A Perspective on Potential Vorticity Dynamics and Jet Structure During South African Cut Off Lows

Cut-off lows (COLs) are recurrent synoptic-scale systems over southern Africa and are frequently associated with high-impact weather, yet their propagation and persistence remain challenging to predict. This study investigates the dynamical structure and evolution of two representative COL events using reanalysis data, with emphasis on geopotential height, sea-level pressure, and three-dimensional potential vorticity (PV) diagnostics. Synoptic identification and PV-based analyses are used to characterise the vertical structure, cold-core development, jet interaction, and propagation of systems. Both events exhibit classical COL characteristics, including closed mid-tropospheric circulation, vertically coherent PV anomalies, and pronounced cold-core structures, but differ substantially in their propagation behaviour. The first event exhibits a deeper and more vertically stacked PV anomaly with limited interaction with the upper-level jet, resulting in quasi-stationary behaviour and prolonged persistence. In contrast, the second event exhibits a more symmetric and comparatively shallower PV structure that remains dynamically coupled to the mid- to upper-tropospheric jet, promoting enhanced downstream advection and a more progressive evolution. Vertical cross-section analyses indicate that the depth and coherence of the cold core, together with the degree of jet–PV interaction, are important factors in modulating COLs propagation and longevity. These results demonstrate the importance of a three-dimensional PV perspective for understanding COLs dynamics over southern Africa and highlight how differences in large-scale dynamical coupling can influence their propagation and predictability.