TITLE: "Downward wave coupling between the stratosphere and troposphere: Comparison with zonal-mean coupling and the impact of ozone depletion"

ABSTRACT: There is increasing evidence that stratospheric processes play an important role in tropospheric weather and climate. The impact of the stratosphere on tropospheric climate traditionally focuses on the impact of planetary wave-mean flow interaction in the stratosphere on the troposphere (called downward zonal-mean coupling). This type of coupling has been shown to be important for seasonal prediction in the troposphere during Northern Hemisphere winter. It is also a mechanism through which Antarctic stratospheric ozone changes are communicated to the troposphere. Another source of downward coupling is planetary wave reflection from the stratosphere (called downward wave coupling).

We investigate the role of downward wave coupling in the seasonal cycle in both hemispheres. In the Southern Hemisphere downward wave coupling occurs from September to December, while in the Northern Hemisphere it occurs from January to March. The wave coupling in both hemispheres occurs when the stratospheric jet has large negative vertical zonal wind shear in the upper stratosphere in high latitudes and large negative vertical and meridional shear in the lower stratosphere in midlatitudes. This configuration of the stratospheric basic state leads to wave reflection in the stratosphere and focuses the reflected wave energy back downward into the troposphere. Downward wave coupling is found to be as important as downward zonal-mean coupling during Northern Hemisphere winter and to dominate stratosphere-troposphere coupling in the Southern Hemisphere during spring. Ozone depletion and recovery are found to significantly impact downward wave coupling. Our results reveal a new mechanism wherein stratospheric ozone changes can affect the tropospheric circulation.

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