Climate simulations with the global coupled atmosphere-ocean model ECHAM2/OPYC Part I: present-day climate and ENSO events

In this study the global coupled atmosphere-ocean general circulation model ECHAM2/OPYC and its performance in simulating the present-day climate is presented. The model consists of the T21-spectral atmosphere general circulation model ECHAM2 and the ocean general circulation model OPYC with a resolution corresponding to a T42 Gaussian grid, with increasing resolution towards the equator. The sea-ice is represented by a dynamic thermodynamic sea-ice model with rheology. Both models are coupled using the flux correction technique. With the coupled model ECHAM2/OPYC a 210-year integration under present-day greenhouse gas conditions has been performed. The coupled model simulates a realistic mean climate state, which is close to the observations. The model generates several ENSO events without external forcing. Using traditional and advanced (POP-technique) methods these ENSO events have been analyzed. The results are consistent with the delayed action oscillator theory. The model simulates both a tropical and an extra-tropical response to ENSO, which are in good agreement with observations.