The purpose of this research is to identify the regional mechanisms by which the Atlantic Multidecadal Oscillation (AMO) influences summer (June-August) precipitation in the central U.S. This was accomplished by running two different sets of simulations using the Weather Research and Forecasting (WRF) regional climate model, one forced by observations and the other forced only by variations in the AMO as obtained via a global climate model (GCM). The results reveal a complex set of mechanisms active in the lower and middle troposphere by which the AMO influences summer circulation and precipitation in the central U.S. During the cold phase of the AMO, much of the central U.S. experiences increased lower tropospheric pressure and precipitation. However, small-scale variability in the pressure increase results in an overall weakening of the pressure gradient, with the greatest reduction occurring in the north-central U.S. This process results in a buildup in the central U.S. of moisture in the lower atmosphere. Additionally, the increased pressure redirects the flow near 700hPa to reduce the moisture contribution from the Gulf of Mexico, producing a more potentially unstable lower atmosphere during the cold phase in which moist air is capped by overlying dry air. In the middle troposphere (500hPa), the increased precipitation is largely supported by increased positive relative vorticity. This increase is produced by cyclonic circulation and effective depth (isentropic thickness) anomalies. Anomalous convergence in the mid- to upper troposphere in the central U.S. appears to be the source for the stretching. The positive relative vorticity anomalies during the cold phase produce favorable conditions for baroclinic development and when combined with the potentially unstable atmospheric moisture profile, produce conditions more favorable for increased precipitation.
Advisor: Qi Hu