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Lunar Tides and Climate Change
References
Egbert, G.D. and Ray, R.D.  2000.  Significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data.  Nature 405: 775-778.

Munk, W.H. and Wunsch, C.  1998.  Abyssal recipes II: Energetics of tidal and wind mixing.  Deep-Sea Research 45: 1977-2010.

Wunsch, C.  2000.  Moon, tides and climate.  Nature 405: 743-744.

Background
There are massive currents that sweep across vast reaches of the global ocean's surface; and there are equally massive currents that sweep across its abyssal plains.  Together, they comprise what is generally termed the thermohaline circulation or oceanic "conveyor belt" system.  The surface currents are driven primarily by wind; while the bottom currents have historically been assumed to be driven by the density-driven sinking of great volumes of surface water made differentially heavy by cooling and salinity increases caused by evaporation and heat loss to the atmosphere in specific locations, such as the region of North Atlantic Deep Water formation.  It has become increasingly evident in recent years, however - in the words of Wunsch (2000) - that "there cannot be a primarily convectively driven circulation of any significance."

The alternative source of power for the thermohaline circulation, or at least the lion's share of it (approximately two terawatts), must therefore come from tidal energy, Munk and Wunsch (1998) have concluded; and they have calculated that about one terawatt of that energy must be derived from mixing-driven tidal dissipation in the deep-ocean.  Thanks to the work of Egbert and Ray (2000), we now have proof that that indeed is what is happening.

What was done
Egbert and Ray used Topex/Poseidon satellite altimeter data to accurately map open-ocean tidal elevations, which allowed them to empirically quantify the spatial distribution of deep-sea tidal energy dissipation.

What was learned
The authors determined that approximately one terawatt of tidal energy was indeed dissipated in the deep-ocean in the general vicinity of rough bottom topography, such as occurs at mid-ocean ridges, verifying the predictions of Munk and Wunsch.

What it means
In the words of Wunsch, "it appears that the tides are, surprisingly, an intricate part of the story of climate change, as is the history of the lunar orbit," which because of the moon's receding from the earth at about four centimeters a year implies a continuing loss of energy from the earth-moon system of three terawatts, which is now accounted for when the newly-discovered one terawatt of deep-ocean tidal energy dissipation is added to the two terawatts of energy estimated to come from shallow-water dissipation.  "There remain," however, in the words of Egbert and Ray, "many questions ? about the implications of these processes for large-scale ocean circulation and climate."  One that comes to our minds concerns the ability of North Atlantic Deep Water formation to play as large a role in governing the strength of the global thermohaline circulation as has been assumed in the past, along with the ancillary assumption that dramatic changes in this phenomenon are responsible for the rapid shifts in climate that have been detected in many ice core studies.  Perhaps of even more importance is the fact that the actual circulation of the global ocean is now known to be, in the words of Wunsch, "much more subtle and interesting" than what is portrayed in today's best climate models, which gives one cause to wonder, as Wunsch does, if the models are up to the task of accurately predicting future climate changes due to any perturbation.


Reviewed 9 August 2000