Diagnosis of northern hemispheric regime behaviour during winter 2013/14

TitleDiagnosis of northern hemispheric regime behaviour during winter 2013/14
Publication TypeTechnical memorandum
Date Published2015
Secondary TitleECMWF Technical Memoranda
AuthorsRodwell, M, Ferranti, L, Magnusson, L


During the northern winter of 2013/14, temperatures over North America were much colder than normal. On the other hand, Eastern Europe was anomalously warm, and strong storms and heavy precipitation affected Western Europe. The temperature signals are evident in the ECMWF ensemble (with smaller amplitude than in reality) at lead-times out to week four. The precipitation anomaly was predicted out to about 10 days. The study suggests that these anomalies were embedded within a hemispheric “regime” that was partly forced by tropical and underlying sea-surface temperatures via “Rossby Wave Source” forcing (associated with convection and its divergent out-flow). It is possible that this forcing may have been responsible for the moderate predictive skill found. Over the North Atlantic, we argue that enhanced baroclinicity associated with the planetary wave regime led to increased storminess, and suggest that the corresponding enhanced precipitation could also have acted to re-enforce the up-stream wave. Finally, we demonstrate the application of some new diagnostics of the data assimilation system (and of the ensemble of data assimilations) that are beginning to shed light on flow-dependent systematic model error and our representation of model uncertainty.


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