The principal mode of interannual variability of the North American Monsoon System

Nonseasonal behavior of the North American Monsoon System (NAMS) during the summer months (June-September) is investigated, based on monthly mean sate llite data and the National Centers for Environmental Prediction-National C enter for Atmospheric Research reanalyses for the period 1979-97. The leadi ng principal component of the monthly mean precipitation over the domain 5 degrees-35 degrees N, 80 degrees-125 degrees W is used as a reference time series. The associated variations in fields other than the rainfall are ide ntified based on linear regression analysis. The reference time series is highly correlated with summer rainfall average d over this broadly defined "NAMS domain" and is therefore referred to as t he (spatially) "integrated precipitation index" (IPI). It is also correlate d with hurricane activity over the tropical and subtropical northeast Pacif ic. In its positive polarity, the IPI is characterized by an intensificatio n and northward expansion of the ITCZ toward the Mexican coast, with enhanc ed rainfall throughout Mexico, but particularly in the south and (in a rela tive sense) in the semiarid northwest. It has a well-defined sea level pres sure and surface wind signature, with negative sea level pressure anomalies to the northwest of the region of enhanced rainfall and westerly wind anom alies converging into the region from the west along 10 degrees-20 degrees N. These features extend upward to the 500-hPa level and are overlaid by we ak anomalies of opposing sign in the upper troposphere. There are indicatio ns of an associated deep barotropic planetary-wave signature over the Unite d States, but the geopotential anomalies are only weakly correlated with th e IPI. The IPI is only weakly correlated with summer rainfall over the sout hwestern United States, with ENSO, and with SST anomalies within the NAMS d omain.