Forecasts longer than the 6 to 9 months typical today, if possible, would rely on two principles: the well-defined relationship between temperatures in the upper one meter of the ocean, so-called sea surface temperatures, and precipitation on land. The second principle is the ocean's thermal inertia. Once an ocean basin begins to cool or warm anomalously, it generally tends to stay that way for several years and even decades. If these slow shifts in ocean regime can be identified in their early stages, then perhaps they can used to assess the probability of disastrous, multiyear droughts across the North American continent and elsewhere.
In this light, the study published today evaluates multidecadal, precipitation variability across a network of 750-year-long tree-ring chronologies from the central and southern Rockies. The study suggests that the Great Plains, the Rockies, and the Southwest are stricken by the same 'megadrought' when for multiple years the tropical Pacific turns cold at the same time that the North Atlantic warms. The geographic scale of such megadroughts is determined by the failure of winter (November-March), early summer (May-June) and mid- to late summer (July-September) precipitation, each of which has specific links to tropical Pacific and North Atlantic sea surface temperatures.
A team of researchers from the U.S. Geological Survey, University of Wyoming and Middlebury College (Vermont) analyzed precipitation changes occurri
Contact: Julio Betancourt
United States Geological Survey