South Georgia Island Wave Patterns Observed in the Stratosphere via Satellite

We describe satellite observations of wing-shaped wave patterns observed as temperature fluctuations in the stratosphere near 40-km altitude above South Georgia Island in the remote southern Atlantic Ocean. Through comparison with a linear mountain wave model, we interpret the observed patterns as vertically propagating mountain waves carrying significant momentum flux vertically from the surface. When mountain wave momentum flux is dissipated, the process drives accelerations of the larger-scale flow, and this mountain wave-driven force is treated via parametrization in most global climate and weather forecasting models in use today. Small island orography is generally neglected in the parametrizations because limited resolution of the models treats the grid cell containing the island as ocean rather than land. A key parameter determining mountain wave effects on the circulation is the wave stress or momentum flux. Through analysis of the satellite observations, we derive high-resolution maps of the momentum flux vector and averages over areas representative of climate model resolutions. The results show large fluxes over South Georgia and suggest mountain waves from island orography may be an important missing source of wave momentum flux and drag forces on the circulation, particularly in the southern hemisphere where there is limited continental mountain orography compared to the northern hemisphere, and westerly wind biases persist in models.