| Recent observational analyses show downward propagation of flow anomalies from the stratosphere to the troposphere. This thesis examines the hypothesis that stratospheric modulation of baroclinic instability in the midlatitude jet is the mechanism responsible for the downward communication of dynarnical influence from the stratosphere to the troposphere.;Chapter 2 reports on initial value lifecycle experiments in a dynarnical core, we show that increased winds in the lower stratosphere result in enhanced poleward momentum fluxes in baroclinic lifecycles, corresponding to a poleward shift in the mean position of the tropospheric jet consistent with observations in which a one standard deviation change in the index of the stratospheric northern annular mode (NAM) corresponds to a 10 hPa dipole in surface pressure. The role of eddy feedback onto stratospheric flow anomalies penetrating the troposphere is also investigated, and feedback is found to enhance the response to stratospherically confined anomalies by up to a factor of 10.;Chapter 3 investigates the tropospheric structure of the annular modes, and finds the ubiquitous dipole structure to be a simple consequence of the meridional movement of the mid-latitude jet, independent of dynamics.;Chapter 4 investigates the role of lower stratospheric vertical shear. Starting with a modified Eady model and proceeding through a hierarchy of models and observations, it is found that increased stratospheric shear increases the poleward momentum flux in baroclinic instability, resulting in a surface annular mode signal again consistent with observations. En passant, it is found that at intermediate synoptic wavenumbers changing the stratospheric shear can induce a dramatic transition from anti-cyclonic to cyclonic wavebreaking when the instability saturates.;Chapter 5 investigates the use of the stratosphere in predicting mid-latitude storms generated by baroclinic instability. Results quantitively consistent with previous simpler statistical models of the stratosphere are found, with the stratosphere able to predict about 10% of the variance of monthly storm transits through 5 degree grid-boxes. |
