Predictability Associated with High-Latitude Retrograde Waves in the 1979-80 Winter Season

Retrograde long waves in the higher latitudes of the Northern Hemisphere can episodically attain large amplitudes and sustain coherent phase propagation for 2 – 3 weeks. The potential influence of such waves on extended-range weather forecast has been conjectured but not systematically quantified. Using a set of ensemble reforecast data, this study examined the predictability associated with an extraordinary retrograde-wave episode in the 1979 – 80 winter. Quantified by the anomaly correlation of the 500 hPa geopotential height in the 40 – 70°N latitudi nal band, increased week-2 predictability was found within the subperiod with the presence of coherent retrograde waves. Some individual forecasts made within the retrograde-wave event exhibited the behavior of “return of skills”. The results suggest a future investigation into the relation between the elevated level of anomaly correlation in week-2 and detailed dynamics of the retrograde waves.