Oscillations in a Simple Equatorial Climate System

Abstract

Coupling between large-scale atmospheric and oceanic equatorial Kelvin waves is shown to be relevant in the climatic time scale related to equatorial ocean/atmosphere processes. The present analyses show that the inclusion of air-sea coupling into the linearized shallow-water equations can result in two types of dispersion relations for the Kelvin waves. The first type (Mode 1) has fast phase speed and is mostly manifest in the atmospheric response. This mode is relatively unaffected by air-sea coupling. The second type (Mode II) has slow phase speed and is the predominant mode in the time-scale of the ocean. A resonant stationary wave is shown to exist as an intrinsic mode in the coupled system, the length scale of which is determined by the strength of the coupling and the magnitude of the damping. The positive feedback mechanism shown to exist between these coupled Kelvin waves in the Mode II regime is suggested to play an important role in relation to observed low-latitude teleconnections. Results of the numerical experiments using the coupled model show that an El Niño-type oscillation can occur in a baroclinic ocean-atmosphere system as a result of a prolonged period of strengthening and subsequent weakening in the barotropic component of the wind. The weakening and the eastward shift of the rising branch of the Walker cell, identified as the atmospheric component of a coupled quasi-stationary Kelvin wave, during El Niño provide a positive feedback favoring warm water formation in the eastern Pacific and contribute to the large amplitude of the oscillation.