ORIGINAL PAPER On the 60-month cycle of multivariate ENSO index Adriano Mazzarella & Andrea Giuliacci & Ioannis Liritzis Received: 12 January 2009 / Accepted: 8 June 2009 / Published online: 8 July 2009 # Springer-Verlag 2009 Abstract Many point indices have been developed to describe El Niño/Southern Oscillation, but the multivariate El Niño Southern Oscillation (ENSO) index (MEI) is considered the most representative since it links six different meteorological parameters measured over the tropical Pacific. Spectral analysis with appropriate data reduction techniques of monthly values of MEI (1950– 2008) has allowed the identification of a large 60-month cycle, statistically confident at a level larger than 99%. The highest values of MEI (typical of El Niño events) and the lowest values of MEI (typical of La Niña events) are concordant with respective maxima and minima values of the identified 60-month cycle. 1 Introduction The monitoring of the El Niño/Southern Oscillation (ENSO) phenomenon is normally based on observations of sea surface temperatures (averaged to single values over large regions within the equatorial Pacific Ocean identified as Niño 4, Niño 3, and even Niño 3.4) and on the difference between Tahiti and Darwin atmospheric pressure in the whole tropical Pacific Basin (Rasmusson and Carpenter 1982; Ropelewski and Jones 1987; Allan et al. 1991; Trenberth 1997; Können et al. 1998). Such point measures are useful indicators of the ENSO phenomenon, but not representative of the coupled ocean–atmosphere phenome- non: the complexity of the climate system cannot be understood on the basis of simple and point measures. A more holistic approach has been developed at NOAA's Climate Diagnostics Center in Boulder Colorado with multivariate ENSO index (MEI) that is derived from tropical Pacific Comprehensive Ocean–Atmosphere Data Set (COADS) (Wolter and Timlin 1993, 1998). MEI is a multivariate measure of the ENSO signal in the first principal component of six main observed variables over the tropical Pacific: sea level pressure, zonal and meridional components of the surface wind, sea surface temperature, surface air temperature, and cloudiness of the sky. The highest values of MEI values represent the warm ENSO phase (El Niño) while the lowest values of MEI represent the cold ENSO phase (La Niña). MEI index time series has not been yet visited on a statistical elaboration, and here, we perform a robust statistical analysis while investigating the underlying physical processes. 2 Data set and analysis The data consist of the monthly values of MEI (period 1950–2008) as computed by Wolter and Timlin (1993, 1998), taken from the web site http://www.cdc.noaa.gov/ people/klaus.wolter/MEI/table.html, where each monthly value is centered between the preceding and subsequent month: for example, the January value represents the value A. Mazzarella (*) : A. Giuliacci Meteorological Observatory, Department of Earth Science, University of Naples Federico II, Largo S. Marcellino, 10 80138 Naples, Italy e-mail: adriano.mazzarella@unina.it A. Giuliacci e-mail: andrea.giuliacci@yahoo.it I. Liritzis Laboratory of Archaeometry, Department of Mediterranean Studies, University of the Aegean, Mytilene 85100, Greece e-mail: liritzis@rhodes.aegean.gr Theor Appl Climatol (2010) 100:23–27 DOI 10.1007/s00704-009-0159-0