| Summary: | Decadal hydroclimate variability in North America and tropical and extratropical South America is analyzed and possible mechanisms for its origin discussed. Focus is on southwestern North America (including Mexico) and the Great Plains, the northeast United States, northeast Brazil and southeastern South America. The varying roles of ocean forcing, internal atmospheric variability and radiatively-forced hydroclimate change are analyzed. In some regions such as southwest North America and the Plains, and northeast Brazil, decadal variations of hydroclimate are quite well understood and can be attributed to variations in tropical Pacific and tropical North Atlantic sea surface temperatures. The mechanisms of tropical ocean influence are reviewed and a case is made that the precipitation anomalies across the Americas associated with the so-called Pacific Decadal Oscillation (PDO) are essentially the same as those associated with the El Niño-Southern Oscillation and also derive from the tropical component of the PDO SST anomalies. In other regions, such as the northeast United States, strong decadal timescale variations are present but cannot be explained in terms of ocean forcing. Both there and in southeast South America decadal variations cannot easily be distinguished from secular wetting trends given the length of the observational record. Finally, it is shown that across much of the Americas near term future radiatively-forced precipitation change will be of the amplitude of historical decadal precipitation variability indicating an important and predictable change.
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