Changes in stratospheric transport and mixing during sudden stratospheric warmings

Abstract

The extreme disruptions of the wintertime stratospheric circulation during sudden stratospheric warmings (SSW) have large effects on tracer concentrations through alterations in transport. This study analyzes the changes in residual circulation and isentropic mixing associated with SSWs, by performing composites using reanalysis (European Centre for Medium-Range Weather Forecasts Re-Analysis Interim) and simulations of the Whole Atmosphere Community Climate Model. The advective Brewer-Dobson circulation accelerates around 15days prior to the wind reversal at 60 degrees N, 10hPa during the onset of SSWs. Soon afterward, it decelerates, leading to reduced advective transport into the vortex and descent over the pole, which persist for more than 2months below 30hPa. The isentropic mixing has a distinct signature in altitude: It is enhanced at the central date of the SSW in the midstratosphere (about 10hPa or 800K), and this signal is delayed and more persistent at lower altitudes. It is shown that sufficiently deep SSWs (particularly those related to Polar-night Jet Oscillation events) have a stronger response in the Brewer-Dobson circulation and mixing. In particular, both the polar downwelling and the tropical upwelling are anomalously weak in the lower stratosphere for 90days after the onset of Polar-night Jet Oscillation events. The redistribution of potential vorticity during the life cycle of SSWs is discussed due to its relevance for the stratospheric circulation. It is shown that the diffusive flux of potential vorticity, calculated in equivalent latitude coordinates, remains anomalously high in the lower stratosphere, a feature that is not seen in more conventional advective eddy fluxes across latitude circles.