Long‐Term Changes in the Northern Midwinter Middle Atmosphere in Relation to the Quasi‐Biennial Oscillation

Long‐term changes in the middle atmosphere due to anthropogenic greenhouse gas emissions are examined in relation to the effect of the equatorial Quasi‐Biennial Oscillation (QBO) on the northern midwinter circulation. The examinations are based on the Coupled Model Intercomparison Project Phase 5 simulations for 1979–2100 with the Earth‐System‐Model MPI‐ESM‐MR that generates the QBO internally. In particular, the three‐dimensional residual circulation is used as proxy for the Brewer‐Dobson circulation, revealing an increasing downwelling in the center of the polar low over Northern Europe/Siberia (~5% per decade). The changes in northern midwinter temperature, zonal wind, and residual circulation are much stronger during westerly (QBO‐W) than easterly (QBO‐E) phase of QBO (e.g., for a moderate increase in greenhouse gases, we find maximum decreases in the zonal mean westerly jet at 60°N and 3 hPa of about −14.8 ± 5.4 m/s for QBO‐W but only −4.7 ± 5.2 m/s for QBO‐E). This is due to a change of the extratropical QBO‐W signature toward QBO‐E signature while the equatorial QBO remains nearly unchanged (i.e., a change toward disappearance of the so‐called Holton‐Tan relationship). Similar to the current change from QBO‐W to QBO‐E signature, the changes during QBO‐W include an increase in amplitude and eastward shift in phase of stratospheric stationary Wave 1 at the cost of Wave 2, with decreasing westerlies over North America and increasing downwelling over Siberia. The eastward shift in phase of stationary Wave 1 is related to the associated increase in meridional transport of planetary vorticity.