Attribution of European Winter 2019/20 Seasonal Forecast Anomalies: Role of Tropical SST Forcing

Abstract:

Seasonal forecasts of the positive NAO and mild European temperatures during DJF 2019/20 were remarkably skilful. This skilful prediction provides a unique opportunity to identify the mechanisms behind the strong seasonal predictability. The strong European forecast signal occurred despite the absence of significant ENSO anomalies. A major anomaly preceding the season was an exceptionally strong positive Indian Ocean Dipole event, whose positive SST anomaly in the west Central Indian Ocean persisted into the DJF season and was well captured by the operational ECMWF seasonal forecasting system SEAS5. A series of attribution experiments are performed to separate the impact of various tropical SST anomalies on the DJF 2019/20 response. In these forced SST experiments, the observed anomalies were replaced by climatology over the tropical Indian, Atlantic and Pacific Ocean basins to isolate the impact of regional SST forcing.

Early results indicate that the strongest impact originates from the Indian Ocean SST anomaly forcing which is consistent with a response to the diabatic heating in the southern Indian Ocean, that drives both a local cross-equatorial Hadley-cell response and northern hemispheric circulation changes mediated by the South Asian mid-latitude jet, with a strong projection onto a positive NAO pattern. The plausible mechanisms for tropical-extratropical pathways will be further explored and discussed.