Fundamental Types And Genesis Of El Nino-Southern Oscillation

The El Nino-Southern Oscillation (ENSO) phenomenon has been a focal point of oceanographic and climatic research over the last few decades. Recent studies reveal that ENSO includes several basic types, and each type may involve different physical processes. However, the present classification of ENSO and suggested physical interpretations are highly debatable, since they cannot explain some of the fundamental characteristics of ENSO, such as the asymmetry between the warm (El Nino) and cold (La Nina) phases of ENSO.Using the rotated empirical orthogonal function analysis, this study first clarifies the problems with previous studies that classify ENSO based on empirical orthogonal function analysis, pointing out the incapability of empirical orthogonal function methods to identify different physical modes of ENSO-like climate variability that contains significant asymmetric signal. Then we apply the fuzzy clustering method to classify the basic types of ENSO, and find three types of El Nino and only one type of La Nina. It is shown that the La Nina type and the canonical El Nifno type contribute to the symmetric part of ENSO, while the warm pool El Nino and the extreme El Nino types are responsible for the asymmetric part of ENSO.This study also presents a brand new idea on the genesis of ENSO diversity, suggesting that the occurrences of different types of warm and cold events is closely related to the westerly wind bursts in the tropical Pacific. The westerly wind bursts can induce anomalous sea surface currents and downwelling equatorial Kelvin waves. The nonlinear interaction of these processes with the tropical Pacific low-frequency oscillation generates three different patterns of El Nino while leaving La Nina pattern unaltered. The idea is verified using LDEO as well as CCSM4ocean-atmosphere coupled models. In the control run, both models produce a regular, symmetric ENSO. When westerly wind bursts are added, the models generate different flavors of ENSO similar to those observed. Our finding provides a new dynamical framework for interpreting the diversity and asymmetry of ENSO.