Mixing of the Weddell Sea continental slope

Abstract

Abstract Antarctic Bottom Water is produced by a mixing of water masses which occurs on Antarctic continental slopes. This paper discusses the mechanisms involved, judging the success or failure of each mechanism by use of two observational criteria. These are (a) that the plume of dense water on the slope should reach the foot of the slope, and (b) that the mass flux should increase by 100 to 300%. Two- and three-dimensional plume models (the latter of which have been successful in describing the properties of the Norwegian and Mediterranean outflows) give results which are incompatible with observations. These plume models are steady , however. The possibility that time variability may be an important factor in the dynamics is examined by modelling the dense water as a thermal. However, this too gives results which do not agree with observations. It is shown that the dominant effect involves the equation of state at low temperatures. In particular, the increase of the thermal expansion coefficient α with depth must be included (nonlinearities, which can produce cabbeling on the small scale, are unimportant here). This effect produces a strange situation. At the top of the slope, the plume is colder—and therefore more dense—than its surroundings. As it falls down the slope it can become even more dense than its surroundings, despite the fact that its environment is stably stratified. The result of this is an effective source of internal energy which accelerates the plume down the slope. At greater depths, where the plume is nearly at the same temperature as its surroundings, this energy source disappears and normal plume dynamics resume. Thus, if the equation of state is included, results in agreement with observations are obtained.