Climatological moisture sources for the Western North American Monsoon through a Lagrangian approach: their influence on precipitation intensity

摘要

This work examines the origin of atmospheric watervapor arriving to the western North American monsoon (WNAM) region over a34-year period (1981–2014) using a Lagrangian approach. This methodologycomputes budgets of evaporation minus precipitation (E−P) by calculating changes inthe specific humidity of thousands of air particles advected into the studyarea by the observed winds. The length of the period analyzed (34 years)allows the method to identify oceanic and terrestrial sources of moisture to theWNAM region from a climatological perspective. During the wet season, the WNAM region itself is on average the mainevaporative source, followed by the Gulf of California. However, water vapororiginating from the Caribbean Sea, the Gulf of Mexico, and terrestrialeastern Mexico is found to influence regional-scale rainfall generation. Enhanced (reduced) moisture transport from the Caribbean Sea and the Gulf ofMexico from 4 to 6 days before precipitation events seems to be responsiblefor increased (decreased) rainfall intensity on regional scales during themonsoon peak. Westward propagating mid- to upper-level inverted troughs (IVs)seem to favor these water vapor fluxes from the east. In particular, a200 % increase in the moisture flux from the Caribbean Sea to the WNAMregion is found to be followed by the occurrence of heavyprecipitation in the WNAM area a few days later. Low-level troughs off the coast ofnorthwestern Mexico and upper-level IVs over the Gulf of Mexico are also relatedto these extreme rainfall events.