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 August 4th, 2015)

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 2010-12 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), after replacing the slightly weaker 2007-09 event with 2010-12 (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 lasted through most 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 a comparison of recently strengthening El Niño conditions with similar historic situations.


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 a comparison of recently strong El Niño conditions with the same seven historic events. Once the 2015-16(?) event is over, that figure will replace the current one in this location.


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: high pressure anomalies in the west and low pressure anomalies in the east correspond to positive MEI values, or El Niño-like conditions. Consistent with P, U has positive loadings centered along the Equator, corresponding to westerly anomalies near the dateline. Negative loadings south of Mexico indicate easterly anomalies during El NiƱo at this time of ear. The meridional wind field (V) features negative loadings north of the Equator across the eastern Pacific basin, denoting the southward shift of the ITCZ so common during El Niño-like conditions, juxtaposed with large positive loadings northeast of Australia (southerly anomalies during El Niño).

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 warm anomalies during an El Niño event. At the same time, total cloudiness (C) tends to be increased over the central and western equatorial Pacific (mainly east of Indonesia), while the easternmost Pacific is often less cloudy than normal east of Galapagos.

The MEI now stands for 20.4% of the explained variance of all six fields in the tropical Pacific from 30N to 30S, one month after its annual minimum of importance. Seventeen years ago, right after the MEI was introduced to the internet, the explained variance of the MEI for June-July 1950-1998 amounted to 23.3%. This drop-off by almost 3% reflects the diminished coherence and importance of ENSO events in much of the recent 17 years, although it has regained 0.5% compared to last year's explained variance. The loading patterns shown here resemble the seasonal composite anomaly fields of Year 0 in Rasmusson and Carpenter (1982).


MEI anomaly maps for the latest season

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With the MEI indicating continued strong El Niño conditions, one can find a long list of key anomalies in the MEI component fields that exceed or equal one standard deviation, or one sigma (compare to loadings figure). Every one of them flags El Niño rather than La Niña conditions.

Significant positive anomalies (coinciding with high positive loadings) indicate high sea level pressure (P) northwest of Australia, strong westerly wind anomalies (U) near the equatorial dateline, southerly wind anomalies (V) northeast of Australia, very high sea surface (S) and air temperatures (A) over the central and eastern equatorial Pacific, as well as strongly enhanced cloudiness (C) over the south-central tropical Pacific. Significant negative anomalies (coinciding with high negative loadings) flag increased easterlies (U) off the Central American coast, increased northerlies (V) over the same region, unusually cold SST (S) east of Australia, and decreased cloudiness (C) over the same region as the enhanced trade winds off the Central American coast. Aside from a weakened Southern Oscillation (P) pressure gradient across the Pacific Basin, all of these anomalies are as strong or stronger than the ones described last 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 El Niño conditions

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In the context of strong El Niño conditions lasting continuously since March-April 2015, this section features a comparison figure with the classic set of strong El Niño events during the MEI period of record.

While the updated (June-July) MEI has dropped slighty (by 0.09 standard deviations in one month) to +1.97, it is now reaching the 2nd highest ranking, surpassed only by 1997 at this time of year. The MEI has hovered around +2 standard deviations for two months running, highest overall since early 1998.

Looking at the nearest 6 rankings (+1/-5) in this season, and excluding cases with declining June-July values compared to earlier in the year gives us five 'analogues' to ponder: 1965, 1972, 1982, 1987, and 1997. All five of them maintained strong El Niño status through at least December of their respective first calendar years. Both 1982-83 and 1997-98 can be classified in a separate 'Super El Niño' category, since they are the only events to reach +3 standard deviations at their respective zeniths, compared to barely above +2 standard deviations for the runner-ups in 1987 (and 1992), as well as 2015 so far.

Positive SST anomalies continue in the eastern equatorial Pacific, all the way from just west of the dateline to the South American coast, as seen in the latest weekly SST map. This includes anomalies in excess of +2C east of 150W, and above +3C near Galapagos.

For an alternate interpretation of the current situation, I recommend reading the NOAA ENSO Advisory which represents the official and most recent Climate Prediction Center opinion on this subject. In its latest update (July 9th, 2015), El Niño conditions were diagnosed, and expected to continue through the rest of 2015 with a greater than 90% chance. I see no reason to disagree with this assessment.

There are a number of 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, but not in time for this go-around. Since October 2014, Niño region 3.4 first hovered around +0.5C, but rose steadily from April onwards, reaching +1.3C in June, and +1.7C in weekly SST since mid-July. Niño region 3 dropped out of weak El Niño conditions from January through March 2015, but has risen to almost +1.7C in June, and weekly SST anomalies at or above +2C in July 2015.

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. In 2015, its value varied from +1 in February (neutral ENSO conditions) down to -11 in March, up again to -4 in April, back down to -14 in May, followed by -12 in June, and -15 in July. The last time this SOI showed three monthly values in a row below -10 was in early 2010. While the SOI remains a noisy ENSO index, its running two-month average has now reached its lowest value since 2005.

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, with the last one to include data through 2014. Extended SST-based ENSO data can be found at the University of Washington-JISAO website, which is now a full four years behind schedule (through January 2011).

Stay tuned for the next update by September 5th (or earlier) to see where the MEI will be heading next. El Niño conditions are guaranteed to persist into the upcoming boreal winter season, most likely at strong levels for much of that period. Whether it will reach the elusive 'Super El Niño' level remains to be seen. In addition, typical El Niño impacts will be supported by positive PDO conditions that have endured since January 2014, reaching record levels from December through February 2015. Daily updates of the ENSO status can be found at the TAO/TRITON website, showing continued westerly wind anomalies near the dateline to strengthen current El Niño conditions even further.


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.