Coherence between seasonal variation in satellite-derived cloud optical depth and boundary layer CCN concentrations at a mid-latitude Southern Hemisphere station

A 7 years record of satellite-derived cloud optical depth for marine stratus clouds upwind of the Cape Grim Baseline Station is found to have a seasonal cycle in phase with that apparent in long-term (10 years) records 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 initialised 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 phytoplankton can influence cloud radiative transfer properties via an influence on CCN concentrations.