A Helicity‐Based Analysis of the Stratospheric Polar Vortex Evolution

Abstract The Arctic atmosphere in the winter months of the Northern Hemisphere is influenced by a stratospheric polar vortex, characterized by strong westerly circumpolar winds and extremely low temperatures. These vortices impact surface weather in various ways, and their dynamics may be affected by recent anthropogenic climate change. However, key aspects of these dynamics, such as the processes of energy and helicity cascading, remain unclear. In this study, we propose a novel approach to studying polar vortex dynamics that considers kinetic helicity. Mainly using ERA5 reanalysis data but also Aeolus Level 2C wind data, we examine the evolution of kinetic helicity in the stratosphere over the Arctic region during winter 2022/23. Our focus is on the formation and stability of the polar vortex, as well as sudden stratospheric warmings (SSWs). We find that the polar vortex is strongly helical, with the sign of kinetic helicity depending on the evolutionary stage of the vortex. Our analysis of the kinetic energy and kinetic helicity spectra reveals the presence of dual cascades during vortex formation. The spectral properties of the stratosphere over the Arctic change seasonally and as a function of the magnitude of the kinetic energy and helicity, with spectra being steeper during polar vortex activity. Similar correlations are absent for the spectra in the troposphere. Finally, we found that the evolution of kinetic enstrophy and helicity can predict the occurrence of SSWs with an accuracy of approximately 8 days.