FY 1995

Oceanic equatorial waves and the 1991–93 El Niño

Kessler, W.S., and M.J. McPhaden

J. Climate, 8(7), 1757–1774, doi: 10.1175/1520-0442(1995)008<1757:OEWATE>2 (1995)

Equatorial Kelvin and Rossby waves associated with the 1991-93 El Niño warm event were detected in temperature observations made by the Tropical Atmosphere-Ocean (TAO) buoy array. Intraseasonal Kelvin waves were a prominent part of equatorial thermocline depth variability and were well represented by a simple model consisting only of first- and second-mode baroclinic Kelvin waves. The second mode was essential to properly represent the observed amplitude. Thermocline depth variability at 5°N and 5°S was dominated by annual and interannual Rossby waves, which were found to have been largely wind forced in midbasin, with little if any signal associated with eastern boundary reflection. An evaluation of the Wyrtki buildup hypothesis and the delayed oscillator hypothesis in connection with the 1991-93 events showed that a long lag (about two years) occurred between the arrival of the downwelling signal in the west and the subsequent initiation of El Niño; this was considerably longer than suggested by delayed oscillator theory. No downwelling Rossby wave reflections occurred at the right time to trigger the onset of the 1991-92 warm event. Termination of the 1991-92 El Niño was due to an upwelling Rossby wave generated during the height of the warm episode, then reflected from the western boundary, consistent with delayed oscillator ideas. However, in early 1993, a second (weaker) warm episode occurred against the background of a very anomalously shallow west Pacific thermocline. This shows that the El Niño-Southern Oscillation cycle cannot be viewed simply as an oscillator mediated by the western boundary reflection of equatorial Rossby waves and that a buildup of a thick warm layer in the west is not a prerequisite to the occurrence of El Niño.