The Misconceptions of El Niño

by Jan Null, CCM

Updated December 2015

Arguably the strongest El Niño since at least 1950 is ongoing in the tropical Pacific with the greatest monthly Sea Surface Temperature Anomaly (SSTA) recorded.  But there are still plenty of questions about how it will impact this winter's weather across the United States.  And clouding the picture even more is the hype wrapped in the many myths, misconceptions and misinformation about what El Niño is and its impacts.

#1. El Niño will soon come to California

No -- El Niño never comes to North America. Rather, it is a phenomenon that periodically occurs in the warm equatorial waters of the Pacific Ocean.  Normally the trade winds along the equator push the warmest waters into the western portions of the Pacific. But on an irregular basis of typically two to seven years, the trades slacken, or sometimes even reverse direction, and warmer-than-normal water accumulates along the equator in the central and eastern Pacific. This warming is called El Niño, referring to the “Christ child'' because its effects are greatest in the winter and often disrupt fishing along the South American coast around Christmas. (The converse case, La Niña, is when the waters of the eastern Pacific are cooler than normal.)

#2. El Niños are all the same

-- While El Niño only occurs in the tropics, its impact is felt in many parts of the world. This happens because the location of the huge mass of warm water causes the location of the jet stream, or storm track, to shift. As a consequence some regions can become warmer or colder, or wetter or drier, than normal. However, not all El Niños have the same strength or location, and consequently their impacts can vary significantly.

In general, the larger the area and the greater the warming of the eastern Pacific's equatorial waters, the greater the impact on other regions. Since 1950 there have been 23 years with during which the equatorial Pacific has warmed enough to be classified as an El Niño. There have been a total of twelve seasons beginning in years (1951, 1952, 1953, 1958,  1968, 1969, 1976, 1977, 1979, 1994, 2004, 2006) classified as “weak” El Niños, six years (1963, 1986, 1987, 1991, 2002, 2009) as “moderate”, three years (1957, 1965, 1972) as “strong” and two years (1982, 1997) as "very strong" El Niños. [see Oceanic Nino Index (ONI)]

click to enlarge

Even within the same category there can be significant differences.  For example, both 1957-58 and 1965-66 were both "strong" El Niños but the winter precipitation was vastly different in many parts of the nation.
[see El Niño and La Niña Years and Intensities Based on Oceanic Niño Index]

Not even all El Niño indices are the same.  See Which El Niño Index is Best.

# 3: El Niño is a storm
No -- El Niño is not some sort of large super storm that sits over an area for the winter.  You will never see it on a satellite , a radar image or with your binoculars at the beach!  It simply causes a pattern change that sometimes leads to more precipiation events through the course of the year. 

And it also does not spawn storms!  El Niño does not directly create any storms over California or anywhere else. It shifts the usual jet stream patterns so that some areas can be more susceptible to storm formation. Consequently, El Niño should not be used as an adjective in phrases such as “El Niño flooding'' or “El Niño storm”. Think of it as the Pacific Ocean and the overlying atmosphere being on a performance -enhancing drug. And just like we don’t know if a great performance from an athlete is the result of natural talent or the drugs, we don’t know whether a particular weather event during an El Nino year would have occurred anyway!

# 4: El Niño guarantees lots of rain in California

No -- The real answer is not always and not everywhere. Historical records for the past six plus decades for Central California (i.e., Climate Division 4), including the SF Bay Area, show that during the twenty-three El Niño events the rainfall has been evenly split; with roughly a third of the years having below normal precipitation  (i.e., < 80%), a third near normal (i.e., 80% to 120% of normal) and the final third of the years being substanially (i.e., >120%) above normal. If just the five strong El Niño events are looked at, then the rainfall has been above normal four of the five seasons, and all four were at least 140% of normal. However, if only the weak and moderate El Niños are examined then it is seen that six of the 16 years received below normal rainfall, five near normal (80%-120%) and five above normal.  [see Climatology of El Niño Events and California Precipitation]

Over the same span, Northern California (Climate Divison 2) had three very wet (> 120%) years years during the five strong events, and only five well above-normal seasons during the 18 weak-to-moderate El Niños.

Southern California (Climate Divison 6) showed more of a wet bias during strong and very strong El Niños with above-normal rain in four of the five seasons and near normal the fifth year.  During weak to moderate events Southern California precipitation was well above normal seven of the 18 seasons, near normal four seasons and well below normal the remaining seven years.

The bottom line is that California can get wet during El Niño, but not always. As a matter of fact, the California drought in the 1976-77 winter was during a weak El Niño. It is important to keep in mind that El Niño is not the only thing happening in the atmosphere and that other patterns can either enhance or detract from its overall impact.

# 5: El Niño means disastrous flooding for California

No -- only occasionally.  It is just as likely that California will have significant flooding in a non-El Niño year. Of the 10 costliest flood years in California since 1950, only four happened during a season when there was an El Niño. Two others occurred during seasons with La Niña, and the final four were when the temperature of the tropical Pacific was near normal. [see El Niño and La Niña...Their Relationship to California Flood Damage]

The major weather pattern that causes flooding in California is when a strong surge of subtropical moisture dumps copious amounts of rain over a portion of California for five to seven days. These are so-called Atmospheric Rivers ("Pineapple connection") and they are actually slightly more prevalent during years when there is no El Niño.

The last very strong El Niño in the winter of 1997-1998 is a good case study of a wet El Niño year, but one with no widespread major flooding. Despite nearly double the normal rainfall over most of California, there were nearly twice the number of days of rain with no huge concentrated deluges, and statewide damage totals were about $500 million. Compare this with the flooding that took place around New Year of 1997, a period with no El Niño, when a week's worth of warm atmospheric river rainfall resulted in $1.8 billion in damage statewide.

# 6: El Niño means warmer than normal sea surface temperatures (SSTs) along the California coast
No -- and it's also not responsible for the variety of warm water species of marine life reported recently. An analyis of SST's along the coast shows that overall they are only warmer than normal along the Southern California coast (i..e, between 30°N and 35°N) during 64% of the El Nino years and warmer than normal only 55% of the time along the northern and central California coast (35°N to 40°N).  [see Don't Blame it on El Niño and  Historical SST Data (1850-2012)]

What does it all mean?

El Niño is "very strong" and already happening. It will continue through the winter of 2015-2016 and if it is similar to other very strong events, California can expect above normal rainfall for the season.  But there are no guarantees as this is climatology, not a forecast. And above normal rainfall is also not a guarantee that the drought will end as some of the deficits in some areas exceed more than two full seasons of rainfall.

Copyright © 2015,
Golden Gate Weather Services
Reproduction in full or part is prohibited without permission.