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 November 6th, 2013)
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 the recent ENSO-neutral conditions with similar historic situations.
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 the recent ENSO-neutral conditions with similar historic situations.
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 near the dateline, corresponding to westerly anomalies along the Equator from Indonesia and the Phillippines to 140W. In contrast, significant negative loadings cover the easternmost Pacific off the Central American coast as well as western Indonesia, denoting easterly anomalies during El Niño at this time of year. This is the peak season of zonal wind contributions to the MEI (more than 25% explained variance). The meridional wind field (V) features scattered negative loadings north of the Equator across the Pacific basin, flagging the southward shift of the ITCZ so common during El Niño-like conditions, juxtaposed with strong 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 the dateline, or warm anomalies during an El Niño event. Strong negative loadings north and east of Australia contribute significantly to the overall temperature pattern. At the same time, total cloudiness (C) tends to be increased over the west-central tropical Pacific and on the northeastern flank of the South Pacific Convergence Zone (SPCZ), sandwiched in between decreased cloudiness over Indonesia and the eastern-most equatorial Pacific.
The MEI now stands for 30.2% of the explained variance of all six fields in the tropical Pacific from 30N to 30S, having regained about 13% since May-June. For comparison, this value is 1.8% lower than the one registered in 1997, attesting to an overall weakening of ENSO variability in the last one and half decades. 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 overall MEI indicating neutral ENSO conditions, one can still find a handful of key anomalies in the MEI component fields that exceed or equal one standard deviation, or one sigma (compare to loadings figure). These may actually flag either El Niño or La Niña.
Significant positive anomalies (coinciding with high negative loadings) indicate moderate westerly (U) anomalies along the Central American coast, as well as anomalously high SST (S) anomalies east of Australia. These anomalies support the diagnosis of La Niña-like conditions. On the other hand, significant southerly wind anomalies (V) are also found east of Australia, juxtaposed with northerly anomalies near the Phillippines (V). In addition, enhanced cloudiness (C) is diagnosed near the equatorial dateline. All of these anomalies support the opposide diagnosis of El Niño conditions.
There are no significant sea level pressure (P) and air temperature (A) anomalies that would flag either ENSO phase in this update. In sum, an overall ENSO-neutral assessment remains reasonable.
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 ENSO-neutral conditions
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In the context of mostly ENSO-neutral conditions since August-September 2012, this section features a comparison figure with persistent ENSO-neutral conditions for at least five bimonthly MEI values from August-September onwards. Longer-lived neutral conditions (such as 1952-54 and 80-82) could only enter once into this comparison figure.
The updated (September-October) MEI has continued its rise, now up to 0.09, an unprecedented two-month increase of +0.7 standard deviations for this time of year. The current ranking has risen to the 36th lowest out of 64, solidly in ENSO-neutral territory. Of the 10 nearest ranked September-October cases since 1950, exactly one case rose up to weak El Niño status (58-59), while one opposite case dropped to weak La Niña rankings (66-67). All eight others remained ENSO-neutral through the following boreal winter. Of course, none of these 10 cases had shown a rise of this caliber leading up to September-October either.
Negative SST anomalies cover only a fraction of the easternmost equatorial Pacific, especially near the coast of South America, as seen in the latest weekly SST map. Positive SST anomalies continue west of the dateline in the equatorial Pacific.
For an alternate interpretation of the current situation, I recommend reading the latest NOAA ENSO Advisory which represents the official and most recent Climate Prediction Center opinion on this subject. In its latest update (October 10th, 2013), ENSO-neutral conditions are diagnosed and expected to continue into the boreal spring of 2014. I agree with this statement for the next few months, perhaps not as far as next spring.
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. In 2012, Niño regions 3 and 3.4 rose from close to -1C in January to 0C in April and June, respectively, followed by a further rise above +0.5C by June and July, respectively. The August values for both index regions peaked at +0.7C. This was followed by a gradual decline below 0C by December 2012, and negative anomalies near -0.5C for both indices during January and February 2013. While both indices eased back towards 0C in March and April, the May Niño 3 value of -0.7C was the lowest since January 2012, while Niño 3.4 only cooled back to -0.3C. Both indices continued near -0.6C and -0.3C, respectively, through the next three months. However, September anomalies had weakened to -0.1C and -0.0C, respectively. Weekly SST data supports further relaxation towards 0C.
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. More recently, this index went negative in April 2012, reached -10 (-1 sigma) in June 2012, but weakened to -2 in July, -5 in August, and even returned to positive values from September to November (all between +2 and +4). However, it became weakly negative between December 2012 and February 2013, raising expectations for a possible transition back into El Niño. However, March 2013 showed a rapid rise to +11, only to fall back to +0 in April, rise back up to +8 in May, even further (+14) in June 2013, then back down to +8 in July and -0.5 in August, +4 in September, and back down to -2 in October. A running 5-month average supports the diagnosis of weak La Niña conditions (+5).
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 (just updated through April 2013). Extended SST-based ENSO data can be found at the University of Washington-JISAO website, which is now more than two years behind in its update (through January 2011).
Stay tuned for the next update by December 7th to see where the MEI will be heading next. El Niño came and went during the summer of 2012, not unlike 1953. This was followed by our first ENSO-neutral winter since 2003-04 (2005-06 was an ENSO-neutral winter, but much closer to La Niña, and dipped into La Niña rankings during March-April). Over the last half-year, La Niña had its turn to come and go again, almost in a mirror-image of last year's sequence. While we have now reached the time of year when drastic transitions are much less common than in the first half of the calendar year, the upward jump of +0.7 standard deviations since July-August indicates unusual volatility. While ENSO-neutral conditions are the safest bet going forward into boreal winter, I would not be surprised if this status were to end by early 2014.
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.