Variability in the upper ocean during MILE. Part II: Modeling the mixed layer response

The Mixed Layer Experiment (MILE) was an examination of the upper-ocean response to meteorological forcing. In Part I of this paper ( DAVIS, DESZOEKE, HALPERN and NIILER, 1981, Deep-Dea Research, 28, 1427-1451) temperature and velocity variability observed at two moorings, and the concurrent atmospheric forcing, were described. The one-dimensional heat budget closed satisfactorily and the low-frequency variability of wind stress and upper-ocean transport agreed qualitatively. In this paper, various aspects of the observed ocean response to wind forcing are modeled. For low frequencies a linear statistical model relating wind stress and currents is found; it describes wind-driven velocities parallel to the expected Ekman transport and decreasing with depth. On time scales long compared with a tidal period the rate of change of vertically integrated potential energy agrees well with 0.5 ϱu _∗³, where u _∗ is the friction velocity in water. The response to wind forcing is also simulated using a one-dimensional mixed-layer model based on parameterizing the potential energy balance in terms of ϱu _∗³ and turbulent energy production by the interaction of velocity shear and the entrainment stress. Simulations of the MILE data indicate that both processes are important, the latter primarily during the start of wind events. This model also reproduces the major features of the observed statistical relationship between winds and currents.