JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. C6, PAGES 10,425-10,430, NOVEMBER 20, 1984 Equatorial Sea Level Response During the 1982-1983 E1 Nifio ROGER LUKAS Joint Institute for Marine and Atmospheric Research, University of Hawaii/NOAA, Honolulu STANLEY P. HAYES Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington KLAUS WYRTKI Department of Oceanography and Hawaii Institute of Geophysics, University of Hawaii, Honolulu During the 1982-83 E1 Nifio/SouthernOscillation event,sea level across the width of the equatorial Pacific adjusted to the reversal of the equatorial trade winds, and by the end of 1982the normal sea level slope across the Pacifichad beeneliminated. The transfer of warm upper-ocean water from the western Pacific to the easternPacific was accomplished by a combination of direct wind forcing as the wind anomaly crossed the basin and by mass flux induced by free equatorial waves.The importance of equatorially trappedKelvin waves of first and second verticalmode during the onsetof the 1982-83 El Nifio is inferred from the cross-correlation statistics between central and eastern Pacific sea level stations and between wind variations in the western Pacific and equatorial sea level stations to the east. The different propagation speeds of these two modes appears to be responsible for the observed change in shapeof the major sea level signals during the 1982-83 event. Tentative evidence for first baroclinic mode,first meridional-mode Rossby waves is also presented. 1. INTRODUCTION During 1982,the trade windsin the tropical Pacificweak- ened, eventuallyreversing direction so that the winds were blowing from west to east along the equator in the central Pacific. In response a zonal redistributionof water masses took place;sea levelfell in the western Pacific and rosein the eastern Pacific. Strongest currents associated with this redistri- bution occurredin the upper layer above the thermocline, although anomalous currents wereseen in and belowthe ther- mocline [Firing et al., 1983; Halpernet al., 1983]. Because of the contrast in temperature of the surface layer between the eastern and western Pacific, largetemperature anomalies [Ras- mussen and Wallace, 1983] were associated with this oceanic response to the anomalous winds [Harrison andSchopf, 1984]. For several years the workinghypothesis for the onset of El Nifio conditions in the eastern Pacific has been that the col- lapse of the equatorial tradewinds in the western Pacific trig- gersequatorially trapped Kelvin waves that propagate along the equator to the coast of South America [Wyrtki, 1975a, 1977]. The first baroclinic mode waves take less than 2 monthsto cross the Pacific; higher modestake longer. The passage of these waves causes changes in the equatorial and coastalcurrents, and E1 Nifio begins. The equatorial Rossby waves generated as part of the Kelvin wave reflection [Moore andPhilander, 1977] propagate backinto the interior, possibly causing further anomalous conditions. Here we briefly describe some important aspects of the re- 'sponse of sealevel in the equatorialPacificOcean to anoma- lous zonal winds during the 1982-83 El Nifio. Then we pres- ent direct evidence of free equatorial waves in the centraland easternPacific during the period prior to and including the onset of the 1982-83 El Nifio. Copyright 1984by the AmericanGeophysical Union. Paper number 4C1035. 0148-0227/84/004C- 1035505.00 2. DATA AND METHODS Tide and subsurface pressure gauges recorded sealevel vari- ations at Nauru (0ø32'S, 166ø54'W), Christmas (1ø59'N, 157ø29'W),Jarvis (0ø2YS, 160øW), Isabela (0øYS, 91ø28'W), and Santa Cruz (0ø27'S, 90ø17'W). The records were low-pass filtered to remove tidal variations, and then daily mean sea level was calculated.These time series(Figure 1) show the onsetof the 1982-83 El Nifio, which is indicatedby the large and rapid increase of sealevel during July 1982 in the central Pacific and by the slowrisein the eastern Pacific, beginning in August.Monthly mean sea levelswere calculated from the daily values, and anomalies from the long-term monthly means at each site were derived.Wyrtki [1984-[discusses fur- ther details of these calculations. In addition, an index of equatorial zonal wind in the west- ern Pacificwas kindly providedto us by James Sadlerof the Universityof Hawaii (Figure 1). This index consists of daily averages of the 6-hourly wind observations at Tarawa, Nauru (Ocean Island), and Beru (Arorae), all near 170øE.Observa- tions from the islands in parentheses were used when the pri- mary site did not report. The averaging of observations from sites north of, on, and south of the equator tendsto emphasize signals in the zonal wind field that are symmetric with respect to the equator.The onset of persistent westerly windsin June 1982 is apparent, but note also the energetic high-frequency variations with periods of several days and the sporadic "bursts" of westerly wind [Luther et al., 1983]. 3. WIND CHANGES According to Sadler and Kilonsky [1983], anomalous and persistent westerly winds began in May-June 1982 in the western Pacific, extending eastward to about 175øE,although several episodes of strong westerlywinds had occurredmuch earlier and farther to the west.Subsequently, these anomalous westerlies moved eastward, eventually reaching the coast of South Americain early 1983. This progression can best be seen in the time-longitude diagram of zonal wind anomalyat the 850-mbar level (approximately 1 km above the sea surface) 10,425