Off-Equatorial Westward Propagating Rossby Waves in the Tropical Pacific during the 1982–83 and 1986–87 ENSO Events

Abstract

Analysis of anomalous vertically averaged temperatures (TAV) in the tropical Pacific Ocean for the eight-year period 1979-86, finds the development during the onset phase, one to two years before the mature phase of the two ENSO events of the period, having dynamically significant similarities both north and south of the equator. During the onset phase of both ENSO events, each beginning in the off-equatorial western tropical Pacific (about 10° latitude), positive anomalies of TAV occurred during northern autumn/winter, one year prior to the mature phase of each ENSO event, in both Northern and Southern hemispheres. This corroborates model results of Pazan et al. and supports the hindcasting/forecasting capability of ENSO events obtained by White et al., though the latter considered only the Northern Hemisphere. It is also demonstrated that the development of positive TAV anomalies in northern autumn/winter along the tropical maritime western boundary of both hemispheres was associated with wind-driven baroclinic long (Rossby) wave activity which had been transmitting positive TAV anomalies from the eastern and central tropical Pacific into the western tropical Pacific over the previous one-to-two years (between 1O° and 15° latitude). The analysis shows that there were positive anomalies on the maritime western boundary in both hemispheres during autumn/winter of the onset phase, not in one hemisphere or the other. During both ENSO events, the dynamical off-equatorial influence was found only to have initiated the ENSO event; off-equatorial influence dominated the first half of the ENSO year, diminishing during the peak phase of the event (i.e., northern summer of both ENSO year), and replaced during the second half of the ENSO year by strictly equatorial influence related to the development of anomalous westerly winds on the equator in the western equatorial Pacific.