Kessler, McPhaden and Weickmann 1995 Abstract

Forcing of intraseasonal Kelvin waves in the equatorial Pacific

Kessler, W.S., M.J. McPhaden and K.M. Weickmann, 1995

Journal of Geophysical Research, 100, 10613-10631.

Ten-year time series of SST, 20[degrees]C depth and zonal winds measured by moored buoys across the equatorial Pacific are used to define the intraseasonal (30-90 day period) Kelvin waves. The Kelvin waves are observed to be forced west of the dateline, and propagate at a speed of 2.4 m/s, with high zonal coherence over at least 10,000 km. They form a major component of thermocline depth variability in the east-central Pacific. The intraseasonal- band variance has a low-frequency modulation both at the annual and ENSO frequencies; higher amplitudes are observed in boreal fall/winter and during the onset phase of El Nino warm events. The oceanic intraseasonal variability and its low-frequency modulation are coherent with atmospheric intraseasonal variations (the Madden-Julian Oscillation; MJO) measured by outgoing longwave radiation (OLR), which are known to propagate eastward into the Pacific from the Indian Ocean as part of a planetary-scale signal. The life cycle of an individual or series of MJOs is determined by a combination of factors including tropical SSTs over the warm pool regions of the Indian and Pacific Oceans and interaction with the planetary-scale atmospheric circulation. Thus the intraseasonal Kelvin waves should be taken as an aspect of a global phenomenon, not simply internal to the Pacific. The oceanic intraseasonal variability peaks at periods near 60-75 days, while the corresponding atmospheric variations have somewhat higher frequencies (35-60 day periods). We show this period offset to be potentially related to the zonal fetch of the wind compared to the frequency-dependent zonal wavelength of the Kelvin wave response. A simple model is formulated which suggests an ocean-atmosphere coupling by which zonal advection of SST feeds back to the atmosphere; the model duplicates the step-like advance of warm water and westerly winds across the Pacific at the onset of the El Nino of 1991-92. The key dynamics of the model is that the atmosphere responds rapidly to the state of the ocean, but the ocean's response to the atmosphere is lagged because it is an integral over the entire wind forcing history felt by the wave. This results in a non-linear ordinary differential equation which allows a net non-zero low-frequency ocean signal to develop from zero-mean sinusoidal forcing at intraseasonal frequencies.
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