Critical levels and mixing layers induced by convectively generated gravity waves during CEPEX

We study data from the Central Equatorial Pacific Experiment, in particular the soundings providing temperature, pressure, wind and ozone profiles, and we link them to the large-scale conditions deduced from the analyses of the European Centre for Medium-Range Weather Forecasts. To the west of the date line, the mean wind shows a strong shear at the tropopause between the easterlies of the upper troposphere and the westerlies of the lower stratosphere. These are conditions propitious to the formation of critical levels for quasi-stationary gravity waves generated by deep convection. The measured wind profiles show the main characteristics of the encounter of a wave with its critical level and, in the ozone and potential-temperature profiles, the existence of a conspicuous mixing layer in the vicinity of this critical level. Our interpretation that the mixing layer is a consequence of the critical level is supported by numerical simulations with a nonlinear time-dependent two-dimensional model. The occurrence of a mixing layer in the tropopause structure seems to be characteristic of large-scale equatorial convective areas during westerly phases of the quasi-biennial oscillation.