Interannual variability of the North American warm season precipitation regime

Interannual variability of the North American warm season-precipitation reg ime is examined in three regions of the United Stares and Mexico: Arizona-N ew Mexico, northwest Mexico, and southwest Mexico. Daily observed precipita tion over the United States and Mexico for a 26-yr (1963-88) period and var ious fields from the National Centers for Environmental Prediction-National Center for Atmospheric Research Reanalysis are used to compare and contras t hydrologic conditions and atmospheric circulation features associated wit h early, late, wet, and dry monsoons in each region. Relationships between anomalous monsoon behavior and the El Nino-Southern Oscillation phenomenon are examined. Some factors associated with the atmosphere's lower boundary conditions that might influence the interannual variability of the warm sea son precipitation regime are discussed. The mean seasonal evolution of the North American monsoon system is charact erized by the regular northward progression of heavy precipitation from sou thern Mexico by early June to the southwestern United States by early July. While the seasonal normal rainfall and its variability are largest in sout hwest Mexico, the mean seasonal percent departure from normal is largest in Arizona-New Mexico. Wet (dry) monsoons in southwest Mexico tend to occur d uring La Nina (El Nino). This association is attributed, in part, to the im pact of local sea surface temperature anomalies on the land-sea thermal con trast, hence the strength of the monsoon. There is also a weak association between dry monsoons in Arizona-New Mexico (northwest Mexico) and La Nina ( El Nino). 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 guile regular in each re gion, the precise dare of onset in a given region is highly variable and li kely to he unrelated to the date of onset in the other regions. Early monso ons in Arizona-New Mexico tend to have heavy seasonal rainfall while late m onsoons in northwest Mexico tend to have deficient seasonal rainfall. The o nset date in southwest Mexico is not related to seasonal rainfall. However, interannual fluctuations in rainfall over the entire monsoon region for th e 2-month period after onset in southwest Mexico are highly correlated, sug gesting that knowledge of the starting dare in southwest Mexico may be usef ul for analyzing, understanding, and possibly predicting these fluctuations .