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.

Key Points

• The statistics of strong and weak vortex events as defined by several easily calculated indices of stratospheric variability are compared

• Statistical and dynamical properties of strong events are found to be more sensitive to the choice of definition than are weak events

• The character of stratosphere-troposphere coupling near the central date of strong events depends strongly on the choice of definition

Plain Language Summary

At times, the stratospheric polar vortex can undergo significant strengthening. Such strengthening is referred to as the polar vortex intensification event. These events are known to improve subseasonal-to-seasonal forecast skills of surface extremes, such as strong winds, and heavy rain. They also have an impact on the Arctic springtime stratospheric ozone concentrations. However, compared to the extensive literature exploring stratospheric sudden warmings, there have been relatively few attempts to establish comprehensive climatology and dynamical benchmarks for strong polar vortex events, and less consensus on how to define strong vortex events. We consider here a range of simple indices of the strength of the polar vortex and evaluate different possible definitions of strong vortex events. To assess these possible definitions, we evaluate how sensitive the climatological statistics of these events are to the choice of definition. We also assess their ability to capture stratosphere–troposphere coupling and meteorological conditions relevant to chemical ozone depletion and show that there are substantial implications for making different choices. Our results highlight the difficulty of settling on a single common definition of strong vortex events and provide useful guidance for selecting an appropriate definition for strong events.