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El Niño (December 1997)

El Niño (December 1997)

1997-98 El Niño Animation

La Niña (December 1998)

La Niña (December 1998)

1998-99 La Niña Animation
    El Niño was originally recognized by fisherman off the coast of South America as the appearance of unusually warm water in the equatorial Pacific ocean, occurring near the beginning of the year. El Niño means The Little Boy or Christ child in Spanish. This name was used for the tendency of the phenomenon to arrive around Christmas. La Niña on the other hand means The Little Girl. La Niña is sometimes called El Viejo, anti-El Niño, or simply "a cold event" or "a cold episode". It is characterized by unusually cold ocean temperatures in the equatorial Pacific.
    El Niño happens when tropical Pacific Ocean trade winds die out and ocean temperatures become unusually warm. The flip side to El Niño called La Niña, occurs when the trade winds blow unusually hard and the sea temperature become colder than normal. El Niño and La Niña are the warm and cold phases of an oscillation we refer to as El Niño/Southern Oscillation, or ENSO, which has a period of roughly 3-7 years. Although ENSO originates in the tropical Pacific ocean-atmosphere system, it has effects on patterns of weather variability all over the world. It also affects Pacific marine ecosystems and commercially valuable fisheries such as tuna, sardines, salmon, and Peruvian anchovetta.
    El Niño/La Niña as a physical occurrence is a proven fact. The way it works is a theory (actually several different theories). It is as real as other weather phenomena: thunderstorms, for instance. We recognize its characteristics as similar to previous occurrences, and note that its life cycle is roughly the same each time. (Of course each one is different, as each thunderstorm is different, but the basic evolution is similar enough that we know an El Niño/La Niña when we see it).
    On the other hand, a difference from thunderstorms is that we have a very good idea what triggers thunderstorms, what conditions make it likely for them to occur, to the point where weather forecast models commonly pinpoint the locations and predicted severity of thunderstorms a day or so in advance. We do not have such knowledge for El Niño/La Niña. Once an El Niño/La Niña has started, we have reasonably good skill in predicting the subsequent evolution over the next 6-9 months, but before it has started we have very little skill in predicting the onset before the event has become obvious. There are a variety of theories for why they start, but none of them has given us real skill in making a forecast in advance, the way we can for thunderstorms.
    However, there is still plenty of social utility in predicting the evolution of an El Niño after it starts, since that gives 6 months or so warning before the effects come to the US. For instance, a weak to moderate El Niño started earlier this year, and that enables forecasters to predict that the coming winter is likely to be warmer than normal across the northern states, and wetter than normal along the Gulf Coast.
    A much more detailed description of El Niño/Southern Oscillation (ENSO) can be found on the Climate Prediction Center ENSO Cycle page.
Current Conditions  (Back to Top)
    The latest assessment of sea surface temperatures in the tropical Pacific by the Climate Prediction Center indicates that strong El Niño conditions are occurring. As seen in the image below, the current anomalies do show indications of El Niño conditions with warmer than normal sea surface temperatures developing from South America westward into the central Pacific Ocean.

NOAA Current Sea Surface Temperature Anomalies
    A good way to monitor the El Niño/Southern Oscillation (ENSO) is through the use of the Multivariate ENSO Index (MEI). This index takes into account six variables including the sea-level pressure, zonal and meridional components of the surface wind, sea surface temperature, surface air temperature, and total fraction of the sky covered by clouds. Positive values of the MEI represent the warm ENSO phase (El Niño) while the negative MEI values represent the cold ENSO phase, (La Niña). As seen below the MEI is currently positive indicating El Niño conditions.

ESRL plot of the Multivariate ENSO Index
    Compared to last month, the updated (March-April) MEI has stabilized (up by 0.11) at +2.07, continuing just below a Top-3 ranking for the second month in a row. The preceding nine-month run in the Top-3 is tied with 1982-83 for its duration, while 1997-98 kept this level going for a full 12 months. No other El Niño since 1950 even exceeded three months at that level. The August-September 2015 MEI of +2.53 represents the peak of the 2015-16 event, and was exceeded only during the 1982-83 and 1997-98 events. The overall evolution of the 2015-16 El Niño has been most similar to 1997-98, as monitored by the MEI. Looking at the nearest 6 rankings (+3/-3) in this season gives us three El Niño cases that more or less kept going into the following calendar year ('58, '87, and '92), two that eased into a neutral state ('83 and '93), and one that switched over to La Niña ('98). Perhaps the odds of a transition to La Niña are still enhanced compared to climatology, given that 1998 appears to be the best analogue year, and considering that this event has now lasted more than one year, but one should not discount the possibility of a neutral outcome.

