The three-dimensional life cycles of potential vorticity cutoffs

The aim of this study is to explore the nature of potential vorticity (PV) cutoff life cycles. While climatological frequencies of such upper-level cyclonic vortices are well known, their life cycle and in particular their three-dimensional evolution is poorly understood. To address this gap, a novel method is introduced that uses isentropic air parcel trajectories to track PV cutoffs as three-dimensional objects. With this method, we can distinguish the two fundamentally different PV cutoff lysis scenarios on isentropic surfaces: complete diabatic decay vs. reabsorption by the stratospheric reservoir.

This method is applied to the ERA-interim dataset (1979-2018) and the first global climatology of PV cutoffs is presented that is independent of the selection of a vertical level and identifies and tracks PV cutoffs as three-dimensional features. More than 140’000 PV cutoff life cycles are identified and analyzed. The climatology confirms known frequency maxima of PV cutoffs and identifies additional bands in subtropical areas in the summer hemispheres and a circumpolar band around Antarctica. The first climatological analysis of diabatic decay and reabsorption shows that both scenarios occur equally frequently – in contrast to the prevailing opinion that diabatic decay dominates.

Further, PV cutoffs are classified according to their position relative to jet streams [equatorward (type I), between two jets (type II), and poleward (type III)]. A composite analysis of PV cutoff genesis shows distinct dynamical scenarios for the three types. Type I forms due to anticyclonic Rossby wave breaking above subtropical surface anticyclones and hardly results in precipitation. Type II results from anticyclonic Rossby wave breaking downstream of the storm tracks and is frequently accompanied by surface cyclogenesis and substantial precipitation. Type III cutoffs preferentially form due to wave breaking within mature extratropical cyclones in the storm track regions. We show that important track characteristics (speed, travel distance, frequency of decay and reabsorption, vertical evolution) differ between the categories, while lifetime is similar in all categories. 

Finally, twelve PV cutoff genesis regions in DJF and JJA are selected to study the regional characteristics of PV cutoff life cycles. We find that many characteristics of these PV cutoffs reflect the preferred regional occurrence of the different life cycle types. However, a few regions are characterized by substantially longer (e.g. central subtropical North Atlantic in summer) or shorter (Mediterranean in summer) lifetimes than most other regions. Furthermore, a remarkable variability in the vertical evolution of PV cutoffs is found. While in some regions, PV cutoffs rapidly disappear at lower levels by diabatic decay, they can grow downward in other regions. We also show that in many regions PV cutoffs can be involved in surface cyclogenesis even after their formation.

This study is an important step towards quantifying fundamental dynamical characteristics and the surface impacts of PV cutoffs. The proposed classification according to the jet-relative position provides a useful way to improve the conceptual understanding of PV cutoff life cycles. However, these life cycles can be substantially modified by specific regional conditions.