Atmospheric Moisture Budget and its Regulation of the Summer Precipitation Variability over the Southeastern United States

The atmospheric moisture budget and its regulation of the summer (June-July-August) precipitation over the Southeastern United State (SE U.S.) are examined during 1948-2007 using PRECipitation REConstruction over Land (Prec/L) and multiple reanalysis datasets. The analysis shows that the interannual variation of the SE U.S. summer precipitation can be largely explained (38% variance) by the leading Empirical Orthogonal Function (EOF) mode showing a spatial homogenous sub-continental scale pattern. Consequently, areal-averaged precipitation is 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 increased precipitation variability is mainly caused by the variability of moisture transport, which exhibits a similarly 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. Note that the summer season is defined here before the hurricane peak season (September). 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 (NASH) 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 increase in the frequency of these two ridge types in the recent 30 years favors the enhancement of summer precipitation variability.