Interaction of the Monsoon and Pacific Trade Wind System at Interannual Time Scales. Part III: A Partial Anatomy of the Southern Oscillation

Abstract

Studies of surface wind fields, sea surface temperature (SST), precipitation and sea level pressure in the tropical band extending ±30° of the equator from Africa to South America led to the following conclusions. The Southern Oscillation, El Niflo, and climatic variations in the Monsoon System are all part of one global scale phenomenon. The new information is that this phenomenon appears in the sea level pressure field to have a strong propagating component that appears first in the northern Indian Ocean and moves eastwardinto the eastern Pacific. Similar propagation of information was found in the surface wind field and equatorial precipitation regimes. These same conditions were amply demonstrated during the 1982-83 event and so it may be concluded that the evolution of that event bears many similarities to those in the historical record studies referred to in this paper. In the surface wind field of the equatorial wave guide, the large-scale signal appears to take the form of a forced Kelvin wave. The mechanism that drives this wave appears to be latent heat release associated with precipitation anomalies that are phase-locked to and propagate with the surface wind anomalies. The long time scales associated with the atmospheric anomalies may be associated either with the slow interaction between the Walker and Hadley cells or with ocean-atmosphere coupling. Variations in the SST in the central equatorial Pacific appear to be due almost exclusively to advective processes and not to local air-sea heat exchange, a result in accord with that of other studies. All SST anomalies in the equatorial region seen to be closely related to earlier variations in the zonal wind over the maritime continent, e.g., perturbations in the Indonesian Low which appear as the eastward propagating node of the Pacific Walker cell. It appears to be this feature of the wind field that eventually forces the observed ocean response. Extremely limited data show a remarkable coincidence between an empirically-derived histogram of recurrence intervals between El Niflo events derived from wind field considerations alone and similar histograms derived by Quinn et al. from completely different data sets. The former distribution function assumes that tropical climate variations can be characterized as frequency-modulated processes.