Assessment of the marine biota-DMS-cloud-climate hypothesis using remotely sensed data and an ocean general circulation model (OGCM)

Dimethylsulfide (DMS) is a sulfur-compound produced naturally in the sea. Once in the atmosphere, it is photooxidized to form non-sea-salt sulfate aerosols which affect the radiative budget indirectly as cloud condensation nuclei (CCN). It has been postulated that the planet's climate may be modulated by variations in DMS production resulting from changes of the sea surface temperature (SST). It is shown that surface seawater DMS concentrations can be diagnosed at a near global scale using (1) satellite observations of Chl a, (2) climatological SSTs, (3) a temperature dependent index of the marine community structure and (4), observational relationships based on cruise data. This procedure is used to characterize the spatio-temporal variations in marine DMS emissions to the atmosphere in the Southern Hemisphere for the spring and summer months. Some coherence is obtained between DMS emissions and the polarization measurements from the spaceborne POLDER instrument used to estimate the effective radius of liquid-phase clouds; higher are the DMS emissions lower is the cloud droplet radius. An inspection of the ISCCP-D products of low-cloud optical depth and DMS emissions at Amsterdam Island and Cape Grim indicates that higher are the DMS emissions higher is the cloud optical depth. The IPSL ocean-atmosphere coupled model, which includes an explicit representation of plankton dynamics and Chl a, predicts significant large-scale changes of DMS fluxes (from-20% in the tropics to +20% in the Southern Ocean for the zonal mean) in response to a doubling of atmospheric CO₂. We envision that the decreased mass of sulfur in the tropical upper troposphere will lead to lower numbers of CCNs in the air subsiding back into the subtropical marine boundary layer. Hence, it is suggested that the sensitivity of mid-latitude clouds to local changes in DMS emissions will increase in the future.