Secular trends and seasonal variations of partial pressure of carbon dioxide in the surface sea water in the Australian secutor of the Southern Ocean

In order to elucidate the secular trends of oceanic CO ₂ uptake in the Indian sector of the Southern Ocean, pCO ₂, the partial pressure of CO ₂ in the ocean surface layer, has been measured since 1987 on board the icebreaker Shirase. Meridional distributions of pCO ₂ along 110δE in early December clearly show steep changes at such fronts as the subtropical front, subantarctic front, and polar front. Although pCO ₂ of each zone shows interannual variation, secular trend is detectable. For example, the estimated rate of increase of pCO ₂ in the permanent open ocean zone between the polar front (around 53°S) and the northern edge of winter ice cover (63°S) is about 1.3 μatm/y, which is slightly lower than the rate of increase of the atmospheric CO ₂ concentration. From the results obtained by multi-ship observations with 4 research vessels in the Southern Ocean in summer, we found that the values of pCO ₂ off the coast of the Antarctic Continent (66°S) varied temporally by 100 μatm for 5 months. We also found that nDIC decreased with time from December 2001 to March 2002 in the upper layer from 100 to 200 m due to biological activity during summer.