Assessment of the predictability of cold-wet-windy Pan Atlantic compound extremes

Occurrence of cold spells in different North American regions has been related to concurrent wet and windy extremes in Western Europe. This link is driven by an anomalous state of the North Atlantic storm track. Two dynamical pathways have been defined as potential origins of the Pan-Atlantic compound extremes. The first pathway is linked to a Rossby wave train propagating from the Pacific toward the Atlantic, associated with a pronounced Alaskan ridge. The second pathway is characterized by the presence of a high west of Greenland, that favors simultaneously a southward displacement of a trough over eastern USA and an upper level trough over South western Europe. This study investigates the predictability of flow associated with cold spells over north America from a dynamical systems perspective, with a focus on the underlying diversity of atmospheric states and wave processes.

We start by assessing the intrinsic predictability of these two pathways using the ERA5 reanalysis and dynamical systems indicators. These indicators can be used as proxies for the predictability of each pathway. We also examine the predictability of those two pathways across different climatological periods. We further explore how variations in Rossby wave behavior and stratospheric anomalies modulate the predictability of these cold spells. We complement this analysis using the ECMWF ensemble reforecasts at different lead times, and computing skill scores for the two pathways. This help to provide new insights into the dynamical precursors and sources of predictability for compound cold and windy extremes across the North Atlantic sector.