NASA - National Aeronautics and Space Administration

The southern Andes region has been clearly identified in previous satellite and balloon observations and in global models as a “hot spot” of small-scale gravity wave activity, with monthly mean momentum fluxes exceeding 10 times background values in fall, winter, and spring seasons. This makes this region a focus of interest for global circulation and climate studies. We analyze a case study on 8 May 2006, combining observations from the Atmospheric Infrared Sounder instrument on the Aqua satellite and the High Resolution Dynamics Limb Sounder instrument of the Aura satellite to form a three-dimensional picture of the wave field. The observations show a widespread wave pattern over the southern Andes extending eastward over the south Atlantic. Simulations with the Weather Research Forecasting model clearly identify the waves as orographic in origin, but the observed wave pattern is far from the simple two-dimensional wave field forced by steady flow over a mountain ridge. The morphology of the pattern is consistent with three-dimensional linear theoretical calculations of downstream propagation and latitudinal focusing of mountain waves into the stratospheric jet. The observations confirm the importance of this process in the stratosphere, and we find the process also occurring in the global analysis and forecasts from the European Centre for Medium-Range Weather Forecasting. Our analysis evaluates some strengths and weaknesses of current orographic wave drag parameterizations in global models and the relevance of parameterization assumptions in global models with high resolution.