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 February 8th, 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 REVISED figure now 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. Click on the "Discussion" button below to find the comparison of the currently rebounding 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 REVISED figure now 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 currently rebounding 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 the Equator, corresponding to easterly anomalies near the dateline. Negative loadings in the far western and eastern Pacific indicate an almost equal area covered by westerly anomalies during La Niña. The meridional wind field (V) features high negative loadings north of the Equator, denoting the northward shift of the ITCZ so common during La Niña conditions, juxtaposed with even stronger 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 the dateline, or cold anomalies during a La Niña event. To its southwest, negative loadings flag warm anomalies during La Niña in the southcentral subtropical Pacific, while weaker loadings over the north-central Pacific complete a horse-shoe-like pattern. At the same time, total cloudiness (C) tends to be decreased over the central equatorial Pacific, juxtaposed with increased cloudiness from northern Australia northwards into the Philippines and towards Japan, as well as the eastern-most equatorial Pacific.

Now just one month past its annual peak, the MEI stands for 31.7% of the explained variance of all six fields in the tropical Pacific from 30N to 30S. Although its temperature components dominate the MEI with over 40% of their possible variance, even P, V, as well as U and C join in with about a third, a quarter, and twice with a fifth of their variance, respectively, at or close to their peak values during the year. Fourteen years ago, when the MEI was in its first year on the Internet, the explained variance for Dec-Jan 1950/51-1997/98 amounted to 32.7%. This slight drop-off reflects the diminished coherence and importance of ENSO events in much of the recent 14 years, bottoming out three years ago with 30.9%. 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 moderate La Niña conditions, there are quite a few key anomalies in the MEI component fields that exceed or equal one standard deviation, or one sigma (compare to loadings figure). During December-January 2011-12, all of them flag La Niña features. Significant positive anomalies (coinciding with high negative loadings) denote increased sea level pressure (P) over the eastern (sub-)tropical Pacific, strong westerly wind anomalies (U) over western Indonesia, southerly wind anomalies (V) northeast of Galapagos, and positive sea surface temperature anomalies (S) east of Australia in the south-central subtropical Pacific. Significant negative anomalies (coinciding with high positive loadings) denote decreased sea level pressure (P) northwest of Australia, reaching strong proportions (+2 sigma or more) over the eastern Indian Ocean, strong easterly anomalies (U) near the dateline, strongly increased northerly anomalies (V) northeast of Australia, and a small remnant of negative air temperature anomalies (A) in the eastern tropical Pacific. There are no significant cloudiness 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 currently unfolding 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 (December-January) MEI continues just below -1.0 sigma, its lowest value since March-April 2011. That particular season had been the last one of 10 in a row to exceed -1 sigma (numerical values are here ). The current rank (11th) of the MEI indicates moderate-size La Niña rankings . 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).

Negative SST anomalies are covering much of the central and eastern tropical Pacific in the latest weekly SST map. Many of these anomalies reach in excess of -1C from about 165E to the coast of South America, with a particular focus on the dateline region since late December 2011. Positive SST anomalies near the Equator remain absent from the eastern and central Pacific.

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 (5 January 2012), La Niñ conditions are diagnosed and expected to continue into the boreal spring of 2012. This is consistent with everything discussed here.

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, but never higher than -2C. One only has to go back to the La Niña winter of 2007-08 to find seasonal Niño 3.4 anomalies that were more extreme than in 2010-11, hence the reluctance of Niño 3.4-based classifications to call this event a 'strong' event. Nevertheless, monthly anomalies for this particular index exceeded -1.5C for five months (between September 2010 and January 2011), one month longer than in 2007-8, and longer than in any La Niña event since 1988-89, or more than two decades ago. In the last three months, Niño 3.4 SST anomalies have remained just slightly below -1.0C, while iño 3 SST anomalies peaked in November with -1.1C, only to drop back to -0.8C by January 2012.

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. After a drop to +2 in June 2010, July rebounded to +20.5, followed by nine monthly values between +16 (November) and +27 (December), including +25 in 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. In May 2011, it dropped by 2.3 standard deviations to +0.2 sigma (or +2), consistent with the rapid weakening of the MEI. This was followed by further drop to +0.0 sigma in June, but a noteworthy rebound to +11 in July 2011, and oscillations around that value through November, followed by a very impressive +23 in December 2011, identical to the December 1950 value, and exceeded since then only in 2010. In January 2012, it fell back to +9, just shy of one sigma. 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 March 10th, or earlier - ICOADS appears to be less threatened for now) to see where the MEI will be heading next. La Niña has staged a comeback similar to 2008-09, and consistent with expectations formulated right here well over a year ago: big La Niña events have a strong tendency to re-emerge after 'taking time off' during northern hemispheric summer. Based on current atmosphere-ocean conditions, I believe the odds for this La Niña event to continue right through early summer (June-July 2012) are just about 50%. Beyond that, it is worth noting that four of the ten two-year La Niña events between 1900 and 2009 ended up as a three-year event, so I would put the odds for this to occur in 2012-13 at 40% right now. The remaining six cases all switched to El Niño, leaving not a single ENSO-neutral case. The year 2012 promises to remain "interesting". If and when something new transpires on the fate of ICOADS and the MEI, I will communicate it right on this webpage.

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.