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 6th, 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 along and just south of the Equator, corresponding to westerly anomalies near the dateline. Negative loadings north of Indonesia indicate easterly anomalies during El Niño at this time of year. The meridional wind field (V) features high negative loadings north of the Equator across the Central Pacific basin, denoting the southward shift of the ITCZ so common during El Niño-like conditions, juxtaposed with high positive loadings northeast of Australia as well as over the South China Sea (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 eastern equatorial Pacific, as opposed to decreased cloudiness from the Philippines to Hawai'i and beyond.

The MEI continues its retreat from its seasonal peak explained variance (now down to 20%) of all six fields in the tropical Pacific from 30N to 30S. This is 0.7% higher than 12 months ago, but 2.6% 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. 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 continued strong El Niño conditions, one can find a shrinking 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 to Australia, strong southerly anomalies (V) over the South China Sea as well east of Australia, very high sea surface (S) and air temperatures (A) anomalies over the central and eastern equatorial Pacific, and enhanced cloudiness (C) over the eastern equatorial Pacific. Significant negative anomalies (coinciding with high negative loadings) flag strong easterly anomalies (U) north of Indonesia, and strong northerly wind anomalies (V) across the northern central equatorial Pacific. Compared to last month, these significant anomalies have either held steady or weakened a bit. All six fields are still showing key anomalies in excess of one standard deviation, three of them still at two sigma or more (P, S, A).

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 (March-April) MEI has stabilized (up by 0.11) at +2.07, continuing just below a Top-3 ranking for the second month in a row. The preceding nine-month run in the Top-3 is tied with 1982-83 for its duration, while 1997-98 kept this level going for a full 12 months. No other El Niño since 1950 even exceeded three months at that level. The August-September 2015 MEI of +2.53 represents the peak of the 2015-16 event, and was exceeded only during the 1982-83 and 1997-98 events. The overall evolution of the 2015-16 El Niño has been most similar to 1997-98, as monitored by the MEI.

Looking at the nearest 6 rankings (+3/-3) in this season gives us three El Niño cases that more or less kept going into the following calendar year ('58, '87, and '92), two that eased into a neutral state ('83 and '93), and one that switched over to La Niña ('98). Perhaps the odds of a transition to La Niña are still enhanced compared to climatology, givent that 1998 appears to be the best analogue year, and considering that this event has now lasted more than one year, but one should not discount the possibility of a neutral outcome. Meanwhile, I believe that general El Niño conditions (rankings in top 30%) are still more likely than not through May-June, in the MEI sense.

Positive SST anomalies cover much of the eastern equatorial Pacific, but cold anomalies have now emerged right along the Equator east of 130W, as seen in the latest weekly SST map. This is one of the more clear-cut cases where the bimonthly assessment in the MEI sense cannot keep up with the faster changes underway now (if they are to continue).

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 (April 14th, 2016), El Niño conditions were diagnosed, but were expected to transition to ENSO-neutral by late boreal spring or early summer. I may disagree with the timing a little, but then Niñ 3.4 SST tends to transition faster than the MEI in similar situations such as 1998.

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. Starting in 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, and peaking at 2.95C in November. It has dropped steadily since then, to +2.8C in December, +2.6C in January, +2.4C in February, accelerating downward in March with +1.7C, and reaching +1.1C in April which has finally dropped below the 'strong' threshold (+1.5C) in CPC's El Niño classification. 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 kept the November Niño 3.4 anomaly quite a bit lower (+2.37C), also a new record, if only by 0.05C. It has dropped to +1.2 in April, more or less catching up with the OI SST. For comparison, Niño 3 OI SST crested at +2.9C in November, dropped slowy to +2.6C in January, followed by a rapid decline all the way to +0.9C in April. Note that this decline still allowed for the full SST to peak at +28.7C in March in this region, the highest temperature for this event. In any case, the Niño 3 anomalies were 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, the current event appears to have been more powerful at its peak than based on the MEI (or Niño 3 SST).

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 2015 (-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 and February, somewhat akin to what happened from late 1997 into early 1998. In March 2016, it weakened to -5, followed by -22 in April. 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 more than five years behind current conditions.

Stay tuned for the next update by about June 4th (if input data is updated in timely fashion) to see where the MEI will be heading next. El Niño conditions in the MEI sense are more probable than not for another month or two, due to off-equatorial anomalies that will take a bit longer to change than right along the Equator. In addition, typical El Niño impacts will be supported by positive PDO conditions that have endured for more than two years, reaching record levels from December 2014 through February 2015, and again in March 2016 (tied with 1941). Daily updates of the ENSO status can be found at the TAO/TRITON website, confirming the recent emergence of eqautorial cold anomalies east of 130W.


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.