Role of Pacific and Atlantic Oceans in Governing the Southeastern US Precipitation Variability

This study investigates the mechanisms driving positive and negative anomalous rainfall events in the southeastern United States (SEUS), focusing on the interaction between upper-level large-scale atmospheric teleconnections and the lower-level North Atlantic Subtropical High (NASH). By employing a conditioned weather regime analysis of geopotential height at both lower and upper levels across the Pacific-North America-Atlantic region, we identify distinct clusters representing persistent and recurring circulation patterns originating from both the Pacific and Atlantic Oceans. Our analysis of lower-level conditioned weather regimes uncovers two distinct phases of the NASH that influence positive and negative rainfall subseasonal patterns in the SEUS. In one phase, the northern boundary of the NASH weakens and shifts eastward, leading to a reduction in the central low-level jet and an increase in cyclonic circulation, which enhances SEUS rainfall. In the other phase, an anomalous trough over the SEUS enhances rainfall by increasing moisture influx and triggering a wave train that strengthens the NASH. Conversely, negative rainfall events are associated with opposite conditions. Additionally, the conditioned weather regime analysis of upper-level geopotential height reveals that a significant portion of SEUS precipitation variability is largely driven by dynamics from the North Pacific and Atlantic Oceans. Furthermore, Rossby wave patterns associated with East Asian summer monsoons also play a substantial role in the SEUS rainfall variability from the upper levels. Thus, our research underscores the importance of considering both global atmospheric teleconnections at upper and lower levels, as well as local pressure systems, in understanding and shaping regional precipitation patterns.