An important goal of the Climate Variability and Predictability (CLIVAR) research on the American monsoon systems is to determine the sources and limits of predictability of warm season precipitation, with emphasis on weekly to interannual time scales. This paper reviews recent progress in the understanding of the American monsoon systems and identifies some of the future challenges that remain to improve warm season climate prediction. Much of the recent progress is derived from complementary international programs in North and South America, namely, the North American Monsoon Experiment (NAME) and the Monsoon Experiment South America (MESA), with the following common objectives: 1) to understand the key components of the American monsoon systems and their variability, 2) to determine the role of these systems in the global water cycle, 3) to improve observational datasets, and 4) to improve simulation and monthly-to-seasonal prediction of the monsoons and regional water resources. Among the recent observational advances highlighted in this paper are new insights into moisture transport processes, description of the structure and variability of the South American low-level jet, and resolution of the diurnal cycle of precipitation in the core monsoon regions. NAME and MESA are also driving major efforts in model development and hydrologic applications. Incorporated into the postfield phases of these projects are assessments of atmosphere-land surface interactions and model-based climate predictability experiments. As CLIVAR research on American monsoon systems evolves, a unified view of the climatic processes modulating continental warm season precipitation is beginning to emerge.

During the last glacial period, large millennial-scale temperature oscillations--the 'Dansgaard/Oeschger' cycles--were the primary climate signal in Northern Hemisphere climate archives from the high latitudes to the tropics. But whether the influence of these abrupt climate changes extended to the tropical and subtropical Southern Hemisphere, where changes in insolation are thought to be the main direct forcing of climate, has remained unclear. Here we present a high-resolution oxygen isotope record of a U/Th-dated stalagmite from subtropical southern Brazil, covering the past 116,200 years. The oxygen isotope signature varies with shifts in the source region and amount of rainfall in the area, and hence records changes in atmospheric circulation and convective intensity over South America. We find that these variations in rainfall source and amount are primarily driven by summer solar radiation, which is controlled by the Earth's precessional cycle. The Dansgaard/Oeschger cycles can be detected in our record and therefore we confirm that they also affect the tropical hydrological cycle, but that in southern subtropical Brazil, millennial-scale climate changes are not as dominant as they are in the Northern Hemisphere.

This paper reviews recent progress made in our understanding of the functioning and variability of the South American Monsoon System (SAMS) on time scales varying from synoptic to long-term variability and climate change. The SAMS contains one of the most prominent summertime climate patterns in South America, featuring a strong seasonal variability in a region lying between the Amazon and the La Plata Basin. Much of the recent progress is derived from complementary international programs, such as the Monsoon Experiment South America (MESA), as well as from ongoing international programs such as the Large Scale Biosphere Atmosphere Experiment in the Amazon Basin (LBA) and the La Plata Basin (LPB) Regional Hydroclimate Project, which includes the CLARIS LPB Europe-South America Network for Climate Change Assessment and Impact Studies in La Plata Basin Project. The latter assesses atmosphere-land surface interactions, the role of land use changes and aerosols from biomass burning considered as sources of variability and change in the SAMS functioning, characteristics and behaviour.The SAMS region is particularly susceptible to variations of climate due to the importance of hydroelectricity generation and the agricultural base of local economies. Also addressed in this report are projections of climate change and extremes, which are important for impact and vulnerability assessments. This discussion includes the need to identify and understand important processes that control the monsoonal climate, how these processes may vary and change, and how they may interact with key societal sectors, including water resource management, hydroelectric generation, agriculture, and agribusiness. This paper reports on the major contributions of MESA to the knowledge of characteristics, functioning and variability of the SAMS, and is based on recent studies and publications, and can be considered as an update of a previous review by C. S. Vera et al. (2006a).

Biomass‐burning plumes and haze layers were observed during the ABLE 2A flights in July/August 1985 over the central Amazon Basin. The haze layers occurred at altitudes between 1000 and 4000 m and were usually only some 100 to 300‐m thick but extended horizontally over several 100 km. They could be traced by satellite imaging and trajectory studies to biomass burning at the southern perimeter of the Amazon Basin, with transport times estimated to be 1–2 days. These layers strongly influenced the chemical and optical characteristics of the atmosphere over the eastern Amazon Basin. The concentrations of CO, CO2, O3, and NO were significantly elevated in the plumes and haze layers relative to the regional background. The NO/CO ratio in fresh plumes was much higher than in the aged haze layers, suggesting that more than 80% of the NOx in the haze layers had been converted to nitrate and organic nitrogen species subsequent to emission. The haze aerosol was composed predominantly of organic material, NH4+, K+, NO3−, SO4=, and anionic organic species (formate, acetate, and oxalate). While the concentrations of most aerosol ions were substantially higher in the haze layers than in the regional background aerosol, the ratios between the aerosol ions in the haze layer aerosols were very similar to those in the boundary layer aerosol over the central Amazon region. Simultaneous measurements of trace gas and aerosol species in the haze layers made it possible to derive emission ratios for CO, NOx, NH3, sulfur oxides, and aerosol constituents relative to CO2. Regional and global emission estimates based on these ratios indicate that biomass burning is an important contributor in the global and regional cycles of carbon, sulfur, and nitrogen species. Similar considerations suggest that photochemical ozone production in the biomass‐burning plumes contributes significantly to the regional ozone budget.

The atmospheric transport of biomass burning emissions in the South American and African continents is monitored through a numerical simulation of the air mass motions using the tracer transport capability of the atmospheric model RAMS (Regional Atmospheric Modeling System) coupled to an emission model. In this method, the mass conservation equation is solved for carbon monoxide (CO) and particulate material