Westward water vapor transport through the Chiricahua Gap: A key to the wettest days in Southern Arizona during the North American Monsoon

F. Martin Ralph, University of California San Diego/Scripps Institution of Oceanography

Abstract for Special HAS Seminar on Friday, February 3, 2017 at 3:00 pm in PAS 224, Refreshments in PAS 546 from 2:30 - 3:00 pm

The southwestern U.S. experiences significant summer rainfall as part of what is referred to as the North American Monsoon.  This is especially true of Arizona.  Past studies have concluded that much of the water vapor needed to fuel Arizona’s monsoon precipitation comes from the Gulf of California, and have hypothesized that transport of water vapor westward across the Continental Divide from New Mexico into Arizona is of less importance.

The study presented here documents that almost all the wettest days in southeastern Arizona are associated with westward transport of water vapor into Arizona from New Mexico.  This transport is enhanced due to the existence of a lesser-known low point in the Continental Divide that is roughly 250 km wide and 1 km deep.  It is the lowest stretch along 3000 km of the Divide stretching from 16-45 N.  Based on its meteorological importance, as described herein, and the apparent absence of a name for the feature, it is referred to here as the “Chiricahua Gap.”

Daily precipitation in the summers of 2009 and 2010 (area averaged over Southeastern Arizona) and water vapor flux through the Chiricahua Gap are analyzed.  Westward zonal flux of vertically integrated water vapor transport (IVT) through the Chiricahua Gap is found to play a key role in modulating daily precipitation in the area.  17 of the 18 wettest days in SE Arizona during the study period (i.e., the 10% wettest days) were found to be associated with westward zonal IVT through the Chiricahua Gap.  This relationship has potential to support forecasting because the westward flux events precede the wettest events.