The Contrasting Roles of Land and Oceanic Moisture Contributions to Summer Rainfall Over the Southeastern United States

Summer precipitation in the Southeast United States (SEUS) is classified into three categories—light, moderate, and heavy—using a Bayesian statistical model. We find that heavy rainfall events explain most of the interannual variance of summertime cumulative precipitation in the region, influencing regional hydroclimate patterns. For each rainfall category, we track the respective moisture sources using the 2-Layers Water Accounting Model driven by reanalysis data. We find that the Atlantic Ocean is the primary moisture source across all rainfall categories and becomes more important with increasing rainfall intensity. Conversely, land moisture contributions decrease with rainfall intensity. In the case of heavy rainfall, the moisture originating from the Atlantic Ocean is transported to the SEUS via a southeastward positioning of the North Atlantic Subtropical High (NASH) Western Ridge. The ample supply of moisture fluxes is further propelled by reduced surface pressure which promotes ascending motion in the SEUS. Analysis of mid-troposphere circulation indicates that this anomalous low-pressure might stem from wave trains originating over the North Pacific Ocean. Thus, heavy rainfall events involve increased Atlantic Ocean moisture fluxes directed by the NASH to the SEUS, which, in turn, is modulated by anomalous atmospheric circulation produced over the Pacific Ocean. Furthermore, we observe that the heavy and moderate (light) rainfall event frequency has increased (decreased) by five days throughout the 1970–2019 analysis period.