Role of ocean evaporation in California droughts and floods

“Ocean evaporation provides moisture for California precipitation but is not the reason for droughts there, although the ocean evaporation is slightly lower during droughts...Ocean evaporation has little direct influence on California precipitation because of its relatively weak variability...

Although [the high-pressure system responsible for the current drought] is a very rare event, the probability of this kind of high-pressure system is likely increasing with global warming.”

Jiangfeng Wei, lead-author and research scientist at The University of Texas at Austin’s Jackson School of Geosciences

• Summarizes that since winter 2011, a record-breaking drought has occurred in California
• States that studies have found the drought is mainly caused by a persistent high-pressure system off the U.S. West Coast, which is linked to Pacific sea surface temperature anomalies
• Stats the water cycles associated with the droughts and floods are still not clearly understood
• Shows that the atmospheric circulation off the West Coast not only controls the atmospheric convergence and formation of precipitation but also largely determines surface wind speed, which further affects the evaporation over the eastern North Pacific, the major evaporative moisture source for California precipitation
• Finds that because of this mechanism, the ocean evaporation over the eastern North Pacific has been reduced during the recent drought; however, the ocean evaporation anomalies have little direct influence on California precipitation, especially during dry years, mainly because of their weak amplitudes
• States the California droughts cannot be readily attributed to the reduced ocean evaporation
• Finds the association between increased Pacific evaporation and floods over California is somewhat stronger