El Nino…realities and myths
June 27,2009 PMSF Weather Examiner
CREDIT: JAN NULL CCM AND RETIRED NATIONAL WEATHER SERVICE FORECASTER
The National Oceanic and Atmospheric Administration (NOAA) recently issued an El Nino Watch about the possible return of El Niño later this summer. And even before the ink was dry on the release, yet another round of El Niño hype was beginning, with the media expectation that California was about to be washed into the Pacific.
But what really is El Niño, and what does it really mean for Californians? Let's try to put some of these myths about this weather phenomenon into perspective.
Myth 1: El Niño will come to California this year. No -- El Niño never comes to California. 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 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.)
Myth 2: All El Niños are 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 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 eighteen 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 (1951, 1963, 1968, 1969, 1976, 1977, 2004, 2006) classified as “weak” El Niños, four years (1986, 1987, 1994, 2002) as “moderate” and six years (1957, 1965, 1972, 1982, 1991, 1997) as “strong” El Niños. [see El Niño and La Niña Years and Intensities: Oceanic Niño Index (ONI)]
Myth 3: There are El Niño-spawned storms. No, El Niño does not actually create any storms over California. It simply shifts the usual pattern so that some areas are 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 home run 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 does occur in the tropical Pacific later this summer and fall, the earliest effects on California would not be felt until the cooler periods of late fall or winter. [see Latest ENSO Advisory from NOAA]
Myth 5: When there is an El Niño, there is lots of rain in California. The answer is not always and not everywhere. Historical records for the past six decades for Central California, including the Bay Area, show that during the eighteen El Nino events the rainfall has been above normal half the time and below normal the other half. If just the six strong El Niño events are looked at then the rainfall has been above normal four of the six seasons, and three of those 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 nine years receive below normal rainfall!
Over the same span, Northern California had three wet years and three dry years during strong events, with five above-normal seasons during the nine weak-to-moderate El Niños. [see Comprehensive Climatology of ENSO Events and California Precipitation]
Southern California showed more of a wet bias during strong El Niños with above-normal rain in five of the six seasons and above-normal rain during five of the nine weak-to-moderate events. The bottom line is that California can get wet during El Niño, but not always. As a matter of fact, the California drought in 1976 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. Occasionally, but 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. This is the so-called pineapple connection, and it is actually 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 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 pineapple-connection rain resulted in $1.8 billion in damage statewide.
So, what does it all mean? From the current forecast there is a possibility of an El Niño later this summer that might have some impact next winter, depending on whether it is a weak or strong event and what else might be occurring at the same time. Stay tuned.
To learn more about El Niño visit the EL NIÑO / LA NIÑA RESOURCE PAGE
Jan Null, CCM
SF Weather Examiner
