SIMULATION OF INTERANNUAL VARIABILITY OF TROPICAL STORM FREQUENCY IN AN ENSEMBLE OF GCM INTEGRATIONS

The present study examines the simulation of the number of tropical st orms produced in GCM integrations with a prescribed SST A g-member ens emble of IO-yr integrations (1979-88) of a T42 atmospheric model force d by observed SSTs has been produced; each ensemble member differs onl y in the initial atmospheric conditions. An objective procedure for tr acking-model-generated tropical storms is applied to this ensemble dur ing the last 9 yr of the integrations (1980-88). The seasonal and mont hly variations of tropical storm numbers are compared with observation s 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 trop ical storms, leading to the conclusion that the potential predictabili ty is particularly strong over the western North Pacific and the easte rn North Pacific, and to a lesser extent over the western North Atlant ic. A set of tools including the joint probability distribution and th e ranked probability score are used to evaluate the simulation skill o f this ensemble simulation. The simulation skill over the western Nort h Atlantic basin appears to be exceptionally high, particularly during years of strong potential predictability.