Tropical and extratropical controls on the sub-seasonal variability of the summertime precipitation over the southwestern U.S.A.

Abstract:

Gulf of California (GoC) moisture surges are transient disturbances that propagate northwards along the Gulf of California during the July-September North American Monsoon (NAM) season. They are typically associated with positive intense northward winds along the GoC and positive precipitation anomalies in the NAM region, with widespread thunderstorms over the Southwestern USA. Their frequency and strength therefore have an important impact on the rainfall summer budget of this area. Previous studies have suggested that the strength of GoC surges depends on large-scale dynamical controls, such as tropical easterly waves (TEWs) transiting over the tropical eastern Pacific ocean, and upper-level midlatitude westerly disturbances (WDs) transiting over Northern America. Here, we perform an EOF analysis of the GoC alongshore wind from reanalysis products to investigate the intra-seasonal variability of GoC surges and its relationship with precipitation anomalies. The first and second principal components, which explain more that 80% of the summertime variability of GoC winds, provide very effective indeces of strong and weak surges, with the former being associated with coherent wind anomalies throughout the GoC and the latter with anomalies primarily confined over its northern half. Lagged linear regressions analyses are used to identify synoptic patterns associated with the development of GoC surges. We identify three modes that modulate GoC surge events on different time scales: 1) fast TEWs and WDs (2-8 days); 2) tropical quasi-biweekly easterly disturbances and quasi-stationary midlatitude Rossby waves (10-20 days) and 3) Madden-Julian oscillation rainfall rearrangements along the ITCZ. The 10-20-day mode is found to be the major control on precipitation anomalies associated with GoC surge events. While not negligible, the synoptic mode (2-8 days) plays a lesser role.