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 12th, 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 positive loadings centered along the Equator, corresponding to westerly anomalies near the dateline. Negative loadings south of Mexico indicate easterly anomalies during El Niño at this time of year. The meridional wind field (V) features moderate 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 20.1% of the explained variance of all six fields in the tropical Pacific from 30N to 30S, one month after its annual minimum of importance. Eighteen years ago, right after the MEI was introduced to the internet, the explained variance for June-July 1950-1998 amounted to 23.3%. 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 having finally dropped into the ENSO-neutral range, 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). Three of them flag El Niño, and three of them flag La Niña conditions.
Significant positive anomalies (coinciding with high positive loadings) indicate high sea surface temperatures (S) east of Tahiti, while enhanced cloudiness (C) hangs in over the central equatorial Pacific. Significant negative anomalies (coinciding with high negative loadings) flag reduced cloudiness (C) east of Galapagos. These three significant anomalies indicate remnant El Niño conditions. Significant positive anomalies (coinciding with high negative loadings) indicate southerly wind anomalies (V) south of Hawai'i, and high sea surface (S) and air temperatures (A) anomalies east of Australia. These three anomalies indicate emerging La Niña conditions. Compared to last month, most El Niño-like anomalies have weakened. Both sea level pressure (P) and zonal wind (U) show no key anomalies at all in excess of one standard deviation.
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 from April-May 2015 through April-May 2016, 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 (June-July) MEI has dropped further to 0.31 (down by 0.69 since last month), which translates into the first ENSO-neutral rankings since January-February 2015. The 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 1.0 in the previous three months gives us exactly one 'analogue': 1998, which quickly transitioned to La Niña a few months later. In fact, that three-month drop in 1998 was the biggest on record for this time of year (-2.34), while this year's drop ranks 3rd with -1.76). Among the other 11 nearest-ranked cases for June-July, three ended up with El Niño conditions by the end of the year (but were already rising from March-April to June-July), seven ended up ENSO-neutral, and one dropped (back) to La Niña by the end of 2008. Based on the continued high similarity between 2015-16 and 1997-98, the odds of La Niña are clearly higher than suggested by those 12 nearest-ranked cases. Another way of picking analogues would be to look at other springs with large three-month drops in the MEI: among the five highest drops since 1950, four ended up as La Niña by the end of the year (in addition to '98, this happened in '73, '88, and 2010), while one (1992) bucked this tendency and recovered to El Niño conditions. Either way, a La Niña appears more likely than discussed earlier this year.
Positive SST anomalies cover much of the off-equatorial tropical Pacific, but cold anomalies are present right along the Equator east of 170W, 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 (11 August 2016), ENSO-neutral conditions were diagnosed and a La Niña watch is in place, with odds pegged at 55-60% for La Niña for the upcoming boreal fall and winter. I am comfortable with those odds for now.
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 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. Since then it has dropped even further to -0.5C in July. 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). For comparison, Niño 3 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, and -0.5C in July. Peak Niño 3 anomalies were quite a bit lower than 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 May (+3), June (+6), and July (+4) for the first time for three months in a row since November 2013 through January 2014.
The next update for the MEI will hopefully be a bit earlier in September than in August 2016 - data ingest via ICOADS has gone through a transition that should make it faster next month. El Niño conditions in the MEI sense have been replaced by ENSO-neutral conditions, although lingering El Niño impacts may be supported by positive PDO conditions that have been in place for more than two years now, reaching record levels from iDecember 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 especially east of 150W, but rather anemic easterly trade wind anomalies (so far).
MEI data access and publications
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.
- 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.