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 March 6th, 2014)
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 the recent ENSO-neutral 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 the recent ENSO-neutral 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: 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 along and just south of the Equator, corresponding to easterly anomalies near the dateline. Negative loadings over Indonesia and east of Australia indicate westerly anomalies during La Niña. The meridional wind field (V) features high negative loadings north of the Equator, 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).
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 horse-shoe pattern of negative loadings (warm anomalies during La Niña) to its north- and southwest. At the same time, total cloudiness (C) tends to be decreased over the central equatorial Pacific, juxtaposed with increased cloudiness north of Australia and over the Philippines.
The MEI continues its retreat from its peak explained variance (now just under 30%) of all six fields in the tropical Pacific from 30N to 30S. Although its temperature components dominate the MEI with at least 36% of their possible variance, even P, V, as well as U and C join in with about a third, a quarter, and twice with about a fifth of their variance, respectively, at or close to their peak values during the calendar year. Sixteen years ago, right after the MEI was introduced to the internet, the explained variance for Jan-Feb 1951-1998 amounted to 31.6%. This slight drop-off reflects the diminished coherence and importance of ENSO events in much of the recent 16 years. 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 overall MEI indicating neutral ENSO 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). For the first time in a long time, these key anomalies all flag La Niña.
Significant positive anomalies (coinciding with high negative loadings) indicate strong westerly (U) anomalies north of Indonesia, moderate southerly (V) anomalies south of Hawai'i and north of the Equator, anomalously high SST (S) and air tempeature (A) anomalies halfway between the Philippines and Hawai'i as well as halfway between Australia and South America, and enhanced cloudiness (C) from Indonesia to the Philippines. Significant negative anomalies (coinciding with high positive loadings) indicate moderate easterly (U) anomalies along the Equator and centered on the dateline, as well as moderate northerly (V) anomalies east of Australia. All of these anomalies support the diagnosis of La Niña-like conditions.
While there are no significant El Niño anomalies to balance this out, the overall assessment remains ENSO-neutral. This is probably related to the fact that sea level pressure anomalies (P) are not flagging either ENSO-phase, while the cardinal anomalies related to the equatorial cold tongue (S, A, C) are small in this go-around.
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 ENSO-neutral conditions
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In the context of ENSO-neutral conditions since August-September 2013, this section features a comparison figure with persistent ENSO-neutral conditions for at least six bimonthly MEI values and ending in close proximity to the ranking of the current situation. Longer-lived neutral conditions (such as 1959-61) could only enter once into this comparison figure.
The updated (January-February) MEI has not changed much since last month, now at -.27. The current ranking has remained the 29th lowest out of 65, solidly in ENSO-neutral territory. Of the 8 nearest ranked January-February cases since 1950, SIX rose to at least weak El Niño status within the next six months (three within the next three months), while the two other cases either stagnated at ENSO-neutral (1960) or even dropped to weak La Niña ranks (1961). This confirms a noteworthy shift in the odds towards El Niño development in 2014 that was first pointed out three months ago. Compared to last month, the odds have increased further to make an El Niño likely in 2014.
Meanwhile, negative SST anomalies still cover much of the eastern equatorial Pacific, as seen in the latest weekly SST map. Positive SST anomalies continue west of the dateline in the equatorial Pacific, slowly making progress towards the east, but have disappeared again from the South American coast.
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 (March 6th, 2014), ENSO-neutral conditions are diagnosed and expected to continue through the boreal spring of 2014, followed by a 50% chance of El Niño developing during the summer or fall. As explained above, I am even more 'bullish' on El Niño development in 2014, but that is a minor disagreement.
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 2013, Niño regions 3 and 3.4 rose from close to -0.5C in January to within +/-0.2C in March and April of that year. While Niño 3 dropped back to -0.5C and even lower from May through August, Niño 3.4 remained less negative (around -0.3C) through this period. During September through December, both indices hovered close to or just under 0C. In January 2014, both indices dropped, down to -0.4C (-0.5C) for Niño 3 (3.4), respectively. This drop continued for Niño 3 in February 2014, now at -0.8C, while Niño 3.4 stabilized at -0.55C.
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 2013, the SOI varied from slightly negative values early in the year (-4 in February) all the way to +14 in June and back down to -2 in October. It rose back up to +9 in November, consistent with potentially re-emerging La Niña conditions. However, the December value dropped right back to +1, only to be followed by a jump to +12 in January 2014, and back to slightly negative values (-1) in February. The SOI remains the noisiest ENSO index that I can think of.
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 (just updated through January 2014). Extended SST-based ENSO data can be found at the University of Washington-JISAO website, which is now a full three years behind in its update (through January 2011).
Stay tuned for the next update by April 12th (probably earlier) to see where the MEI will be heading next. El Niño came and went during the summer of 2012, not unlike 1953. This was followed by two ENSO-neutral winters, and occasional dips into weak La Niña conditions over the last 1.5 years. It appears that this is about to change, although we have a long way to go to reach full-fledged El Niño conditions.
MEI data access and publications
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.
- 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.