Kelvin waves excited by intraseasonal wind forcing

The Kelvin wave excited by an intraseasonal wind forcing with a 40-day period over the western Pacific Ocean was simulated using an ocean general circulation model, and was investigated by the use of spectral analysis. The amplitude of the temperature has two peaks north and south of the equator at the depth of the thermocline, and the amplitude of zonal velocity also has two peaks on the equator above and below the thermocline. The phase shows the upward propagation of the wave. It was queried why this wave, which appears to be transient rather than modelike, is formed quickly and always propagates with a phase velocity of about 3 m/s. The vertical one- dimensional forcing problem was studied, where the external forcing of up and down motions moving eastward is imposed at the surface. The growth time is estimated from the resonant solution. The first mode can resonate quickly, but the second cannot. The response in the infinitely deep ocean was also studied to focus on the transiency, where the reflection from the bottom is inhibited. The wave response to the forcing with a speed of about 3 m/s has a large amplitude, i.e. quasi-resonance occurs. In this case, the thermocline plays the role of a reflector, and the upper ocean between the sea surface and the thermocline behaves as a duct. Here, the small resonant cavity explains why the wave is formed so quickly, and the special value of the wave velocity is interpreted as a resonance condition in the duct. The wave corresponding to the second baroclinic mode could not be excited easily by the short-lived forcing at the surface, since this mode is mainly structured under the thermocline. It was found that the wave damps in consequence of leaking energy downward, and the damping rate depends on the period of the wave.