Wednesday, March 29, 2017
What can drought-stricken California expect from the El Niño winter forecast?
A brief science assessment by a subgroup of the NOAA Drought Task Force

Key Points
Recognizing the sensitivity of likely impacts on California winter precipitation to El Niño intensity, and also recognizing the spread of possible outcomes even for a very strong El Niño (see Fig. 3), the outlook must be expressed probabilistically. Nonetheless, this brief assessment leads to the following key points:

  • Impacts are likely to be greater in late winter than early winter.
  • Southern California has a stronger chance of wet conditions than northern California.
  • In case of a very strong El Niño, heavy precipitation is more likely across the entire state.
During 2011-15, California experienced the driest four successive winters since 1895. Dry conditions have been widespread and, according to the U.S. Drought Monitor for August 2015, all of California is in severe to exceptional drought. Recent research has demonstrated that sea surface temperature (SST) anomalies - cool conditions in the central to eastern equatorial Pacific and warm conditions in the west Pacific and Indian Ocean - were important factors contributing to the drought. This SST pattern has now changed. A developing El Niño, with strong warming of the east equatorial Pacific and cooling of the tropical west Pacific and North Pacific, reverses many of the anomalies prevailing during 2011-15. This El Niño ranks among the strongest in the historical record for this time of year and forecast models predict it to last into 2016.

Photo courtesy California Department of Water Resources --

Frequently Asked Questions
This FAQ covers questions asked regarding El Niño and California Drought.

What drives SST patterns to change?

Natural cycles of ocean - atmosphere interaction lead to recurring swings between anomalously warm (El Niño) and cold (La Niña) sea surface temperature states in the equatorial Pacific. A tension is always present between the climatological warm-water volume of the west Pacific and the cold-water volume of the east. This is maintained by trade winds blowing westward across the tropical Pacific, acting to pile up warm water in the west. The periodic weakening of the trade winds relaxes that tension, and is coupled with a movement of the warm water mass eastward, often resulting in El Niños.

How can SSTs in the Equatorial Pacific impact weather in California, thousands of miles away?

As warm water shifts eastward and El Niño develops, persistent rain and thunderstorms follow suit and shift into the usually dry equatorial east Pacific. Atmospheric convection responsible for thunderstorms generates waves in the atmosphere that spread over long distances much like ripples moving away from a rock dropped in a pond. The waves arch northward and eastward through the Earth’s atmosphere and create a low pressure system over the North Pacific Ocean that guides storm systems into California. Most noticeable in mid-winter, this effect can appreciably increase California precipitation during its wet season.

Will this El Niño end the ongoing California drought?

An above-normal rainy season with heavy snowpack likely won’t replenish aquifers and wells sufficiently to lift some water users out of drought, though most will greatly benefit. Also it must be noted that El Niño events have not reliably delivered above-average precipitation across the entire state, especially not in the northern watersheds. The few very strong El Niños of 1877-78, 1982-83, and 1997-98 have been notable exceptions, each spreading much above normal moisture across the entire state—often leading to flooding and landslides. It is virtually certain that El Niño will prevail this winter and in the next several months it will become clearer if the current event has the intensity and attributes of those rare historical very strong cases to which California responded strongly statewide.

If California received precipitation in amounts similar to what it received during the El Niño of 1997-98, would it end the drought? Conversely, if California were as dry as it was during the 1965-66 El Niño, what would happen to the drought?

The past 4 years have been the driest consecutive string in California since record keeping began in 1895. Statewide, the cumulative moisture deficit during 2011-2015 has approached a full year’s precipitation. High temperatures have likely further eroded water supply. Thus, on a purely accounting basis, statewide precipitation would have to be at least double the normal to fully recover. But the drought story is much more complicated than this simple numeric implies. As of July 31, California’s statewide reservoirs were 54% of average, reflecting the dry and warm conditions since 2011, but ground water levels have been drawn down even more dramatically. Even normal precipitation and mountain snowpack will go a long way to replenishing reservoirs, and sustaining stream flow in 2016. But severe deficits in the environment, land surface, and soils must also be considered and recharged. Even in normal years, groundwater is tapped to meet demand, for example in the Los Angeles Basin. Continued ground water extraction without replenishment is unsustainable, and it is unclear whether a heavy precipitation year would greatly recharge ground water to conditions 5 years ago. Further it must be recognized that California precipitation is influenced by many factors other than El Niño and in the past some El Niños have gone along with dry winters. The lesson of past events is that a dry winter could happen again, even if the risks are reduced given the strength of the current El Niño. A fifth dry year would be most unwelcome but there have been two prior occurrences in which California statewide precipitation ran below average for at least five consecutive years, the last being 1987-1992.

Do the models used to understand and predict climate faithfully capture the link between El Niño and California precipitation?

Significant advances in climate models have been made in the last decade. Models used for climate forecasting couple the ocean to the atmosphere, and are able to predict skillfully the emergence of El Niño events often more than a season in advance. They realistically capture the manner in which weather patterns over the United States respond to El Niño, and they furthermore provide a new tool for understanding how El Niño strength matters for California. It is from these models that research evidence points to significant increases in the probabilities for wet conditions across the entire state under the influence of El Niños of the 1982-83 and 1997-98 caliber, while much more modest effects occur for garden variety El Niños.

Most of California's water resources are received and stored in the northern part of the state and consumed in the southern part. How does that usage behavior mesh with potential El Niño impacts?

Moisture conditions linked to weather patterns affecting northern California and the Sierras are most relevant for the state’s overall water supply variations from year to year. A large portion of that water is conveyed to central and southern California for consumption. El Niño impacts have been unreliable in the north historically, and only Southern California has tended to be consistently wet during El Niños. The increase in the south, though beneficial, is insufficient to meet the overall demand by agriculture in the Central and Imperial Valleys, and the large urban center needs. History reveals that, since 1877, three very wet years occurred over northern California (and the state overall) in tandem with the three strongest El Niños. Research using improved climate models is indicating such a relation has unlikely been random, but appears to be symptomatic of a greater statewide influence exerted by very strong El Niños compared to the more typical events. For this reason, monitoring and forecasting the eventual strength of the current developing El Niño is important for water year precipitation outlooks and planning.


What can drought-stricken California expect from the forecast El Niño winter?

August 2015

An assessment by a sub-group of the NOAA Drought Task Force

Richard Seager1, Mingfang Ting1, Bor-Ting Jong1, Martin Hoerling2, Siegfried Schubert3, Hailan Wang3, Bradfield Lyon4, Arun Kumar5

1Lamont Doherty Earth Observatory of Columbia University, Palisades, New York 

2NOAA Earth System Research Laboratory, Boulder, Colorado
3NASA Goddard Space Flight Center, Greenbelt, Maryland 

4International Research Institute for Climate and Society, Palisades, New York
5NOAA Climate Prediction Center, College Park, Maryland

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