Propagation and reflection of long equatorial waves in the Pacific Ocean during the 1992–1993 El Niño

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Abstract
The TOPEX/POSEIDON satellite, together with the Tropical Ocean and Global Atmosphere‐Tropical Atmosphere Ocean (TOGA‐TAO) array, provides oceanographic and atmospheric observations which allow a detailed study of the equatorial Pacific variability. During the November 1992 to December 1993 El Niño period, sea level, dynamic height, wind stress, sea surface temperature, and surface zonal current data derived from TOPEX/POSEIDON and TOGA‐TAO measurements were used to describe the Pacific ocean‐atmosphere system and to understand the role played by long equatorial waves. A potentially important mechanism of the El Niño‐Southern Oscillation (ENSO), commonly referred to as the delayed action oscillator, involves Kelvin and long Rossby waves and their reflections at the Pacific western boundary. In order to investigate if this process was at work during the period under study, a method for projecting TOPEX sea level, TOGA‐TAO dynamic height, and zonal wind stress onto meridional wave structures was designed both in unbounded and bounded regions. The Kelvin and first three Rossby waves of the first baroclinic mode are propagating at theoretical wave speeds in all data sets. Zonal wind stress projections show that oceanic propagating wave features are strongly linked to wind variability. Reflections are then examined at both boundaries. At the eastern boundary most of the signal reflected from incoming Kelvin waves is either counteracted by unfavorable wind forcing or strongly reinforced and therefore does not seem to play a significant role for generating the major Rossby wave signals during the period under study. In the western Pacific, wind forcing, rather than western boundary reflections, appears to be the main trigger for returning Kelvin waves from the western Pacific to the eastern Pacific. Simultaneously with the weakening of the extended 1991–1993 ENSO event, an upwelling Kelvin wave is observed propagating from the western Pacific in September 1993 to the eastern Pacific in November 1993. This scenario is consistent with some features of the delayed action oscillator mechanism, where an upwelling Kelvin wave is systematically seen returning from the western boundary to the east at the end of warm events. However, here, contrary to the delayed action oscillator, most of this returning Kelvin wave seems to be forced by a strong easterly anomaly located in the western Pacific, rather than by reflection of an upwelling first Rossby wave at the western boundary.