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Comparing ENSO Measures

By Joe D'Aleo
Monday, July 28, 2008

We have just come off a strong La Nina and are back in a neutral ENSO state. I have been asked by some of you to discuss what measures are used to determine ENSO state and strength. There are three primary measures, one official.

NINO 3.4 ANOMALIES

On February 23, 2005, NOAA announced that the NOAA National Weather Service, the Meteorological Service of Canada and the National Meteorological Service of Mexico reached a consensus on an index and definitions for El Niño and La Niña events (also referred to as the El Niño Southern Oscillation or ENSO). Canada, Mexico and the United States all experience impacts from El Niño and La Niña.

The index is defined as a three-month average of sea surface temperature departures from normal for a critical region of the equatorial Pacific (Niño 3.4 region; 120W-170W, 5N-5S). This region of the tropical Pacific contains what scientists call the "equatorial cold tongue," a band of cool water that extends along the equator from the coast of South America to the central Pacific Ocean. Departures from average sea surface temperatures in this region are critically important in determining major shifts in the pattern of tropical rainfall, which influence the jet streams and patterns of temperature and precipitation around the world.

North America's operational definitions for El Niño and La Niña, based on the index, are:
El Niño: A phenomenon in the equatorial Pacific Ocean characterized by a positive sea surface temperature departure from normal (for the 1971-2000 base period) in the Niño 3.4 region greater than or equal in magnitude to 0.5 degrees C (0.9 degrees Fahrenheit), averaged over three consecutive months.

La Niña: A phenomenon in the equatorial Pacific Ocean characterized by a negative sea surface temperature departure from normal (for the 1971-2000 base period) in the Niño 3.4 region greater than or equal in magnitude to 0.5 degrees C (0.9 degrees Fahrenheit), averaged over three consecutive months.

You can see in the above plot of the NINO34 temperatures that shows the brief El Nino in the early winter of 2007 and the strong La Nina in the winter of 2007/08. The NINO 34 is updated both monthly and daily at the CPC site here.

There are other measures of the El Nino La Nina (ENSO) oscillation.

THE SOUTHERN OSCILLATION INDEX (SOI)

The Southern Oscillation Index (SOI) is the oldest measure of the large-scale fluctuations in air pressure occurring between the western and eastern tropical Pacific (i.e., the state of the Southern Oscillation) during El Niño and La Niña episodes. Traditionally, this index has been calculated based on the differences in air pressure anomaly between Tahiti and Darwin, Australia. In general, smoothed time series of the SOI correspond very well with changes in ocean temperatures across the eastern tropical Pacific. The negative phase of the SOI represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin. Prolonged periods of negative SOI values coincide with abnormally warm ocean waters across the eastern tropical Pacific typical of El Niño episodes. Prolonged periods of positive SOI values coincide with abnormally cold ocean waters across the eastern tropical Pacific typical of La Niña episodes.
Being an atmospheric observation based measure, it is subject not only to underlying ocean temperature anomalies in the Pacific but also the intraseasonal oscillations like the MJO discussed last week. The SOI often shows month-to-month swings even if the ocean temperatures remain steady due to these atmospheric waves. This is especially true in weaker El Nino or La Ninas and La Nadas (neutral ENSO).  Indeed even the changes week-to-week can be significant. See the last 30 day table here. The monthly SOI since 1950 is here.

THE MULTIVARIATE ENSO INDEX (MEI)

Wolter developed a multivariate approach to monitor ENSO by using a Multivariate ENSO Index (MEI) using six main observed variables over the tropical Pacific. These six variables are: sea-level pressure (P), zonal (U) and meridional (V) components of the surface wind, sea surface temperature (S), surface air temperature (A), and total cloudiness fraction of the sky (C). After spatially filtering the individual fields into clusters (Wolter, 1987), the MEI is calculated as the first unrotated Principal Component (PC) of all six observed fields combined. This is accomplished by normalizing the total variance of each field first, and then performing the extraction of the first PC on the co-variance matrix of the combined fields (Wolter and Timlin, 1993).

In order to keep the MEI comparable, all seasonal values are standardized with respect to each season and to the 1950-93 reference period. Negative values of the MEI represent the cold ENSO phase, a.k.a. La Niña, while positive MEI values represent the warm ENSO phase (El Niño). The MEI can be found here, updated monthly usually within the first 10 days, often sooner.

 

Here is a plot of the three indices the last two years.

On the graph of the three indices above, you can see how well correlated the NINO 34 is to the MEI. You can also see the SOI is much more variable month-to-month than the MEI and NINO34. The MEI and NINO are more reliable determinants of the true state of ENSO especially in weaker ENSO events.

 

To access historical data series of these parameters and other teleconnection indices go here. 

ENSO EFFECTS

El Nino and La Nina effects are well known since the late 1980s after research after the super El Nino of 1982/83 became published.

The El Nino and La Nina effects are nearly mirror opposites as can be seen in the chart.

Since more regions are warm during El Nino and cold during La Nina it is not surprising that ENSO correlates well with global temperatures. You can see how well that correlation has worked since 1998.

 

A page which shows ENSO response by region and season can be found here.

 

Other factors often modulate the actual effects on a localized basis. More sophisticated forecast operations like WSI utilize a multivariate approach to improve on ENSO predictability.

 

ENSO UPATE

 

The strong La Nina has weakened the last 3 months to neutral.

 

 

This is not uncommon in most stronger La Ninas which take a break in the spring into the following summer. A few have gone on to El Ninos but those have been brief and La Ninas have often returned. Most of the ENSO models both dynamical and statistical maintain more or less a neutral state through the upcoming fall and winter.

 

 

 

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