Thermohaline circulation (THC) refers to ocean currents driven by fluxes of heat and salinity gradients throughout the seas of the world.The phenomenon of thermohaline circulation includes formation of cold deepwater masses, deep-sea movement of this cold water, subsequent ocean interior mixing of heat and salt, and return flow of surface warm currents.
Thermohaline circulation can be affected by variations in climate, but any alteration in thermohaline circulation can conversely have an affect on global climate. The feedback mechanisms between ocean and climate are incompletely understood, but generally the oceans are a damping effect on the faster frequency atmospheric meteorological oscillations.
The word "thermohaline" is a combination of the word "thermo" refering to heat and the word "haline" refering to saltiness (salt is the chemical compound Sodium Chloride and the element chlorine in one of a group of elements known as "halides"). Thus "thermohaline circulation" refers to circulation driven by variations in heat and salitiness.
Modern scientific understanding of thermohaline circulation is generally traced to an experiment conducted in 1751 by a slave ship captain Henry Ellis using apparatus provided to him by the Reverend Stephen Hayes. Lowering Hayes' "bucket sea-gauge," Ellis was able to measure the temperature of water at various depths. He discovered that near the surface of the ocean existed a layer of warm water with a relatively uniform temperature. Going deeper into the ocean revealed a rapid decline in temperature (a transition now known as the "thermocline.") It was striking that the deep water in tropical regions of the ocean was far colder that it ever got in those regions. Thus, the question was raised as to whether the cold deep water was coming from polar regions as part of some vast current deep in the ocean.
Initial Formation of Cold Deepwater Masses
Creation of deepwater masses of intensely cold water is an initial step in the entire thermohaline cycle. The cold dense water masses that sink into the deep basins are formed in distinct areas of the North Atlantic, Arctic Ocean and the Southern Ocean. In these polar regions, seawater at the surface of the ocean is intensely cooled by high velocities of the polar wind. Winds moving over sea surfaces also produce a great flux of evaporation, leading to a decrease in temperature, which process is termed evaporative cooling. Evaporation removes only water molecules, resulting in an increase in the salinity of the seawater left behind, and thus an increase in the density of the water mass.
In the Norwegian Sea evaporative cooling is the dominant mechanism for refrigeration, and the sinking water mass, the North Atlantic Deep Water (NADW), fills the basin and spills southward through crevasses in the submarine sills which connect Greenland, Iceland and the United Kingdom. It then flows quite slowly into the deep abyssal plains of the Atlantic, always in a southerly direction. Flow from the Arctic Ocean basin into the Bering Sea, however, is largely blocked by the narrow shallows of the Bering Strait.
Movement of Deepwater Masses
Vertical movement of the deep water masses at the North Atlantic Ocean, creates sinking water masses that fill the basin and flows very slowly into the deep abyssal plains of the Atlantic. This high latitude cooling and the low latitude heating drives the movement of the deep water in a polar southward flow. The deep water flows through the Antarctic Ocean Basin around South Africa where it is split into two routes: one into the Indian Ocean and one past Australia into the Pacific basin.
Schematic global oceanic thermohaline circulation. Blue lines are deep coldwater flow; red lines are warmer surface flow. Source: IPCC
In the Indian Ocean, some of the cold and salty water from the Atlantic Ocean causes a vertical exchange of dense, sinking water with lighter water above, termed overturning. This exchange is induced by the movement of warmer, fresher upper ocean water from the tropical Pacific. In the Pacific basin, the remainder of the cold, saline water from the Atlantic undergoes Haline forcing and gradually becomes warmer and less saline.
The outflowing deepsea cold, saline water makes the sea level of the Atlantic Ocean slightly lower than the Pacific and salinity or halinity of water at the Atlantic higher than the Pacific. This generates a large, slow moving transport of warmer and fresher upper ocean water from the tropical Pacific Ocean to the Indian Ocean through the Indonesian Archipelago, which flow replaces the cold, highly saline Antarctic Bottom Water. This process is also termed Haline forcing (net high latitude freshwater gain and low latitude evaporation). This warmer, fresher water from the Pacific flows up through the southern Atlantic Ocean to Greenland, where this water mass cools and also undergoes evaporative cooling, thence sinking to the ocean bottom, establishing a continuous thermohaline circulation.
Relation to Meteorology
The first order relation of ocean circulation to meteorology is fairly straightforward: winds drive surface currents and solar insolation causes near surface ocean warming. The depth influence of these first order forcing factors is generally agreed to be no greater than 100 meters. More complex and even more potent factors are the indirect phenomena caused by sea ice and atmospheric gas exchange. In the case of sea ice and micro-topographic changes of wave height variations, the sea surface albedo can change, giving rise to the alteration of percent reflected sunlight, and hence change in heat flux entering the sea surface from the atmosphere.
The oceans represent a vast heat-sink and carbon sink relative to the atmosphere. For example, there is a greater heat capacity in the top three meters of the seas of the world compared to the entire atmosphere. ,
Surface ocean currents are driven by atmospheric prevailing winds. Some of these currents are systematic and consistent throughout the year. In particular, prevailing easterlies in both the Arctic and Antarctic regions drive surface currents extending as deep at 1000 to 1500 meters. Further, prevailing westerly winds drive surface currents at similar depths at moderate latitudes.
Atlantic Meridional Overturning Circulation
Sometimes a specific example, like the Atlantic Meridional Overturning Circulation (AMOC) is used interchangeably with thermohaline circulation, but they have distinctly separate meanings. The AMOC, by itself, holds no information on what drives ocean circulation. In contrast, THC implies a specific driving mechanism related to creation and destruction of buoyancy.
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