GCM simulation of the tropical mixing barriers in the lower stratosphere

Abstract

The high-resolution SKYHI general circulation model is used to simulate tropical mixing behavior and to investigate mixing barriers in the tropical lower stratosphere. Results show that during the solstitial seasons a tropical weak-mixing zone is present in the SKYHI simulations. The weak-mixing zone is relatively isolated from the surf zones in the middle latitudes by two mixing barriers, one weak and one strong. Lagrangian trajectory calculations show that the tropical mixing barriers are quasi-penetrable, and the weak mixing zone allows more tropical air to mix into the winter hemisphere than into the summer hemisphere. The tropical mixing barriers have seasonal variations in location and strength. The variations are closely coincident with the annual cycle of the tropical easterly zonal flow and subtropical wave spectra. Wave spectrum analysis indicates that during the solstitial seasons there exists a zone in the tropics, which exactly matches the weak mixing zone, where there is relatively absence of westward-moving waves. Calculations of meridional diffusion coefficients with filtered and anti-filtered winds suggest that mixing in the tropical easterlies occurs near the critical layers for the large-scale westward-moving waves, where the waves would be expected to break. The presence of the tropical mixing barriers is a result of the absence of such waves. Experiments with wave phase-speed filters indicate that stationary and quasi-stationary waves have significant effects on tropical mixing. The Rossby-gravity waves are less important for tropical mixing, and the Kelvin waves make no contribution to tropical mixing. Analysis indicates that equatorward propagation of the extratropical waves has important influences on tropical mixing. An experiment with a modified zonal flow shows a tropical mixing configuration of the westerly QBO phase. Finally, the SKYHI simulation results are compared with the observations and other simulations.