New Data on Deep Sea Turbulence Shed Light on Vertical Mixing: Rough Seafloor Topography Has Far-Reaching Effect

Article excerpt

The global thermohaline circulation is basically a wholesale vertical overturning of the sea, driven by heating and cooling, precipitation and evaporation. (Changes in temperature=thermo, changes in salinity=haline.) Bottom waters move equatorward from their high-latitude regions of formation (the cold limb of the circulation), upwell, and return poleward at intermediate depth and/or the surface (the warm limb). As the bottom waters are colder than the overlying waters, this circulation is responsible for a large fraction of the ocean's poleward heat transport. In addition, these flows often redistribute fresh water, as the northward and southward moving waters generally have different salinities.

These oceanic heat and water transports play a significant role in Earth's climate. The earth gains heat from the sun at low latitude, and radiates heat back to space about the poles. To maintain a quasi-steady state, the ocean-atmosphere system must carry heat from low to high latitude. At mid-latitudes, where the poleward heat flux is maximum, the oceanic and atmospheric contributions are about equal. One component of the atmospheric heat transport involves evaporation, water vapor transport, and its subsequent condensation. Net north/south water vapor transport in the atmosphere is balanced by liquid water transport by rivers and ocean currents.

For almost 200 years, since the writing of Count Rumford in 1797, there has been a basic understanding of the cold limb of the thermohaline circulation. The combination of atmospheric cooling, evaporation, and, in some cases, salt rejection during the formation of sea ice causes surface waters at high latitudes to become sufficiently dense that they sink to the ocean bottom. These newly formed deep waters subsequently spread horizontally within the constraints of the seafloor's bathymetry to renew the deep waters found in the interiors of the world's oceans. There are two principal formation sites for dense bottom water: the Greenland and Norwegian Seas of the northern North Atlantic Ocean, and around the Antarctic continent, particularly within the Weddell Sea. Together, these source regions export some 20 to 30 million cubic meters per second of bottom water to the other ocean basins. (For comparison, the chiefly wind-driven Gulf Stream, Kuroshio, and Agulhas Currents carry in excess of 100 million cubic meters per second within horizontal circulations.)

The processes involved with the return limb of the thermohaline circulation - the transformation of these bottom waters to lower density, and their upwelling and eventual return to the high-latitude cooling zones - are less well understood. An upwelling of deep and bottom waters is believed to be fed by the continual supply of new bottom water: Dense new waters intrude below older waters and force them upwards. The bottom water source strength of 20 to 30 million cubic meters per second translates into a globally averaged upwelling rate at mid-ocean depth of about 3 meters per year. This upwelling has both dynamical and thermodynamical implications.

To maintain a steady-state temperature distribution in the face of this upwelling of cold water, a compensating warming is required. This warming may be accomplished by internal mixing of the deep ocean. Models exploring the thermodynamic balance between the downward diffusion of heat associated with mixing by turbulent eddies and the upwelling of cold water were published by Klaus Wyrtki (University of Hawaii) and Walter Munk (Scripps Institution of Oceanography) in the mid 1960s. At about the same time Wyrtki's and Munk's papers appeared, Henry Stommel, considering the dynamical effects of deep upwelling, proposed the existence of abyssal gyre circulations involving poleward deep flow in the ocean interiors fed by a series of western boundary currents. These boundary flows ultimately connect to the high-latitude bottom water formation sites. Twenty years later Frank Bryan (National Center for Atmospheric Research) published a study of an idealized, three-dimensional ocean model showing a direct relationship between the intensity of the vertical mixing and the strength of the thermohaline overturning circulation. …