Thermohaline circulation is a process in the world's oceans involving fluxuations in temperature and salinity. Such variations create massive currents that move huge volumes of seawater around the world every day. Differences in the temperature and salinity of oceanwater have effects on its density, which is what drives these currents.

Global Oceanic Currents

Colder water that has a higher salt content from the North Atlantic is flushed southward along the eastern coasts of the Americas and south of Africa and Australia, where it turns northward east of Asia and wamer water with lower concentration of salt is pushed along the current back toward the North Atlantic. The freezing ocean water of the North Atlantic is the primary drive behind thermohaline circulation. This is due to the fact that its density is much greater than any other water along the currents. As water is brought up from the south, it chills very quickly and freezes, but the salt is not, making the water that is still in liquid form have a relatively high concentration of salt. The extremely low temperature of the water, combined with the high salt levels, makes this water much more dense. It sinks to ocean floor, and is known as North Atlantic Deep Water (NADW). As it flows southward, it encounters Antarctic Bottom Water (AABW) which is similarly quite cold and high in salinity, making the current even stronger as it turns eastward. Once it reaches the Pacific Ocean, it has warmed considerably, and has risen closer to the surface as a result. Here a second current is generated, travelling westward along the same path, except north of the coldwater current, hugging the coasts.

Components of Thermohaline Circulation

• Deep Water Formation
  
  
  • Norwegian Sea: The Norwegian Sea is one localized area of deep water formation, having salinity levels ranging from 34.90 to 34.94 and temperatures ranging from -0.5°C to -1.1°C resulting in a very dense water mass. This deep water formation contributes to the North Atlantic Deep Water, creating a slow moving current along the abyssal plains of the Atlantic Ocean floor southward.
    
    
  • Labrador Sea: The Labrador Sea also has currents that are a part of the North Atlantic Deep Water. However, currents out of the Labrador Sea are much more shallow, as the temperatures of the water range from 1.5°C to 3.0°C, and salinities from 34.80 to 34.86, causing the water to have a lower overall density.
    
    
  • Mediterranean Sea: The latest data collected from the Mediterranean deep water shows a remarkable increase in overall density in the last ten years. Salinity levels range from 38.48 to 38.49 and temperatures range from 12.89°C to 12.92°C.
    
    
  • Ross Sea: The Ross Sea is another area of deep water formation, with salinity ranging from 34.20 to 34.72 and temperatures ranging from 0.11°C to 0.18°C. The currents from the Ross Sea contribute greatly to thermohaline circulation in the southeast Pacific. Its currents extend from the shelf to a depth of nearly 3 km.
    
    
  • Weddell Sea: Temperatures in the Weddell Sea range from -0.9°C to 0.5°C and salinity levels range from 34.64 to 34.70. The Weddell Sea is the primary source for Antarctic Bottom Water currents.
  
  
  
• Deep Water Fluctuation: Thermohaline circulation is thought to have been an ongoing process since the end of the last ice age, however, many changes have occured in the strength of the currents over time. As water densities change, some currents can become stronger than others. For instance, during the last ice age, Antarctic Bottom Water may have been the strongest force of the circulation process, causing a cooling of surface waters in the North Atlantic. This may have led to a cooling trend in North America and Northern Europe and a general tendency toward climate instability. Warming trends, conversely, can cause a slowdown or even a halt to thermohaline circulation. The polar regions of the earth warm much more quickly, causing less ice to be created, and so salinity declines, lowering overall density. Warming at the equator causes more water to evaporate leaving the remaining water with a higher concentration of salt. The disparity between the ocean regions is less, and so the strength of the currents drops and may even cease altogether.
  
  
• Upwelling: There are many types of upwelling, some that occurs near the coasts, but some that occurs in the open ocean. Open ocean upwelling occurs as a response to deep water formation. The dense water flowing toward the ocean floor displaces a large volume of water, which must travel somewhere. In this case, it flows upward toward the surface. This process is accelerated by strong ocean winds that dictate the direction of this flow of water. Due to the increased circulation, these waters are often very nutrient-rich, containing high concentrations of phytoplankton. These waters are often very critical to supporting life, both at the coasts and in the ocean's interior regions. Upwelling is very diffuse in the open ocean, making it difficult to target exact locations. However, most research indicates that a large amount of upwelling occurs in the Southern Ocean, and smaller portions of it occur in the North Pacific.

Thermohaline Circulation and Climate Change

Throughout history, the direction and strength of thermohaline circulation has changed. The concern today is that with the increased warming trend going on that has been accelerated by the various industrial efforts of humans, the process of thermohaline circulation may again change drastically. The most obvious way in which this may occur is through the increased melting at the poles. The release of fresh water into the North Atlantic may lead to a desalinization point that could cause the density of the North Atlantic Deep Water to lower, and eventually might slow the deep water formation there. This will slow the currents of cold, dense water flowing southward, and also may slow the northward surface currents of warm water that contribute to the moderate climate enjoyed by eastern Canada, the eastern United States, Great Britian, and western Europe. Wind-driven ocean currents undoubtedly also help contribute to this, but without the effects of thermohaline circulation, change would likely be drastic and irreversible. Some reasearch even indicates that given the recent trends, there is a 70% likelihood that a slowing or even a shutdown of thermohaline circulation will occur between now and 2205.¹

Sources

Smith, R. O. and Bryden, H. L.: Observations of new western Mediterranean deep water formation using ARGO floats 2004–2006, Ocean Sci. Discuss., 4, 733-783, 2007
Locarnini, R.: The Ross Sea, Quarterdeck 3.1, Texas A&M University, 1995.
Robertson, R.; Visbeck, M; Gordon, A.L. and Fahrbach, E., Long-term temperature trends in the deep waters of the Weddell Sea. Deep Sea Res. II 49, 2002.
The Oceanic Thermohaline Circulation. van Aken, Hendrik M., Atmospheric and Oceanographic Sciences Library, Vol. 39., 2007.

Notes

¹James Kloeppel, University of Illinois at Urbana-Champaign, Global warming could halt ocean circulation, with harmful results, 2005.