CIRCULATION REGIME FLUCTUATIONS AND THEIR EFFECT ON INTRASEASONAL VARIABILITY IN THE ECHAM CLIMATE MODEL

Interannual variability of the observed extratropical atmosphere is known to be arranged in well-defined spatial patterns (teleconnection patterns). The representation of these patterns in multi-year runs with the ECHAM2/T21 general circulation model is investigated for Northern Hemisphere winter, applying the techniques of teleconnectivity analysis and EOF-analysis. Simulations either including or neglecting the forcing effect of interannual sea surface temperature variability (SST) are considered. Prominent modes of interannual atmospheric variability like the Atlantic oscillation, the West Pacific pattern, and the Pacific/North American pattern are all reproduced by the model. If interannual SST variations are included, the simulation of the patterns is far more realistic. The features of the Atlantic Oscillation appear to be particularly sensitive in this respect. The fluctuations of the interannual variability patterns (or regimes) have a substantial influence on the strength and the spatial structure of intraseasonal transient activity, both on the cyclonic and the low-frequency (weekly) time scale. This is consistently pointed out by conventional transient eddy statistics and by consideration of individual synoptic events. The simulated relationship between interannual variability and intraseasonal variability compares favourably with observational evidence. Hence, the results suggest that methods of deducing local climate changes from large-scale response patterns of the model may be successful.