Trends and Variability in the Northern Hemisphere Stratospheric Polar Vortex over the Last 100+ Years

Emerging evidence shows that the Northern Hemisphere (NH) stratospheric polar vortex has undergone significant weakening in magnitude and a shift in its mean seasonal position over the last two decades. These changes in the stratosphere polar vortex have importance on seasonal timescales (through its connections to tropospheric winter weather regimes) and longer timescales (i.e., how a potentially weaker vortex may impact winters in a future warmer world). While several hypotheses have been suggested for this weakening trend, we still do not fully understand the magnitude of natural or internal variability that the vortex possesses. This study attempts to address this issue by exploring several metrics of the strength and position of the NH stratospheric polar vortex using long (100+ year-long) reanalysis products and climate models. Using regime detection techniques, we identify periods of strong and weak vortex winters in the past, which occur on quasi-decadal timescales. Potential dynamical forcings on the stratospheric circulation, including vertical wave activity flux, tropical and North Pacific sea surface temperature variability, and Arctic cryospheric variables are also related to these strong/weak vortex periods to understand the potential mechanisms yielding this variability. Spatial statistics describing vortex variability (e.g., spatial trends in geopotential heights, changes in the centroid latitude of the vortex, and the maximum displacement of the vortex from the pole) are also quantified. The trends from reanalysis are then compared and contrasted to NH polar vortex variability diagnosed in long control runs of several climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) models. We also relate the findings to how the models handle stratosphere-troposphere coupling dynamics, as models have known biases in these dynamics. While we are able to replicate the recent weakening trend and spatial shift of the vortex, these trends exhibit strong variability over the 20th century. As such, preliminary results suggest that the range of internal variability in the NH polar vortex may be large enough such that the recent weakening trend is not statistically distinguishable from its natural or internal variability - at least, not yet. Relations between these findings and the nature and characteristics of NH winters in the middle and high latitudes in the recent past are also discussed.