In this study, we examine the South American Monsoon System (SAMS) variability with an emphasis on three-day periods or longer during which time the low-level wind in central Amazonia is consistently from the west or the east (westerly and easterly wind regimes, WWRs and EWRs, respectively). We use observational and Reanalysis datasets for the southern summer, as in previous studies on SAMS variability. Our research methodology involves the addition of model data that consists of the Coordinated Enhanced Observing Period (CEOP) Model Output Location Time Series (MOLTS) at locations in the SAMS region, and ensemble atmospheric general circulation model (AGCM) simulations of the southernsummer season.
For both wind regimes, the observed and simulated composites of anomalous precipitation over South America show a dipole pattern with poles in northwestern and central-southeastern Brazil. During WWRs (EWRs), precipitation anomalies at the central-southeastern pole are positive (negative), indicating a stronger (weaker) SACZ. In addition, during WWRs the upper-level monsoon high is stronger and the subtropical jet in the South American sector is stronger and closer to the equator.
We also examine the diurnal cycle of rainfall from three CEOP MOLTS for Rondonia and Brasilia, which are situated at different poles of the rainfall anomalies dipole. The diurnal cycle in precipitation during both wind regimes shows an early-afternoon maximum at the selected locations. At Rondonia (Brasilia), there is a second early-morning (nocturnal) maximum on EWR (WWR) days. The diurnal mean precipitation in the regime that shows this maximum value, which is primarily due to enhanced cumulus convection, is higher than that in the other regime. In the simulation, WWRs and EWRs occur with approximately equal frequency. There is also a dipole pattern in anomalous precipitation during the days of each regime, although the poles are displaced relative to the observations. The simulated diurnal cycles of rainfall for the different wind regimes are nearly identical. Possible explanations for these AGCM difficulties are briefly discussed.
Lag composites of geopotential height anomalies at 700 mb prior to the establishment of the wind regimes in Amazonia show the development of structures in the South Pacific that resemble the principal modes of variability in that region. This indicates the existence of links between the wind regimes and the modes of intraseasonal variability over South America. It is proposed that the wind regimes are associated with low-frequency variability in the extratropical circulation.