Statistical features of mergers between potential vorticity cutoffs using a three-dimensional tracking algorithm considering various connection types

A stratospheric air mass with high potential vorticity (PV) can intrude the troposphere and result in severe weather phenomena. The air mass can be observed as vortex-like high PV anomalies disconnected equatorward from the stratospheric high-PV reservoir in the upper troposphere. This paper refers to this high-PV region as a PV cutoff. Motivated by case studies, this study aims to investigate the importance of mergers of a PV cutoff with another cutoff on maintenance and background conditions contributing to the mergers. For this purpose, we developed a new method to detect and track three-dimensional PV cutoffs. This new method can constrain the evolution of cutoffs after a merger or a split. Firstly, we investigated frequency of PV cutoffs and found the increasing linear trend in the number of events during 1958–2021. We also revealed that they are located on poleward of the climatological strong wind zone in all seasons, whereas in summer several local maxima are identified on equatorward. Secondly, we investigated the lifetime of PV cutoffs and obtained results suggesting that mergers contribute significantly to the maintenance of PV cutoffs by extending their lifetime. We also found out that the extension would be longer if they have similar areas. Lastly, we made composite maps of the background field and showed that stronger cyclonic shear is favorable for PV cutoff mergers. This condition generally comes from the subtropical jet, but in some extratropical areas in summer, the condition might be related to background strong wind split from the jet.