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 September 8th, 2017)

This webpage consists of seven main parts, three of which are updated every month:

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. UPDATED MEI loading maps for the latest season;

5. UPDATED MEI anomaly maps for the latest season;

6. UPDATED 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).

NEWSFLASH: Processing of ICOADS was delayed by more than three weeks in December 2016. We are working with NCEI to reduce the risk of similar delays in the future. Subsequent updates for monthly data have typically become available before the 10th of the month. The extra long delay in August 2017 had nothing to do with NCEI, but personal reasons..

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 seven), 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 seven rankings) just once. Click on the "Discussion" button below to find a comparison of 2015-17 mostly El Niño conditions with historic strong El Niño events.


Historic El Niño events since 1950

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How does the 2015-16 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 seven), and 2015-16 has replaced the 2009-10 event that reached the top seven ranking only twice. Visual inspection reveals that the 2015-16 event was slightly weaker than 1982-83 and 1997-98, resembling the latter more than the former in its evolution. Click on the "Discussion" button below to find a comparison of recent conditions with analogous situations.


MEI loading maps for the latest season

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The six loading fields show the correlations between the local anomalies of each field 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 west of the dateline. In contrast, significant negative loadings cover the easternmost Pacific off the Central American coast, denoting easterly anomalies during El Niño at this time of year. The meridional wind field (V) features its biggest 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. Negative loadings north and east of Australia contribute to the overall temperature pattern (for S and A). 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 23.1% of the explained variance of all six fields in the tropical Pacific from 30N to 30S, having regained almost 6% since May-June. Twenty years ago, right after the MEI was introduced to the internet, the explained variance for July-August 1950-1997 amounted to 26.5%. This drop-off by more than 3% reflects the diminished coherence and importance of ENSO events in much of the recent two decades. 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 now showing ENSO-neutral conditions, four out of eight key anomalies in the MEI component fields presently flag El Niño, while the other four flag La Niña. Key anomalies refer to values in excess of one standard deviation, or one sigma in support of either ENSO phase (compare to loadings figure).

Starting with the lingering El Niño features, significant positive anomalies (coinciding with high positive loadings) flag anomalously high sea level pressure (P) over the Maritime Continent, while anomalously warm air temperatures (A) are still found along the Equator south of Hawai'i. Significant negative anomalies translate into northerly wind anomalies (V) south of Hawai'i and reduced cloudiness (C) near Galapagos. On the other side of the ledger (La Niña), negative zonal wind anomalies (U) near the Equator and northeast of Australia flag enhanced easterlies, while positive anomalies south of Mexico indicate anomalous westerlies. Unusual warmth northeast of Australia (both in A and S) also flag emerging La Niña conditions.

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

If you prefer to look at anomaly maps without the clustering filter (which is most limiting for the cloudiness field), 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 yet another aborted El Niño event, this section features a comparison figure with six other short-lived events during the MEI period of record. Only one of them occurred before 1980, while 2017 joins both 2014 and 2012 in a recent flurry of events that lasted five bimonthly seasons or less, with all of them ending before September-October.

Compared to last month, the updated (July-August) MEI has dropped another 0.43 standard deviations from 0.46 to 0.03, reaching just about the most ENSO-neutral a href="rank.html"> rankings possible.

Looking at the nearest 12 rankings (+6/-6) in this season, and allowing only for cases that showed similar one-month and three-month change behavior, eliminates all but seven cases: 1953, 1969, 1981, 1984, 1990, 1995, and 2016 (three of these are also included in the comparison figure). Subsequently, most of these years stayed ENSO-neutral, but two became borderline La Niña events a few months later: 1984-85 and 1995-96. Note that 2016-17 never reached La Niña status in the MEI sense. All in all, ENSO-neutral appears to be the safest bet for the near future, as measured by the MEI.

Positive SST anomalies have been replaced by weak negative SST anomalies over much of the central and eastern equatorial tropical Pacific, as seen in the latest weekly SST map.

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 (10 August 2017), ENSO-neutral conditions are diagnosed, and favored to continue through boreal winter 2017-18, in agreement with the MEI-based assessment above.

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, in time for this one. After a short-lived stretch of Niño region 3.4 SST anomalies near +0.5C in May and June 2017, this index dropped to +0.4C in July, and further to -0.15C in August. Whie a major drop in temperature, this is not as dramatic as last year's drop from +0.3C in June to -0.5C in August. For comparison, Niño region 3 SST nurtured +0.5C anomalies from February through May 2017, dropping to -0.2C in August. p>For extended Tahiti-Darwin SOI data back to 1876, and timely monthly updates, check the Australian Bureau of Meteorology website. In 2017, this index oscillated around 0 through May, only to drop to -10 (-1 sigma) in June, corresponding to El Niño conditions for just one month, but rose to +8 in July and back down to +3 in August, thus indicating ENSO-neutral once again.

The next update for the MEI is expected on or before Ocotber 7. ENSO-neutral conditions have reasserted themselves at a time of year when persistence often takes over. Meanwhile, the PDO shows its lowest value since December 2013 (+0.1) in July, flagging PDO-neutral conditions rather than the prolonged positive mode that dominated the last 3.5 years. Daily updates of the ENSO status can be found at the TAO/TRITON website, showing weak La Niña-like conditions over the equatorial eastern Pacific, promising an interesting update in October.


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., 1074-1087. Available from Wiley Online Library.


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