Defining Arctic Stratospheric Polar Vortex Intensification Events

Abstract

Using the ERA5 reanalysis data, we identify seven easily calculable indices of the strength of the Arctic stratospheric vortex: zonal winds at 10 hPa and temperature or height anomalies at 10 , 50, and 100 hPa. We then compare the climatological statistics and meteorological properties of strong and weak events based on these indices. We particularly consider the sensitivity of the event statistics to the choice of thresholds, the use of these indices in capturing stratosphere–troposphere coupling, and meteorological conditions relevant to chemical ozone depletion. The frequency, seasonal distribution, and interdecadal variability of strong events is more sensitive to threshold or index choice compared to weak events. Composites of polar‐cap geopotential height anomalies are found to differ significantly based on the choice of index. In particular, height‐based events reveal a strong and immediate barotropic response near the central date due to surface pressure fluctuations, making it more difficult to regard central dates of height‐based events as purely stratospheric in origin. We further characterize the relationship of all indices to conditions relevant to chemical ozone depletion, finding that temperature‐based indices in the lower stratosphere perform best. Finally, we present four dynamical benchmarks used to assess and compare the representation of strong events in climate models. Our results highlight the challenges in determining the optimal definition for strong events and emphasize the implications of different choices, providing valuable insights for guiding future studies in defining strong events.