Moon, tides and climate

1 by laser reflectors left there by astronauts. What does this motion have to do with the ocean circulation? By Kepler's laws, the recession implies that there is a continuing loss of energy in the Earth–Moon system of 3 ǂ10 12 watts, or 3 terawatts, mostly in the ocean. But where in the ocean does this energy go, and what are its effects? On page 775 of this issue, Egbert and Ray 2 produce evidence for the seemingly lunatic conclusion 3 that dissipation of tidal energy in the deep sea, and the resulting mixing, are controlling features of the overall ocean circulation. In the conventional picture of the oceans, there is a wind-driven upper circulation that gives rise to massive, near-surface flows such as the Gulf Stream and the Kuroshio and Antarctic Circumpolar currents. Superimposed upon this circulation is one often labelled, unhappily, the 'thermohaline' circulation. This is supposedly driven by surface ocean density contrasts arising from temperature and salt variations produced by strong atmospheric cooling and wind-induced evaporation. In this process, dense water sinks at high latitudes through con-vection, driving a 'meridional overturning circulation' , which many believe dominates the heat and freshwater budgets of the climate system. (The terminological problem with 'thermohaline' circulation arises because, for example, half the heat transport in the North Pacific Ocean is in the wind-driven upper circulation 4 .) As formulated in almost all models of the Earth's climate, both theoretical and numerical, the dense water sinking in the meridional overturning circulation at high latitudes then flows, close to the ocean bottom , throughout the world, returning to the surface by a uniform upwelling through the 'interior' ocean. Under the simple assumption that a uniform upwelling of cold water is balanced by a uniform downward mixing of warmer water throughout the water column, a steady state is achieved. Almost all numerical models of the ocean and climate systems represent this process through spatially constant vertical 'eddy'-mixing coefficients, as do the textbook theories. It has become evident, however, that the actual circulation is much more subtle and interesting. Consideration of the stability and energetics of a fluid being heated and cooled at the surface 5 shows that the resulting motion would be extremely weak — a 'diffusive creep'. Such a fluid system is stable, and in a steady state it cannot produce the vigorous flow we observe in the deep oceans. There cannot be …