Energetics of the global ocean overturning circulation

Graham O. Hughes, Andy McC. Hogg, Ross W. Griffiths

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

The time-averaged overturning circulation obtained in 2-D numerical simulations of a model ocean basin forced only by surface heating and cooling (varying smoothly from 200 W/m2 at the left end to -200 W/m2 at the right end, but with no net heat input to the basin). The simulation was non-hydrostatic, was conducted at high resolution (10-75 m vertical resolution and 0.75 -7.5 km horizontal resolution), and was run with a vertical diffusion coefficient of 10-4 m2/s. The coldest waters are coloured blue and warmer waters towards the surface are shown as yellows and reds. The maximum overturning streamfunction is 28 x 103 kg/s per unit width.

The overturning circulation of the global oceans regulates Earth's climate, and oceanographers have been interested in which energy sources maintain that circulation. Energy input from surface winds and tides is important, but whether or not surface heating and cooling also contribute has been the subject of considerable debate. An understanding of the energetics is essential in addressing problems such as the response of the overturning circulation to forcing changes and in highlighting processes that need to be addressed in the development of general circulation models and climate models.

This year we have developed a theoretical framework that can be used to study the energy budget of the ocean overturning circulation. The concept of available potential energy, which is a measure of how far the density field is from equilibrium, underpins this framework. We have demonstrated that surface buoyancy forcing generates available potential energy, and is indeed an important energy source for the overturning circulation. In particular, only mixing and surface buoyancy forcing act to change the density of waters and, in a steady circulation, the energy transports associated with these two processes must balance. We have further clarified how the sources of kinetic energy associated with the winds and tides are simultaneously necessary to maintain the observed ocean overturning circulation.

We have in conjunction with our theoretical work undertaken a series of high-resolution numerical computations using a general circulation model of an ocean basin (figure 1). The results confirm our theoretical findings. Importantly, they also show that deductions regarding the energy budget of the ocean overturning circulation are strongly influenced by parameterizations of small-scale processes that are in common use in numerical models.