North American Monsoon Sensitivity to Soil Moisture in a Regional Climate Model

Abstract Past work suggests a possible connection between spring soil moisture conditions in the southwestern United States and the onset of the North American Monsoon (NAM). We test the NAM's response to both moist and dry spring soil moisture perturbations under different synoptic circulation regimes using high-resolution numerical simulations. Anomalously high soil moisture led to a robust increase in latent heat and a decrease in sensible heat fluxes, whereas anomalously low soil moisture led to a robust decrease in latent heat and an increase in sensible heat fluxes. There was no clear, regionally coherent 500-hPa geopotential height (Z 500 ) response. While the magnitude of Z 500 responses aligned with expectations of atmospheric thickening as a function of surface temperature change near the forcing region, simulations forced with small random soil moisture anomalies generated as large downstream Z 500 responses (outside the forcing region) as simulations with larger soil moisture forcing. We repeated the experiment both with a larger region of soil moisture perturbation and in larger-domain simulations to test if atmospheric boundary conditions were limiting the atmospheric circulation response, but still found no clear circulation response to the imposed soil moisture anomalies. This result held over a range of nine objectively identified circulation patterns (e.g., phase of El Niño and position of monsoonal ridge). We conclude that, for realistic soil moisture anomalies, there is a robust surface response (latent heat flux and sea level pressure) over a range of climate circulation patterns, but no clear atmospheric circulation response was detected. Precipitation responses appear chaotic, with no clear pattern emerging across domain size or soil moisture forcing area. While soil moisture anomalies in this experimental configuration predictably affected the surface energy balance locally, their influence on broader circulation patterns was weak or easily masked by internal variability.