Interannual and decadal variations of blocking activity in a quasi-geostrophic model

Blocking regimes simulated in a spectral 3-level quasi-geostrophic model have been examined and compared with observations. A control experiment and perturbation experiments have been performed using different constant and time-varying external forcing fields. The main emphasis of this paper is to determine the signal to noise ratio of the effects of anomalous external forcing and the natural variations in blocking activity. Forcing the model with the climatological winter mean external forcing, a control experiment has been carried out for 500 winters. The model simulates accurately the structure of the blocking pattern, as well as its effect on the transient weather systems. It also produces approximately the same number of blocking days as in the observations. However, the average duration of the simulated blocking regimes is about one day shorter than the observed one. The interannual variability in blocking activity is similar to the observations. The model also produces substantial long-term variations in blocking activity on a time-scale of decades. The observed climatological intraseasonal variations in the external forcing field appear to have a negligible effect on the mean blocking activity and its interannual variability. Observed interannual changes in the forcing configuration significantly affect the probability for a blocking circulation to occur. However, if we include the year-to-year changes in external forcing, the model generates only slightly larger interannual variations in blocking activity as compared to the control experiment with constant climatological forcing. So it seems that the observed interannual variability of blocking in winter is predominantly internally generated and changes in external forcing fields are of secondary importance.