Long-term Variation of Oceanic Carbon Dioxide and Possible Acidification in the Indian Sector of the Southern Ocean

Long-term monitoring of the partial pressure of CO2 in the surface seawater (pCO2) on-board Japanese icebreaker SHIRASE has been carried out in the Indian sector of the Southern Ocean since 1987 as a part of Japanese Antarctic Research Expedition (JARE). Meridional distributions of pCO2 along 110°E in early December and along 150°E in late March clearly show steep changes at such fronts as Subtropical Front, Subantarctic Front, and Polar Front. pCO2 of each zone divided by the fronts can be distinguished from the others by the difference of averaged pCO2 in the zone. Although pCO2 of each zone shows interannual variation, secular trend is detectable. For example, estimated increasing rate of pCO2 in the permanent open ocean zone between polar front (around 53°S) and northern edge of winter ice cover (63°S) is about 1.5 μatm/y which is almost same as the increasing rate of atmospheric CO2 concentration. Oceanic acidification corresponding to pCO2 increase is one of the most direct effects of increasing atmospheric carbon dioxide. Preliminary analysis of pH which has been observed on board SHIARASE and her predecessor FUJI shows gradual decrease from 1980 to 2005.