Multivariate ENSO Index (MEI)

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

MEI

Page Outline (updated on November 7th, 2018)

This web page 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 on 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, denoting easterly anomalies during El Niño at this time of year. 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 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 the dateline, or warm anomalies during an El Niño event. Substantial negative loadings north and east of Australia contribute to the overall temperature pattern (for S and A). 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 in decreased cloudiness over Indonesia and the eastern-most equatorial Pacific.

The MEI now stands for 29.9% of the explained variance of all six fields in the tropical Pacific from 30N to 30S, having regained almost 13% since May-June. Twenty-one years ago, right after the MEI was introduced to the internet, the explained variance for September-October 1950-1997 amounted to 31.8%. This drop-off by 2% 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), but is less dramatic than at other times 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 El Niño-like conditions, six key anomalies in the MEI component fields flag El Niño, while La Niña anomalies are still found in four locations/variables, so this is not a settled affair. 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 positive loadings) denote anomalously high sea level pressure (P) over the Maritime Continent, and warm sea surface (S) and air temperature anomalies (A) along the Equator from the dateline to 120W (peaking right over Niño 3.4). Significantly lowered sea level pressure (P) coinciding with high negative loadings is found near Hawai'i and near the Equator at 140W, while easterly wind anomalies (U) over the Maritime Continent, and northerly wind anomalies (V) west of Hawai'i round out the list of key anomalies that are consistent with El Niño.

On the other hand, there is a rather unusual patch of positive sea level pressure anomalies (P) near 130W and 10S, southerly wind anomalies (V) along and north of the Equator south of Hawai'i and close to Central America, and warm temperature anomalies (S, A) northeast of Australia still flag lingering La Niña conditions. These temperature anomalies are also consistent with a long-term warming trend that has made it very hard for this region to show negative anomalies in recent years.

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, as well as this year's repeated attempts at crossing into El Niño territory, 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 at most five bimonthly seasons, with all of them ending before September-October.

Compared to last month, the updated (September-October) MEI remained steady at +0.47, ending up a couple of ranks below last month and below the lowest El Niño ranking. While not a single season has reached El Niño conditions in 2018, it has come awfully close.

Looking at the nearest 12 rankings (+6/-6) in this season, and removing cases with three-months drops rather than rises leading up to September-October, we are left with seven 'analogue' years: 1951, '52, '68, '69, '79, '90, and 2003. All of them remained in either high neutral or weak El Niño conditions through the following few seasons - this is the time of year when persistence is hard to beat. Compared to last month the odds for full-fledged El Niño conditions at some point through the following six months is still about or just higher than 50/50, since early 1953, early and late '69, and late '79 into '80 reached that threshold.

Negative SST anomalies south of the Equator and along the South American coast have decreased further since last month, while positive SST anomalies are now found along the Equator from west of the dateline to Galapagos, 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 (11 October 2018), ENSO-neutral conditions are diagnosed, and predicted to transition to El Niño later this year with at least 70% odds. The latest MEI assessment is more or less in agreement with this outlook.

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. 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 around -0.4C early last fall, just shy of the official La Niña threshold of -0.5C, but then decreased dramatically in November (-0.86C) to hover near that level (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, only to flatline from July through September near +0.3C. In October 2018, this index jumped to just under +0.9C. For comparison, Niño region 3 SST nurtured +0.5C anomalies from February through May 2017, dropping to -0.2C in August, and between -0.6C and -0.7C in the next two months, 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 near that value through the next three months, only to jump to almost +0.9C in October.

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 that event, only to yoyo back to -1 in December, and back up to +9 in January, -6 in February, all the way up to +11 in March, its final La Niña peak for the last event. Since then it declined steadily and reached -6 by June 2018, a hick-up to +2 in July, back down to -7 in August, and finally to -10 in September. True to form, it jumped back up to +3 in October 2018. In sum, the SOI is back to wild fluctuations that do not really match any other ENSO index, although the last months were 'ahead' of the SST (and MEI) data, if only for a month or two.

The next update for the MEI is planned by December 8th (crossing fingers that the input data is not as late again as in early October). Compared to last month, the odds for El Niño in 2018 have settled into 50% or better for two months in a row - not a guarantee, but doubled odds compared to climatology. Daily updates of the ENSO status can be found at the TAO/TRITON website, showing continued weak-to-moderate warm SST anomalies, with unremarkable wind anomalies overlaying the sea surface.


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.