Aspects of the intraseasonal oscillation simulated by the Hadley Centre Atmosphere Model

The period and amplitude of the tropical intraseasonal oscillation in the Hadley Centre Global Climate Model have been examined by applying time filtering and spectral analysis techniques to time series of daily velocity potential. Results from an AMIP integration of the model, forced with observed sea surface temperatures for the decade 1979-88 show that the model has a reasonable degree of skill in simulating the main features of the oscillation, although the signal in the model is rather noisy compared to NWP analyses. The model also shows interannual variability of the oscillation similar in magnitude to that in the analyses. The impact on the oscillation of including a parametrization of momentum transport by cumulus convection is assessed. This parametrization significantly improves the mean circulation of the model, reducing systematic errors particularly in the tropics. However, the strength and coherence of the tropical intraseasonal oscillation are significantly reduced by this parametrization. It is speculated that this may be related to a reduction in westerly flow in the tropical central and east Pacific and a reduction in the strength of the SPCZ in DJF.