Orographic Wave Drag over the Southern Ocean: A Linear Theory Perspective

Abstract

ecent studies suggest that stratospheric wind biases in global and climate models in the Southern Hemisphere may result from insufficient orographic wave drag, particularly over the Southern Ocean in the latitude belt centered near 60°S. In this study, contributions to the stratospheric wave drag along 60°S from three neighboring orographic wave sources are evaluated using a multiple-layer linear wave model with large-scale wind and stratification profiles derived from the Interim ECMWF Re-Analysis (ERA-Interim) between the years 1991 and 2010. The orographic wave sources include the Patagonian peaks in the southern Andes, the Antarctic Peninsula, and the island of South Georgia. The climatological and dynamical aspects of the wave drag and its dependence on tropospheric winds are investigated.The results suggest that these orographic wave sources may have significant contributions to the stratospheric drag over the Southern Ocean through meridional spreading of the wave momentum flux aloft associated with three-dimensional wave propagation. Among the three locations considered, the wave drag from the Antarctic Peninsula is substantially larger than that from Patagonia and nearly two orders of magnitude larger than that from South Georgia island. The orographic wave drag is in general proportional to the westerly component of the surface winds and becomes virtually zero when the surface winds have an easterly component, associated with critical level absorption between the tropospheric easterlies and prevailing westerlies in the stratosphere. The derived wave drag exhibits substantial temporal variations, including synoptic-scale, month-to-month, and interannual variations.