Tropical mixing barriers in the lower stratosphere in the Geophysical Fluid Dynamics Laboratory SKYHI model

Large-scale isentropic mixing in the tropical lower stratosphere is investigated by computing Lagrangian trajectories using winds from the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model, The use of model-simulated winds avoids some problems with analyzed winds in the tropics, During the solstitial seasons a weak-mixing zone is present in the tropics in the model simulations. The weak-mixing zone is relatively isolated from the midlatitude surf zones in the northern and southern hemispheres by two mixing barriers: a strong one on the summer hemisphere side and a weak one on the winter side, The mixing barriers vary seasonally in response to seasonal variations of the tropical zonal-mean zonal flow and wave spectrum, The roles of the zonal-mean flow and of different parts of the wave spectrum in meridional transport are tested using filter techniques. Effective meridional diffusion coefficients are calculated by using filtered and antifiltered winds (i.e., winds with selected spatial scales either attenuated or amplified), The results suggest that mixing in the tropical easterlies occurs near the critical layers for the large-scale (zonal wavenumber <10) westward moving waves, which is where the waves would be expected to break, Wave spectrum analysis indicates that during the solstitial seasons there is a zone in the tropics, exactly matching the weak-mixing zone, where there is relatively little power in westward moving waves, The tropical mixing barriers are a result of the relative lack of such waves.