Outflows and deep water production by marginal seas

Abstract

We examine some of the processes that determine the properties of marginal sea outflows by reviewing historical data and by an analysis of two simple models. Our numerical simulation model makes several of the streamtube approximations of Smith (1975), but goes on to include a Froude number-dependent entrainment parameterization, an Ekman number-dependent parameterization of broadening, and it can accept real bottom topography and real oceanic temperature and salinity profiles. This numerical model reproduces some of the main features of the four major outflows considered here (Mediterranean Sea, two from the Norwegian-Greenland Sea, and Weddell Sea) including the bulk properties of the product water. A notable feature of these four outflows is that the density ordering of the source waters and product waters is reversed; for example, the densest source water comes from the Mediterranean Sea, the Mediterranean outflow makes the least dense product water. The reason is that intense entrainment of North Atlantic Central Water decreases the density of the Mediterranean outflow by about 1kg m −3 as it begins to descend the continental slope. The three high-latitude outflows are more or less similar in that strong entrainment generally occurs over short segments of the path where the bottom topography is relatively steep, typically just beyond the shelf-slope break. However, the densities of these outflows are decreased much less by entrainment (about 0.1kgm −3 in the Denmark Strait outflow, about 0.2kg m −3 in the Faroe Bank Channel outflow and 0.03kg m −3 in the Weddell Sea outflow) as they descend to the bottom in their respective basins. Entrainment causes much less decrease in density in these outflows primarily because of the lower density differences between these outflows and their overlying oceanic waters. High latitude outflows are more likely to reach the bottom because the oceanic water column in polar and subpolar seas is weakly stratified. These and other results indicate that the temperature and salinity of the oceanic water column are of considerable importance in determining the product water of a marginal sea outflow. Numerical experiments suggest that the density of the product water is remarkably insensitive to variations in the density of the source water. For example, if the density of the Mediterranean source water is arbitrarily increased by 1kg m −3, then the increased density difference between outflow and overlying oceanic water results in even more vigorous entrainment. This increases the transport of product water by about 10 6m 3s −1 and limits the increase in density of the product water to only about 0.15kg m −3. These and other results are obtained also from a simple end-point model of a descending outflow.