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 August 9th, 2018)

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. MONTHLY UPDATED MEI loading maps for the latest season;

5. MONTHLY UPDATED MEI anomaly maps for the latest season;

6. MONTHLY 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. Subsequent updates for monthly data have typically become available before the 10th of the month, so I have been able to update by about the 10th in most cases.

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 recent conditions with analogous situations.


Historic El Niño events since 1950

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How does the 2015-16 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), and 2015-16 has replaced the 2009-10 event that reached the top seven ranking only twice. Visual inspection reveals that the 2015-16 event was slightly weaker than 1982-83 and 1997-98, resembling the latter more than the former in its evolution. Click on the "Discussion" button below to find a comparison of recent conditions with analogous situations.


MEI loading maps for the latest season

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The six loading fields display 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 centered along the Equator, corresponding to westerly anomalies near the dateline. Negative loadings south of Mexico indicate easterly anomalies during El Niño at this time of year. The meridional wind field (V) features moderate negative loadings north of the Equator across the eastern Pacific basin, denoting the southward shift of the ITCZ so common during El Niño-like conditions, juxtaposed with moderate 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 a 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 19.4% of the explained variance of all six fields in the tropical Pacific from 30N to 30S, one month after its annual minimum of importance. Twenty years ago, right after the MEI was introduced to the internet, the explained variance for June-July 1950-1998 amounted to 23.3%. This drop-off by almost 4% reflects the diminished coherence and importance of ENSO events over the last two decades (and/or more subtle changes in the expression of ENSO during this time of year). 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 showing ENSO-neutral conditions, four key anomalies in the MEI component fields flag La Niña, compared to only three for El Niño. Key anomalies refer to values 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 posiive loadings) denote high sea level pressure (P) anomalies over the Maritime Continent, and westerly wind anomalies (U) along the Equator near the dateline. Significant negative anomalies (coinciding with high negative loadings) flag northerly wind anomalies (V) south of Mexico. All of these anomalies are consistent with emerging El Niño conditions.

On the other hand, significant positive anomalies (coinciding with high negative loadings) translate into high sea level pressure (P) over the eastern equatorial Pacific, and southerly wind anomalies (V) just north of Galapagos (just south of the opposing anomalies mentioned in last paragraph). Significant negative anomalies (coinciding with high positive loadings) flag negative SST anomalies (S) east of Tahiti, as well as negative cloudiness anomalies (C) northeast of Australia. All of the last four anomalies are indicators of La Niña.

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 short-lived El Niño conditions and their aftermath.

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In the context of last year's aborted El Niño event, this section features a comparison figure with six other short-lived events during the MEI period of record. Only one of them occurred before 1980, while 2017 joined both 2014 and 2012 in a recent clustering of such events that lasted five five bimonthly seasons or less, with all of them ending before September-October.

Compared to last month, the updated (June-July) MEI dropped rapidly to +0.07, ending up right in the middle of ENSO-neutral ranking. This means that not a single season has reached El Niño conditions in 2018. Looking at the nearest 12 rankings (+6/-6) in this season, and excluding all cases that departed by more than 0.6 standard deviations in the changes from the previous month as well as three months earlier (March-April), there are only four analogues to the situation this season: 1985, '00, '01, and '08. All four of these cases either continued with ENSO-neutral conditions (2001) or dropped into at least intermittent La Niña conditions (especially in 2008, but also in 2000, and very briefly in 1985). Even among the other eight cases, El Niño was 'not on the menu' (2003 came closest). Compared to last month, the likelihood of El Niño conditions later this year has changed dramatically (from "inevitable" to "very unlikely").

Negative SST anomalies south of the Equator and along the South American coast have survived compared to last month, while positive SST anomalies are hanging in along the Equator just west of the dateline, 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 (9 August 2018), ENSO-neutral conditions are diagnosed, and predicted to transition to El Niño later this year with 70% odds by boreal winter. The latest MEI assessment is in disagreement with this, but also with itself just one month ago.

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 one. Note that I am referring to the OISST.v2 data, not ERSSTv5. After a short-lived run of Niño region 3.4 SST anomalies near +0.5C in May and June 2017, this index dropped to -0.43C in September and -0.45C in October, just shy of the official La Niña threshold of -0.5C, but then decreased dramatically in November (-0.86C), and hovered near that level for the next four months, between -0.73C in March and -0.90C in February. In April 2018, this index rose to -0.4C, continued warming in May with -0.1C, +0.2C by June, and +0.3C by July. For comparison, Niño region 3 SST nurtured +0.5C anomalies from February through May 2017, dropping to -0.2C in August, between -0.6C and -0.7C in September and October, followed by a similar dramatic drop in November (-1.05C), and continued anomalies just below -1.0C in the following three months, only to weaken decisively to -0.8C in March, -0.4C in April, -0.15C in May, +0.3C in June, and +0.4C in July.

For extended Tahiti-Darwin SOI data back to 1876, and timely monthly updates, check the Australian Bureau of Meteorology website. In 2017, this index oscillated around 0 through May, only to drop to -10 (-1 sigma) in June, corresponding to El Niño conditions for just one month, rose back up to +8 in July, continued between +3 and +9 through October, hit +12 in November, the highest value since September 2016, flagging the strongest La Niña conditions for this event, only to yoyo back to -1 in December, and back iup to +9 in January, -6 in February, all the way up to +11 in March, its final (?) La Niña peak for this event. Since then it declined steadily and reached -6 by June 2018, only to bounce back up to +2 in July. In sum, the SOI is back to wild fluctuations that do not really match any other ENSO index, although the longer-term average was in weak La Niña territory for much the last year.

The next update for the MEI is planned for September 8th or earlier (crossing fingers that the input data is not as late as this month). Compared to last month, the odds for El Niño in 2018 have dropped dramatically, not unlike previous aborted events in 2017, 2014, and 2012. Daily updates of the ENSO status can be found at the TAO/TRITON website, showing weak anomalies, or mostly ENSO-neutral conditions in early August over the equatorial Pacific.


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., 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.