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 July 3rd, 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 recent 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 recent El Niño conditions with the same seven historic El Niño 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 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 moderate positive loadings centered along the Equator, corresponding to westerly anomalies near the dateline. Moderate negative loadings over Indonesia and northeast over the Maritime Continent as well as off the Colombian coast indicate easterly anomalies during El Niño at this time of year. The meridional wind field (V) features moderate negative loadings north of the Equator in the central and eastern Pacific basin, denoting the southward shift of the ITCZ so common during El Niño-like conditions, juxtaposed with moderate 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 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 east of Hawai'i and northeast of Galapagos.

The MEI now just stands for 17.6% of the explained variance of all six fields in the tropical Pacific from 30N to 30S, right at its annual minimum of importance. Eighteen years ago, right after the MEI was introduced to the internet, the explained variance for May-June 1950-1998 amounted to 21.0%. This drop-off by more than 3% reflects the diminished coherence and importance of ENSO events in much of the recent 18 years. 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 still hanging on to El Niño conditions, one can still find several key anomalies in the MEI component fields that exceed or equal one standard deviation, or one sigma (compare to loadings figure). All but one of them flags El Niño rather than La Niña conditions.

Significant positive anomalies (coinciding with high positive loadings) indicate high sea level pressure anomalies (P) from the Maritime Continent to Australia, high sea surface (S) and air temperatures (A) anomalies south of the Equator in the tropical eastern Pacific, and enhanced cloudiness (C) over the central equatorial Pacific. Significant negative anomalies (coinciding with high negative loadings) flag decreased cloudiness (C) east of Hawai'i and northeast of Galapagos. Compared to last month, most anomalies have weakened. Only four fields (P,S,A,C) are still showing key anomalies in excess of one standard deviation, with only one of them (C) still at two sigma or more.

On the other hand, decreased cloudiness (C) over the southeastern tropical Pacific, while small in areal coverage, is more consistent with La Niña than El Niño.

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 (May-June) MEI has dropped to +1.03 (down by 0.67 since last month), still a moderate El Niño ranking. The recent nine-month run in the Top-3 from May-June 2015 through January-February 2016 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 continues to be most similar to 1997-98, as monitored by the MEI.

Looking at the nearest 12 rankings (+6/-6) in this season AND requiring that the MEI has dropped by at least 0.5 in the previous three months gives us exactly one 'analogue': 1998, which quickly transitioned to La Niña a few months later. All other 11 nearest-ranked cases remained either El Niño or ended up ENSO-neutral by the end of the calendar year. While the 2015-16 El Niño is clearly weakening fast, an ENSO-neutral outcome is still not completely off the table during the remainder of 2016.

Positive SST anomalies cover much of the off-equatorial tropical Pacific, but cold anomalies are present right along the Equator east of 160W, as seen in the latest weekly SST map. This remains 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 (9 June 2016), El Niño conditions were considered replaced by ENSO-neutral conditions as of the end of May, and a "La Niña" watch was hoisted for boreal fall and winter 2016-17 with 75% odds. This is consistent with most forecast models, although only a few of them reach the moderate La Niña threshold (-1.0C) for Niñ 3.4 SST anomalies.

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. Unless otherwise noted, I refer to the OISSTv2 anomalies in this discussion. 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, reaching +1.1C in April, and dropping below +0.5C in May with +0.3C. 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 dropped to +1.1 in April, catching up with the OI SST, but is now a bit higher than OI SST in May with +0.6C. For comparison, Niño 3 OI SST crested at +2.9C in November and December, dropped slowly to +2.6C in January, followed by a rapid decline all the way to +0.0C in May. 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, and back above 0 in both May (+3) and June 2016 (+6) for the first time for two months since April-May 2014.

The next update may be as late as mid-August 2016 - data ingest via ICOADS is going through a transition right now, even though I was able to get this one in unexpectedly early. El Niño conditions in the MEI sense are waning fast, but lingering El Niño impacts may 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 from March 2016 (tied with 1941) through May 2016. 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, but a lack of easterly trade wind anomalies (so far).


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.