Stress on the Mediterranean Outflow Plume: Part I. Velocity and Water Property Measurements

Abstract

In September 1988 six sections were occupied across the Mediterranean outflow plume in the Gulf of Cadiz within 100 km of the Strait of Gibraltar. Vertical profiles of temperature and salinity were collected at CTD stations. Velocity and temperature profiles were collected with expendable current profilers at a subset of these stations. At the channel base, the plume undergoes geostrophic adjustment and turns northwest to flow along the continental slope. There it decelerates and spreads gradually down the slope as friction slows the current and allows it to cross isobaths. Within the plume, downstream velocity and density increase rapidly in the interfacial layer with depth to the velocity maximum, or nose, 5?150 m above the bottom. Below the nose, in the bottom layer, downstream velocity decreases rapidly toward the bottom, but the stratification is weak. Ekman-like veering occurs in the interfacial layer. Local bottom stresses on the plume are estimated by fitting the near-bottom velocity profiles to a log-layer model. These stresses are compared with bulk estimates of total stresses from momentum budget residuals and of interfacial stresses from combining the mean vertical shear with bulk turbulent dissipation estimates. The downstream pattern of the sum of the local bottom stresses and the bulk interfacial stresses agrees well in magnitude and distribution with that of the bulk total stresses. The largest stresses reach a mean of 5 Pa where the plume is flowing rapidly westward down a channel after exiting the strait, thinning, and accelerating. These stresses are an order of magnitude larger than mean wind stress values over the ocean gyres and exceed most bottom stress estimates in other regions.