A strong El Niño developed in the Pacific Ocean during late 2015, and the question is how this event affects the prospects for a substantial recovery from one of the most intense 4-year droughts on record. A warming of the east central equatorial Pacific Ocean, El Niño events recur about every 2 to 7 years and significantly influence weather patterns worldwide. El Niño has a checkered history with regard to California rainfall, however—both the wettest winter (1982–1983) and the driest winter (1976–1977) on record since 1895 occurred during El Niño.
To address this discrepancy, Hoell et al. used historical climate simulations from multiple models to examine how El Niño events of varying intensities affect the chance of rainfall during the November–April rainy season. There are far too few observational samples from which to draw strong conclusions, a fact that motivated their alternate approach, known as ensemble modeling, in which the historical record is simulated a multitude of times. In their case, they repeated the 1979–2014 period in their atmospheric models over 100 times.
The model data indicate California rainfall patterns to be highly sensitive to the strength of El Niño events. Strong El Niños were found to greatly increase the odds of wet conditions across the entire state, especially over the important watershed regions of northern California and the Sierra Nevada. Neither weak nor moderate El Niños materially increased the odds of wet conditions in this region, implying such weaker-caliber events (the vast majority of El Niños) would not be expected to cause appreciable recovery from the recent drought.
Because the results show an 85% greater chance of above-average precipitation across the state’s major northern watersheds during strong El Niños (such as those in 1982–1983 and 1997–1998), the authors suggest that forecasts of the intensity of El Niño events could improve water resource planning in California. To increase the confidence of their results, a better understanding of the physical mechanisms by which El Niño events affect rainfall in the state will be necessary, according to the authors. (Geophysical Research Letters, doi:10.1002/2015GL067102, 2016)
—Terri Cook, Freelance Writer
Citation: Cook, T. (2016), Does El Niño intensity affect precipitation in California?, Eos, 97, doi:10.1029/2016EO046627. Published on 24 February 2016.