The Myths and Realities of El Niño
by Jan Null, CCM
 updated June, 2015

The National Oceanic and Atmospheric Administration (NOAA) continues their El Niño Watch for the possible return of El Niño later this fall and winter.  Along with this forecast there have already been considerable misconceptions and misinformation about what the upcoming winter may hold.  So what really is El Niño, and what does it really mean for the United States and other parts of the world?  Below are some talking points to hopefully debunk some of these myths and to put this weather phenomenon into perspective.

Myth 1: El Niño will come to California and the United States this year. 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.)  

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Myth 2: All El Niños are the same. No -- 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 are 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 eight seasons beginning in years (1952, 1953, 1958, 1969, 1976, 1977, 2004, 2006, 2013) classified as “weak” El Niños, eight years (1951, 1963, 1968, 1986, 1991, 1994, 2002, 2009) as “moderate”, four years (1957, 1965, 1972, 1987) as “strong” and two years (1982, 1997) as "very strong" El Niños. [see Oceanic Nino Index (ONI)]
Even within the same category there can be significant differences.  For example, both 1976-77 and 1977-78 were both "moderate" 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]  

Myth 3: There are El Niño-spawned storms. No -- El Niño does not actually create any storms over California or anywhere else. It simply 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 steroids. And just like we don’t know if a homerun from a baseball player on steroids is the result of natural talent or the “roids”, we don’t know whether a particular weather event during an El Nino year would have occurred anyway!

Myth 4: We will see the impacts from El Niño any day now. No -- the long-range ocean and atmosphere forecasts from NOAA and other agencies are just that, forecasts. If the forecast warm-up in the tropical Pacifici does occur later this fall, the earliest effects would not be felt until the cooler periods of winter. [see Latest ENSO Advisory from NOAA and Forecast Array]
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Myth 5: When there is an El Niño, there is lots of rain in California. No -- The answer is not always and not everywhere. Historical records for the past six plus decades for Central California, including the SF Bay Area, show that during the twenty-two El Niño events the rainfall has been roughly above normal (i.e., > 120%) half the time and below normal (<80%) the other half. If just the six 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 had three wet years years during the five strong events, with five above-normal seasons during the seventeen weak-to-moderate El Niños.

Southern California showed more of a wet bias during strong El Niños with above-normal rain in four of the five seasons, near normal the fifth year.  During weak to moderate events Southern California precipitation was above normal six of the 17 seasons, near normal six seasons and below normal the remaining five yearss.

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.

Myth 6: 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 slightly more prevalent during years when there is no El Niño.

The last strong El Niño in the winter of 1997-1998 is a good case study of a wet El Niño year, but one with no major flooding. Despite nearly double the normal rainfall over most of California, there was 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 rain resulted in $1.8 billion in damage statewide.

Myth 7: El Niño means warmer than normal sea surface temperatures (SSTs) along the California coast.
  No -- 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 Historical SST Data (1850-2012)]

What does it all mean? From the current forecast there is a 90% possibility of El Niño continuing through 2015.  And more and more indications are that it may be a stong event and possibily and "very stong" event but historically there can still be a very broad spectrum of outcomes.