The three-dimensional life cycles of potential vorticity cutoffs: a global and selected regional climatologies in ERA-Interim (1979–2018)
Authors: Portmann Raphael , Sprenger Michael , Wernli Heini 
Published: June 2021

Abstract. The aim of this study is to explore the nature of potential vorticity (PV)
cutoff life cycles. While climatological frequencies of such near-tropopause 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 150 000 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. Then, 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 shows distinct dynamical scenarios for the genesis of 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 in mid-latitudes in regions with split-jet conditions and is frequently accompanied by surface cyclogenesis and substantial precipitation. Type III cutoffs preferentially form due to cyclonic Rossby wave breaking within extratropical cyclones in the storm track regions. We show that important track characteristics (speed, travel distance, frequency of decay and reabsorption, isentropic levels) differ between the categories, while lifetime is similar in all categories. Finally, 12 PV cutoff genesis regions in DJF and JJA are selected to study the regional characteristics of PV cutoff life cycles. As a particularly novel aspect, the vertical evolution of PV cutoffs along the life cycle is investigated. We find that, climatologically, PV cutoffs reach their maximum vertical extent about one day after genesis in most regions. However, while in some regions PV cutoffs rapidly disappear at lower levels by diabatic decay, they can grow downward in other regions. In addition, regional differences in lifetimes, the frequencies of diabatic decay and reabsorption, and the link to surface cyclones are identified that cannot be explained only by the preferred regional occurrence of the different cutoff types as defined above. Finally, 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 in different regions of the globe. However, these life cycles can be substantially modified by specific regional conditions.