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 April 6th, 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: low pressure anomalies in the west and high pressure anomalies in the east correspond to negative MEI values, or La Niña-like conditions. Consistent with P, U has positive loadings mostly along and just south of the Equator, corresponding to easterly anomalies near the dateline. Negative loading over Indonesia indicate westerly anomalies during La Niña at this time of year. The meridional wind field (V) features high negative loadings north of the Equator across much of the Pacific basin, denoting the northward shift of the ITCZ so common during La Niña-like conditions, juxtaposed with high positive loadings northeast of Australia (northerly anomalies during La Niña) and over the South China Sea.

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 cold anomalies during a La Niña event. They are flanked by a weak horse-shoe pattern of negative loadings (warm anomalies during La Niña) to the north- and southwest. At the same time, total cloudiness (C) tends to be decreased over the central equatorial Pacific, juxtaposed with increased cloudiness from northern Australia to the Philippines.

The MEI continues its retreat from its peak explained variance (now down to 25.5%) of all six fields in the tropical Pacific from 30N to 30S. This is 2.0% lower than twenty years ago during the extra strong El Niño of 1998, showing a lingering effect of the overall reduction in ENSO activity during the last two decades. The loading patterns shown here resemble the seasonal composite anomaly fields of Year 1 in Rasmusson and Carpenter (1982).


MEI anomaly maps for the latest season

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With the MEI showing weakened La Niña-like conditions, seven key anomalies in the MEI component fields flag La Niña, compared to none 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 negative loadings) flag anomalously high sea level pressure (P) near Tahiti, anomalous westerlies (U) over Indonesia, anomalous southerlies (V) just north of the Equator and south of Hawai'i, and above-normal sea surface (S) and air (A) temperatures from east of the Philippines to Hawai'i. Significant negative anomalies (coinciding with high positive loadings) indicate anomalously low sea level pressure (P) over the western equatorial Pacific, as well as enhanced northerly flow (V) over the South China Sea.

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 yet another aborted El Niño event in 2017, 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 joins both 2014 and 2012 in a recent clustering of events that lasted five bimonthly seasons or less, with all of them ending before September-October.

Compared to last month, the updated (February-March) MEI rebounded back up to -.50, dropping just out of La Niña rankings.

Looking at the nearest 12 rankings (+6/-6) in this season, and excluding the three cases that showed a three-month change of 0.6 or more, we end up with the following nine 'analogues': 1962, 63, 68, 85, 96, 97, 01, 06, and 12 (five of these were flagged as analogues last month). Subsequently, three of these analogues transitioned to El Niño ('63, 97, and 09), two hung on to La Niña rankings through early fall ('62 and '85), and the rest of them remained mostly ENSO-neutral. Not exactly a strong indication one way or another, but the odds for continued La Niña appear diminished compared to last month.

Negative SST anomalies can be found along the Equator from the dateline to South America, weaker than last month, as seen in the latest weekly SST map. The coldest anomalies stretch along the Equator from 160-110W, and right against the South American coastline.

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 (8 March 2018), weak La Niña conditions are diagnosed, and predicted to transition to ENSO-neutral during March-May with a 55% chance. The latest MEI assessment is more in line with this than last month.

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, not in time for this one, although the weekly data is available through most of March. i Note that I am referring to the OISST.v2 data, not ERSSTv5 (which are currently in sync). 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, i just shy of the official La Niña threshold of -0.5C, but decreased dramatically in November (-0.86C), and hovered near that level for the next three months, between -0.75C in January and -0.90C in February. The latest weekly SST has weakened slightly to -0.7C. 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 between -1.01C and -1.14C in the following three months, only to weaken to just -0.6C by late March.

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, but rose to +8 in July, back down to +3 in August, +7 in September, +9 in October, +12 in November, the highest value since September 2016, only to yoyo back to -1 in December, bouncing back to +9 in January, -6 in February, and all the way up to +11 in March, returning to the strongest indications for La Niña since November. In other words, the SOI is back to wild fluctuations that do not match any other ENSO index, although the longer-term average is in weak La Niña territory.

The next update for the MEI is expected by May 12th or earlier. Compared to last month, the odds for continued La Niña conditions for the next few months are now worse than 50% in the MEI sense, while the odds for El Niño appear to gain the upper hand during the second half of the year, but only slightly. Meanwhile, the PDO showed its lowest value since January 2014 in October (+0.05), followed by rise back up to +0.70 in January 2018 and back down to +0.37 in February. At least for now, the PDO is still projecting onto its positive phase. Daily updates of the ENSO status can be found at the TAO/TRITON website, showing weakened La Niña conditions in early April over the equatorial Pacific, along with weak anomalous southeasterlies near the dateline that might just be sufficient to maintain status quo (weak La Niña) for a little while longer.


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.