ESRL Comparison of recent conditions with other similar historic El Niño events
Forecast  (Back to Top)
    The latest assessment of sea surface temperatures in the tropical Pacific by the Climate Prediction Center indicates that the neutral conditions will continue to into the upcoming Summer with El Niño possible by this Autumn. This forecast is based on predictions from the latest NCEP Climate Forecast System (CFS), first image below, as well as from more than two-thirds of the other statistical and coupled model predictions, second image below.

    In order to be considered an El Niño/La Niña episode the sea surface temperature anomalies in the Niño 3.4 region have to at least meet the criteria listed below, and then remain at or above/below these levels for at least five consecutive months to be considered an El Niño/La Niña event.
  • El Niño = SST anomaly greater than or equal to +0.5 degrees C
  • Neutral = SST anomaly between -0.5 and +0.5 degrees C
  • La Niña = SST anomaly lesser than or equal to -0.5 degrees C
    A more defined definition of El Niño, although not totally etched in stone is listed below.
  • Strong El Niño = SST anomaly greater than or equal to +2.0 degrees C
  • Moderate El Niño = SST anomaly between +1.0 and +1.9 degrees C
  • Weak El Niño = SST anomaly between +0.5 and +0.9 degrees C
    Therefore, using this criteria, it can be seen in the images below that ENSO Neutral conditions are expected to continue through the upcoming Summer followed by Weak El Niño or Neutral conditions later in the year.

NCEP Climate Forecast System (CFS) SST Anomaly Predictions for the Niño 3.4 Region

Model Forecasts of SST Anomalies in the Niño 3.4 Region from the International Research Institute
Effect on U.S. Weather  (Back to Top)

  What Effect Does El Niño/La Niña Have On Our Weather?
    In general, winter El Niño episodes (top map) feature a strong jet stream and storm track across the southern part of the United States, and less storminess and milder-than-average conditions across the North. La Niña episodes (bottom map) feature a very wave-like jet stream flow over the United States and Canada, with colder and stormier than average conditions across the North, and warmer and less stormy conditions across the South.
El Niño Related Winter Features Over North America
La Niña Related Winter Features Over North America

    This information can be verified by looking at the Composite Temperature and Precipitation Anomaly maps for the strongest El Niño and La Niña events displayed below.
El Niño Events
El Niño Composite Temperature Anomaly Graphic
El Niño Composite Precipitation Anomaly Graphic

La Niña Events
La Niña Composite Temperature Anomaly Graphic
La Niña Composite Precipitation Anomaly Graphic
Yearly Anomalies  (Back to Top)
    Sea surface temperature anomalies in the Niño 3.4 region (5°N-5°S, 120°-170°W) of the eastern and central equatorial Pacific Ocean based on the 1971-2000 base period were obtained from the Climate Prediction Center back through 1950. They were then averaged over the five month period from December to April each year. Local analysis of the anomalies led to breaking the data into the five groups listed below.
  1. Strong El Niño  (Average anomaly greater than +0.65 degrees C)
  2. Weak El Niño  (Average anomaly between +0.30 and +0.65 degrees C)
  3. Neutral  (Average anomaly between -0.29 and +0.29 degrees C)
  4. Weak La Niña  (Average anomaly between -0.30 and -0.65 degrees C)
  5. Strong La Niña  (Average anomaly less than -0.65 degrees C)

