Stratosphere-Troposphere Coupled Variability in the Wintertime Northern Hemisphere

Two aspects of coupled variability in the stratosphere and troposphere are studied. The first concerns tropospheric precursors of Northern Hemisphere wintertime stratospheric polar vortex variability. Regional extratropical tropospheric variability in the North Pacific and Eastern Europe is well correlated with variability in the polar vortex in both the European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis record and in WACCM (Whole Atmosphere Community Climate Model). In order to explain this correlation, we analyze the link between stratospheric vertical EP flux variability and tropospheric variability. Simple reasoning shows that variability in the North Pacific and Eastern Europe can deepen or flatten the wintertime tropospheric stationary waves, providing a physical explanation for the correlation between these regions and vortex weakening. Several implications of this effect are explored.;The second concerns the effect of the Quasi-Biennial Oscillation (QBO) of the tropical stratosphere on the extratropical tropospheric circulation in wintertime. El-Nino events appear to have a significantly stronger teleconnection when the equatorial stratospheric Quasi-Biennial Oscillation (QBO) is in its westerly phase relative to its easterly phase at 70hPa in both the reanalysis record and in the Whole Atmosphere Community Climate Model (WACCM). Composites of the QBO at 50hPa or 70hPa in the reanalysis and WACCM show easterly anomalies at the climatological subtropical jet position and westerly anomalies in the deep tropics under EQBO. A shallow water model linearized about the zonal wind anomalies associated with the EQBO gives a weaker extratropical response to a deep tropical vorticity anomaly, suggesting a dynamical explanation of how the QBO can influence the extratropical response to anomalous ENSO convection. These results suggest that modification of wave propagation is likely an important cause of the observed differences in El-Nino teleconnections between easterly and westerly QBO phases, but sampling variability and differences in underlying tropical convection are also likely contributors.;Finally, the dynamics behind the downward arch of QBO wind anomalies into the troposphere are explained. QBO momentum anomalies require a meridional circulation to establish thermal wind balance. This circulation extends downwards into the troposphere and induces zonal wind anomalies in the subtropical troposphere. In the presence of extratropical eddies, the zonal wind anomalies are intensified and extend downward to the surface. This pathway is found to be important in both the presence and absence of vortex and convective variability, and explains much of the tropospheric North Pacific response to the QBO.