Dianeutral Upwelling Without Dissipation of Kinetic Energy

Estimates of the upwelling of bottom and deep water have recently been used to deduce the magnitude of area-averaged mixing activity and the required dissipation of kinetic energy that is input by winds and tides. Here we raise questions about this link between upwelling and the dissipation of energy. Several processes are discussed, each of which lead to the upwelling of water across neutral density surfaces, but none of which have a signature in the dissipation of kinetic energy. Some of these processes seem too large to ignore while others remain to be properly quantified. Until these processes are either quantified or dismissed as unimportant, it is premature to use a mixing efficiency (such as 0.2) to relate global dianeutral upwelling to the dissipation of kinetic energy. The influence of the nonlinear nature of the equation of state on the evolution of gravitational potential energy is also discussed. We raise serious doubts about the suggestion that the contraction-on-mixing feedback effect involving the gravitational potential energy of the whole water column, gives rise to an upper limit on the vertical gradient of potential temperature.