83rd Annual

Thursday, 13 February 2003
Impact of the Northwest Mexican Monsoon on Precipitation in the Central United States: A Moisture and PV Transport Perspective
Stephen M. Saleeby, Colorado State University, Fort Collins, CO; and W. R. Cotton
Poster PDF (3.0 MB)
The Colorado State University - Regional Atmospheric Modeling System (CSU-RAMS) is being used to examine linkages between the Mexican monsoon and precipitation on the Great Plains. Seasonal simulations have been performed for June-August of the 1988 drought year, 1993 mid-west flood year, and the 1997 El Nino year.

From vertical profiles of mixing ratio and moisture transport over the monsoon region, it appears there are two sources from which moist air is being advected into the U.S. At low-levels, the Gulf of California LLJ pushes moisture into southern AZ and NM, though this moisture remains contained to the southern half of these states. A moisture and wind convergence zone forms along the northern extent of the low-level moisture surge, and convection and precipitation form along this region. The increase in vertical moisture transport from the convection, along with southwesterly flow from the Pacific, aids in transporting this moisture into the mountain west at mid-levels.

During surge events, flow from the SMO tends to be southwesterly and most influential at mid-levels. Plumes of moisture "break off" from the SMO, advect into the mountain states, and strengthen the mid-level moisture gradient in central to eastern NM and CO. The increase in moisture, along with the daily upslope flow and elevated heating in the mountains, aids in convective system formation that often focuses along the tight moisture gradient. These developing precipitation systems often experience further strengthening once they reach the high plains of CO. Following development, these systems propagate across the Great Plains over a period of 1-2 days. Their ability to maintain themselves depends on their local dynamics and the dynamics downstream.

During the monsoon, the vertical motion over the central U.S. is in a subsidence pattern, and the overall dynamics is less likely to support or generate its own convection. Many of the systems generated in the lee of the Rockies contain high PV air that has been transported from the convective regions along the SMO. The resulting strong PV gradient tends to induce localized upper-level jet streaks imbedded within the large-scale westerly flow over the U.S. The associated jet streak circulations increase vertical motion and precipitation formation along the right entrance region of the jet. From this, it may be possible for these convective systems to generate their own dynamical support to maintain themselves as they propagate downstream.

Sensitivity tests to variations in soil moisture and SSTs have also been performed. Decreased (increased) values in soil moisture and SSTs tend to decrease (increase) the surface latent heat flux and overall accumulated precipitation. The greatest impact of soil moisture variation is localized in the regions with the largest initial soil moisture values. A similar pattern holds true for changes to SSTs, such that the greatest impact is located in regions where the low-level flow advects air off of the warm waters of the Gulfs of Mexico and California. There are several locations, though, that appear to be affected in the opposite sense.

Supplementary URL: