El Niño & La Niña Misconceptions

by Jan Null, CCM

Updated October 2018

Once again El Niño is in the news and there is already lots of hand-waving and excited predictions about how it will impact this winter's weather across the United States and elsewhere.  And clouding the picture even more is the hyperbole wrapped in the many myths, misconceptions and misinformation about what El Niño and La Niña are and their impacts.

#1. El Niño or La Niña will soon arrive in California

No -- Neither ever comes to North America. Rather, they are phenomena 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 five 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 Christmastime. (The naming of El Niño)  When the trades strengthen and the waters of the eastern Pacific are cooler than normal it is called La Niña.
Neutral El Niño La Niña

#2. All El Niños and La Niñas, and their impacts, are the same

-- While El Niño and La Niña only occur in the tropics, their impacts are felt in many parts of the world. This happens because the location of the huge mass of warm water for El Niño, and cold water for La Niña, may cause 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 historic normals. However, not all ENSO (i.e., El Niño - Southern Oscillation, the collective term for both El Niños and La Niñas) events 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. The most common metric for classifying ENSO events is the Oceanic Nino Index (ONI) which identifies how much above or below normal the sea surface temperatures (SSTs) are for a specific region (i.e., Niño 3.4) of the eastern tropical Pacific.

Since 1950 there have been 25 years with during which the equatorial Pacific has warmed enough to be classified as an El Niño. There have been a total of ten seasons beginning in years (1952, 1953, 1958,  1969, 1976, 1977, 1979, 2004, 2006, 2014) classified as “weak” El Niños, seven years (1951, 1963, 1968, 1986, 19941, 2002, 2009) as “moderate”, five years (1957, 1965, 1972, 1987, 1991) as “strong” and three years (1982, 1997, 2015) as "very strong" El Niños.

Conversely, there have been 22 years with during with equatorial Pacific SSTs cooler than normal enough to be classified as an La Niña. A total of ten seasons beginning in years (1954, 1964, 1971, 1974, 1983, 1984, 2000, 2005, 2008, 2016, 2017) are classified as “weak” La Niñas, four years (1955, 1970, 1995, 2011) as “moderate” and seven  years (1973, 1975, 1988, 1998, 1999, 2007, 2010) as “strong” La Niñas. (List)

[And it is extremely important to note that ENSO is not the only large scale climatological phenomena that impacts the world's weather. ENSO's impact in any given year is dependent on not only their strength but also on the location and extent of the event along the Equator in the Eastern Pacific Ocean. It's also a function of ENSO's interaction with the whole "alphabet-soup" of other Indices and Oscillations; such as the PDO (Pacific Decadel Oscillation), NAO (North American Oscillation), (AO) Arctic Oscilation (AO), (MJO) Madden-Julian Oscillation and other.]  

Even within the same ENSO category there can be significant differences.  For example, the most recent weak La Niñas in 2008-09 and 2016-17 had very different winter precipitation patterns in many parts of the nation, especially California.

Similarly, the two most recent very strong El Niños, 1997-98 and 2015-16 were quite dissimilar.

See also the extensive 1950 to 2017 ENSO US Winter Precipitation & Temperature Climatologies categorized by strength: El Niño or La Niña

One additional takeaway is the need to look beyond more than just the ONI. A larger view is presented in Differences Between 2015-16 El Niño and Previous Strong and Very Strong  Events and Comparative El Niño and La Niña Climatology(1997-1998 to 2017-2018).  And not even all ENSO indices are the same and seen in
Which El Niño Index is Best.

# 3: El Niño and La Niña are storms
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 variations from normal precipiation and temperature patterns. 

And they also does not spawn storms!  El Niño and La NIña do 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, they should not be used as an adjective in phrases such as “El Niño flooding'', “El Niño storm” or "La Niña Drought". 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 a drugs, we don’t know whether a particular weather event during an El Nino or La Niña 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-five El Niño events the rainfall has been evenly split; with 10 of 25 years having below normal precipitation  (i.e., < 80%), 5 of 25 near normal (i.e., 80% to 120% of normal) and the final 10 of the years being substanially (i.e., >120%) above normal. See Climatology of El Niño Events and California Precipitation and Climatology of La Niña Events and California Precipitation.

Over the same span, Northern California (Climate Divison 2) had three very wet (> 120%) years years during the eight strong or very strong events, and three well below average.

Southern California (Climate Divison 6) showed more of a wet bias during strong and very strong El Niños with above-normal rain in five of the eight seasons, near normal for two, but one well below also. 

The bottom line is that California can get wet during El Niño, but it is near or below normal almost as often. 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)]

The Bottom Line...

As we transiton into another winter season the perceived effects of a (possibly) developing ENSO event is already being seen in the the plethora of forecasts and outlooks. While historic teleconnections between what is going in the eastern tropical Pacifc and the evental winter weather across North America should not be ignored, it is important to keep in mind that those relationships are not as linear as once thought and that many other factors, including climate change, need to be part of the equation. The bottom line is that there are no guarantees, implicit or implied, in ENSO climatologies, and they are definitely NOT intended as forecasts.

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