AN ALTERNATE METHOD FOR DIFFERENTIATING EASTERN- AND CENTRAL-PACIFIC ENSO EVENTS USING PRECIPITATION ANOMALIES

In recent years, researchers have noticed ENSO events appear to occur in two distinct modes, with some geographically centered over the Central Pacific Ocean and others over the Eastern Pacific Ocean. At least three methods have been proposed for classifying the proposed different types of El Niño: the El Niño Modoki index of Ashok et al. (2007), which takes the difference between sea surface temperature (SST) anomalies in the central and those in the eastern and western equatorial Pacific; the Niño method of Yeh et al. (2009), which compares the magnitudes of the averages of the Niño 3 and Niño 4 SST indices for boreal winter; and the EP/CP-index of Kao & Yu (2009), which combine SST, subsurface ocean temperature, surface wind stress, and precipitation data using empirical orthogonal function (EOF) analysis and linear regression. When looking at the 21 El Niño events since 1950, these three methods agree unanimously on which type of El Niño event occurred for 13 of the events, but disagree on the other 8.

We propose a fourth method relying primarily on precipitation anomalies, as significant differences within a coupled ocean-atmosphere system should be manifest in precipitation patterns. Our analysis identifies El Niño events by the definition of the National Oceanic and Atmospheric Administration (NOAA), namely that the Ocean Niño Index (ONI) equal or exceed 0.5℃ over at least 5 consecutive and overlapping three-month seasons. Precipitation data is taken from the Global Precipitation Climatology Project (GPCP) and SST data from NOAA’s Extended Reconstructed Sea Surface Temperature (ERSST) V3b dataset for years 1970 to 2013. Monthly and seasonal anomalies are calculated as the deviation from the 1980-2010 mean. Results are compared to those of the three preexisting methods to determine if a precipitation-based method can help elucidate differences between or, alternatively, complicate the proposed Eastern/Central Pacific binary.