Stratospheric influence on tropospheric circulation through altered tropospheric eddy phase speeds

We discuss the assertion that the stratosphere influences the surface wind field and the associated tropospheric eddy fluxes in large part by modifying tropospheric eddy phase speeds. As motivation for this hypothesis, we first describe recently published work, in collaboration with Walter Robinson, on the effects of changes in the strength of surface friction on the latitude of the surface westerlies. It is argued that the increase in phase speed of midlatitude eddies is the key to the poleward displacement of the westerlies that one sees in models when surface friction is reduced in strength. We describe observational evidence for an increase in phase speed in the Southern Hemisphere. Several recent life-cycle studies of baroclinic instability are reviewed which indicate sensitivity of upper tropospheric wave breaking to lower stratospheric winds. A stochastically forced shallow water model of the upper troposphere, in which the phase speed of the eddies can be controlled, is used to make plausible that an increase in phase speed can cause a poleward displacment of the surface westerlies.