The North American warm season precipitation regime is characterized by a continental-scale "monsoon-like circulation regime" over Mexico and the U.S. that exerts a fundamental control on the summertime precipitation climatology of the region. It is hypothesized that there is a link between the onset date of the monsoon in the southwest U.S.and summertime precipitation in the Great Plains of the U.S. and that this link may have predictive value at the seasonal timescale.
The following observations have been made in support of this hypothesis. First, the summer precipitation regime is dominated by a continental-scale precipitation pattern characterized by an out-of-phase relationship between precipitation in the southwest U.S. and that in the Great Plains of the U.S. and by a weaker in-phase relationship between precipitation in the southwest U.S. and that along the southeast coast of the U.S. Second, the onset date of the monsoon rains in the southwest U.S. is highly correlated with interannual fluctuations in rainfall over the southwest such that early (late) monsoons tend to be relatively wet (dry). Third, wet (dry) monsoons in the southwest U.S. often follow winters characterized by dry (wet) conditions in the southwest U. S. and wet (dry) conditions in the northwest U. S. Finally, the onset date of the monsoon in the southwest U.S. is significantly correlated with SST during the preceding winter yielding a pattern that resembles the North Pacific Decadal Oscillation. The mechanism relating the North Pacific wintertime SST pattern to the summer monsoon onset date appears to be via the impact of variations in the Pacific jet on west coast precipitation regimes during the preceding winter. The onset date of the monsoon in the southwest U.S. is also influenced by springtime variations in the SST/ITCZ/local Hadley circulation in the eastern tropical Pacific and by variability in the local boundary forcing, which reflects the cumulative effects of precipitation regimes during the preceding winter and spring