Multivariate ENSO Index

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The El Niño/Southern Oscillation (ENSO) phenomenon is the biggest player in the game of year-to-year climate variability. "El Niño" is named after the Christ child (in spanish) because of its early (~December) manifestation in the local waters off Peru, where abnormally warm sea surface temperatures (SST) often signal the beginnings of positive SST anomalies over a much larger domain, namely the eastern half of the equatorial Pacific. "La Niña" is the opposite phenomenon, referring to abnormally cold SST in the eastern half of the equatorial Pacific. The "Southern Oscillation", as originally named and described by Sir Gilbert Walker, consists of an east-west atmospheric pressure see-saw that directly affects tropical weather around the globe. As we have now come to appreciate, both "El Niño/La Niña" and swings of the "Southern Oscillation" typically occur in conjunction. They are often called "ENSO events", and can last several months to over one year. The influence of ENSO events is profoundly felt outside the tropics as well.

Here we attempt to monitor the coupled oceanic-atmospheric character of ENSO by basing the Multivariate ENSO Index (MEI) on the main observed variables over the tropical Pacific. These surface marine observations have been collected and published in COADS for many years. The MEI can be understood as a weighted average of the main ENSO features (details) contained in the following six variables: sea-level pressure, the east-west and north-south components of the surface wind, SST, surface air temperature, and total amount of cloudiness. The MEI is extended during the first week of the following month (discussed on the MEI home page) based on near-real time marine ship and buoy observations (courtesy of R.W. Reynolds at NCEP) summarized into "COADS-compatible 2-degree monthly statistics at 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. Positive values of the MEI represent the warm ENSO phase (El Niño).

ENSO Index: Comparison of this event with other El Niño events

For comparison, the seven biggest El Niño events since 1950 are shown here, including 1997/98. The first three events (1957/58, 65/66, and 72/73) all featured an early warming in the far eastern Pacific and reached their standardized peaks before the end of the first year. The more recent El Niño events (1982/83, 86/87, and 91/92) took longer to mature, typically reaching their peaks in the spring of the second year. Early 1983 saw the peak of the biggest El Niño of the century, while 1997/98 featured two peaks just below the 1982/83 El Niño, one in July/August 1997, and one in February/March 1998.

ENSO Index: Comparison of current conditions with La Niña events

The 1997/98 El Niño event is over. Last year's descent of the MEI into La Niña conditions was every bit as dramatic as its rise towards El Niño conditions in 1997. Therefore, a comparison figure of the seven biggest La Niña events since 1949 is shown above, with the most recent MEI values included as well.

Although previous strong La Niña events got off to an earlier start, the present La Niña came on so strong so fast that the MEI almost caught up with the 'pack' of historic events. During the last two decades, only the 1988/89 La Niña has seen lower MEI values.

Text and figures prepared by Klaus Wolter [klaus.wolter@noaa.gov] (303) 497-6340
Source: Wolter and Timlin (1993, 1998).

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