The Importance of Background State for the Climatology of Equatorial Kelvin Wave Propagation into the Stratosphere

We analyse the propagation of equatorial Kelvin waves from the troposphere to the stratosphere using a new filtering technique applied to ERA-Interim temperature data (results for COSMIC temperatures very similar) that allows separation of wave activity into number of waves and wave amplitude. A detailed climatology of Kelvin waves in ERA-Interim data is presented. We show that ray tracing calculations, using climatological winds and temperatures, can explain the observed patterns of wave activity, and that non-local ray convergence plays a central role. As a consequence, there is no simple local relationship to zonal wind. We find that easterlies amplify and deflect the eastward travelling waves upwards, and that westerlies have the opposite effect, but these effects are also dependant on the entire tropical background wind field. The success of the ray tracing approach in qualitatively explaining the climatology shows that seasonal and interannual variability of Kelvin wave propagation is dominated by the variability in the wind field rather than by tropospheric convectively coupled wave activity.