OCEANIC KELVIN WAVES AND THE MADDEN-JULIAN OSCILLATION

The relationship between the Madden-Julian oscillation (MJO), the domi nant mode of intraseasonal variability in the tropical troposphere, an d the Kelvin waves that dominate the variability of the equatorial the rmocline in the central and eastern Pacific Oceans is explored. The Ke lvin waves have period near 70 days, which is distinctly longer than t he dominant period of the MJO (40-50 days). Their zonal wavelength is roughly the width of the Pacific basin, which is about twice the zonal scale of the zonal stress anomalies produced by the MJO across the we stern Pacific. Their eastward phase speed is about 2.3 m s(-1). which is indistinguishable from the gravest baroclinic mode using the observ ed stratification in the Pacific. The stress anomalies that force the Kelvin waves are shown to be associated with the lower-frequency compo nents of the MJO (i.e., periods greater than about 60 days). These str ess anomalies move eastward at less than 5 m s(-1) from the Indian Oce an to the date line, where their local wavelength is about 15 000 km. East of the date line, where the convective component of the MJO weake ns, the phase speed of the stress anomalies increases to greater than 10 m s(-1). The similarity of the phase speeds of the MJO west of the date line and of the gravest baroclinic Kelvin wave is shown to result in near-resonant forcing by the relatively weak. but zonally broad, s tress anomalies induced by tile MJO. Despite the large increase in pha se speed east of the date line, the MJO-induced stress anomalies are s hown to continue to positively project onto the Kelvin waves to about 130 degrees W, which is where the observed thermocline perturbations a re the largest. East of this longitude, the MJO-induced stress anomali es detract from the amplitude of the Kelvin waves. The large spatial s cale of the zonal stress anomalies produced by the MJO and the near-re sonant forcing west of the date line helps explain the observed spectr al peak near 70 days for the Kelvin waves despite the higher central f requency of the MJO.