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Multivariate ENSO Index (MEI)

Last update: 4 January 2009

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

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 COADS 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. The MEI is extended during the first week of the following month based on near-real time marine ship and buoy observations (courtesy of Diane Stokes at NCEP) summarized into COADS-compatible 2-degree monthly statistics at NOAA-ESRL PSD. Caution should be exercised when interpreting the MEI on a month-to-month basis, since the input data for updates are not as reliable as COADS, and 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).

You can find the numerical values of the MEI timeseries under this link, and historic ranks under this related link. 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 (NOW available online as pdf files!), and/or this webpage.

If you have trouble getting at the data, please contact me under (Klaus.Wolter@noaa.gov)

How does the 1998-2000 La Niña event compare against the seven previous biggest La Niña events since 1949? Only strong events (with a peak value of at least -1.2 sigma) are included in this figure. Note that some events last through the full three years shown here (for instance, 54-56), while others revert to "normal" or El Niño conditions by the second or third year (especially in 64-66). The 1998-2000 La Niña does not resemble any previous event in this comparison figure. It started late (about three months later than the previous latest case), and it featured a superimposed annual cycle (peaking around May and troughing around November) that does not match the other events displayed in this figure. However, the weak La Niña period after the 1982-83 El Niño had similar characteristics. Click on the "Discussion" button below to find a comparison of recent MEI conditions against a large subset of these La Niña events.

  • Discussion and comparison of recent conditions with historic El Niño events

    • How does the 2002-04 El Niño event compare against the seven previous biggest El Niño events since 1949? Aside from 2002-04, only strong events (with a peak value of at least +1.4 sigma) are included in this figure. The 2002-03 El Niño event peaked below that threshold, with just over +1.2 sigma in early 2003. Overall, I would rank it just barely in the top 10 El Niño events of the last half century. In its evolution, it bears some resemblance to the 1965-67 event (highest temporal correlation), but shared with 1991-93 its reluctance to drop below the zero line once it had run its course. The most recent El Niño event of 2006-07 reached a similar peak as the 2002-03 event, but lacked 'staying power', and collapsed rather early in 2007. I have discontinued its comparison plot against other weaker events in favor of a new comparison with other La Niña events below (click on "Discussion" to jump to the bottom of the page).

    The six loading fields show the correlations between the local anomalies and the MEI time series. Land areas are flagged in green, and 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 the Equator, corresponding to easterly anomalies near the dateline. Negative loadings in the far western and eastern Pacific, as well as to the south of the positive loading center, show that westerly anomalies are almost equally pervasive in these regions during La Niña. The meridional wind field (V) features moderate negative loadings north of the Equator, denoting the northward shift of the ITCZ so common during La Niña conditions, juxtaposed with strong positive loadings northeast of Australia.

    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 conditions) to its northwest and, in particular, to its southwest. During La Niña conditions, total cloudiness (C) tends to be decreased over the central equatorial Pacific, sandwiched in between increased cloudiness over Indonesia and the eastern-most equatorial Pacific.

    Now at its annual peak, the MEI explains 32.3% of the total variance of all six fields in the tropical Pacific from 30N to 30S, mostly due to its temperature components that crest near one half of the possible explained variance. Eleven years ago, when the MEI was introduced to the Internet, the explained variance for Nov-Dec 1950-1997 peaked at 34.0%. This slight drop-off reflects the diminished coherence and importance of ENSO events in the last eleven years. The loading patterns shown here resemble the seasonal composite anomaly fields of Year 0 in Rasmusson and Carpenter (1982).

    Consistent with re-emerging La Niña conditions, there are several of the observed key anomalies in the MEI component fields that exceed or equal one standard deviation, or one sigma (compare to loadings figure) that flag typical La Niña features, while one 'rogue' El Niño-like feature remains present as well. Significant negative anomalies (coinciding with high positive loadings) denote anomalously low sea level pressure (P) east of Australia, very strong easterly anomalies (U) along the Equator and near the dateline, as well northerly wind anomalies (reduced southerly flow) east of Australia (V). Significant positive anomalies (coinciding with high negative loadings) flag anomalously high sea level presssure over the eastern equatorial Pacific (P), positive sea surface and air temperature (S and A) northeast of Australia and west of Hawaii (mainly for S), and enhanced cloudiness north of Australia (C). While the enhanced easterlies near the dateline exceeded two standard deviations, SST anomalies in the equatorial eastern Pacific did not reach one standard deviations just yet in November-December, even though the most recent SST data shows a clear cooling trend through the month of December. All of these anomalies are consistent with La Niña. On the other hand, large negative anomalies (coincident with high negative loadings) denote moderately strong easterly anomalies (U) as far west as the Philippines, which is more typical for El Niño than for 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, check out the climate products map room.

