Shear-flow turbulent mixing in the tropical tropopause layer

It has been suggested that the tropical tropopause layer (TTL) may be affected by mixing from shear-flow instability in connection with Kelvin waves, with implications for the tracer, momentum and heat budgets. Using data from the European Centre for Medium-range Weather Forecast, it is shown that the revised Louis-scheme (used in operational analyses and ERA-40 and ERA-Interim) indeed strongly responds to Kelvin wave perturbations, and average mixing maximises where Kelvin wave amplitudes are largest (Indian ocean/Maritime continent region). Conversely, the Monin-Obukhov scheme (used for operational analyses after 2005) predicts fewer, but more intense mixing events that maximise further East over the entire western tropical Pacific region. For the TTL as a whole, the amount of mixing predicted by the two schemes is similar, and is likely a process of secondary importance. Locally, however, the models predict shear-flow mixing that is sufficient to yield net diabatic descent over the aforementioned regions. The data analysed here remains inconclusive about which scheme captures reality better, rendering the role of Kelvin waves for shear-flow instability uncertain. In turn, the uncertainty of the mixing schemes in their sensitivity to Kelvin waves has implications for the dissipation of Kelvin waves in models and analysed meteorological data.