Changes in predictability associated with the PNA pattern

Numerical experiments are performed to determine whether the predictability of a model atmosphere is influenced by the presence of the Pacific/North American (PNA) pattern. The predictions are made with a T21 3-level quasi-geostrophic model. The relationship between the forecast errors and the ''interannual'' variation of the PNA anomaly is investigated. Comparison of the error growth for the forecasts made during the positive and negative PNA phases indicates that little difference in the error growth can be realized before about one week. After that period the forecast skill over the North Pacific, the North American and the North Atlantic regions is higher during the positive PNA phase than that during the negative PNA phase. A global signal of this relationship is also observed. The physical mechanism for the difference of error growth is discussed. For the forecasts made during the positive PNA phase, the pattern of systematic error has a negative PNA structure, while for the forecasts made during the negative PNA the systematic error shows a positive PNA distribution. An explanation is given for the nature of the systematic error. This result implies that by selecting a set of particular anomalous observed cases, a bias is necessarily introduced into the model forecasts verified for these periods.