Topographic Forcing from East Asia and North America in the Northern Winter Stratosphere and Their Mutual Interference

Abstract

This study uses the stratosphere-resolved Whole Atmosphere Community Climate Model to demonstrate the “independent” and “dependent” topographic forcing from the topography of East Asia (EA) and North America (NA), and their “joint” forcing in the northern winter stratosphere. The mutual interference between the EA and NA forcing is also demonstrated. Specifically, without EA, an independent NA can also, like EA, induce a severe polar warming and weakening of the stratospheric polar vortex. While EA favors a displacement of the polar vortex toward Eurasia, NA favors a displacement toward the North America–Atlantic region. However, the independent-EA-forced weakening effect on the polar vortex can be largely decreased and changes to a location displacement when NA exists, and the interference the other way around is even more critical, being able to completely offset the independent-NA-forced effect, because EA can substantively obstruct NA’s effect on the tropospheric wave pattern over the Eurasia–Pacific region. The much stronger (weaker) interference of EA (NA) is associated with its stronger (weaker) downstream weakening effect on the zonal flow that impinges on NA (EA). The mutual interference always tends to further destruct the upward wave fluxes over the eastern North Pacific and enhance the downward wave fluxes over North America. The overall changes in upward wave fluxes, as well as that in the Rossby stationary wavenumber responsible for the stratospheric changes, are related to changes in the zonal-mean flow pattern. The joint effects of EA and NA, rather than being a linear superimposition of their independent effects, are largely dominated by the effects of EA.