The relation between Rossby wave-breaking events and low-level weather systems

Abstract

Rossby wave-breaking events describe the last stage in the life cycle of baroclinic atmospheric disturbances. These breaking events can strongly influence large-scale circulation and are also related to weather extremes such as heat waves, blocking, and extreme precipitation events. Nonetheless, a complete understanding of the synoptic-scale dynamics involved with the breaking events is still absent. For example, it is not clear how well the theoretical life cycle experiments, which use a specified initial perturbation with a single zonal wavenumber and a prescribed simplified initial zonal jet, capture the life cycle of real-atmosphere weather systems. Here we combine a storm-tracking technique together with a wave-breaking detection algorithm to examine how upper-level wave breaking and surface weather systems are related in the North Atlantic during winter. These datasets allow us to examine whether upper-level wave breaking and low-level weather systems always occur simultaneously and if we can identify preferred relations between the surface weather system type (cyclone or anticyclone) and the type of the upper-level breaking event (cyclonic or anticyclonic wave breaking denoted CWB or AWB, respectively). We find that in the North Atlantic, most weather systems are associated with AWB and/or CWB at some point during their lifetime, while only few cyclones and anticyclones do not involve any upper-level wave breaking (roughly 11 % and 15 %, respectively). Our results imply that composites of cyclones and anticyclones involve a mixture of different types of life cycles, depending on whether they involve CWB or AWB, as well as their position relative to the Rossby wave-breaking (RWB) center. Moreover, the system characteristics (including actual and relative positions, intensities, and displacements) differ depending on the associated breaking type. We distinguish between “same-pairing” cases (i.e., cyclone with CWB and anticyclones with AWB) and “opposite-pairing” cases (i.e., cyclones with AWB and anticyclones with CWB). Compositing the cyclones and anticyclones based on this criterion, we find that in similar pairings 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), but, for opposite pairings, 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. A better understanding of the different life cycles of real-atmosphere cyclones and anticyclones and the upper-level breaking they involve is important for exploring the relation between storm tracks and slowly varying weather regimes and how they are mediated by RWB events.