New Research:

ENSO Impact on Extreme Daily Rainfall Probabilities in South Texas

Kenneth J. Tobin and Pablo H. De La Garza 5201 University Blvd., Department of Natural Science, Texas A&M International University, Laredo, TX 78041 (ktobin@tamiu.edu; 956-326-2417)

Previous studies indicate significant modulation of precipitation in response to El Nino Southern Oscillation (ENSO) in the southeastern and western United States especially during the winter season. Rainfall variability is an inherent characteristic of the semi-arid climatic regime characteristic of South Texas, a region experiencing rapid human population growth with finite water resources. Twenty-six stations from South Texas that have a thirty continuous year record (1963-1994) of precipitation data were selected. South Texas precipitation data were related to ENSO warm (El Nino), cold (La Nina), and neither (neutral) episodes of surface seawater temperature from the equatorial Pacific Ocean (PAC3). This preliminary study focuses on wintertime precipitation (DJF). At all stations the percentage of dry days during DJF is consistent (La Nina > neutral > El Nino). Daily, precipitation data for three ENSO modes were fit to a gamma distribution and precipitation probabilities at the 90, 95, and 99th percentile were generated to characterize the extreme ENSO response. Precipitation is generally higher (lower) during El Nino (La Nina) episodes compared with neutral years especially towards the south and west. The increase (decrease) in precipitation is most likely the result of the increased (decreased) influence of the subtropical jet stream to the south and west that advected moisture from the tropical Pacific. Significance testing is planned that will provide validation of the above results. In conclusion, this preliminary study quantifies the spatial variability of climatic modulation associated with ENSO over a highly populated semi-arid region and these results have the potential to be used by water resource managers and planners.

Current Funding Record:

Tobin, K.J., and Bennett, M.E., Oct. 2002 to Sept. 2005, $268,350, South Texas Borderlands Center for Hydrometeorology. National Atmospheric and Oceanographic Adminstration, Educational Partnership Program with Minority Serving Institutions.