|Title||Impact of increasing greenhouse gas concentrations in seasonal ensemble forecasts.|
|Publication Type||Technical memorandum|
|Secondary Title||ECMWF Technical Memoranda|
|Authors||Doblas-Reyes, F, Hagedorn, R, Palmer, TN, Morcrette, J-J|
|Place Published||Shinfield Park, Reading|
A set of 44-year 6-month long seasonal ensemble coupled model forecasts performed with annually updated greenhouse gas concentrations is compared to a standard seasonal ensemble forecast experiment performed with fixed concentrations. The former has more realistic temperature variability and clearly better forecast quality. The improvement in model variability is due to a better simulation of climate trends. Differences indicate that realistic initial conditions are not enough to reproduce this long-term variability and provide new evidence of the climatic effects of anthropogenic changes in atmospheric composition. The improvement in probabilistic forecast quality is mostly due to an increase in the ability to reliably discriminate the occurrence of events and non-events. The relevance of these results for the improvement of operational seasonal predictions is discussed.