High-frequency wind forcing of a channel model of the ACC: normal mode excitation

Abstract

The response of the Antarctic circumpolar circulation to wind stress variability is studied in a simple model of the Southern Ocean. The model consists of a zonally reentrant channel with mid-ocean barrier, forced by surface heat flux and stochastic wind stress. The MITgcm code is used to solve for temperature, sea level, and the velocity field. The channel transport responds actively to the stochastic wind forcing through the excitation of several eigenmodes. The fundamental basin mode is excited, which has a period of about 20 days. However, the response is dominated by a topographic mode with a period of 3 days. This barotropic mode is related primarily to topographic Rossby waves propagating on the submarine sill, and Kelvin waves propagating westward on the southern boundary. The mode shares characteristics with numerical predictions of normal modes in the Southern Ocean, as well as with recently observed modes of variability around Antarctica, with periods between 1 and 2 days. These results show that both topographic normal modes and Rossby basin modes can play a role in the adjustment of the ACC to high-frequency wind stress variability. The presence of the resonance in the system leads to interesting phase behavior between the wind stress forcing and the ocean transport: for a considerable frequency range, the transport seems to lead the wind stress variability. This shows that the classical notion of transport passively lagging wind forcing is inadequate when eigenmodes are excited.