Simulation of El Niño-Southern Oscillation-like Variability in a Global AOGCM and its Response to CO2Increase

Abstract

A 75-year integration of a coupled atmosphere–ocean model is examined for tropical interannual variability. The atmospheric model has interactive cloud and a seasonal cycle. The fluxes of heat and salinity into the ocean component of the model are flux corrected. The model has tropical variability that is qualitatively similar to that of the observed El Niño/Southern Oscillation (ENSO). The maximum amplitude of the model Nino3 signal is approximately half that observed and the modeled ENSO timescale is greater than that observed. In the first 50 years of the integration the model has eight warm events. Each event is one of two types: one characterized by a standing SST anomaly in the central and eastern Pacific and the other by a westward propagating sea surface temperature anomaly. The majority of the model warm events are of the first type. The first type of event is triggered by the eastward propagation of Kelvin waves across the Pacific, and the second by westward propagation of warm temperature anomalies through the atmospheric response to a warm anomaly causing the suppression of equatorial upwelling. There is a coupling to the seasonal cycle for the first type of event. A positive feedback through changes in marine stratocumulus in the east Pacific is an important factor in some simulated warm events. Another integration was carried out in which atmospheric CO2 was increased at a rate of 1% (compounded) per annum. There is no significant change in the one to ten year interannual variance of SST in the east Pacific, and this suggests that the size of the SST anomalies during warm or cold events in the “greenhouse” world may not be significantly different from those of today.