Decadal Buoyancy Forcing in a Simple Model of the Subtropical Gyre

Abstract

The decadal response of the ventilated thermocline to surface buoyancy forcing is investigated, using simplified mixed layer thermodynamics and two-layer planetary geostrophic dynamics. The model is forced by a stationary Ekman pumping and a fluctuating sea surface density, while the associated surface buoyancy flux is diagnosed from the buoyancy budget of ventilated columns. The decadal forcing is represented as a slow modulation of the seasonal cycle, rather than a slow periodic forcing as in Liu and Pedlosky. It is found that the amplitude and the degree of nonlinearity of the interior response depend on the anomalies of the yearly averaged surface buoyancy flux, but that omitting the seasonal cycle leads to an overestimation of the thermocline variability by a factor of 2. At periods longer than 10 years, the interior response becomes linear and frequency independent so that in the presence of stochastic buoyancy forcing the spectrum of potential vorticity or interface displacement is white at low frequency and red at high frequency. A statistical signature of the buoyancy forcing is the asymmetric shape of the cross-covariance function between sea surface density and interface displacement anomalies in lead and lag conditions.