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, 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. In fac, subsequent updates for monthly data became available by the 7th of the month, hopefully to be sped up further in the future.
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 2015-16 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 seven), with the exception of the 2009-10 event that reached the top seven 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 2015-16 El Niño conditions with the same seven historic El Niño events. By August 2017, 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 mostly 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 peak explained variance (now down to 20.1%) of all six fields in the tropical Pacific from 30N to 30S. This is 2.5% lower than 19 years ago during the extra strong El Niño of 1998, 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 showing a sudden foray into El Niño territory, one would expect a clear majority of key anomalies in the MEI component fields to indicate the same. Instead, we are seeing an equal number of key anomalies in excess of one standard deviation, or one sigma in support of either ENSO phase (compare to loadings figure).
Significant positive anomalies (coinciding with high positive loadings) flag warm sea surface (S) and air (A) temperature anomalies in the eastern equatorial Pacific flag emerging El Niño conditions, quite similar to last month. On the other hand, significant positive anomalies (coinciding with high negative loadings indicate high sea level pressure (P) anomalies over the eastern equatorial Pacific and highly increased cloudiness (C) near the Philippines. Both of these anomalies indicate 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 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 (March-April) MEI has increased by 0.85 standard deviations from -.08 up to 0.77, jumping up from ENSO-neutral to moderate El Niño rankings. In fact, this increase is the largest on record for this time of year (and since 1950), bringing the recent ENSO-neutral spell to at least a temporary end.
Looking at the nearest 12 rankings (+6/-6) in this season, and eliminating those cases when the previous three months saw a drop in the MEI, we find that five of the remaining eight cases ended up with an El Niño event by the end of the calendar year, or more than twice the climatological odds (30%). The other three cases (1953, '80, and '81) dropped back to ENSO-neutral, while none of them flipped into La Niña conditions (1953 did that in the following year).
Positive SST anomalies cover the eastern equatorial tropical Pacific, especially near the South American coast, while cold anomalies (below -0.5C) have disappeared altogether from the 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 (13 April 2017), ENSO-neutral conditions were diagnosed, were favored to continue through boreal spring, with increasing chances for El Niño by late summer and fall. It will be interesting to see what the May update says.
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. Starting in May 2016, Niño region 3.4 SST anomalies dropped below +0.5C for the first time since late 2014. They reached -0.5C in July, and appear to have peaked with -0.7C in October, continued below -0.5C through November (5 months total), weakened further to -0.3C in January, and finally crossed over to +0.1C in both February and March, rising to +0.3C in April. Meanwhile, Niño region 3 SST never crossed over i the -0.5C threshold, but moved up above +0.5C in February, hovering around +0.6C since then.
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 2016, its value varied from -22 in April to +14 in September (the only month with full-blown La Niña conditions in 2016), fell again to -4 in October, and oscillated between -2 (in February) and +5 (March) since then, only to drop back to -6 in April, hinting at developing El Niño conditions.
The next update for the MEI will hopefully take place before June 10. ENSO-neutral conditions may have been replaced by a fledgling El Niño, too early to tell. Meanwhile, the PDO continues in mild positive territory (0.7-0.8 from January through March), thus not interfering with El Niño-typical impacts, should it take hold. Daily updates of the ENSO status can be found at the TAO/TRITON website, confirming the recent disappearance of equatorial cold anomalies, and the replacement of easterly wind anomalies by weak westerly anomalies near the dateline.
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.