In recent years the WCRP/CLIVAR/VAMOS and U.S. CLIVAR programs have made major contributions to our understanding of the American Monsoon systems via focused research activities in South, Central, and North America. In this paper we will review these CLIVAR achievements, with emphasis on common features of the American monsoon systems and intersections amongst the activities. Over tropical and subtropical South America, there has been considerable effort to understand the diurnal cycle and mesoscale variability of the precipitation and atmospheric flow. Precipitation in both regions is strongly controlled by the continentalscale gyre that transports moisture from the tropical Atlantic Ocean, first westward across the Amazon Basin, and then southward across the extratropical continent. That gyre displays a regional intensification just to the east of the Andes Mountains, usually in the form of the South American Low-Level Jet (SALLJ). The VAMOS/SALLJEX field experiment (Nov 02-Feb 03) provided a unique dataset for improved understanding and more realistic simulations of the jet and related precipitation patterns. Variability on both intraseasonal and interannual time scales produces a strong modulation of the low-level circulation mainly through zonal-wind changes at the SA tropics and meridional-wind changes at the subtropics, both associated with a dipole-like precipitation anomaly structure, being one of its centers associated with the SACZ activity. Over the core region of the North American monsoon similar efforts are underway to understand the diurnal cycle of convection, intraseasonal variability, and the influence of oceanic and continental boundary forcing on the atmospheric circulation and precipitation patterns in the region. As in South America, a low-level jet is an important feature for transporting moisture onto the continent, so a common research goal on both continents is to improve understanding and predictability of such jet circulations. The diurnal cycle in this region is larger than the amplitude of the annual cycle. There are large-scale shifts in the regions of deep convection during the day from over high topography on the continent (western Mexico) to over the eastern Pacific Ocean. The intraseasonal and interannual fluctuations of monsoon precipitation are in turn linked to continent-scale precipitation patterns. The North American Monsoon Experiment (NAME) field campaign (summer 2004) will provide a unique dataset for improved understanding and more realistic simulations of warm season precipitation and atmospheric circulation patterns over the region. Associated NAME modeling, data assimilation and predictability studies will serve to accelerate progress towards measurably improved climate models that predict North American monsoon variability out to months to seasons in advance.Pages: 13-1
