Possible causes for an earlier demise of the North American Monsoon

Monsoon precipitation over North America and northwest Mexico has important implications in both society and economy. Our preliminary analysis suggests an earlier demise of the North American Monsoon System (NAMS) over northwest Mexico. During the NAMS decay phase, the midlatitude westerly regime shifts equatorward and precipitation events are more frequently associated with synoptic-scale frontal systems rather than with localized convective instability [Vera et al., 2006]. On the other hand, lower surface temperature and precipitable water decrease rainfall intensity during wet season and high loss of soil moisture increase dryness. This study aims to identify which possible causes could produce a shorter NAMS season due to an earlier decay phase. We will discuss the relative influences of these processes on the change of monsoon demise using precipitation, daily maximum and minimum surface temperature, and atmospheric circulation fields from NARR (North American Regional Reanalysis) and the extended reconstructed mean sea surface temperature (SST) from the National Oceanographic and Atmospheric Administration (NOAA) Climate Diagnostic Center (CDC). Vertical profiles from NARR are used since this reanalysis considers radiosonde data over North America as an input. Rainfall rate, derived from NARR daily data, is used to determine the NAMS decay following Li and Fu [2004].