Cloud-Radiation Feedback as a Leading Source of Uncertainty in the Tropical Pacific SST Warming Pattern in CMIP5 Models

The role of the intermodel spread of cloud-radiation feedback in the uncertainty in the tropical Pacific SST warming (TPSW) pattern under global warming is investigated based on the historical and RCP8.5 runs from 32 models participating in CMIP5. The large intermodel discrepancies in cloud-radiation feedback contribute 24% of the intermodel uncertainty in the TPSW pattern over the central Pacific. The mechanism by which the cloud-radiation feedback influences the TPSW pattern is revealed based on an analysis of the surface heat budget. A relatively weak negative cloud-radiation feedback over the central Pacific cannot suppress the surface warming as greatly as in the multimodel ensemble and thus induces a warm SST deviation over the central Pacific, producing a low-level convergence that suppresses (enhances) the evaporative cooling and zonal cold advection in the western (eastern) Pacific. With these processes, the original positive SST deviation over the central Pacific will move westward to the western and central Pacific, with a negative SST deviation in the eastern Pacific. Compared with the observed cloud-radiation feedback from six sets of reanalysis and satellite-observed data, the negative cloud-radiation feedback in the models is underestimated in general. It implies that the TPSW pattern should be closer to an El Nino-like pattern based on the concept of observational constraint. However, the observed cloud-radiation feedback from the various datasets also demonstrates large discrepancies in magnitude. Therefore, the authors suggest that more effort should be made to improve the precision of shortwave radiation observations and the description of cloud-radiation feedback in models for a more reliable projection of the TPSW pattern in future.