Intraseasonal variations in the upper equatorial Pacific Ocean prior to and during the 1997-98 El Nino

Time series data from the Tropical Atmosphere-Ocean (TAO) array of moored buoys and data from other sources are used to document the oceanic dynamical response to intraseasonal (periods centered near 60 days) wind stress forcing in the equatorial Pacific. We focus on the period from October 1996 to December 1997, encompassing the time just prior to and during the onset of the 1997-98 El Nino, when both atmospheric forcing and ocean response were exceptionally strong. Analysis reveals that the oceanic response to intraseasonal wind forcing is characterized by two district regimes. West of the international date line, the response is dominated by reversing zonal jets in both time and depth in the upper 200 m. East of the date line, variability is controlled primarily by first baroclinic mode equatorial Kelvin waves propagating eastward at phase speeds of 2.4-2.7 m s(-1). However, amplitude structures in the eastern Pacific deviate from those expected for first baroclinic Kelvin waves in a resting ocean, suggesting that the wave field may be sensitive to the presence of a zonally sloping thermocline and the Equatorial Undercurrent. In addition, upward phase propagation, consistent with downward energy propagation, suggests the presence of some higher baroclinic mode wind-forced wave energy in the eastern Pacific. The implications of these results for understanding the evolution of the 1997-98 El Nino are discussed.