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 5th, 2016)

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).

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 strong 2015 El Niño conditions with historic strong El Niño 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 a comparison of strong 2015 El Niño conditions with the same seven historic events. Once the 2015-16(?) event is over, the comparison figure with 2015-16 will replace the current one with 2009-10.


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 mostly along the Equator, corresponding to westerly anomalies near the dateline. Negative loadings in the far western and eastern Pacific indicate an almost equal area covered by easterly anomalies during El Niño. The meridional wind field (V) features high negative loadings north of the Equator, flagging the southward shift of the ITCZ so common during El Niño-like conditions, juxtaposed with even stronger 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 the dateline, or warm anomalies during an El Niño event. To the southwest, negative loadings flag cold anomalies during El Niño in the southcentral subtropical Pacific, while weaker negative loadings over the northcentral Pacific complete a horse-shoe-like pattern. At the same time, total cloudiness (C) tends to be increased over the central equatorial Pacific, sandwiched in between decreased cloudiness from northern Australia northwards into the Philippines and towards Japan, and the easternmost equatorial Pacific.

Now just one month past its annual peak, the MEI stands for 32.0% of the explained variance of all six fields in the tropical Pacific from 30N to 30S. This is 1.3% higher than 12 months ago, and 0.7% lower than 18 years ago during the last extra strong El Niño, showing a lingering effect of the reduction in ENSO activity during the first decade of this century. Although the temperature components dominate the MEI with well 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 explained variance, respectively. 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 MEI indicating continuing 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 very high sea level pressure anomalies (P) from the Phillippines down to Australia, strong westerly anomalies (U) near the equatorial dateline, very high sea surface (S) and air temperatures (A) anomalies over the central and eastern equatorial Pacific, and enhanced cloudiness (C) over the central equatorial Pacific. Significant negative anomalies (coinciding with high negative loadings) flag low sea level pressure (P) near Tahiti, very strong easterly anomalies (U) over the South China Sea, as well as near the Central American coast, strong northerly wind anomalies (V) over the northern central equatorial Pacific, and much reduced cloudiness (C) over the western tropical Pacific, as well as over the easternmost equatorial Pacific. The biggest change compared to last month is a reduction in southerly flow anomalies east of Australia, but four fields (P, U, S, and A) still feature key anomalies in excess of two standard deviations.

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 strong El Niño conditions 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.

Compared to last month, the updated (December-January) MEI has increased slightly (by 0.08) to +2.20, continuing at the 3rd highest ranking, and 0.3 to 0.5 sigma behind 1998 and 1983, respectively, for this season. The August-September 2015 value of +2.53 remains the third highest overall at any time of year since 1950. The evolution of the 2015-16 El Niño remains very similar to 1997, as monitored by the MEI, including a first peak in August-September and subsequent weakening during the remainder of the calendar year. In 1998, this was followed by a second peak in late boreal winter 0.2-0.3 sigma higher than in December-January.

Looking at the nearest 6 rankings (+2/-4) in this season gives us four 'analogues' already identified three months ago: 1965-66, 1972-73, 1982-83, and 1997-98, plus 1957-58 and 1991-92. Two of these six analogues evolved into La Niña events one year later (1973-74 and '98-99), two of them ended up neutral ('83-84 and '65-66), and two hung onto weak El Niño conditions ('58-59 and '92-93). If you expand the set of analogue cases to include the six more (weaker) cases, you find a big increase in the odds for La Niña one year from now: four out of six switched to La Niña, one ended up neutral, and one continued as El Niño. Thus, it is fair to say that La Niña is more likely than not by the end of 2016, it is by no means a guaranteed outcome. Meanwhile, strong El Niño conditions (top 10% ranking) are quite likely through March-April (four of the top six cases remained in that category), while general El Niño rankings (top 30%) are still common (four out of six) through May-June, in the MEI sense.

Positive SST anomalies cover 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 above +2C from about 90-170W, and even +3C anomalies within that 'warm tongue', but not as widespread as a few months ago.

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 (January 14th, 2016), strong El Niño conditions were diagnosed, and were expected to weaken through boreal spring, with a transition to ENSO-neutral anticipated during late spring or early summer 2016. No major disagreement there.

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, just in time for this go-around. Unless otherwise noted, I refer to the OISSTv2 anomalies in this discussion, they tend to be bigger than the ERSSTv4 anomalies that are currently used by CPC. Since October 2014, Niño region 3.4 first hovered around +0.5C, but rose steadily from April 2015 onwards, reaching +1.3C in June, 2.1C in August, peaking at 2.95C in November, and dropping since then, to +2.8C in Decemberi, and +2.6C in January. The November 2015 value appears to be the highest on record for any month since 1982, exceeding December 1997 (2.7C) and January 1983 (2.8C). The ERSST4 version keeps the November Niño 3.4 anomaly quite a bit lower (+2.37C), also a new record, if only by 0.04C For comparison, Niño 3 crested at +2.93C in November and dropped to +2.85C in December, and +2.57C in January. These values are quite a bit lower than what was recorded in December 1997 or January 1983 (by 0.7C and 0.4C, respectively). Based on Niño 3.4 SST alone, the current event appears to have been more powerful at its peak than based on the MEI.

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, and back down below -10 from May through October, reaching -20 both in August and October. The running five-month average peaked in June-October (-16.5), which was the lowest since early 1998. However, the November and December 2015 SOI weakened considerably (-5 and -9), only to rebound back to -20 in January, somewhat akin to what in late 1997 and early 1998. This serves as a reminder that this index is noisier from month to month than any other ENSO index.

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 all of 2015. Extended SST-based ENSO data can be found at the University of Washington-JISAO website, which is now exactly five years behind current conditions.

Stay tuned for the next update by March 12th (probably earlier, although I will be on travel in early March) to see i where the MEI will be heading next. El Niño conditions are guaranteed to persist through the next few months, but probably slightly weaker than in 1983 or 1998. Typical El Niño impacts will be supported by positive PDO conditions that have endured since January 2014, reaching record levels from December 2014 through February 2015. Daily updates of the ENSO status can be found at the TAO/TRITON website, showing reduced coverage of +3C SST anomalies at the beginning of February 2016 compared to just a month or two ago.


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.