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El Niño expected to drench California

Shasta Lake in 2009. El Niño precipitation may help refill the lake after four years of drought. (Photo: CC BY 3.0 by Apaliwal via Commons)
Brace yourself for El Niño. All major climate models indicate that the current El Niño will be the strongest on record in terms of sea surface temperature departures from normal.

Climate scientists refer to the anomaly as ENSO, for El Niño Southern Oscillation. The term describes the fluctuations in temperature between the ocean and atmosphere in the east-central Equatorial Pacific, just west of the Peruvian coast. The area is roughly between the International Date Line and 120 degrees west.

The ENSO cycle has three distinct phases: El Niño, La Niña and neutral. El Niño is defined when sea surface temperature is unusually warm for an extended period of time. La Niña is declared when equatorial Pacific is unusually cool for an extended period of time. Neutral phase is defined when the sea surface temperature is considered normal.

These large-scale changes in the surface water temperatures are linked to changes in the strength of the trade winds blowing from east to west across the region, which impacts weather patterns across the globe.

According to the NOAA Climate Prediction Center and International Research Institute for Climate and Society, a strong El Niño will continue through the Northern Hemisphere during the winter of 2015-16, followed by weakening and a transition to ENSO-neutral during the late spring or early summer.

What does that mean for California? In general, during a strong El Niño, California experiences a wetter than normal winter. During six strong El Niño events (1957-58, 1965-66, 1972-73, 1982-83, 1991-93 and 1997-98), California received 120 to 160 percent of normal precipitation from October through March. In addition to the wetter than average conditions forecast for the 2015-16 rainy season, the winter is also has a warmer than normal temperature outlook.

Some of the expected outcomes are:

  • Increased risk of flooding for California, since most of the precipitation is expected in the form of rainfall, rather than snow. Increasing streamflow in undammed rivers and quick filling of reservoirs that come with the potential for reservoir releases for flood control.

  • Early bud breaking in many agricultural crops due to warmer than usual conditions. This can have significant yield impact on crops that rely on sufficient chill for proper development, such as citrus, apples, tree nuts and grapes.

  • Increased aquifer recharge through so-called groundwater banking.

UC Agriculture and Natural Resources researchers will continue to monitor the El Niño Southern Oscillation and how it influences weather patterns in California. Future articles will provide detailed discussions on El Niño, with a focus on water resources impacts, effects on crops and potential for groundwater recharge.

An initiative to improve California water quality, quantity and security is part of the UC Division of Agriculture and Natural Resources Strategic Vision 2025.

Authors: Tapan Pathak, UC ANR Cooperative Extension specialist in climate adaptation in agriculture and Samuel Sandoval Solis, UC ANR Cooperative Extension specialist in water resources

Posted on Thursday, November 19, 2015 at 8:34 AM


This article does not get it quite right. A strong El Nino will deliver higher than average precipitation somewhere along the U.S. West Coast, but that does not necessarily mean CA will receive higher than average precipitation and it certainly does not mean that N. CA (or S. CA) is guaranteed higher than average precipitation this winter. For example, atmospheric blocking can have profound effects on how El Nino may affect CA. During 6 El Nino events CA received higher than average precipitation. Ok, but how about other El Nino events? You are making inferences on less than 60 years of data which is more weather- than climate-related. Even worse, the NCDC/NOAA figure only uses an average period of 30 years. As scientists we need to be more careful when we discuss weather and climate.

Posted by Bob Smith on November 19, 2015 at 11:12 AM

Thanks for commenting on this article. We are not commenting that this El Nino is guaranteed to bring higher than average precipitation. But rather stating that El Nino is LIKELY to bring higher than normal precipitation based on climatology. Of course, 6 strong El Nino events is not ideal, but that is what we have available for comparing strong El Nino years with climate normal (1980-2010). Article from (ño-impacts-0) provides comprehensive view of winter precipitation pattern during strong, moderate, and weak El Nino events since 1950. Thirty-year average is a standard and is widely used for defining climate normal in climate science community.

Posted by Tapan Pathak on November 19, 2015 at 12:32 PM

I think the first comment was referring to the name of your article, "El Niño expected to drench California," which sure makes it sound like California is going to get plenty of rain this winter because of El Niño. Ask any entry-level weatherman with a slight background in meteorology and they would tell you to take those predictions with a grain of salt. The fact is, California has experienced El Niño before where substantial parts of the state get lower than average precipitation.  
If data is "not ideal," then it is best not to make strong statements that are no more than a best guess, or at least use words suggesting this is your best guess. You make it sound like we should all prepare for a wet winter. You may want to rethink how you present information to your audience. At least when I was in graduate school, we were taught to use caution when using strong statements, especially if there were insufficient data to back them up.

Posted by Marcia Lewis on November 19, 2015 at 1:54 PM

Dear Bob. Thanks for your comment on the article. The information that we provided in the previous blog was carefully written, it is based on science, and backed up by climatologist experts. Last summer, meteorologist start following the development of El Niño (, and discussions focused on the precipitation effects of a regular or strong El Niño for California ( Strength matter, and this strong El niño is very LIKELY to bring wetter than normal conditions. Once El Niño was established, conversation turned into discussions about the potential persistence the Buoyant Blob (AKA Ridiculous Resilient Ridge) or the predominance of el Niño ( As we have seen, a strong El Niño has prevailed over the Buoyant Blob. Recently, even though there is a slow start of the rainy season, it is predicted a wetter than average conditions for later in the winter season ( This analysis is based on statistical analysis of climate data, as well as model predictions. We are not the only ones mentioning the expected events that can happen due to El Niño, there are other experts mentioning the same events ( As always, Mother Nature can surprise us, but we wanted to share our most informed and science based prediction.

Posted by Sam Sandoval on November 19, 2015 at 4:47 PM

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