COHERENCE BETWEEN SEASONAL-VARIATION IN SATELLITE-DERIVED CLOUD OPTICAL DEPTH AND BOUNDARY-LAYER CCN CONCENTRATIONS AT A MIDLATITUDE SOUTHERN-HEMISPHERE STATION

A 7 year record of satellite-derived cloud optical depth for marine st ratus clouds upwind of the Cape Grim Baseline Station is found to have a seasonal cycle in phase with that apparent in long-term (10 year) r ecords of CCN concentration measured in marine boundary layer air. The amplitude and phase of the mean seasonal cycle in cloud optical depth was reproduced reasonably well by a semi-quantitative cloud model ini tialised with observed monthly mean CCN size distributions. While this coherence does not prove a causal relationship, it is consistent with the hypothesis that emissions of reduced sulfur gases by oceanic phyt oplankton can influence cloud radiative transfer properties via an inf luence on CCN concentrations.