THE BIRTH AND EVOLUTION OF AN EASTWARD PROPAGATING AIR-SEA COUPLED DISTURBANCE IN AN AQUA-PLANET

The birth and evolution of an air-sea coupled disturbance relevant to the El Nin-approximately-o/Southern Oscillation (ENSO) events is investigated using a simple coupled aqua-planet model composed of the Gill's moist atmosphere and the Anderson-McCreary ocean. A coherent air-sea coupled disturbance with the zonal wavenumber 1 emerges from different initial disturbances either in the atmosphere or the ocean and propagates eastward as observed in the 1982/83 event. In the case of an initial westerly wind burst, oceanic Kelvin waves generated by the winds cause weak but long-lasting ocean temperature anomalies which trigger the air-sea coupled disturbance. When the initial disturbance is in the oceanic mixed-layer temperature, the coupled disturbance is excited more easily because the oceanic relaxation time is long compared to the atmospheric one. This is consistent with the result that the coupled disturbance collapses when the disturbance is forced to vanish on the oceanic side rather than on the atmospheric side.