Multivariate ENSO Index (MEI)

The views expressed are those of the author and do not necessarily represent those of NOAA.

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Outline for MEI webpage (updated on May 5th, 2012)

This webpage consists of seven main parts:

1. A short description of the Multivariate ENSO Index (MEI);

2. Historic La Niña events since 1950;

3. Historic El Niño events since 1950;

4. MEI loading maps for the latest season;

5. MEI anomaly maps for the latest season;

6. Discussion of recent conditions;

7. Publications and MEI data access.

El Niño/Southern Oscillation (ENSO) is the most important coupled ocean-atmosphere phenomenon to cause global climate variability on interannual time scales. Here we attempt to monitor ENSO by basing the Multivariate ENSO Index (MEI) on the 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). These observations have been collected and published in ICOADS for many years. The MEI is computed separately for each of twelve sliding bi-monthly seasons (Dec/Jan, Jan/Feb,..., Nov/Dec). 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.

IMPORTANT CHANGE: The MEI used to be updated every month during the first week of the following month based on near-real time marine ship and buoy observations (courtesy of Diane Stokes at NCEP). However, this product has been discontinued as of March 2011 (ICOADS-compatible 2-degree monthly statistics). Instead, the MEI is now being updated using ICOADS throughout its record. The main change from the previous MEI is the replacement of 'standard' trimming limits with 'enhanced' trimming limits for the period from 1994 through the current update. This leads to slightly higher MEI values for recent El Niño events (especially 1997-98 where the increase reaches up to 0.235 standard deviations), and slightly lower values for La Niña events (up to -.173 during 1995-96). The differences between old and new MEI are biggest in the 1990s when the fraction of time-delayed ship data that did not enter the real-time data bank was higher than in more recent years. Nevertheless, the linear correlation between old and new MEI for 1994 through 2010 is +0.998, confirming the robustness and stability of the MEI vis-a-vis input data changes. Caution should be exercised when interpreting the MEI on a month-to-month basis, since the MEI has been developed mainly for research purposes. 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).

IMPORTANT ADDITION: For those interested in MEI values before 1950, a 'sister' website has now been created that presents a simplified MEI.ext index that extends the MEI record back to 1871, based on Hadley Centre sea-level pressure and sea surface temperatures, but combined in a similar fashion as the current MEI. Our MEI.ext paper that looks at the full 135 year ENSO record between 1871 and 2005 is available online at the International Journal of Climatology (Wolter and Timlin, 2011).

Historic La Niña events since 1950

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How does the 2007-09 La Niña event compare against the six previous biggest La Niña events since 1949? This figure includes only strong events (with at least three bimonthly rankings in the top six), as well as the 2007-09 event which reached this threshold twice (rankings are listed here). La Niña events have lasted up to and over three years since 1949, in fact, they do tend to last longer on average than El Niño events. The longest two events included here covered much of 1954-56 and 1973-75. The longest event not included here occurred in 1999-2001 which reached the 'strong' threshold (top six rankings) just once. Once this year is over (2012), I will update this figure to replace 2007-09 with the 2010-12 event. Click on the "Discussion" button below to find the comparison of the recent La Niña event with historic events.

Historic El Niño events since 1950

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How does the 2009-10 El Niño event compare against the seven previous biggest El Niño events since 1950? This figure includes only strong events (with at least three bimonthly rankings in the top six), with the exception of the 2009-10 event that reached the top six ranking twice. Compared to the previous version of this figure, 1997-98 now reaches very similar peak values to the 1982-83 event, just above the +3.0 sigma threshold. Click on the "Discussion" button below to find the comparison of the recent La Niña event with historic events.

MEI loading maps for the latest season

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The six loading fields show the correlations between the local anomalies and the MEI time series. Land areas as well as the Atlantic are excluded and flagged in green, while typically noisy regions with no coherent structures and/or lack of data are shown in grey. Each field is denoted by a single capitalized letter and the explained variance for the same field in the Australian corner.

The sea level pressure (P) loadings show the familiar signature of the Southern Oscillation: low pressure anomalies in the west and high pressure anomalies in the east correspond to negative MEI values, or La Niña-like conditions. Consistent with P, U has positive loadings along and just south of the Equator, corresponding to easterly anomalies near the dateline. The meridional wind field (V) features negative loadings north of the Equator across the Central Pacific basin, denoting the northward shift of the ITCZ so common during La Niña conditions, juxtaposed with high positive loadings northeast of Australia (northerly anomalies during La Niña).

