The three-way feedback between North-Atlantic circulation regimes, Rossby wave breaking, and surface weather.

Rossby Wave Breaking (RWB) events describe the last stage in the life cycle of baroclinic atmospheric disturbances. These breaking events can strongly influence the large-scale circulation and are tightly related to low-frequency weather regimes. In addition, RWBs are often associated with weather extremes such as heat waves, blockings, and extreme precipitation events. Here we examine the three-way interaction between RWB, weather regimes, and surface weather in the North Atlantic. This is done by combining a RWB detection algorithm, a storm-tracking routine, and a clustering technique to identify low-frequency circulation regimes in the North-Atlantic. We find that regardless of weather regime, most cyclones and anticyclones are associated with an Anticyclonic Wave Breaking (AWB) and/or a Cyclonic Wave Breaking (CWB) at some point during their lifetime, while very few storms do not involve any upper-level wave breaking (~11%). Moreover, storm characteristics (e.g., actual and relative positions, intensities, and displacements) differ depending on the associated breaking type. In “same-pairing” cases (i.e., cyclones with CWB and anticyclones with AWB) the surface system is positioned so that its associated upper-level winds would enhance the breaking (the anomalous circulation is in the same direction as the background shear). In “opposite-pairing” cases (i.e., cyclones with AWB and anticyclones with CWB), the upper-level winds associated with the surface system do not act to enhance the breaking which occurs in the direction of the background shear. In addition, we find that the surface storm characteristics are significantly altered with the weather regime, with distinct and clearly preferred storm paths in each cluster. We suggest a picture in which the resulting RWB frequencies and positions in each cluster are modified by the corresponding tracks of cyclones and anticyclones, with the maximum breaking found where there is a constructive interaction with the low-frequency flow. The positions of RWBs, in turn, shape the overall cluster structure and contributes to the persistence of the weather regime. An improved understanding of the relation between weather systems, RWB events, and weather regimes can also help us improve our understanding of and confidence in projected future circulation changes (e.g., by relating changes in the frequency and positions of RWB events, storm-tracks, and the North-Atlantic jet).