Weakening and Shift of the Arctic Stratospheric Polar Vortex: Internal Variability or Forced Response?

Several recent studies have proposed that the Arctic stratospheric polar vortex has weakened and shifted away from the North Pole during the past three decades. Some of these studies suggest that this trend has been driven by a decline in Arctic sea-ice leading to enhanced zonal wavenumber-1 waves propagating into the stratosphere, and that it has in turn contributed to a recent wintertime surface cooling over North America and some parts of Eurasia. Here trends in several measures of the location and strength of the stratospheric polar vortex from 1980-2016 are examined in two reanalysis products. All measures show weakening and equatorward shift trends, but only one measure, the vortex centroid latitude, has a trend which is statistically significant at the 95% level. By comparing large ensembles of historical simulations with pre-industrial control simulations for two coupled climate models (CESM1 and CanESM2), the ensemble mean response of the vortex is found to be small relative to the variability between ensemble members. There is also no relationship between sea-ice decline and trends in either vortex location or strength. Despite this, individual ensemble members are found to have vortex trends similar to those observed, indicating that these trends may be primarily a result of natural internally-generated climate variability.