The Impact of El Niño on an Ensemble of Extended-Range Forecasts

Abstract

Two ensembles of 90-day forecasts for 1982–83 have been made with the UK Meteorological Office 11-layer atmospheric general circulation model. Each ensemble comprised three integrations initialized one day apart, using analyses from December 1982. The first ensemble used observed SSTs and the second used climatological SSTs. Our objectives were to compare the skill of the two ensembles and to compare the results with a longer climate sensitivity experiment. The skill of the forecasts for 10-, 30- and 90-day means was assessed using root-mean-square wind errors at 200 mb. In the tropics, the skill was improved with observed SSTs on all time scales. In the extratropics, the skill was improved on the 30-day time scale except at the initial stages of the forecasts, and the skill was also improved for 90-day means. On the 10-day time scale, however, the improvement was not consistent, and there were periods in which the errors were larger with observed SSTs. The response of the model to the observed SSTs was found to be similar to that of a 540-day perpetual January integration with the winter mean SST anomaly for 1982–83.