Oceanic Kelvin Waves and the Madden–Julian Oscillation

Abstract

The relationship between the Madden?Julian oscillation (MJO), the dominant mode of intraseasonal variability in the tropical troposphere, and the Kelvin waves that dominate the variability of the equatorial thermocline in the central and eastern Pacific Oceans is explored. The Kelvin waves have period near 70 days, which is distinctly longer than the 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 western Pacific. Their eastward phase speed is about 2.3 m s?1, which is indistinguishable from the gravest baroclinic mode using the observed stratification in the Pacific. The stress anomalies that force the Kelvin waves are shown to be associated with the lower-frequency components of the MJO (i.e., periods greater than about 60 days). These stress anomalies move eastward at less than 5 m s?1 from the Indian Ocean to the date line, where their local wavelength is about 15000 km. East of the date line, where the convective component of the MJO weakens, 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, stress anomalies induced by the MJO. Despite the large increase in phase speed east of the date line, the MJO-induced stress anomalies are shown to continue to positively project onto the Kelvin waves to about 130°W, which is where the observed thermocline perturbations are the largest. East of this longitude, the MJO-induced stress anomalies detract from the amplitude of the Kelvin waves. The large spatial scale of the zonal stress anomalies produced by the MJO and the near-resonant forcing west of the date line helps explain the observed spectral peak near 70 days for the Kelvin waves despite the higher central frequency of the MJO.