Numerical simulations of mesoscale flood environment

Summary The transition from a cold to a warm state of the E1 Niño-Southern Oscillation (ENSO) cycle is studied using Comprehensive Ocean-Atmosphere Data Sets (COADS) for the period 1950–1992.The onset of El Niño (November to December of the year preceding the El Niño) is characterized by an occurrence of minimum sea-level pressure anomalies in the subtropics around the node line of the Southern Oscillation. This pressure fall favors the formation of the anomalous cyclonic circulations over the western Pacific and leads to the establishment of anomalous westerlies in the western equatorial Pacific during the boreal spring of the El Niño year. The westerly anomalies then intensify and propagate into the central Pacific by the end of the El Niño year. This is an essential feature of the development of a basin-wide warming.It is argued that the development of the equatorial westerly anomalies over the western Pacific may result from the thermodynamic coupling between the atmosphere and ocean. In boreal winter and spring the mean zonal winds change from westerly to casterly over the western equatorial Pacific. A moderate equatorial westerly anomaly initially imposed on such a mean state may create eastward SST gradients via changing rates of evaporational cooling and turbulent mixing. The equatorial SST gradients would, in turn, induce differential heating and zonal pressure gradients which reinforce the westerly anomalies. The feedback between the eastward SST gradients and westerly anomalies promotes the eastward propagation of the westerly anomalies.With 9 Figures