Coupling of the quasi-biennial oscillation and the extratropical circulation in the stratosphere through planetary wave transport

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'. With the same forcing but with tropical winds representative of the westerly phase of the QBO, the vortex preserves a high degree of polar symmetry in that case. 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. On the other hand, when tropical winds are westerly the zero wind line is removed into the summer hemisphere, resulting in reduced eddy transport and a more orderly circumpolar flow. -from Authors