Impacts of wave forcings on the stratospheric residual meridional circulation in an idealized atmospheric model

The impacts of synoptic and planetary wave forcings on different branches of stratospheric residual meridional circulation are investigated in an idealized 3d atmospheric model. The model is forced by Newtonian relaxation to the prescribed equilibrium temperature, which is imposed with a seasonal cycle in the stratosphere but is time-independent in the troposphere. Additional surface topography is included in the Northern Hemisphere to generate tropospheric planetary wave forcing.

The impacts of wave forcings are diagnosed in a zonally symmetric version of the full model. Daily eddy forcing is calculated from the full model and the effects of different wave numbers and different regions of the forcings are separately diagnosed. The mean wintertime stratospheric temperature response to total eddy forcings displays a pattern of cooling in the tropics and warming in the middle and high latitudes, as expected from the Brewer-Dobson circulation. This pattern is primarily due to the planetary wave forcing in the stratosphere, and partly due to synoptic wave forcing in the lower stratosphere. Among the stratospheric temperature response to planetary wave forcing, eddy momentum flux and resultant subtropical wave drag are more important for tropical cooling, whereas eddy heat flux is more important for high latitude warming. This suggests the meridional wave propagation to low latitudes is necessary for extratropical waves to induce tropical upwelling. We will also present to what extent the daily evolution of planetary wave forcing during transient events (such as stratospheric sudden warming or stratospheric final warming) can induce a residual circulation and associated tropical cooling.