Interpreting wind-driven Southern Ocean variability in a stochastic framework

A stochastic model is derived from wind stress and bottom pressure gauge data to examine the response of the Antarctic Circumpolar Current (ACC) transport to wind stress forcing. A general method is used to estimate the drift and diffusion coefŽ cients of a continuous stationaryMarkovian system. As a Ž rst approximation, the response of the ACC to wind stress forcing can be describedby a multivariate Ornstein-Uhlenbeck process: Gaussian red noise wind stress drives the evolution of the ACC transport,which is damped by a linear drag term. The model indicates that about 30(610)% of ACC variability is directly driven by the wind stress. This stochastic model can serve as a null hypothesis for studies of wind drivenACC variability. A more accurate stochastic description of the wind stress over the Southern Ocean requires a multiplicative noise component. The variability of the wind stress increases approximately linearly with increasing wind stress values. A multiplicative stochastic process generates a power-law distribution rather than a Gaussian distribution.A simple stochasticmodel shows that non-Gaussian forcing could have a signiŽ cant impact on the velocity (or transport) probability density functions (PDFs) of the wind-driven circulation. The net oceanic damping determineswhether the distribution of the oceanic  ow is Gaussian (small damping) or resembles the distribution of the atmospheric forcing (large damping).