California droughts caused mainly by changes in wind, not moisture

The findings were published in Geophysical Research Letters, a journal of the American Geophysical Union, on June 30. The research increases the understanding of how the water cycle is related to extreme events and could eventually help in predicting droughts and floods, said lead author Jiangfeng Wei, a research scientist at The University of Texas at Austin's Jackson School of Geosciences.

"Ocean evaporation provides moisture for California precipitation but is not the reason for droughts there, although the ocean evaporation is slightly lower during droughts," Wei said.

The researchers analyzed 30-year data sets that recorded precipitation, ocean evaporation, surface wind speed and atmospheric pressure on and near the west coast of the United States. These are all factors that influence the water cycle in California. One of the difficulties of studying the water cycle, Wei said, is that the water sources for precipitation cannot be directly observed, so the team also used a mathematical moisture-tracking method and high-resolution model simulations.

Their analysis showed that although moisture evaporated from the Pacific Ocean is the major source for California precipitation, the amount of water evaporated did not strongly influence precipitation in California, except in the cases of very heavy flooding. That's because the amount of water evaporated from this ocean region does not change much year by year, researchers found, and did not cause rain to occur more or less often.

"Ocean evaporation has little direct influence on California precipitation because of its relatively weak variability," Wei said.

Instead, the researchers found that disturbances in atmospheric circulation, the large-scale movement of air, have the most effect on drought because they can affect factors that will cause it to rain more or less.

The study co-authors are Qinjian Jin, a postdoctoral researcher at the Massachusetts Institute of Technology who earned his Ph.D. at the Jackson School; Zong-Liang Yang, a professor in the Jackson School's Department of Geological Sciences; and Paul Dirmeyer, a professor at George Mason University.

Most of California has been in a severe drought since 2011, although a strong El Niño in the winter of 2015 helped diminish the drought. The current drought is caused by a high-pressure system that disturbs the atmospheric circulation. The development of the high-pressure system is related to a sea surface temperature pattern in the Pacific Ocean, according to research cited by the study.

"Although this is a very rare event, the probability of this kind of high-pressure system is likely increasing with global warming," the authors said.

Yang said that the research could aid in the prediction of droughts and floods by improving scientific understanding of the intricate factors that influence rainfall the most.

"The topic is extremely timely as current and future climate change would mean more changes in extreme events such as droughts and floods," Yang said. "Understanding this asymmetric contribution of ocean evaporation to drought and flooding in California will ultimately help us make better predictions."