Downwelling and deep water formation – what drives the THC

As we have seen in previous posts, ocean water reaches higher densities the colder and saltier it gets. Thus, to find significant areas of downwelling, we have to look for places on the globe, where ocean water is made particularly cold and salty. This leads us to the poles.

Both the Arctic and Antarctic have the potential to cool water to minimal temperatures. In the Antarctic, waters under ice sheets may lose so much heat, that the process is referred to as super-cooling. Through ocean gyres [info box], water is brought to the surface and cooled via convection by releasing heat to the atmosphere. Due to its lower temperature, the water mass will increase its density and sink to the bottom, where it is pushed away from the creation center by following water masses. The conveyor belt is moving.

Simply cooling water down will not lead to particularly dense waters. Parallel to the temperature loss, salinity needs to be increased. This is mainly possible by taking away water, but leaving the salts behind. Hence, the left behind water mass will become more saline.

There are two main processes that will accomplish the above: evaporation and ice formation.

Through evaporation, water will be removed from the oceans and enters the atmosphere as vapor. Since most salt particles are too heavy, they will be left behind. Ice formation leads to a similar process. By freezing ocean water, fresh water is taken out of the water mass, while the salts stay behind. This process is referred to as brine rejection.

Deep water formation in the Antarctic: Antarctic Bottom Water (AABW)

There are several places around the Antarctic continent where deep water formation takes place. The most famous one is the Weddell Sea, where the Atlantic Ocean hits Antarctica. Deep water formation in Antarctica is mainly connected with heat loss and brine rejection. Large year round ice sheets cool the surrounding ocean water to up to minimum temperatures of -2.2°C and increase their salinity by constantly freezing more water. This leads to the AABW being the coldest and densest water mass on earth.

Deep water formation in the Arctic: North Atlantic Deep Water (NADW)

In the Arctic, most downwelling is happening in the Barents Sea, Greenland Sea and Labrador Sea. Here, convection and mixing are the two most important processes. As explained above, gyres transport water to the surface and cool it there. In difference to the Antarctic, the Arctic ice is purely sea ice with no underlying continental mass. Thus, many areas experience a great fluctuation in ice amount with no ice during summer and little during winter. The cold open oceans lead to extreme heat loss (no sea ice that protects the upper water layer) that rapidly cools down water masses. In a complicated mixing process, many different water masses with different densities form the NADW which leaves the Arctic to flow southwards as the Atlantics deep water flow. When it reaches Antarctica, it mixes with the Antarctic Circumpolar Current, which flows all around the South Pole and the Antarctic’s AABW. From there, the new water mass intrudes other ocean basins and connects the Atlantic with other world’s oceans.

You might have noticed that there is no particular process to enhance salinity in the Arctic. Under certain circumstances the mixing of all those different water masses may lead to higher salinity, but the most important process is actually happening long before the water reaches the Arctic: Evaporation of large amounts of water at the equator and the subtropics.

Due to the Hadley Cell and the Coriolis force [info box], these large amounts of water vapor are transported east across the Atlantic and across Middle America. The flat topography of Middle America allows the water masses to be exported straight into the Pacific, which means that the Atlantic loses large amounts of water which are not coming back (Richter & Xie, 2010). The only way to counterattack this water export is by importing fresh water through river outflows. However, looking at the Atlantic, only few large rivers (e.g. the Amazon) enter the Atlantic with significant freshwater inputs. When calculating the difference of input and output, we see that the Atlantic is losing more than it gains. Thus, the water masses flowing northwards become saltier. 

By the time they reach the Arctic deep water formation places, the salt content is high enough to form deep water merely by lowering temperature.

[All of the above named processes are well described in van Aken (2007) The OceanicThermohaline Circulation, An Introduction. Atmospheric and Oceanic SciencesLibrary, Vol. 39 or Talley et al. (2011) Descriptive Physical Oceanography: An Introduction, 6.Ed.]