Relationship Between Spring Soil Moisture and Snow and the North American Monsoon.

The North American Monsoon (NAM) is a regional circulation pattern that drives atmospheric moisture into the southwest United Stated (US) and northern Mexico (“Desert Southwest”), often resulting in frequent precipitation and thunderstorms. The onset timing and associated precipitation from the NAM provides local communities with rainfall for agriculture, drinking water, and other life-quality aspects, while thunderstorms can create environmental conditions that can threaten lives and property. The factors that control the interannual variability of the NAM are highly debated, and much of the current literature disagrees on the relative importance of the primary moisture source, El Niño Southern Oscillation (ENSO), local topography, synoptic-scale circulation, and surface conditions prior to the NAM season. Using a common approach to defining a monsoon’s onset date along with investigating reanalysis and observational data, we find that anomalies of spring soil moisture content (SM) and snow water equivalent (SWE) in the Colorado Plateau (CP) negatively correlate with the NAM onset date over much of the Desert Southwest. The physical process we propose is that anomalously high spring SM or SWE in the vicinity of the CP decreases the local sensible heat flux, which inhibits growth of the 1000-500 hPa geopotential height thickness through spring. This results in a weakened and delayed migration of high pressure into the southwest US, reducing moisture transport from the south and east, delaying NAM onset, and decreasing precipitation over much of the Desert Southwest. Each of these process steps are significantly correlated at p < 0.1, and the coefficients of determination tend to at least match the magnitude of other NAM-influencing factors (e.g., ENSO). The physical processes that influence the NAM and the results of our analysis are presented here to gain a deeper understanding of the environmental factors that influence the NAM’s development. Numerical simulation sensitivity tests will be conducted to quantify the relative impact that varying spring SM or SWE in the southwest US has on the NAM.