Both sea (S) and air (A) surface temperature fields exhibit the typical ENSO signature of a wedge of positive loadings stretching from the Central and South American coast to just east of the dateline, or cold anomalies during a La Niña event. At the same time, total cloudiness (C) tends to be decreased over the central and eastern equatorial Pacific, as opposed to increased cloudiness from the Philippines to Hawai'i and eastward.

The MEI continues its retreat from its peak explained variance (now at just 20%) of all six fields in the tropical Pacific from 30N to 30S. The loading patterns shown here resemble the seasonal composite anomaly fields of Year 1 in Rasmusson and Carpenter (1982).

MEI anomaly maps for the latest season

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With the overall MEI indicating now indicating ENSO-neutral conditions, one can find opposing key anomalies in the MEI component fields that exceed or equal one standard deviation, or one sigma (compare to loadings figure). During March-April 2011-12, three each flag emerging El Niño and lingering La Niña features, respectively. Significant positive anomalies (coinciding with high negative loadings) denote enhanced southerly flow (V) northeast of New Guinea, as well as enhanced cloudiness west of the Philippines (C), both in support of lingering La Niña conditions. Enhanced easterly flow (U) along and just south of the Equator near the dateline falls into the same category. On the other hand, enhanced southerly flow (V) east of Australia, positive sea surface temperature anomalies (S) southwest of Galapagos, as well as increased cloudiness near the Peruvian coast (C) are all consistent with emerging El Niño conditions.

There are no significant sea level pressure (P) or air temperature (A) anomalies that project onto key MEI features this month.

Go to the discussion below for more information on the current situation.

If you prefer to look at anomaly maps without the clustering filter, check out the climate products in our map room.

Discussion and comparison of recent conditions with historic La Niña events

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In the context of the rapid transition from El Niño into La Niña in 2010, this section features a comparison figure with strong La Niña events that all reached at least minus one standard deviations by June-July of the first year, and a peak of at least -1.4 sigma over the course of the event. It turns out that this selection covers exactly the same strong La Niña events that were introduced earlier (1949-51, 1954-56, 1964-65, 1970-72, 1973-76, and 1988-89), in addition to monitoring the lingering current event. The most recent moderate La Niña events of 1998-2001 and 2007-09 did not qualify, since they either did not reach the required peak anomaly (the first one) or became strong too late in the calendar year (both).

The updated (March-April) MEI registers at +0.06, a large one-month increase of +0.47 sigma that is the 7th highest such increase for this time of year since 1950, which also constitutes the 3rd monthly rise of this caliber in a row. In the MEI record, there are only two years with similar or higher-ranked sequences of three consecutive monthly rises: 1972 and 1997, both ending up with strong El Niño events in those years. The current rank (32nd) of the MEI corresponds to an ENSO-neutral ranking. The 2010-11 portion of this La Niña event had been classified as strong (top 6 rankings since 1950) from July-August 2010 through March-April 2011, tied with 1975-76 for 2nd place in terms of strong duration, and only behind 1955-56 (15 months). The 2011-12 portion of this event only reached moderate levels (top 12 ranking) one time three months ago.

Negative SST anomalies are still covering some of the central tropical Pacific in the latest weekly SST map. However, they have weakened further, and are now smaller in amplitude than positive SST anomalies of over +1C from about 110W eastward to the South American coast, possibly indicating the early stages of a developing El Niño event.

For an alternate interpretation of the current situation, I highly recommend reading the latest NOAA ENSO Advisory which represents the official and most recent Climate Prediction Center opinion on this subject. In its latest update (May 3rd, 2012), La Niñ conditions are officially declared over, with ENSO-neutral conditions expected to continue through northern summer of 2012. This is consistent with everything discussed above.

There are several other ENSO indices that are kept up-to-date on the web. Several of these are tracked at the NCEP website that is usually updated around the same time as the MEI, just in time for this go-around. In 2010, Niño regions 3 and 3.4 dropped below -0.5C by early June, reaching -1.5C by September 2010, and remaining below that threshold for five months, one month longer than in 2007-08, and longer than in any La Niña event since 1988-89. After a full three months closer to 0C than -0.5C from May through July 2011, La Niña conditions re-emerged and peaked from November 2011 through January 2012, with Niño 3.4 SST anomalies around -1.0C, while Niño 3 SST anomalies peaked in November with -1.1C. Since then, Niño 3.4 (3) SST anomalies have weakened to -0.4C (+0.1C).

