The Principal Mode of Interannual Variability of the North American Monsoon System

Abstract

Nonseasonal behavior of the North American Monsoon System (NAMS) during the summer months (June?September) is investigated, based on monthly mean satellite data and the National Centers for Environmental Prediction?National Center for Atmospheric Research reanalyses for the period 1979?97. The leading principal component of the monthly mean precipitation over the domain 5°?35°N, 80°?125°W is used as a reference time series. The associated variations in fields other than the rainfall are identified based on linear regression analysis. The reference time series is highly correlated with summer rainfall averaged over this broadly defined ?NAMS domain? and is therefore referred to as the (spatially) ?integrated precipitation index? (IPI). It is also correlated with hurricane activity over the tropical and subtropical northeast Pacific. In its positive polarity, the IPI is characterized by an intensification and northward expansion of the ITCZ toward the Mexican coast, with enhanced rainfall throughout Mexico, but particularly in the south and (in a relative sense) in the semiarid northwest. It has a well-defined sea level pressure and surface wind signature, with negative sea level pressure anomalies to the northwest of the region of enhanced rainfall and westerly wind anomalies converging into the region from the west along 10°?20°N. These features extend upward to the 500-hPa level and are overlaid by weak anomalies of opposing sign in the upper troposphere. There are indications of an associated deep barotropic planetary-wave signature over the United States, but the geopotential anomalies are only weakly correlated with the IPI. The IPI is only weakly correlated with summer rainfall over the southwestern United States, with ENSO, and with SST anomalies within the NAMS domain.