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.
In fac, subsequent updates for monthly data became available by the 7th of the month, hopefully to be sped up further in the future.
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.This figure includes only strong events (with at least three bimonthly rankings in the top seven), with the exception of the 2009-10 event that reached the top seven 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 2015-16 El Niño conditions with historic strong El Niño events.How does the 2009-10 El Niño event compare against the seven previous biggest El Niño events since 1950?
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This webpage consists of seven main parts, three of which are updated every month: 1. The MEI is computed separately for each of twelve sliding bi-monthly seasons (Dec/Jan, Jan/Feb,..., Nov/Dec).A short description of the Multivariate ENSO Index (MEI); 2. 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 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. These observations have been collected and published in ICOADS for many years.
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).Click on the "Discussion" button below to find a comparison of 2015-16 El Niño conditions with the same seven historic El Niño events.By August 2017, the comparison figure with 2015-16 will replace the current one with 2009-10.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).