Simulation of interannual variability of tropical storm frequency in an ensemble of GCM integrations

Abstract

The present study examine the simulation of the number of tropical storms in GCM integrations with a prescribed SST. A 9-member ensemble of 10-yr integrations (1979-88) of a T42 atmospheric model forced by observed SSTs has been produced: each ensemble member differs only in the initial atmospheric conditions. An objective procedure for tracking-model-generated tropical storms in applied to this ensemble during the last 9yr of the integrations (1980-88). The seasonal and monthly variations of tropical storm numbers arer compared with observations for each ocean basin. Statistical tools such as the Chi-square test, the F test, and the t test are applied to the ensemble number of tropical storms, leading to the conclusion that the potential predictability is particularly strong over the western North Pacific and the eastern North Pacific, and to a lesser extent over the western North Atlantic. A set of tools including the joint probability distribution and the ranked probability score are used to evaluate the simulation skill of this ensemble simulation. The simulation skill over the western North Atlantic basin appears to be exceptionally high, particularly during years of strong potential predictability.