Forcing of intraseasonal Kelvin waves in the equatorial Pacific

Abstract

Ten‐year time series of sea surface temperature (SST), 20°C depth, and zonal winds measured by moored buoys across the equatorial Pacific are used to define the intraseasonal (30‐ to 90‐day period) Kelvin waves. The Kelvin waves are observed to be forced west of the date line and propagate at a speed of 2.4 m s−1, 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 interannual frequencies; higher amplitudes are observed in boreal fall/winter and during the onset phase of El Niño warm events. The oceanic intraseasonal variability and its low‐frequency modulation are coherent with atmospheric intraseasonal variations (the Madden‐Julian Oscillation (MJO)), 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‐ to 60‐day periods). We show that this period offset is 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 that suggests an ocean‐atmosphere coupling by which zonal advection of SST feeds back to the atmosphere; the model duplicates the steplike advance of warm water and westerly winds across the Pacific at the onset of the El Niño of 1991–1992. 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 nonlinear ordinary differential equation that allows a net nonzero lowfrequency ocean signal to develop from zero‐mean sinusoidal forcing at intraseasonal frequencies.