Sensitivity of the boreal winter circulation in the middle atmosphere to the quasi-biennial oscillation in MAECHAM5 simulations

Abstract

The polar vortex in the Northern Hemisphere exhibits high intraseasonal and interannual variability which, to some degree, may be controlled by the quasi-biennial oscillation (QBO) in the tropical stratosphere. Here we analyze the QBO signal in the Northern Hemisphere polar vortex in a model simulation using the general circulation model MAECHAM5 ( Middle Atmosphere European Center Hamburg Model), which simulates the QBO as an internal mode of variability. Composites have been computed for the westerly and easterly QBO phases from early winter to late winter ( November December, December - January and January - February) of a 50-year experiment containing 20 complete QBO cycles. This method identifies tropical and midlatitude patterns in wind and temperature that are related to the secondary meridional circulation of the QBO extending towards the winter pole. In the tropics, significant QBO signature is observed in the mesosphere up to 0.05 hPa only in early winter and mainly in the easterly QBO phase forced by the parameterized gravity waves-mean flow interaction. At high latitudes, MAECHAM5 shows a significantly warmer (colder) polar stratosphere in the easterly ( westerly) QBO phase at 30 hPa accompanied by a weaker ( stronger) polar vortex in the late winter months, from December to February ( December to January) in the easterly ( westerly) phase of the QBO. In early winter there is no significant change in temperature and zonal mean zonal wind in the polar stratosphere. The analysis of EP fluxes shows a different behavior between QBO phases, with changes in the waveguide. Wave propagation occurs upwards and polewards in the easterly QBO phase at 30 hPa, while waves are refracted equatorward in the westerly phase. No relationship has been found between the tropical QBO and the final warming at the end of the boreal winter.