    Using these breakups for the December through April time period, the years since December 1950 that fall into each category are listed below (the average anomaly based on 1971-2000 normals is in parentheses):
Strong El Niño
1983 (+2.23)
1998 (+1.99)
1992 (+1.78)
1958 (+1.36)
1987 (+1.28)
1973 (+1.14)
1966 (+1.14)
2003 (+0.89)
1969 (+0.89)
1995 (+0.79)
Weak El Niño
1970 (+0.63)
2005 (+0.50)
2007 (+0.46)
1988 (+0.46)
1977 (+0.44)
1993 (+0.43)
1959 (+0.42)
1980 (+0.39)
1964 (+0.33)
1952 (+0.32)
1991 (+0.31)
1978 (+0.30)
1953 (+0.28)
2004 (+0.19)
1990 (+0.17)
1982 (+0.14)
1979 (+0.14)
1954 (+0.14)
1994 (+0.12)
2002 (+0.07)
1957 (+0.05)
1960 (-0.06)
1961 (-0.11)
1997 (-0.22)
1981 (-0.23)
1972 (-0.28)
Weak La Niña
1968 (-0.59)
2006 (-0.55)
2001 (-0.54)
1951 (-0.54)
1984 (-0.53)
1986 (-0.52)
1975 (-0.51)
1967 (-0.46)
1965 (-0.40)
1963 (-0.33)
1962 (-0.31)
Strong La Niña
1989 (-1.54)
1974 (-1.44)
1971 (-1.37)
2000 (-1.33)
1999 (-1.26)
1976 (-1.19)
1985 (-1.08)
1956 (-0.94)
1955 (-0.85)
1996 (-0.67)
Note: 1951 represents December 1950 through April 1951, etc.
    The data shows that since 1950 the winter/early spring with the strongest El Niño was 1983, while the strongest La Niña was 1989.
Local Effect on Temperature  (Back to Top)
    Temperature data was broken up by month and by event, based on the locally derived El Niño/La Niña yearly anomalies, for four stations in west central and southwest Florida: Tampa, Lakeland, Sarasota-Bradenton, and Fort Myers. Average temperatures are linked below:
    The results show that:
  • Strong El Niño events, temperatures were above average in December then below average for January through April.
  • Weak El Niño events, temperatures were genearlly slightly below average December through April.
  • Weak La Niña events, temperatures were slightly below average in December then near to slightly above average for January through April.
  • Strong La Niña events, temperatures were generally slightly above average December through April.

To get a better picture, the anomalies were separated into high temperature anomalies and low temperature anomalies. These graphs are linked below.
    These charts show that the high temperatures skew the average temperature. The high temperatures are much farther below normal than the low temperatures during strong El Niño years. This result indicates significant cloudiness is present, keeping daytime temperatures low while preventing overnight temperatures from becoming too cold. In fact, Tampa has on average 34 percent more cloud cover during strong El Niño events than during strong La Niña events. The additional cloudiness is likely due to a combination of more storms affecting the area as well as a stronger jet stream carrying high clouds across the area from the Pacific Ocean.

