Tropical and Extratropical Controls of Gulf of California Surges and Summertime Precipitation over the Southwestern U.S

In this study we use ERA-Interim and MERRA data to study the influence of Gulf of California (GoC) moisture surges during the North American Monsoon (NAM) season and the monsoonal precipitation over Arizona and western New Mexico (AZWNM), as well as their connection with larger-scale variability in the tropics and extra-tropics. To identify GoC surges, we introduce an improved index based on principal component analyses of the near-surface GoC winds. We find that GoC surges explain up to 70-80% of the summertime rainfall over AZWNM. The number of surges that lead to enhanced rainfall in this region varies from 4 to 18 and is positively correlated with its annual summertime precipitation.

Regression analyses are performed to explore the relationship between Goc surges, and hence AZWNM precipitation, and tropical and extratropical atmospheric variability at the synoptic (2-8 days), quasi-biweekly (10-20 days) and sub seasonal (25-90 days) time scales. It is found that tropical and extratropical waves, responsible for intrusions of moist tropical air into midlatitudes, interact on all three time scales, with direct impacts on the development of GoC surges and positive precipitation anomalies over AZWNM. Strong precipitation events in this region are, however, found to occur on time scales longer than synoptic, with the quasi-biweekly and subseasonal modes playing a dominant role in the occurrence of these more extreme events.