Impact of greenhouse gas concentrations on tropical storms in coupled seasonal forecasts

The impact of using realistic varying instead of fixed greenhouse gas concentrations is investigated in an ensemble of 6-month integrations of a coupled general circulation model (CGCM) from 1958 to 2001. Results suggest that an increase of greenhouse gas concentration is conducive to a decrease of tropical storm activity over all basins except the western North Atlantic and the eastern North Pacific. This result seems consistent with the impact of an increase of greenhouse gas concentration on the thermodynamical and dynamical variables that have an impact on the tropical cyclone activity. The interdecadal variability of tropical storm frequency is more realistic when using realistic varying GHG concentration than when using constant GHG. The improvement seems to be due to a more realistic trend of tropical storm frequency than to an improvement of the interdecadal variability around this trend. Therefore, it is important for seasonal forecasting systems to take account of the variation of GHG in the hindcast period used to calibrate the system.