Influence of the North Atlantic Subtropical High on Summer Precipitation over the Southeastern United States
The Southeastern United States (SE US) is one of the fastest developing regions of the nation, where summer precipitation becomes increasingly important to sustain population and economic growth. In recent decades, the variability of SE US summer precipitation has significantly intensified, leading to more frequent and severe climate extremes. However, the processes that have caused such enhanced climate variability have been poorly understood. By analyzing atmospheric hydrological cycle, diagnosing atmospheric circulation dynamics, and performing regional climate simulations, this dissertation investigates the mechanisms responsible for SE US summer precipitation variability.
Analysis of regional moisture budget indicates that the variability of SE US summer precipitation is primarily controlled by moisture transport processes associated with the variation of the North Atlantic Subtropical High (NASH) western ridge, while local water recycling is secondary. As the ridge moves northwestward (NW) into the US continent, moisture transport pathway is away from the SE US and the upward motion is depressed. Thus, rainfall decreases over the SE US, leading to dry summers. In contrast, when the ridge moves southwestward (SW), moisture convergence tends to be enhanced over the SE US, facilitating heavier rainfall and causing wetter summers. However, as the ridge is located relatively eastward, its influence on the summer precipitation is weakened. The intensified precipitation variability in recent decades is attributed to the more frequent occurrence of NW- and SW-type ridges, according to the "NASH western ridge - SE US summer precipitation" relationship.
In addition, the "NASH western ridge - SE US summer precipitation" relationship acts as a primary mechanism to determine general circulation model (GCM) and regional climate model (RCM) skill in simulating SE US summer precipitation. Generally, the state-of-the-art GCMs that are capable of representing the abovementioned relationship perform better in simulating the variability of SE US summer precipitation. Similarly, the RCM simulated summer precipitation bias over the SE US is largely caused by the errors in the NASH western ridge circulation, with the physical parameterization playing a secondary role.
Furthermore, the relationship between the NASH western ridge and SE US summer precipitation well explains the projected future precipitation changes. According to the projection by the ensemble of phase-5 of Coupled Model Intercomparison Project (CMIP5) models, summer precipitation over the SE US will become more variable in a warming climate. The enhancement of precipitation variability is due mainly to the atmospheric circulation dynamics, resulting from the pattern shift of the NASH western ridge circulation. In a warming climate, the NASH circulation tends to intensify, which forces its western ridge to extend further westward, exerting stronger impact on the SE US summertime climate. As the ridge extends westward, the NW- and SW-type ridges occur more frequently, resulting in an increased occurrence of extreme summers over the SE US.
In summary, the studies presented in this dissertation identify the NASH western ridge as a primary regulator of SE US summer precipitation at seasonal scale. The "NASH western ridge - SE US summer precipitation" relationship established in this study serves as a first order mechanism for understanding and simulating processes that influence the statistics of extreme events over the SE in the current and future climate.
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