Rainfall and Its seasonality over the Amazon in the 21st century as assessed by the Coupled Models for the IPCC AR4

The global climate models for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) predict very different changes of rainfall over the Amazon under the SRES A1B scenario for global climate change. Five of the eleven models (CNRM-CM3, GISS-EH, GISS-ER, IPSL-CM4, and NCAR-CCSM3) predict an increase of annual rainfall, two models (UKMO-HadCM3 and GFDL-CM2.1) predict a decrease of rainfall, and the other four models (INM-CM3.0, ECHAM5/MPI-OM, MIROC3.2-medium resolution and MRI-CGCM2.3.2) predict no significant changes in the Amazon rainfall. Such discrepancies would potentially lead to strong disagreements in global atmospheric CO2 predicted for the 21st century if the ecosystem feedbacks were included. We have examined two models that are among the best in representing the current rainfall seasonality over the Amazon but predict opposite rainfall change. The UKMO-HadCM3 model predicts an El Niņo-like sea surface temperature (SST) change and warming in the northern tropical Atlantic which appear to enhance atmospheric subsidence and consequently reduce clouds over the Amazon. The resultant increase of surface solar absorption causes a stronger surface sensible heat flux and thus reduces relative humidity of the surface air. These changes decrease the rate and length of wet season rainfall and surface latent heat flux. This decreased wet season rainfall leads to drier soil during the subsequent dry season, which in turn can delay the transition from the dry to wet season. GISS-ER predicts a weaker SST warming in the western Pacific and the southern tropical Atlantic which increase moisture transport and hence rainfall in the Amazon. In the eastern Amazon where the strongest rainfall increase occurs, the resultant higher soil moisture supports a higher surface latent heat flux during the dry and transition season and leads to an earlier wet season onset in the eastern Amazon. The UKMO-HadCM3 prediction suggests a higher risk of forest fire, a climate more accessible for land use in large parts of the western equatorial Amazon, and a savanna-type climate in the eastern equatorial Amazon. The GISS-ER prediction suggests more river discharge and a greater flooded area in the Amazon and perhaps a wetter and greener Nordeste region.