Arctic Amplification of Stratospheric Variability (Invited Presentation)

At mid-to-high latitudes, the stratosphere contains >25% of the column of atmospheric mass. Large-scale vertically-propagating waves drive a synchronised meridional circulation that moves mass into and out of the polar cap, modulating adiabatic warming of the stratospheric polar air column, and altering the strength of the stratospheric polar vortex. These stratospheric changes are associated with substantial effects on surface weather and climate, especially on Northern Annular Mode (NAM) with associated long-lasting shifts in the jet streams, storm tracks, precipitation, and likelihood of blocking events. Despite unambiguous observations of this phenomenon, as well as numerical simulations, a clear physical explanation of this downward coupling remains elusive. Here we view the stratospheric polar vortex as an isolated potential vorticity (PV) anomaly, and we show that PV theory accounts for the observations that the meridional circulation extends below the tropopause, modulating the day-to-day thickness of the troposphere, especially over the Arctic. We argue, based on PV theory, that Arctic temperature anomalies at 100 hPa can be used as a proxy for the integrated effect of polar stratospheric PV anomalies above. We introduce a polar cap pressure index that provides a quantitative vertical measure of stratosphere–troposphere coupling that can be used to compare atmospheric climate/weather models to each other, as well as to observations. The stratospheric pressure anomaly signal is a minimum near the tropopause, and is amplified in the troposphere, where the signal is largest at the surface near the North Pole. This Arctic amplification of stratospheric variability cannot be accounted for by a stratospheric PV anomaly. Rather, it is consistent with self-amplification of the NAM by temperature advection, with polar surface PV (potential temperature) anomalies induced by variations of the strength of the stratospheric PV anomaly.