Essay on Dynamical Long-Range Forecasts of Atmospheric Circulation

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Abstract
The feasibility of monthly and seasonal forecasts is considered. The gross features of departures of meteorological variables from climatology (anomalies) are the targets of forecasts, and the anomalies can be divided into two modes, i.e., free modes and forced modes. The free modes are the anomalies that are predicted under the specification of climatological external forcings for the surface temperature, that are free from the anomalous forcings, whereas the forced modes are the anomalies that correspond to the anomalous components of external forcings. The GCM (general circulation model) is, in some cases, capable of predicting the free mode at least one month ahead (particularly the most extraordinary blocking event in January, 1977), and is, in other cases, marginal. However, the capability could be increased further by improving the GCM. In addition, recent studies have revealed that there are growing evidences for the feasibility of prediction of forced modes over the United States through the teleconnection process from the sea surface temperature anomalies over the equatorial Pacific. Yet the GCM approach is expensive and may be limited in improving mathematical accuracy, to a satisfactory extent. As a remedy, the possibility of anomaly models are being investigated.