Response of the Separated Western Boundary Current to Harmonic and Stochastic Wind Stress Variations in a 1.5-Layer Ocean Model

Abstract

Abstract A time-dependent version of the Parsons model (geostrophic 1.5-layer model of the ventilated thermocline) has been developed to investigate the response of the midlatitude ocean to wind stress variations in a simple configuration. In this model, the total amount of water is kept constant and the eastern boundary thermocline depth can vary in time so as to maintain mass balance. Here, basin modes are not investigated, in contrast to many recent studies, but the emphasis is on the line where the motionless second layer outcrops, which represents the separated western boundary current. It is shown that the position of this line only depends on the wind stress, the earth rotation, and the thermocline interior solution. The position is not influenced by the parameterization of the dissipative processes. This generalizes previous results established in the stationary case. The displacement of the outcrop line in the case of harmonic or stochastic wind stress variations is computed numerically, showing a lag of 0–4 yr that results from a combination of the instantaneous Ekman response and the delayed response due to Rossby wave propagation. Such delay is in satisfactory agreement with observations of Gulf Stream adjustment to wind stress changes, considering the limitations of the model, and is in good agreement with intermediate-resolution OGCM models. Although inertial effects and buoyancy forcing also need to be considered, this suggests that the outcropping mechanism plays a role in the variability of the separated boundary currents and may be dominant in non-eddy-resolving ocean models.