Multivariate ENSO Index (MEI)

The views expressed are those of the author and do not necessarily represent those of NOAA.

Click to enlarge

Outline for MEI webpage (updated on June 14th, 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

Click to enlarge

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

Click to enlarge

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

Click to enlarge

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 just south of the Equator, corresponding to westerly anomalies near the dateline. Modest negative loadings just north and northeast of Australia indicate easterly anomalies during El Niño at this time of year. The meridional wind field (V) features 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 modest 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 eastern equatorial Pacific, as opposed to decreased cloudiness east of Hawai'i.

The MEI now stands for just under 18% of the explained variance of all six fields in the tropical Pacific from 30N to 30S. 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

Click to enlarge

With the overall MEI indicating ENSO-neutral 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), that may flag either El Niño or La Niña. Significant positive anomalies (coinciding with high positive loadings) indicate anomalously high sea level pressure (P) over northern Indonesia. Similarly, increased cloudiness (C) near Galapagos coincides with high positive loadings. Significant negative anomalies (coinciding with high negative loadings) flag easterly wind anomalies (U) northeast of Australia. All of these anomalies are typical for El Niño conditions.

On the other hand, high pressure anomalies over the eastern tropical Pacific (P) and strong easterly wind anomalies (U) along and south of the Equator near the dateline both indicate La Niña-like conditions in significant regions.

There are no significant meridional wind (V), SST (S), or air temperature (A) anomalies that would flag either ENSO phase in this update.

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 events

Click to enlarge

In the context of the persistent 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. I only included the beginning stretch of longer-lived cases, keeping subsequent years out of this comparison (such as 1953-4 and 1981-2).

The updated (April-May) MEI has risen to +0.07, now in its nineth month in a row with ENSO-neutral rankings, at present the 31st lowest out of 64 cases. The MEI remains as solidly neutral as it has ever been. However, of the 10 nearest ranked April-May cases since 1950, only four remained ENSO-neutral for the rest of the calendar year, while five ended up in La Niña territory by the end of those years (and only one moved up to El Niño conditions).

Negative SST anomalies cover the easternmost equatorial Pacific, especially near the coast of South America, as seen in the latest weekly SST map. Positive SST anomalies have almost disappeared from the equatorial Pacific, even west of the dateline.

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 (June 6th, 2013), ENSO-neutral conditions are diagnosed and expected to continue through the boreal summer of 2013.

There are several other 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 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. While both indices eased back towards 0C in March and April, the May Niño 3 value of -0.7C is the lowest since January 2012, while Niño 3.4 only cooled back to -0.3C.

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 went 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, and rise back up to +8 in May. It remains a very 'noisy', but useful ENSO index.

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 (currently through July 2012). Extended SST-based ENSO data can be found at the University of Washington-JISAO website, last updated through January 2011.

Stay tuned for the next update by 9 July (hopefully earlier) to see where the MEI will be heading next. El Niño came and went last summer, not unlike 1953. We have just witnessed 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). Of the six such ENSO-neutral cases before 2012-13 shown here, two ended up as full-blown El Niño events by the end of the second year shown in this graph (1991 and 2002), while three remained more or less ENSO-neutral, and one drifted into weak La Niña territory (1967). However, during boreal spring four of these cases showed at least a brief foray into El Niño conditions, with two of them (1953 and 1981) quickly returning to ENSO-neutral later that year. Given a recent preference for La Niña, I would not be surprised if we see a gradual drift of the MEI into negative territory later this year.

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.