Observed Influence of Soil Moisture on the North American Monsoon: An Assessment Using SGEFA

Abstract:

Soil moisture has an important influence on precipitation associated with the North American Monsoon (NAM). The purpose of this study is to isolate and evaluate both local and synoptic scale feedback of soil moisture to summer (June - September) precipitation in the NAM region using observational data. Although the observation-based study is essential for investigating soil moisture feedback processes, it is difficult to separate the soil moisture forcing signal from the internal atmospheric variability and the sea surface temperature (SST) forcing. This study applied a multivariate statistical method: Stepwise Generalized Equilibrium Feedback Assessment (SGEFA) method to eliminate undesired forcing signals and quantify soil moisture feedback in the NAM region.

Results show significant but different feedback pathways of soil moisture in the NAM region and the Southern Rocky Mountains (SRM) region to the NAM precipitation. The wet soil conditions in SRM north of the NAM region can result in lower-than-normal local surface temperature, weaker water vapor transport from eastern Pacific into the NAM region, and eventually less monsoon precipitation. On the other hand, anomalously wet soil within the NAM region will increase NAM precipitation by increasing local moist static energy and intensifying the monsoonal circulation. Our results highlighted the crucial role of land-atmosphere interactions for understanding the NAM variability.