The climatology and interannual variability of the North American Monsoon as revealed by the NCEP-NCAR Reanalysis

The North American Monsoon (NAM) is a seasonal shift of upper level winds which brings summertime moisture into the Southwest United States. The climatology and interannual variability of the NAM are examined using the daily NCEP-NCAR Reanalysis (1948-98). The diurnal and seasonal evolutions of pertinent meteorological variables, such as moisture flux, moisture convergence, and pressure heights, are constructed using a five-day running mean. All of the years are used to calculate a daily Z-score which removes the seasonal and diurnal cycles. This new reanalysis climatology reveals: a diurnal cycle in moisture convergence, the Gulf of Mexico is the primary source for NAM moisture, and the time of maximum northward extent of the NAM is late July.

Interannual variability of the NAM is most likely explained by the El Nino Southern Oscillation (ENSO) and the North Pacific Oscillation (NPO). Specific combinations of these cycles induce predictable dynamic responses which affect the distribution of summer moisture across the western United States. A cold north Pacific and El Nino conditions favor a trough over the western United States which directs moisture into the Great Plains and Midwest, such as in 1993. A warm north Pacific and La Nina conditions favor a ridge over the Plains which causes dry conditions in the Great Plains and a wet and early monsoon in Arizona, such as in 1988. An evolving correlation time series of moisture convergence with an ENSO-NPO index shows these dynamic responses are dominant in early summer before the onset of significant tropical Pacific activity. The spatial patterns in correlation in the western United States clearly show moisture in Arizona and the Great Plains has a maximum summer response to Pacific sea surface temperatures. Years are composited according to phases of ENSO and NPO to determine statistically significant interannual differences in meteorological variables.