The interaction between the stratosphere and the troposphere as revealed by singular vectors

Stratospheric singular vectors (S-SV's) have been calculated with one of the latest versions of the ECMWF model, for the months January and July 2004-2006. Basic properties as amplification, wavenumber spectra and preferred geographical position are studied. In January, the majority of these S-SV's amplify by growing on the background wind shear of the polar vortex. For July, the amplification of the S-SV's is mainly associated with the tropospheric summer jet, that extends above 100 hPa. The S-SV's are located lower (100 to 50 hPa) in the stratosphere than the January SV's and their amplification is less. We have also calculated S-SV's during the stratospheric sudden warming (SSW) of January 2006. The amplification factor of the SSV's does not seem to be affected by the SSW. The breaking of the polar vortex is however clearly reflected in the preferred geographical location of the S-SV's. In addition to these 'pure' stratospheric SV's, perturbations have been calculated that initially are located in the stratosphere, but that are optimized to maximize the perturbation energy growth in the low troposphere (below 500 hPa) on a time scale of 48 and 120 hours. Basic properties of this type of SV's are studied. First results, for these Stratosphere-Troposphere SV's (STSV's), show that they mainly seem to rely on the tropospheric jet to maximize their amplification. From a non-modal point of view, small-scale structures with maximal amplitude around total wavenumber 10 seem to be important in the interaction between the stratosphere and troposphere.