Coupling of the Quasi-biennial Oscillation and the Extratropical Circulation in the Stratosphere through Planetary Wave Transport

Abstract

A significant fraction of the interannual variability of the wintertime stratosphere in the Northern Hemisphere is related to the quasi-biennial oscillation (QBO) in tropical winds. Disturbed conditions of the polar night vortex are favored in years when the winds over the equator are easterly. More quiescent conditions prevail at high latitudes when the phase of the tropical QBO is westerly. The effects of tropical winds on the extratropical circulation are studied in the equivalent barotropic framework on the sphere. Calculations are performed with the tropical flow representative of easterly and westerly phases of the QBO. With planetary wave forcing representative of conditions in the lower stratosphere, the wintertime polar vortex is highly disturbed during the easterly phase of the QBO, the evolution taking the form of a ?Canadian warming.? In contrast, more quiescent conditions prevail with the same forcing but with tropical winds representative of the westerly phase of the QBO, the vortex preserving a high degree of polar symmetry in that case. The dependence of the extratropical circulation on tropical winds and the QBO in these calculations originates in planetary wave transport. When tropical winds are easterly, the zero wind line is shifted into the winter hemisphere. Under such conditions, disturbance amplitudes at middle and high latitudes are amplified as is eddy transport which acts to displace and degrade the vortex. On the other hand, when tropical winds are westerly the zero wind line is removed into the summer hemisphere, in particular away from the heart of the vortex. Under these conditions disturbance amplitudes at middle and high latitudes are weaker, resulting in reduced eddy transport and a more orderly circumpolar flow. Modulation of planetary wave activity by the QBO is interpreted in terms of a latitudinal shift of the critical region, where eddy displacements are large. Both eddy stirring and displacement of the vortex out of polar symmetry appear to be involved in modifying the extratropical flow, the latter resulting in nonconservative behavior through thermal dissipation. Modulation of planetary wave transport by tropical winds may also explain interannual variability of ozone at middle and high latitudes.