Thermohaline Circulation

The warm surface waters of the tropical oceans occupy a very shallow layer, approximately 328 ft. (100 m) deep that floats on the far colder water of the deep ocean that reaches to depths of 3.1 mi. (5 km). The temperature and salinity of the deep ocean is so uniform that its water must originate in cold, high latitudes where surface waters sink into the deep ocean and then spread across the globe. Ultimately, this water must rise back to the surface and return to the regions of sinking, thus constituting a conveyor belt. Because the motion has strong north–south and up–down components, it is referred to as a meridional overturning cell. For the water to sink, it must be dense, which means that it must be cold and saline. Hence, the circuit away from and then back to the regions of sinking, depending on density and salinity gradients, is known as the thermohaline circulation. Its depiction in documentary films such as Al Gore’s An Inconvenient Truth has brought the oceanic conveyor belt to the public’s attention.

At first, oceanographers speculated that the thermohaline circulation is symmetrical about the equator, with water sinking in polar regions and rising to the surface in lower latitudes. In reality, the circulation is very asymmetrical. Sinking in the northern hemisphere is limited to the northern Atlantic Ocean and is absent from the Pacific and Indian oceans because those two oceans are insufficiently saline at the surface. Confirmation of this asymmetry is available in carbon-14 measurements of the age of a parcel of seawater, the time since it was last at the ocean surface. The results show that the deep water is youngest in the northern Atlantic and oldest in the northern Pacific, where its age approaches 1,000 years. The surface flow toward the northern Atlantic affects a northward transport of heat that contributes to the temperate climate of western Europe.

Oceanic density’s dependence on both temperature and salinity means that the increase in density associated with low temperatures in high latitudes could be countered by a flux of freshwater onto the ocean’s surface, for example, when glaciers melt. If the water becomes too buoyant, it no longer sinks, the conveyor belt stops, and oceanic circulation experiences radical changes.

Such changes, which may have altered climate in

Earth’s very distant past, are shown vividly in the movie The Day After Tomorrow. Geochemical measurements provide a wealth of information about the thermohaline circulation. Unfortunately, there is also much misinformation concerning the role of that circulation in Earth’s climate. The reason is a lack of information about key aspects of the thermohaline circulation.

Although it is known where the surface waters sink into the deep ocean, where that water returns to the surface is at present a puzzle. The water has to be heated as it rises into warmer layers close to the surface. The required rate of heating is far greater than that which measurements show to be available in much of the oceans. Hence, the cold water probably rises in relatively small regions of strong turbulent mixing, for example over ridges on the ocean floor. It is possible that the waters that sink in high latitudes rise back to the surface in high latitudes where they can do so without requiring significant heat. 

Once in the surface layers, the motion of the water is strongly under the influence of the winds that drive intense currents shallow, swift currents, which would be present even in the absence of a thermohaline circulation, are also involved in the transport of heat to high latitudes, and therefore in the maintenance of temperate climates in high latitudes. Do fluctuations in the intensity of the Gulf Stream indicate imminent changes in the climate of western Europe? What is the relative importance of the wind-driven and thermohaline circulations in the poleward transport of heat today? Did changes in that transport contribute to climate changes in Earth’s distant past, and contribute to the recurrent ice ages, for example? 

The disagreements among scientists regarding answers to these questions reflect the uncertainties in what is known about the thermohaline circulation. This circulation is of central importance to the global climate because it maintains remarkably uniform conditions in the deep ocean, which has a high concentration of the greenhouse gas carbon dioxide. It is so slow that water particles take approximately 1,000 years to travel from the North Atlantic to the deep northern Pacific; measurements of current conditions provide only limited information about possible changes in that circulation. Observations of past climates similarly provide limited information. As a result, predictions of future changes in this circulation that will accompany global warming are also uncertain.