Balaji Rajagopalan
University of Colorado

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Analysis is performed on the spatio-temporal attributes of North American Monsoon rainfall in the southwestern USA. Results show a significant delay in the initiation, peak and closing stages of the monsoon in recent decades. Consequently, there is a decrease in rainfall during July and a corresponding increase in rainfall during August and September. Relating these attributes of the summer rainfall to antecedent winter/spring land and ocean conditions leads us to propose the following hypothesis: warmer tropical Pacific SSTs and cooler northern Pacific SSTs (i.e., a warm Pacific Decadal Oscillation (PDO)/ El Niño Southern Oscillation (ENSO) pattern) in the antecedent winter/spring leads to wetter than normal conditions over the desert southwest (and drier than normal conditions over the Pacific Northwest). This enhanced antecedent wetness delays the seasonal heating of the North American continent that is necessary to establish the monsoonal land-ocean temperature gradient.

The delay in seasonal warming in turn delays the monsoon initiation, thus reducing rainfall during the typical early monsoon (July) period and increasing the rainfall during the later months (August and September) of the monsoon season. While the rainfall during the early monsoon appears to be most modulated by antecedent winter/spring Pacific SST patterns (PDO/ENSO), the rainfall in the later part of the monsoon seems to be driven largely by the SST conditions surrounding the monsoon region along the coast of California and the Gulf of California. The role of antecedent land and ocean conditions in modulating the following summer monsoon

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5 October 2005
2 PM/ DSRC 1D 403
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