Direct and indirect effects of land surface processes on climate (Invited Presentation)

It is generally agreed that land surface processes exert a profound influence on climate. Numerous studies using atmospheric general circulation models (AGCMs) with either realistic or idealized modifications in the surface conditions over land and/or different parameterizations of surface processes support this concept. AGCMs prescribe sea surface temperatures (SST), which excludes atmosphere/ocean interactions. In the present study we use a coupled atmosphere-ocean GCM (CGCM) to demonstrate that interactive soil moisture and vegetation biophysical processes can have significant impacts on the mean climate and interannual variability over both land and ocean. In turn, changes in the mean climate of the oceans can feedback on conditions over the continents. The latter influence is referred to as the indirect effect of land surface processes.

The perturbations in surface conditions we apply correspond to the differences between simulations by the same CGCM with two distinct land surface parameterizations. It is shown that such perturbations have strong impacts on the seasonal mean states and seasonal cycles of global precipitation, clouds, and surface air temperature. The impact is especially significant over the tropical Pacific Ocean, and hence on the interannual variability of the coupled atmosphere-ocean system. To explore the mechanisms for impact, model experiments are performed with the perturbations confined to selected continental-scale regions where strong interactions of climate-vegetation biophysical processes are present. The largest impacts are found over the tropical Pacific for perturbations in the tropical African continent, where convective heating anomalies associated with perturbed surface heat fluxes trigger global teleconnections through equatorial wave dynamics. In the equatorial Pacific, the remote impacts of these African anomalies are further enhanced by strong air-sea coupling between surface wind stress and upwelling, as well as by the effects of ocean memory. Perturbations over South America and Asia-Australia have much weaker global impacts. Our results suggest that a correct representation of land surface processes, land use change, and associated changes in the deep convection are crucial to reducing the uncertainty of future climate projections with global climate models under various climate change scenarios. Furthermore, it is suggested that the processes discussed are particularly important in tropical Africa.