89th American Meteorological Society Annual Meeting

Sunday, 11 January 2009
The Relationship of Transient Upper-Level Troughs to Variability of the North American Monsoon System
Phoenix Convention Center
Stephen W. Bieda III, University of Arizona, Tucson, AZ; and C. L. Castro, S. L. Mullen, A. C. Comrie, and E. S. Pytlak
Relationships between transient upper tropospheric troughs and warm season convective activity over the Southwest U.S. and northern Mexico are explored. Analysis of geopotential height and vorticity fields from the North American Regional Reanalysis and cloud-to-ground lightning data indicates that the passage of mobile inverted troughs (IVs) significantly enhances convection when it coincides with the peak diurnal cycle (1800 UTC – 0900 UTC) over the North American Monsoon (NAM) region. The preferred tracks of IVs during early summer are related to the dominant modes of Pacific sea surface temperature (SST) variability. When La Niña-like (El Niño-like) conditions prevail in the tropical Pacific and the eastern North Pacific has a horseshoe shaped negative (positive) SST anomaly, IVs preferentially track further north (south) and are slightly (typically one IV) more (less) numerous. These results point to the important role that synoptic-scale disturbances play in modulating the diurnal cycle of precipitation over the NAM region and the significant impact that the statistically supported low-frequency Pacific SST anomalies exert on the occurrence and track of these synoptic transients.

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