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

Abstract

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 extendedrange 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 latitudinal 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.