North American Monsoon Response to Antecedent Soil Moisture and Snow in the Colorado Plateau

Abstract This study explores the relationship between spring soil moisture and snowpack in the Colorado Plateau (CP) and the onset of the North American Monsoon (NAM) using reanalysis data. We find that increased spring soil moisture in the CP corresponds to later NAM onset dates throughout much of the southwestern United States (US) and northwestern Mexico. Anomalously moist spring soils in the CP increase the seasonal‐average surface latent heat flux and decrease sensible heat flux, resulting in lower late spring and early summer surface temperatures and a weakened thermal gradient between the land and ocean. Decreased spring sensible heat flux also corresponds to decreased early summer geopotential heights over the southwestern US, a weaker NAM circulation, and reduced moisture flux into the NAM region. NAM onset dates in years with the highest spring soil moisture or snow depth content in the CP are delayed 1 week on average in the NAM region. Conversely, NAM onset dates in years with the lowest spring soil moisture or snow depth content in the CP occur 1 week earlier on average. While increased spring snow in the CP is correlated with a delayed NAM onset, the relationship between spring albedo and NAM onset lacks statistical significance, suggesting that CP snow anomalies affect the NAM by contributing soil moisture via snowmelt, rather than directly affecting the surface energy budget. Results indicate a mechanistic pathway that links spring soil moisture and snowpack anomalies in the CP to NAM development, which will be tested in future numerical simulations.