 Temperature anomalies during strong La Niña years are linked below:
    These graphs show the opposite tendencies of strong El Niño years: i.e. high temperatures are more above normal than low temperatures. This indicates much less cloudiness during strong La Niña years, allowing for significant solar warming during the day and radiational cooling at night.
Local Effect on Freezes  (Back to Top)
    Freeze data was broken up by month and by event, based on the locally derived El Niño/La Niña yearly anomalies, for four stations in west central and southwest Florida: Tampa, Lakeland, Sarasota-Bradenton, and Fort Myers. Freezes per year for the stations are linked below.
    As the results show, freezes occur more often than normal in February during strong El Niño years. Interestingly enough there are also above normal freezes in February during strong La Niña events in the central part of the state.
    For the strong El Niño events, the likely cause for the greater than normal amount of freezes in February is the abnormally strong storms that pass through and to the north of Florida. This causes cold air to be dragged southward behind the storms into the state (advective freezes).
    For the strong La Niña events, the likely cause is radiational cooling under clear skies with calm wind conditions (radiational freezes).
Local Effect on Rainfall  (Back to Top)
    What stands out most clearly is that the rainfall is well above normal in December through March during strong El Niño years, and well below normal during the same months in strong La Niña years.
    Looking more closely, the links below show total rainfall from December through March for all strong El Niño years since 1951.
    Tampa shows 7 of the 10 strong El Niño years above normal, Sarasota-Bradenton 8 of 10, with Lakeland and Fort Myers showing 9 of 10. Clearly, rainfall is likely to be above normal during strong El Niño events.
    Next are the December through March rainfall totals for strong La Niña years.
    Without fail, all but one La Niña year had below normal rainfall for the December through March period, with some years well below normal.
Local Effect on River Flooding  (Back to Top)
    Obviously, if there is more rain during El Niño years, there is also likely to be more flooding on area rivers. The December through April river flooding graph below does indicate that during strong El Niño years vs. all other years (since December 1950) that most locations do flood more, some up to twice as often, in the winter and spring of strong El Niño years than all other conditions combined.
December through April Flooding Weather
Local Effect on Severe Weather  (Back to Top)
      Across Florida El Niño causes increased storminess which can also mean an increase in the amount of severe weather thanks to the frequent cold fronts moving across the state along with the stronger upper level jet stream.
      Based on the locally derived El Niño/La Niña yearly anomalies, the graph below of December through April Severe Weather during strong El Niño years vs. strong La Niña years (since December 1950) shows that west central and southwest Florida does experience severe weather (i.e. tornadoes, strong damaging winds, and hail) up to twice as often in the winter and spring during strong El Niño conditions than during strong La Niña conditions.
December through April Severe Weather
Local Effect on Tropical Systems  (Back to Top)
    To examine the correlation between El Niño/La Niña events and tropical systems that affect west central and southwest Florida, we need to calculate the anomalies for the Atlantic hurricane season (May-November). Using the same El Niño/La Niña category break down as listed on the yearly anomalies page, the results were as follows:
Strong El Niño
1997 (+2.00)
1987 (+1.55)
1982 (+1.44)
1972 (+1.30)
1965 (+1.15)
2002 (+1.10)
1991 (+0.83)
1957 (+0.77)
Weak El Niño
1969 (+0.64)
1963 (+0.64)
2004 (+0.63)
1994 (+0.62)
2006 (+0.58)
1986 (+0.51)
1993 (+0.50)
1951 (+0.42)
1977 (+0.41)
1953 (+0.40)
1976 (+0.34)
1992 (+0.24)
1990 (+0.23)
1968 (+0.23)
2005 (+0.22)
2003 (+0.22)
1979 (+0.22)
1958 (+0.22)
1980 (+0.13)
1966 (+0.06)
2001 (+0.04)
1983 (-0.03)
1960 (-0.11)
1952 (-0.13)
1981 (-0.21)
1961 (-0.21)
1959 (-0.23)
1996 (-0.24)
1967 (-0.26)
1962 (-0.29)
Weak La Niña
1971 (-0.63)
1974 (-0.59)
1956 (-0.57)
1984 (-0.53)
2000 (-0.50)
1985 (-0.43)
1995 (-0.41)
1989 (-0.41)
1978 (-0.35)
Strong La Niña
1988 (-1.58)
1955 (-1.28)
1975 (-1.17)
1973 (-1.10)
1999 (-0.98)
1964 (-0.88)
1998 (-0.87)
1950 (-0.86)
1970 (-0.84)
1954 (-0.76)
Note: 1950 represents May 1950 through November 1950, etc.
    Notice that the 1997 hurricane season saw the strongest El Niño during this study while 1988 saw the strongest La Niña. Based on these categories, a graph of the tropical systems that affect west central and southwest Florida per year for each category is linked below.     There is clearly less activity during strong El Niño years than other years in this study. However, the overall relationship between El Niño/La Niña events and the tropical activity over west central and southwest Florida is not as clear cut. Even though from 1951 to 2005 a total of 16 hurricanes, 20 tropical storms, and 10 tropical depressions crossed over or near part of the area, the 55 year period of record is relatively short. While there is certainly a trend evident in the data, it would need to be done over a longer time period to be more statistically significant. Tropical landfalls were also compared to the event of the previous winter/spring (i.e. comparing the category of El Niño/La Niña in December to April to the following hurricane season). The results are shown in the graph below.     Interestingly enough, there is little overall difference on whether the previous winter was a strong/weak El Niño to weak La Niña, but there is a lower occurrence following a strong La Niña. Note: All tropical systems whose center made landfall and/or crossed our area were included in this study, along with those that had some effect on any part of west central and southwest Florida. For example, although Hurricane Wilma in October of 2005 made landfall south of our area, it was included in the study because it had some effects within our area of interest.
Other Links  (Back to Top)
Climate Prediction Center
ESRL (Earth System Research Laboratory) formerly Climate Diagnostics Center