The impact of simulated climate change on the air-sea flux of dimethylsulphide in the subantarctic Southern Ocean

Abstract

Dimethylsulphide is an important sulphur-containing trace gas produced by enzymatic cleavage of its precursor compound, dimethylsulphoniopropionate (DMSP) which is released by marine phytoplankton in the upper ocean. After ventilation to the atmosphere, DMS is oxidised to form sulphate aerosols which in the unpolluted marine atmosphere are a major source of cloud condensation nuclei (CCN). Because the microphysical properties of clouds relevant to climate change are sensitive to CCN density, it has been postulated that marine sulphur emissions may play a role in climate regulation. Here we examine the DMS cycle in the subantarctic Southern Ocean west of the atmospheric baseline station at Cape Grim, NW Tasmania, where a long time series of atmospheric data has been collected. The Southern Ocean is relatively free of anthropogenic sulphur emissions and thus sulphate aerosols will be mainly due to the biogenic source of DMS. An atmospheric general circulation model has been used to provide meteorological forcings under current and doubled atmospheric CO₂ conditions. We have used an existing DMS production model to investigate the sensitivity of the sea-to-air flux to the simulated changes in temperature and wind speed at the ocean surface. Under doubled CO₂ conditions sea-surface temperature is simulated to increase by 4 °C throughout the year. Annual mean surface wind speed is simulated to decrease by about 3%. The changes in temperature and wind speed cause a net increase of DMS transfer velocity in the range 3-11%. The increase in temperature will also increase the growth rate of phytoplankton. The annual integrated DMS flux is simulated to increase between 2% and 8% under doubled CO₂ conditions. A median change of + 5% in DMS flux corresponds to an increase in the range 2-4% in cloud condensation nuclei (CCN) concentration and a perturbation to radiative forcing of — 0.29 W m^-2, confirming a minor role for DMS-derived aerosols in climate regulation.