Aerosol-cloud semi-direct effect and land-sea temperature contrast in a GCM

Simulations with the CAM3 climate model show that prescribed heating by anthropogenic aerosols significantly affects cloud cover. Globally the dominant effect is a decrease in mid-level clouds, which contributes to a semi-direct effect (SDE) that warms the surface by 0.5 W m(-2). The SDE negates some of the impact of absorbing aerosol on surface fluxes, up to 50% over the Northern Hemisphere extratropical (NHE) land during JJA. The SDE-not direct effects-drives NHE JJA warming and a land-sea contrast in surface temperature that may help explain recent trends. This behavior is mainly due to 1. the trapping of near-surface moisture associated with aerosol induced enhanced lower tropospheric stability, which preferentially increases low cloud over the sea; and 2. a burn-off of cloud, especially over land, due to reduced relative humidity in the low and middle troposphere. These effects are due to vertical redistribution of energy rather than to the horizontal heterogeneity of aerosol forcing. Citation: Allen, R. J., and S. C. Sherwood (2010), Aerosol-cloud semi-direct effect and land-sea temperature contrast in a GCM, Geophys. Res. Lett., 37, L07702, doi:10.1029/2010GL042759.