Tropical extratropical interaction and systematic errors of climate models

The nature of the Quasi-Stationary Waves (QSW) observed in an aquaplanet model, which consists of an Atmospheric General Circulation Model (AGCM) whose lower boundary is given by a swamp ocean, is investigated in detail. In this experimental setting, using Hayashi spectra of atmospheric energetics, QSW are identified as being marginally stable baroclinic waves. This novel theoretical paradigm is then applied for studying QSW in the real world. The examination of NCEP-DOE reanalysis reveals that a coherent QSW, of zonal wavenumber four, forms in the Southern Hemisphere austral summer. Its growth seems to be favoured by El Nino tropical conditions, and mechanisms of interaction consistent with the aquaplanet theory are proposed. The ECHAM5 AGCM, set-up in AMIP configuration, does not satisfactorily reproduce the observed QSW activity, and its misrepresentation is analysed as a function of the systematic errors of the model. The role played by the model resolution for better capturing QSW is extensively discussed.