An Explanation of the Poleward Mass Flux in the Stratosphere

This paper offers a new perspective on the explanation of the poleward mass flux in the stratosphere. This mass flux represents the upper leg of the so-called Brewer–Dobson circulation. This new perspective is based on the following hypothesis. A positive potential vorticity anomaly, centered over the North Pole, exists in the stratosphere during the winter half-year. This positive potential vorticity anomaly is associated with a negative isentropic density anomaly, which forms due to cross-isentropic downwelling associated with radiative cooling. Isentropic potential vorticity mixing due to breaking planetary waves weakens this potential vorticity anomaly while zonal-mean thermal wind balance is maintained. This requires a weakening of the negative Polar cap isentropic density anomaly, which in turn requires a poleward isentropic mass flux. Support for this hypothesis is found in a case study of a major Sudden Stratospheric Warming event, as an example of intense potential vorticity mixing. It is shown that the stratosphere, both before and after this event, is very close to zonal-mean thermal wind balance, despite the disruptive potential vorticity mixing, while mass is shifted poleward during this event. Solutions of the potential vorticity-inversion equation, which is an expression of thermal wind balance, for zonal-mean potential vorticity distributions before and after the Sudden Stratospheric Warming, demonstrate that mass must shift poleward to maintain zonal-mean thermal wind balance when the positive potential vorticity anomaly is eliminated by mixing. This perspective on the reasons for the poleward stratospheric mass flux also explains the observed isobaric warming as well as the Polar cap zonal-mean zonal wind reversal during a major Sudden Stratospheric Warming.