The Neutral Phases of ENSO: Are They Really Neutral?

Research concerning the propagation and evolution of the El Niño-Southern Oscillation (ENSO) has shown shifting warm and cold phases within the coupled ocean-atmospheric system throughout much of the Pacific Ocean. While the teleconnected climate conditions for the warm El Niño phase and the opposite cold La Niña phase are well understood, the mechanisms that drive the oscillating nature of the phenomenon continue to be debated. Without this understanding, forecasting the onset and demise of El Niño events will continue to be a critical problem that plagues forecasters around the world. This study examines this problem by utilizing the neutral phases that occur before both warm and cold events. It is common practice to combine these two neutral phases when analyzing the ENSO life cycle, which undoubtedly disregards the concurrent climate teleconnections associated with transition periods. ENSO theories attempt to explain the complex nature of El Niño and the 4-year periodicity of the cycle, stressing the importance of free oceanic equatorial waves propagating and reflecting within the Pacific Ocean. The processes that drive these waves, the general structure, and the feedback mechanisms are studied in detail. The investigation of neutral ENSO phases hinges on these equatorial waves and can clearly be represented in correlated ocean-atmospheric conditions. The Bjerknes' Hypothesis, a positive feedback mechanism that describes the integrated ocean-atmospheric system along the equatorial Pacific, and the synthesis of free equatorial waves drive the development of warm (and cold) ENSO episodes. The creation of a transitional phase index and NCEP reanalysis from 1945-2007 show the resulting neutral episodes that occur climatologically one year before an El Niño event. The representation of equatorial Kelvin and off-equatorial Rossby waves, along with anomalous cyclones that cause westerly wind anomalies on the equator can be seen within the reanalysis. Suggested teleconnected atmospheric conditions show anomalously high pressure centers over the southeastern United States, along with increasingly low precipitation rates along the western portions of North America. These teleconnected patterns prove to be useful in forecasting conditions before both warm and cold ENSO events, in turn offering new direction for long term El Niño forecasts.