Summer Rainfall Variability over the Southeastern United States in the 21st Century as Assessed by the CMIP5 Models

The variability of the Southeast (SE) United States (U.S.) summer precipitation in current and future climate is analyzed using phase 5 of the Coupled Model Intercomparison Project (CMIP5) models. By comparing the simulated historical precipitation variability with the observations, we categorize the CMIP5 models into two groups: Group 1 (G1) models that simulate the summer precipitation variability reasonably well, and Group 2 (G2) models that need further improvements. Our analysis suggests that the relatively higher skill of the G1 models results from their ability to accurately represent the dynamical linkage between the SE U.S. summer precipitation variability and the North Atlantic Subtropical High (NASH) western ridge position. In contrast, the inability of the G2 models to represent such linkage leads to biases in their simulations of the variability in the SE U.S. summer precipitation. According to our analysis, the ensemble projection of the CMIP5 models suggests that under the Representative Concentration Pathway (RCP) 4.5 scenario, the variability in the SE U.S. summer precipitation will intensify and that this intensification is more pronounced among the G1 models. Our analysis further suggests that this intensification is most likely because of the projected pattern shift of the NASH western ridge in a warming climate. Under the RCP4.5 scenario, the NASH western ridge will extend further westward and lead to more frequent occurrences of the Northwestward and Southwestward ridge patterns that are respectively related to the dry and wet summers in the SE U.S. The projected enhancement of the summer precipitation variability will likely drive the SE U.S. towards a more extreme climate in the future.