An examination of NCEP reanalysis anomaly data for neutral El Niño-Southern Oscillation (ENSO) phases is conducted in order to portray a three-dimensional view of the global teleconnections associated with this phenomenon. The neutral phase, which occurs when Niño 3.4 sea surface temperatures (SSTs) lack either warm or cold anomalies, precedes both warm and cold ENSO events. Few studies have examined the significance of these phases, usually grouping both warm-to-cold and cold-to-warm phases into one mean state, when in reality these transitional periods are far from neutral. Because of a lack of neutrality seen within NCEP reanalysis data from 1950-2007, the title of “developmental phase” will be given for apparent neutral events leading up to an extreme phase.
During a developmental phase one year before a mature El Niño (December-January-February [-1]), warm SST anomalies bisecting the equator/date line and along 30°S within the eastern Pacific are seen. Increased SSTs also appear in the northern Pacific Ocean, southern Atlantic Ocean, Baffin Bay off the west coast of Greenland, Gulf of Mexico and the eastern Mediterranean Sea. Cold SST anomalies are seen predominantly throughout the Tropical Pacific. Enhanced precipitation and upward motion above warm SST anomalies occurs over the western Pacific, surrounding Indonesia and Papua New Guinea.
Analysis of the zonal mean circulation provides a vertical representation of how the neutral phase affects the three latitudinal circulation cells, along with the horizontal Walker circulation. Symmetric cooling of the tropical troposphere and Northern Hemisphere (NH) midlatitudes can be seen during this phase, with a mid-level warm core above the equator. More vertical warming around the polar regions, especially in the Southern Hemisphere (SH) can be seen during the boreal winter one year before an El Niño. Combining these vertical and horizontal cross-sections shows a weakening and displacement of Hadley cells 20°N and S, with a strong westerly tropical jet positioned directly along the warm core in the mid-to-upper troposphere. Critical in the development and decay of ENSO events, the Walker Circulation appears to be deteriorating, a foretelling sign of an impending El Niño. Even in the polar regions, critical weakening of the polar vortex correlates with neutral conditions.
The teleconnections seen during the neutral phases of ENSO provide new findings that help explain the complex dynamics associated with a coupled ocean-atmospheric system. The timescale of a developmental phase aids in predicting warm and cold events that are potentially one year away from fruition. With the completion of a three dimensional schematic and the corresponding teleconnections associated with a developmental episode, this study provides new methodology in forecasting extreme events throughout the entire ENSO cycle.