Intercomparison of the Principal Modes of Interannual and Intraseasonal Variability of the North American Monsoon System

Time series of seasonal-, monthly, and pentad-mean precipitation are subjected to empirical orthogonal function analysis, regression analysis, and compositing techniques to study the principal modes of interannual and intraseasonal variability of the North American Monsoon System (NAMS). The leading principal component (PC) from the summertime seasonal-mean data is associated with El Niño–Southern Oscillation (ENSO) variability while the leading PC from the pentad-mean data is associated with 30–60-day intraseasonal (Madden–Julian) oscillations (MJOs). The leading PC from the monthly mean data is a hybrid of the two above-mentioned modes, capturing aspects of both. The leading PCs are used as reference time series for regressions and composites that reveal the structure of the principal modes and their manifestation in the NAMS. The leading PCs are also used to estimate the fraction of the variance of summer precipitation that is explained by ENSO and by interannual variations of MJO activity. ENSO-related impacts on the NAMS are linked to meridional adjustments of the ITCZ. In its positive polarity, the leading PC of interannual variability is associated with warm (ENSO) episodes and is characterized by an expansion of the ITCZ toward the south, increased precipitation in a zonally oriented band just north of the equator, and decreased precipitation over Mexico and portions of the Caribbean. MJO-related impacts on the NAMS are linked to more regional meridional adjustments in the precipitation pattern over the eastern tropical Pacific. In its positive polarity, the leading PC of intraseasonal variability is associated with an intensification and northward adjustment of the precipitation pattern in the eastern tropical Pacific, with increased precipitation over the warm pool to the west of Mexico and over portions of Mexico and the southwestern United States. Both the interannual and intraseasonal modes have well-defined, but distinct, sea level pressure and surface wind signatures in the eastern tropical Pacific. These features extend to the middle troposphere and are capped by circulation features in the opposite sense in the upper troposphere. The relationship of the MJO to the NAMS is examined in more detail using the leading PC of intraseasonal variability and an objective procedure to identify the phase of MJO events. The leading PC is strongly related to the eastward progression of centers of enhanced (reduced) convection around the global Tropics o