The genesis and predictability of persistent Pacific–North American anomalies in a model atmosphere

The setup process of Pacific—North American (PNA) pattern anomalies that last more than 10 days and the role played therein by synoptic-scale transients are investigated using a T21,3-level quasi-geostrophic model. As there is no time-dependent forcing in the model, the lowfrequency PNA anomalies are generated entirely by the internal dynamics. From a 300-winter integration, 100 cases of positive PNA anomalies and 118 cases of negative PNA anomalies lasting at least 10 days are identified. The PNA composites reveal that 5 days before the setup of a positive (negative) PNA anomaly there is a negative (positive) height anomaly over Canada. This anomaly moves westward and stops over the North Pacific. It then intensifies and a wave train develops downstream. The roôe played by the synoptic-scale transients is explored by prediction experiments starting from day -5 (5 days before the appearance of a PNA anomaly). A 5-day average centered at day -5 is used to remove the synoptic-scale transients from the initial state. The results indicate that even without these transients in the initial conditions, the setup process of a positive PNA anomaly can be reproduced quite well. On the other hand, the setup of a negative PNA pattern cannot be predicted if the synoptic-scale transients are absent from the initial conditions. In this case, the pattern evolves systematically, but differently from the way it evolves in the control run.