The Arctic Polar Vortex and Its Impacts

During northern winter, the circulation of the stratosphere is highly variable, with occasional reversals of the usually cyclonic circulation known as sudden stratospheric warmings. Variations in the strength of the polar vortex, with a time scale of a few weeks, are associated with substantial effects on surface weather and climate, especially on sea-level pressure (SLP) and the Northern Annular Mode (NAM), with associated shifts in the jet streams, storm tracks, and precipitation. Despite unambiguous observations of this phenomenon, as well as numerical simulations, a quantitative physical explanation remains elusive. The SLP response to stratospheric variability is an order of magnitude larger than would be expected by the remote effects of a stratospheric potential vorticity anomaly. This necessitates a tropospheric amplification mechanism in which variations in the strength of the polar vortex trigger a leading-order feedback mechanism that amplifies polar SLP anomalies. In observations, the tropospheric heat flux into the Arctic is modulated by stratospheric variability, and that leads to low-level Arctic temperature anomalies. Cold Arctic anomalies induce higher SLP through the mechanism of radiative cooling-induced anticyclogenesis, and warm anomalies induce lower SLP. The net effect is to amplify the effects of stratospheric variability.