Annual cycle of wave driving in the tropical tropopause layer

Abstract:

Wave forcing in the tropical tropopause layer is an important driver of the cross-tropopause upwelling and troposphere-to-stratosphere transport. In this study, we analyze the zonal wavenumber spectrum of atmospheric wave forcing in the lower stratosphere to understand the annual cycle of upwelling at the tropical tropopause. Tropopause upwelling is derived based on the wave forcing computed from ERA-Interim reanalysis using the momentum and mass conservation equations in the transformed Eulerian-mean framework. The calculated upwelling agrees well with other upwelling estimates and successfully captures the annual cycle, with a maximum during Northern Hemisphere (NH) winter. The spectrum of wave forcing reveals that zonal wavenumber-3 component drives a large portion of the annual cycle in upwelling with a secondary contribution from wavenumber-4 component. The wave activity flux (Eliassen-Palm flux) shows that both tropical and extratropical waves are equally important for the annual cycle. The quasi-geostrophic refractive index suggests that the NH extratropical wavenumber-3 component can enhance tropopause upwelling because these planetary-scale waves are largely trapped in the vertical, while being refracted towards the subtropical upper troposphere and lower stratosphere. Regression analysis based on interannual variability suggests that the wavenumber-3 waves are related to tropical convection and wave breaking along the subtropical jet in the NH extratropics.