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 September 3rd, 2015)
This webpage consists of seven main parts:
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. MEI loading maps for the latest season;
5. MEI anomaly maps for the latest season;
6. 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 recently strengthening 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 recently strong El Niño conditions with the same seven historic events. Once the 2015-16(?) event is over, that figure will replace the current one in this location.
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 centered along the Equator, corresponding to westerly anomalies mostly west of the dateline. In contrast, significant negative loadings cover the easternmost Pacific off the Central American coast, denoting easterly anomalies during El Niño at this time ofyear. The meridional wind field (V) features its biggest negative loadings north of the Equator across the eastern Pacific basin, flagging the southward shift of the ITCZ that is 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 23.8% of the explained variance of all six fields in the tropical Pacific from 30N to 30S, having regained about 6% since May-June. Seventeen years ago, right after the MEI was introduced to the internet, the explained variance of the MEI for July-August 1950-1997 amounted to 26.5%. This drop-off by almost 3% reflects the diminished coherence and importance of ENSO events in much of the recent 17 years, although it has regained 1.0% compared to last year's minimum. 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 indicating even stronger 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) northwest of Australia, strong westerly wind anomalies (U) along the Equator, especially close to the dateline, southerly wind anomalies (V) northeast of Australia, and very high sea surface (S) and air temperatures (A) over the central and eastern equatorial Pacific. Significant negative anomalies (coinciding with high negative loadings) have become reestablished over the eastern equatorial Pacific as well as near Hawai'i for sea level pressure (P). They also flag increased easterlies (U) off the Central American coast, strongly increased northerlies (V) over the same region, anomalously cold air temperatures (A) east of Australia, and decreased cloudiness (C) near Galapagos. Compared to last month, both SLP and SST anomalies over the eastern Pacific have grown in the normalized sense.
Go to the discussion below for more information on the current situation.
If you prefer to look at anomaly maps without the clustering filter, 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 (July-August) MEI has increased significantly by 0.39 standard deviations to +2.37, or the 2nd highest ranking, surpassed only in 1997 at this time of year. This new peak value of the current event ranks third highest overall at any time of year since 1950, with 1982-83 and 1997-98 remaining in a 'Super El Niño' club of their own (for now), with peak values around +3 standard deviations.
Looking at the nearest 6 rankings (+1/-5) in this season, and excluding cases with declining July-August values compared to earlier in the year gives us the same five 'analogues' as last month: 1965, 1972, 1982, 1987, and 1997. All five maintained strong El Niño status through at least December of their respective calendar years. However, three of them ('65, '72, and '97) peaked already in July-August (the current season), followed by minor declines by the end of the year. Only 1982 showed continued growth right into the following year, while 1987 had already peaked back in May of that year.
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 90W to 170W, with a few peak values in excess of +3C.
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 (August 13th, 2015), El Niño conditions were diagnosed, and are expected to continue through the rest of 2015 with a greater than 90% chance. I see no reason to disagree with this assessment.
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. Since October 2014, Niño region 3.4 first hovered around +0.5C, but rose steadily from April onwards, reaching +1.3C in June, +1.6C, and +2.1C in August. Niño region 3 dropped out of weak El Niño conditions from January through March 2015, but quickly rose from +0.7C in April to +2.3C in August 2015.
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 since May, reaching -20 in August, its lowest value since February 2005, and its first four-month run below -10 since early 1998.
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 2014. Extended SST-based ENSO data can be found at the University of Washington-JISAO website, which is now more than four years behind current conditions.
Stay tuned for the next update by October 5th (probably earlier) to see where the MEI will be heading next. El Niño conditions are guaranteed to persist into the upcoming boreal winter season, most likely at strong levels for much of that period. Whether it will reach the elusive 'Super El Niño' level remains to be seen. In addition, 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 a renewed westerly wind surge near the dateline as of the beginning of September.
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., in press. Available from Wiley Online Library.
Questions about the MEI and its interpretation should be addressed to:
(Klaus.Wolter@noaa.gov), (303) 497-6340.