Defining Arctic Stratospheric Polar Vortex Intensification Events

 Using the ERA5 reanalysis data, we identify seven easily calculable indices of the strength of theArctic 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 basedon these indices. We particularly consider the sensitivity of the event statistics to the choice of thresholds, theuse of these indices in capturing stratosphere–troposphere coupling, and meteorological conditions relevant tochemical ozone depletion. The frequency, seasonal distribution, and interdecadal variability of strong eventsis more sensitive to threshold or index choice compared to weak events. Composites of polar-cap geopotentialheight anomalies are found to differ significantly based on the choice of index. In particular, height-basedevents reveal a strong and immediate barotropic response near the central date due to surface pressurefluctuations, making it more difficult to regard central dates of height-based events as purely stratosphericin origin. We further characterize the relationship of all indices to conditions relevant to chemical ozonedepletion, finding that temperature-based indices in the lower stratosphere perform best. Finally, we presentfour 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 emphasizethe implications of different choices, providing valuable insights for guiding future studies in defining strong events.