melting in the austral summer in the SO. Thus, cold polar wind enhanced the sea-ice extent whereas warm subtropical winds reduced its formation. The Weddell Sea region showed the highest monthly averaged sea-ice extent of 6.2 × 106km2 during September (austral spring), whereas monthly averaged sea-ice extent was less than 2.9 × 106km2 for any other region. The largest sea-ice extent of 2 × 104km2 was observed in the Ross Sea during the austral summer (December, January, and February) owing to the katabatic winds. All other regions reported sea-ice extents below 1.15 × 104km2. Melting of sea ice during the austral summer was responsible for enhanced Chl-a in the SO through increased availability of nutrients in the near surface waters. Considering Chl-a as a proxy for productivity in the SO, the western sector (Weddell, Bellam and Ross Sea regions) was found to be more productive than the eastern sector (Indian and Pacific Ocean regions) during the study period. To understand the vertical variation of physical and biogeochemical variables and their influence on Chl-a variability, data from a few active Biogeochemical-Argo floats in the region were analysed. It was concluded that while the surface Chl-a was directly influenced by SST variations and sea-ice melting, the nutricline influenced the depth of Deep Chlorophyll Maximum (DCM) and Chl-a vertical distribution in the SO.