Relationships between spring snow mass and summer precipitation in the Southwestern United States associated with the North American monsoon system

Abstract

This paper provides a detailed description of the relationship between spring snow mass in the mountain areas of the western United States and summertime precipitation in the southwestern United States associated with the North American monsoon system and examines the hypothesis that antecedent spring snow mass can modulate monsoon rains through effects on land surface energy balance. Analysis of spring snow water equivalent (SWE) and July-August (JA) precipitation for the period of 1948-97 confirms the inverse snow-monsoon relationship noted in previous studies. Examination of regional difference in SWE-JA precipitation associations shows that although JA precipitation in New Mexico is significantly correlated with SWE over much larger areas than in Arizona, the overall strength of the correlations are just as strong in Arizona as in New Mexico. Results from this study also illustrate that the snow-monsoon relationship is unstable over time. In New Mexico, the relationship is strongest during 1965-92 and is weaker outside that period. By contrast, Arizona shows strongest snow-monsoon associations before 1970. The temporal coincidence between stronger snow-monsoon associations over Arizona and weaker snow-monsoon associations over New Mexico (and vice versa) suggests a common forcing mechanism and that the variations in the strength of snow-monsoon associations are more than just climate noise. There is a need to understand how other factors modulate monsoonal rainfall before realistic predictions of summertime precipitation in the Southwest can be made.