For extended Tahiti-Darwin SOI data back to 1876, and timely monthly updates, check the Australian Bureau of Meteorology website. This index has often been out of sync with other ENSO indices in the last decade, including a jump to +10 (+1 sigma) in April 2010 that was ahead of any other ENSO index in announcing La Niña conditions. It continued around +20 from July 2010 through April 2011. The last (and only) time that this index showed higher values for the average of any ten months running was during 1917(!), so any SOI-based classification would classify this event as the second-strongest event of the last century. Since July 2011, it oscillated around +10 through January of 2012, reaching a very impressive +23 in December 2011, but falling all the way to +3 in February and remaining there in March 2012, its lowest value since last August. In April 2012, this index dropped another standard deviation, and now stands at -7, its lowest value since March 2010. Overall, the reputation of the SOI as being a 'noisy' ENSO index remains well deserved.

An even longer Tahiti-Darwin SOI (back to 1866) is maintained at the Climate Research Unit of the University of East Anglia website, however with less frequent updates (currently stuck in March 2010). Extended SST-based ENSO data can be found at the University of Washington-JISAO website, currently updated through January 2011, showing the 6th month in a row running below -1C at that time.

Stay tuned for the next update by mid-June 2012 (this is in flux, hopefully sooner) to see where the MEI will be heading next. La Niña has gone through a second-winter stage similar to 2008-09, and consistent with expectations formulated right here in late 2010: big La Niña events have a strong tendency to re-emerge after 'taking time off' during northern hemispheric summer. Based on the evolution of recent atmosphere-ocean conditions, the MEI has obviously already reached ENSO-neutral conditions, with little chance of dropping back into La Niña conditions any time soon. As stated two months ago, there is a distinct possibility that we could see a switch to El Niño during the next few months. However, all multi-year La Niña events of the last 13 years have shown a tendency to weaken or even disappear during this time of year (as in 2000, 2001, 2008, 2009, and 2011), with only 2009 showing a clear-cut switch to El Niño by the summer of that year.

As noted before, all of the ten two-year La Niña events between 1900 and 2009 either continued as a La Niña event for a third year (four out of ten), or switched to El Niño (six out of ten), with none of them ending up as ENSO-neutral. The year 2012 promises to remain "interesting".

MEI data access and publications

You can find the numerical values of the MEI timeseries under this link, and historic ranks under this related link.

If you have trouble getting the data, please contact me under (Klaus.Wolter@noaa.gov)

You are welcome to use any of the figures or data from the MEI websites, but proper acknowledgment would be appreciated. Please refer to the (Wolter and Timlin, 1993, 1998) papers below (available online as pdf files), and/or this webpage.

In order to access and compare the MEI.ext against the MEI, go here.

Publications

  • Rasmusson, E.G., and T.H. Carpenter, 1982: Variations in tropical sea surface temperature and surface wind fields associated with the Southern Oscillation/El Niño. Mon. Wea. Rev., 110, 354-384. Available from the AMS.
  • Wolter, K., 1987: The Southern Oscillation in surface circulation and climate over the tropical Atlantic, Eastern Pacific, and Indian Oceans as captured by cluster analysis. J. Climate Appl. Meteor., 26, 540-558. Available from the AMS.
  • Wolter, K., and M.S. Timlin, 1993: Monitoring ENSO in COADS with a seasonally adjusted principal component index. Proc. of the 17th Climate Diagnostics Workshop, Norman, OK, NOAA/NMC/CAC, NSSL, Oklahoma Clim. Survey, CIMMS and the School of Meteor., Univ. of Oklahoma, 52-57. Download PDF.
  • Wolter, K., and M. S. Timlin, 1998: Measuring the strength of ENSO events - how does 1997/98 rank? Weather, 53, 315-324. Download PDF.
  • Wolter, K., and M. S. Timlin, 2011: El Niño/Southern Oscillation behaviour since 1871 as diagnosed in an extended multivariate ENSO index (MEI.ext). Intl. J. Climatology, 31, 14pp., in press. Available from Wiley Online Library.

Questions about the MEI and its interpretation should be addressed to:
(Klaus.Wolter@noaa.gov), (303) 497-6340.