The Atlanticis the only ocean where heat is transported north across the equator. Here warm surface water from the tropics reaches further north than in anywhere else. The relatively warm, salty water of the Gulf Streamsystem remains at the ocean surface west of Svalbard to a latitude of about 80 degrees before it dips underneath the much fresher and less dense polar water. The heat released by this warm water makes the climate in regions bordering the eastern North Atlantic warmer than at similar latitudes elsewhere. (reference)
The North Atlantic loop of the thermohaline circulation (THC), from the Greek words thermos(heat) and halos(salt), is controlled by the sinking of dense (cold and salty) water at high latitudes. The density of seawater is a result of both temperature and salinity (salty water is denser than fresh water, and cold water denser than warm water). Although the Gulf Stream water is saltier than the deep water below, it is much warmer, so its density is lower, and it remains on the surface. On its journey north, the water releases heat to the atmosphere, and cools gradually, until it is cold enough for its density to match that of the deep layer. Sinking can begin.
In the seas that ring the northern fringe of the Atlantic, the Labrador, Irminger, and Greenland Seas, the ocean releases large amounts of heat to the atmosphere and then a great volume of cold, salty water sinks to the abyss. This water flows slowly at great depths into the South Atlantic and eventually throughout the world’s oceans. Thus, the North Atlantic is the source of the deep limb of the Ocean Conveyor. The plunge of this great mass of cold, salty water propels the global ocean’s conveyor-like circulation system. It also helps draw warm, salty tropical surface waters northward to replace the sinking waters. If cold, salty North Atlanticwaters did not sink, a primary force driving global ocean circulation could slacken and cease. Existing currents could weaken or be redirected. The resulting reorganization of the ocean’s circulation would reconfigure Earth’s climate patterns. (reference)
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Climate tipping points: Amazon Rainforest, Arctic Sea-Ice, Atlantic Thermohaline Circulation, Greenland Ice Sheet, Sahara/Sahel