Equatorward breaking Rossby waves over the North Atlantic and Mediterranean region in the ECMWF operational Ensemble Prediction System

The equatorward breaking of Rossby waves is a frequent feature of the synoptic-scale circulation over the North Atlantic. It often creates upper-level disturbances at low latitudes which can cause heavy precipitation in the Mediterranean region and Saharan dust outbreaks. The present study is the first to systematically explore the enormous dynamical information content of 12 years of data from the operational Ensemble Prediction System (EPS) of the European Centre for Medium-Range Weather Forecasts for this particular atmospheric feature. Dynamical precursors, forecast quality and predictability are investigated using a range of verification and analysis tools based on potential vorticity (PV). The main conclusions from this work are: (i) the EPS shows an underdispersive behaviour in the subtropics during PV streamer events; (ii) there is a tendency for too weak Rossby wave breaking and therefore a northward shift of the streamers in the forecast; (iii) strong PV streamers in the medium-range forecasts are preceded by an active wave train in the Subtropics, strongly positive PV anomalies in the Extratropics, and latent heating upstream of the PV streamer. There are no clear indications that blocking downstream is an important factor, in contrast to other studies. Analysis tools developed specifically for EPS data in this study such as ensemble correlation techniques could be applied to other atmospheric phenomena in the future.