    Discussion and comparison of recent conditions with historic El Niño events

    In the context of La Niña-like conditions for much of the period since about September 2007, this section displays a comparison figure of historic La Niña events since 1949 vs. current conditions. The most recent (November-December) MEI value has settled on -.63, nearly unchanged from last month's -.56. While still running higher than in the 1974 'analog year', it is noteworthy that the 1973-74 period remains the most similar stretch to 2007-08 in the MEI record, correlating at +0.90 for the last 24 months. In 1974, this was followed by continued La Niña conditions well into 1976. The most recent MEI rank has remained the same, at 17th, still exceeding the weak La Niña threshold in the MEI rankings for this season. The MEI has thus continued the status of weak La Niña conditions that it lost between May-June and July-August of this year.

    Moderate negative SST anomalies cover most of the equatorial Pacific from just west of the dateline to the South American coast, with no 'stubborn pockets of positive anomalies' left - conventional Niño-SST indices now show values well below -0.5C, at least as reflected in the latest weekly SST map.

    For an alternate interpretation of the current situation, I highly 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 (11 December 2008), ENSO-neutral conditions were diagnosed (based on Niño 3.4), but were given equal chances of being replaced by La Niña by early 2009 as their continuation. It will be interesting to see what this week's ENSO discussion will state - there may be a 'convergence of opinion' in the works...

    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, not in time for this go-around. Niño regions 3 and 3.4 showed persistent temperature anomalies of -1.3C or lower from September (region 3) and October 2007 (region 3.4) through February 2008, but hovered much closer to 0C anomalies between about June and November. A dramatic cool-off ensued, leading to weekly SST anomalies of around -1C in both regions by the end of December. For extended Tahiti-Darwin SOI data back to 1876, and timely monthly updates, check the Australian Bureau of Meteorology website. This index belatedly caught on to the current La Niña event last November, reaching +12 to +14 (1.4 sigma) in December, January and March, and higher yet (+21) in February. The average for February-March (+16.8) was ranked fourth highest for that season (since 1950, but this ranking is true for the full record as well). It then hovered near 0 from April through July, while the August SOI rebounded right up to +9, followed by +14 in September, +13 in October, +17 in November, and +13 in December. The five-month average SOI since August (+13) is the highest since 1988, ranked in the top 10 values for this five-month season since 1876(!). These last five months of SOI conditions thus indicate that the atmospheric Southern Oscillation is clearly back in La Niña territory, while the oceanic conditions are just now catching up. An ever 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 more than one year behind). Extended SST-based ENSO data can be found at the University of Washington-JISAO website, currently updated through April 2007.

    Stay tuned for the next update (by February 7th) to see where the MEI will be heading next. While we currently have the time of year when persistence will be hard to beat, the recent transition back to La Niña in the MEI shows that changes are still possible even this time of year. Given that most atmospheric circulation components of the MEI are shifted towards La Niña (as is the SOI), while sea surface temperature anomalies in the equatorial belt are just now cooling off, it is not surprising that an overall ENSO index like the MEI shows weak La Niña conditions. If December's SST anomalies persist (or even deepen) into January, watch for a more negative MEI by next month's update. The odds for the opposite to occur are clearly less than 50%. I have pointed out most of 2008 that this would be in keeping with earlier behavior of strong La Niña events that have tended to linger beyond a single year. Note that I have discontinued my monthly e-mail announcements in favor of keeping the discussion right here on this webpage.

    REFERENCES

    • 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.

    Questions about the MEI and its interpretation should be addressed to:
    (Klaus.Wolter@noaa.gov), (303) 497-6340.