Sea Level Variations in the Tropical Pacific Ocean and the Madden-Julian Oscillation

Satellite observations of sea level and surface wind from the tropical Pacific Ocean, and their relationship to the Madden-Julian oscillation (MJO), are analyzed using a combination of statistical techniques and a simple, physically based model. Wavenumber-frequency analysis reveals that sea level variations at the equator contain prominent eastward-propagating signals as the intraseasonal Kelvin waves. The component of sea level variation that is coherent with the MJO (eta(MJO)) is concentrated in a narrow strip along the equator between 150 degrees E and 110 degrees W. To explain the physical forcing of eta(MJO), the component of zonal wind stress that is coherent with the MJO (tau(x)(MJO)) is also calculated. It is shown that tau(x)(MJO) is strongest in the western Pacific, but the MJO accounts for a higher percentage of the wind variance in the central equatorial Pacific. A simple linear model of the Kelvin waves, based on a first-order wave equation forced by tau(x)(MJO) and with a linear damping term included, successfully reproduces eta(MJO). It is also shown that zonal wind variations to the east of the date line act to increase the apparent propagation speeds of the Kelvin waves.