Variability in the upper ocean during MILE. Part I: The heat and momentum balances

Abstract

The Mixed Layer Experiment (MILE) was an examination of the upper ocean carried out near Ocean Weather Station P during a 20-day period in the autumn of 1977 characterized by two wind events. In this paper the variability of temperature and velocity observed at two moorings are described and related to the surface heat flux and wind forcing. A one-dimensional upper layer heat budget is found to close acceptably and it is shown that this success depends on having well-sampled temperature records and a method of accounting for vertical velocities in the seasonal thermocline. A similar budget of momentum is also reasonably accurate, but it is necessary to account for the effects of quasi-geostrophic velocities in the upper layer inferentially. Internal waves are found to be a major source of variability, dominating velocity records even at 5-m depth, well within the mixed layer. In the seasonal thermocline there is a high-frequency cut-off of internal wave energy well below the local buoyancy frequency. High-frequency internal wave ‘events’ are identified in the seasonal thermocline. Shear in the upper layer, and its response to wind, have some features in accord with models taking slab flow in the mixed layer, but the agreement is far from complete. Observations in the thermocline indicate that velocity differences in the vertical are maintained only if there is sufficient density stratification to make the Richardson number exceed a critical value.