Scientists Look to the Past to Predict Future Rainfall Patterns in the American West

The American Southwest now has a 50 percent chance of entering a 30-year "megadrought." Scientists have examined the past in order to better understand rainfall patterns of today and have found that while the southwest is headed toward a drought, the Pacific Northwest will continue to warm with slightly drier summers and even wetter winters.

"Most of the previous research of the past climate in this region is based on detailed studies of specific sites," said Jessic Oster, lead author of the new study, in a news release. "We combined these records to create a detailed map of past climate change in the American West. We then compared this map to computer climate models to understand what caused these changes.

The researchers chose to map this particular region because the mid-latitudes, in general, and the western United States, in particular, are regions where the climate models tend to disagree on the magnitude and even the direction that precipitation patterns will change in the future.

In order to reconstruct past variations in precipitation patterns, the researchers combined records of ancient lake levels, location and extent of glaciation, variations in the composition of stalagmites in caves and evidence for changes in vegetation and subsurface soil deposits associated with water table depth.

"People hypothesized that the transition between the two climate zones ran along a straight east-west line, but that didn't work very well," said Oster. "Our study indicates that the transition zone is angled from the northwest to the southeast."

During the Last Glacial Maximum, Canada was covered by the massive Laurentide Ice Sheet and while some studies found that the climate was drier, others found it was wetter. This latest research, though, explains why there were drier conditions in Utah and Colorado. The scientists found that the wetter sites in the north were situated next to large inland lakes that existed at the time, and so were experiencing local effects.

"According to these models, it is the high pressure cells that are really important in steering winter storms, and in determining the shape and location of the transition zone," said Oster. "Some models do hint at an increase in subtropical winter moisture, but we don't see evidence of an enhanced summer monsoon."

There is no chance that this ancient weather pattern will return with continued global warming. However, a similar pattern re-emerges periodically during the warm phase of El Niño-Southern Oscillation, which produces drier than normally winters in the Northwest and wetter than normal winters in the Southwest. These findings reveal a bit more about the precipitation patterns seen in these regions.

The findings are published in the journal Nature Geoscience.

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