Changes to the North Atlantic Subtropical High and Its Role in the Intensification of Summer Rainfall Variability in the Southeastern United States

We have investigated the changes of the North Atlantic Subtropical High (NASH) and its impact on summer precipitation over the Southeast US (SE US) using the 850hPa geopotential height field in the NCEP, ERA-40 reanalysis, long-term rainfall data and IPCC AR4 model simulations during the past six decades. Our results show that the western ridge of the NASH has moved westward and the intensity of the NASH's center has increased in recent decades. The westward extension of the ridge enhances the effect of the NASH on the SE US precipitation through the ridge's meridional movement. Our analysis of the IPCC AR4 models' simulations for the 21st century suggests that with an increase of the atmospheric CO2 concentration, the center of the NASH tends to intensify and the western ridge of the NASH shifts further westward, increasing the likelihood of both strong anomalously wet and dry summers over the SE US in future.