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 June 4th, 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 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 recently strengthening El Niño 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 moderate positive loadings centered just south of the Equator, corresponding to westerly anomalies near the dateline. Moderate negative loadings over Indonesia and northeast of Australia as well as off the Peruvian coast indicate easterly anomalies during El NiƱo at this time of year. The meridional wind field (V) features moderate negative loadings north of the Equator across the Pacific basin, denoting the southward shift of the ITCZ so common during El Niño-like conditions, juxtaposed with moderate positive loadings northeast of Australia and 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 east of Hawai'i.

The MEI now just stands for 18.0% of the explained variance of all six fields in the tropical Pacific from 30N to 30S. Seventeen years ago, right after the MEI was introduced to the internet, the explained variance for Apr-May 1950-1998 amounted to 21.3%. This drop-off by more than 3% reflects the diminished coherence and importance of ENSO events in much of the recent 17 years, although it has regained 0.3% compared to last year's explained variance. 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 further strengthening of El Niño conditions, one can find quite a few 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 flag El Niño conditions indeed.

Significant positive anomalies (coinciding with high positive loadings) indicate very high sea level pressure (P) northwest of Australia, southerly wind anomalies (V) northeast of Australia and over the South China sea, high sea surface (S) and air temperatures (A) over the eastern equatorial Pacific, and increased cloudiness (C) near Galapagos. Significant negative anomalies (coinciding with high negative loadings) flag anomalously low sea level pressure (P) over the eastern equatorial Pacific as well as increased easterlies (U) northeast of Australia and along the South American coast. All of these anomalies are consistent with El Niño.

On the other hand, persistent southerly wind anomalies (V) northeast of New Guinea are more consistent with La Niña than with 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, 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 rapidly strengthening El Niño conditions over the last few months, this section features a comparison figure with analogous events since 1950.

The updated (April-May) MEI has risen by 0.61 standard deviations in one month to +1.57, for a high ranking above the 'strong' El Niño threshold (upper 10%ile). This is the highest MEI value in 17 years, surpassing the peak of the 2009-10 El Niño by a few 1/100. The current El Niño has ranked above the weak El Niño threshold for four months in a row, and above the strong threshold for two months running. If it were to remain above the strong threshold for just one more month, it would be the first strong El Niño event since 1997-98 with at least three months registering in the upper 10%ile. By this definition, six other El Niño events occurred since 1950: 1957-58, '65-66, '72-73, '82-83, '86-87, and '91-92, or just over once a decade.

Looking at the nearest 12 rankings (+5/-7) in this season, and excluding cases with declining April-May values compared to earlier in the year gives us six 'analogues' to ponder: 1957, 1987, 1993, 1997, 2002, and 2014 (all but 2014 are included in the MEI comparison figure of this section). Except for 2002-03, peak values of the remaining four events reached the strong threshold and maintained it from three months (1993) to 13 months (1997-98). None of the comparison figure cases dropped back to ENSO-neutral rankings before the end of the calendar year. Thus, if it were not for last year's unusual evolution, it would be safe to state that El Niño conditions are virtually guaranteed through 2015. Nevertheless, the odds for a substantial El Niño during the next six months appear better than since at least 2009.

Positive SST anomalies have consolidated over the equatorial Pacific, all the way from the dateline to the South American coast, as seen in the latest weekly SST map. While most of these anomalies reach +1 to +2C, one can find anomalies in excess of +3C near Galapagos.

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 (May 14th, 2015), El Niño conditions were diagnosed, and expected to continue through the rest of 2015 with a greater than 80% 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, in time for this go-around. In early 2014, Niño regions 3 and 3.4 rose from around -0.5C for both indices to +0.9C for Niño 3, and just shy of +0.5C for Niño 3.4 by June. This first stage of El Niño-like conditions was interrupted in July and August with barely positive values, only to rise again to +0.9C for Niño 3 and +0.85C for Niño 3.4 in November 2014. The following four months saw Niño 3 values as low as +0.2C, and Niño 3.4 hanging in around +0.5C. The Niño 3 anomaly rose by a full degree Celsius since then, reaching +1.2C in May, while Niño 3.4 rose to +1.0C in May. Weekly SST anomalies show a continual rise thru last month, reaching +1.4C and 1.3C by the end of May, respectively.

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 2014, its value has varied from +12 in January (moderate La Niña conditions) to -13 in March and -11 in August (full-blown El Niño conditions). April and May were positive, indicating mild La Niña conditions in opposition to all other ENSO indices, but between August and November negative SOI values between -8 and -11 indicated at least weak El Niño conditions. Since December, it rose all the way to +1 in February 2015, only to drop back to -11 in March, up again to -4 in April, and back down to its lowest value so far in May: -14. While the SOI remains a noisy ENSO index, it was more clearly in the El Niño 'camp' than others from August 2014 through March 2015 with an eight-month average of almost -8.

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, although the last one was just completed to include data through 2014. Extended SST-based ENSO data can be found at the University of Washington-JISAO website, which is now a full four years behind schedule (through January 2011).

Stay tuned for the next update by July 12th (travel, sorry!) to see where the MEI will be heading next. El Niño went through a four-month 'dress rehearsal' last boreal summer, disappeared for two months, returned for two months, went back to an El Niño-flavored neutral status in January, but has steadily grown to its highest value in 17 years as of April-May 2015. If it stays above +1.2 for just one more month, it will have exceeded the upper decile threshold for three months running, thus joining the somewhat exclusive 'club' of strong events according to the MEI. Meanwhile, typical El Niño impacts will be supported by positive PDO conditions that have endured since January 2014, reaching record levels from December through February 2015. Daily updates of the ENSO status can be found at the TAO/TRITON website.


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., 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.