Not long ago, conventional wisdom was that you couldn’t predict the climate for more than a few days in advance. Then came the awareness of El Niño and La Niña and the forecast window increased to as much as 6 to 9 months, depending on the region and season. But a forthcoming study in the Geophysical Research Letters suggests that opposing shifts in Tropical Pacific and North Atlantic Ocean temperatures may foretell persistence of disastrous, multiyear droughts across the North American continent.
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 occurring over decades and recorded in 750-year-old tree-ring chronologies in the central (Colorado, Montana, Wyoming) and southern Rockies (Arizona, New Mexico, Utah). The researchers found that the tree-ring records exhibit significant oscillations in precipitation that last between 40 and 70 years. In general, multidecadal oscillations in the tree-ring record are not cyclical (they don’t occur at regular intervals in time) and are not always in phase across Rockies, suggesting complex linkages between seasonal precipitation and ocean temperatures.
Occasionally, these oscillations can synchronize across the Rockies, particularly in times of "megadroughts" that affect large regions of the country for a decade or more, such as occurred in the late 1500s and the 1950s, said Julio Betancourt, of the U.S. Geological Survey and the University of Arizona’s Desert Laboratory in Tucson, Ariz., and one of the paper’s authors. The 1950s drought was associated with a persistently cold tropical Pacific and warm North Atlantic, and similar conditions have been in place since 1998, when once again the Rockies, Great Plains and Southwest have been stricken by drought. Because no comparable 750-year-long-proxy records exist of sea surface temperatures in the Tropical Pacific and North Atlantic, the demonstrated link between ocean temperatures and precipitation in the Rockies is limited to the last century.
But according to Betancourt, "In the context of shifting ocean climate, the current drought should give water and other resource managers in the Rockies and Southwest little cause for optimism about the drought ending any time soon."
Could we have predicted the 1950’s or current drought, and all of their consequences for the Southwest?
Says Betancourt: "Since climate on these time scales is obviously not cyclical, the next best hope for long-term drought prediction lies with identifying precursor states in oceanic climate, similar in fact to the way we use Tropical Atmosphere Ocean moorings to predict and monitor El Niño or La Niña." (The Tropical Atmosphere Ocean array consists of about 70 moorings in the tropical Pacific Ocean that transmit oceanographic and meteorological data in real-time via satellites.)
"What we hope to be able to do eventually," said Betancourt, "is use the information on the relationship between sea surface temperatures and North American climate to help guide us in more effective and long-term water management and to anticipate climatic effects on ecosystems."
For example, he said, the current ‘megadrought’ is playing a major role in resetting plant demographic clocks across the Rockies through wildfires, insect outbreaks, and tree mortality from physiological stress. Given the longer growing season associated with global warming, the species present in the region now would be more likely to be replaced by other native and non-native species, producing long-term vegetation changes.
The paper published today is "Patterns and sources of multidecadal oscillations in drought-sensitive tree-ring records from the central and southern Rocky Mountains," by Steve Gray (University of Wyoming), Julio Betancourt, U.S. Geological Survey, Chris Fastie, (Middlebury College), and Steve Jackson, University of Wyoming).