The Role of Topography in Shaping Southeast US Winter Precipitation

During the winter months (December-March), a distinct area of relatively high mean precipitation is present in the Southeast US north of the Gulf of Mexico. The cause of this area of high precipitation has not been thoroughly explored in existing literature. In this work, we explore the influence of topography on this precipitation pattern by running climate simulations with a Geophysical Fluid Dynamics Laboratory (GFDL) general circulation model with 50 km horizontal resolution. In these simulations we use both observed and altered North American topography. With observed topography, the model simulates a realistic relative maximum of precipitation in the southeast US, whereas the feature is not present in a flattened North America experiment. Here, simulated precipitation is as much as 75-100% greater with realistic topography as it is with flat North American topography, showing a critical role of topography. Analyses of simulated largescale circulation and further regional topography flattening simulations suggest that the combined effect of deflection of westerlies by the Rockies and blocking of surface easterlies by Central American orography leads to the observed southerly low-level flow from the Gulf of Mexico and vertical ascent associated with this precipitation maximum, with a local enhancement by the Appalachians. The topographic enhancement of southerly flow and precipitation is similar to that in southeast South America, which has been shown by previous work to arise via poleward low-level flow generated via blocking of tropical easterlies by the Andes.