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

Interannual variability of the observed extratropical atmosphere is kn own to be arranged in well-defined spatial patterns (teleconnection pa tterns). 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 analysi s and EOF-analysis. Simulations either including or neglecting the for cing effect of interannual sea surface temperature variability (SST) a re considered. Prominent modes of interannual atmospheric variability like the Atlantic oscillation, the West Pacific pattern, and the Pacif ic/North American pattern are all reproduced by the model. If interann ual 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 inter annual variability patterns (or regimes) have a substantial influence on the strength and the spatial structure of intraseasonal transient a ctivity, both on the cyclonic and the low-frequency (weekly) time scal e. This is consistently pointed out by conventional transient eddy sta tistics and by consideration of individual synoptic events. The simula ted relationship between interannual variability and intraseasonal var iability compares favourably with observational evidence. Hence, the r esults suggest that methods of deducing local climate changes from lar ge-scale response patterns of the model may be successful.