Atmospheric moisture budget and its regulation of the summer precipitation variability over the Southeastern United States

Atmospheric moisture budget and its regulation of the summer (June-July-August) precipitation over the Southeastern United State (SE U.S.) were examined during 1948-2007 using PRECipitation REConstruction over Land and multiple reanalysis datasets. The analysis shows that the interannual variation of SE U.S. summer precipitation can be largely explained by the leading Empirical Orthogonal Function mode showing a spatially homogenous sub-continental scale pattern. Consequently, areal-averaged precipitation was investigated to focus on the large-scale rainfall changes over the SE U.S. The wavelet analysis identifies an increased 2-4 year power spectrum in recent 30 years (1978-2007), suggesting an intensification of the interannual variability. Analysis of the atmospheric moisture budget indicates that the increase in precipitation variability is mainly caused by moisture transport, which exhibits a similar increase in the 2-4 year power spectrum for the same period. Moisture transport, in turn, is largely controlled by the seasonal mean component rather than the subseasonal-scale eddies. Furthermore, our results indicate that dynamic processes (atmospheric circulation) are more important than thermodynamic processes (specific humidity) in regulating the interannual variation of moisture transport. Specifically, the North Atlantic Subtropical High western ridge position is found to be a primary regulator, with the ridge in the northwest (southwest) corresponding to anomalous moisture divergence (convergence) over the SE U.S. Changes in moisture transport consistent with the increased frequency of these two ridge types in recent 30 years favor the intensification of summer precipitation variability.