A Modeling Study of Stratospheric Waves over the Southern Andes and Drake Passage

Abstract

Large-amplitude stratospheric gravity waves over the southern Andes and Drake Passage, as observed by the Atmospheric Infrared Sounder (AIRS) on 8–9 August 2010, are modeled and studied using a deep (0–70 km) version of the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model. The simulated tropospheric waves are generated by flow over the high central Andes ridge and the Patagonian peaks in the southern Andes. Some waves emanating from Patagonia propagate southeastward across Drake Passage into the stratosphere over a horizontal distance of more than 1000 km. The wave momentum flux is characterized by a tropospheric maximum over Patagonia that splits into two comparable maxima in the stratosphere: one located directly over the terrain and the other tilting southward with altitude. Using spatial ray-tracing techniques and flow conditions derived from the numerical simulation, the authors find that waves that originate from the high ridge in the Central Andes are absorbed by a critical level in the lower stratosphere. The three-dimensional waves originating from Patagonia could be separated into three families—namely, a northeast-propagating family, which is absorbed by a critical level between 15 and 20 km; a localized family, which breaks down in the stratosphere and lower mesosphere directly above Patagonia; and a southeast-propagating family, which forms the observed linear stratospheric wave patterns oriented across Drake Passage. The southward group propagation, assisted by lateral wave refraction due to persistent meridional shear of the zonal winds, leads to stratospheric wave breaking and drag near 60°S, well south of the parent orography.