Second and first baroclinic Kelvin modes in the equatorial Pacific at intraseasonal timescales

Abstract

TOPEX/Poseidon sea level and time series from the TAO (Tropical Atmosphere Ocean) array of moorings over 1992–1999 are used to investigate the spatiotemporal characteristics of equatorial Kelvin waves in the Pacific at period shorter than 180 days. Spectral analyses show high oceanic energy in two separate bands of period, around 70 days and 120 days. Signals are coherent in both bands all along the equator. Ocean General Circulation Model (OGCM) simulations forced by different wind stress fields are also analyzed and exhibit the same features. Observed and modeled sea level, dynamic height and 20°C isotherm depth, band‐pass filtered around 70 days, show an eastward propagation at phase speed of 2.4–2.9 m s−1, typical of the first baroclinic Kelvin mode. The meridional and vertical structures of the equatorial waves at the onset of the 1997 El Niño are also those expected for a first Kelvin mode. At 120‐day period, phase speeds, meridional and vertical structures are closer to those expected for a second baroclinic mode. Both modes are nevertheless present in the two frequency bands. A simple Kelvin wave model shows that the wind at 120‐day period seems to force the Kelvin waves at the same period. However, the predominance of second baroclinic mode at 120‐day period cannot be explained solely by the characteristics of the wind. OGCM simulations rather suggest the importance of internal oceanic processes such as vertical redistribution of energy from first to second baroclinic mode as the waves propagate eastward. Model outputs over 1984–1999 indicate that these equatorial Kelvin waves at 70‐ and 120‐day period may have an effect on El Niño since they seem to be stronger at its onset.