THE IMPACT OF SIMULATED CLIMATE-CHANGE ON THE AIR-SEA FLUX OF DIMETHYLSULFIDE IN THE SUB-ANTARCTIC SOUTHERN-OCEAN

Dimethylsulphide is an important sulphur-containing trace gas produced by enzymatic cleavage of its precursor compound, dimethylsulphoniopro pionate (DMSP) which is released by marine phytoplankton in the upper ocean. After ventilation to the atmosphere, DMS is oxidised to form su lphate aerosols which in the unpolluted marine atmosphere are a major sourer of cloud condensation nuclei (CCN). Because the microphysical p roperties of clouds relevant to climate change are sensitive to CCN de nsity, it has been postulated that marine sulphur emissions may play a role in climate regulation. Here we examine the DMS cycle in the suba ntarctic Southern Ocean west of the atmospheric baseline station at Ca pe Grim, NW Tasmania, where a long time series of atmospheric data has been collected. The Southern Ocean is relatively free of anthropogeni c sulphur emissions and thus sulphate aerosols will be mainly due to t he biogenic source of DMS. An atmospheric general circulation model ha s been used to provide meteorological forcings under current and doubl ed atmospheric CO2 conditions. We have used an existing DMS production model to investigate the sensitivity of the sea-to-air flux to the si mulated changes in temperature and wind speed at the ocean surface. Un der doubled CO2 conditions sea-surface temperature is simulated to inc rease by 4 degrees C throughout the year. Annual mean surface wind spe ed is simulated to decrease by about 3%. The changes in temperature an d wind speed cause a net increase of DMS transfer velocity in the rang e 3-11%. The increase in temperature will also increase the growth rat e of phytoplankton. The annual integrated DMS nux is simulated to incr ease between 2% and 8% under doubled CO2 conditions. A median change o f +5% in DMS flux corresponds to an increase in the range 2 to 4% in c loud condensation nuclei (CCN) concentration and a perturbation to rad iative forcing of -0.29 W m(-2), confirming a minor role for DMS-deriv ed aerosols in climate regulation.