Interannual Variability of the North American Warm Season Precipitation Regime

Interannual variability of the North American warm season–precipitation regime is examined in three regions of the United States and Mexico: Arizona–New Mexico, northwest Mexico, and southwest Mexico. Daily observed precipitation over the United States and Mexico for a 26-yr (1963–88) period and various fields from the National Centers for Environmental Prediction–National Center for Atmospheric Research Reanalysis are used to compare and contrast hydrologic conditions and atmospheric circulation features associated with early, late, wet, and dry monsoons in each region. Relationships between anomalous monsoon behavior and the El Niño–Southern Oscillation phenomenon are examined. Some factors associated with the atmosphere’s lower boundary conditions that might influence the interannual variability of the warm season precipitation regime are discussed. The mean seasonal evolution of the North American monsoon system is characterized by the regular northward progression of heavy precipitation from southern Mexico by early June to the southwestern United States by early July. While the seasonal normal rainfall and its variability are largest in southwest Mexico, the mean seasonal percent departure from normal is largest in Arizona–New Mexico. Wet (dry) monsoons in southwest Mexico tend to occur during La Niña (El Niño). This association is attributed, in part, to the impact of local sea surface temperature anomalies on the land–sea thermal contrast, hence the strength of the monsoon. There is also a weak association between dry monsoons in Arizona–New Mexico (northwest Mexico) and La Niña (El Niño). Wet summer monsoons in Arizona–New Mexico tend to follow winters characterized by dry conditions in the southwestern United States and vice versa. Although the onset and duration of the monsoon are quite regular in each region, the precise date of onset in a given region is highly variable and likely to be unrelated to the date of onset in the other regions. Early monsoons in Arizona–New Mexico tend to have heavy seasonal rainfall while late monsoons in northwest Mexico tend to have deficient seasonal rainfall. The onset date in southwest Mexico is not related to seasonal rainfall. However, interannual fluctuations in rainfall over the entire monsoon region for the 2-month period after onset in southwest Mexico are highly correlated, suggesting that knowledge of the starting date in southwest Mexico may be useful for analyzing, understanding, and possibly predicting these fluctuations.