This article is part of the 4sea Project.
The sinking of water masses forms the “motor” of the thermohaline circulation. Warm saline water masses flow from the equatorial regions to the north, where they cool down, lose volume and sink to the bottom. Along the ocean floor those water masses flow back to the south. However, these water masses must come up at some point to form a circulation and to follow the simple law of physics of mass continuity. The process of these re-emerging water masses is called upwelling.
Upwelling does not just happen anywhere but in certain regions of wind-sea interaction. Along the west coasts of continents in the northern hemisphere northerly winds (coming from the north) are responsible for this phenomenon while in the southern hemisphere southerly winds cause upwelling.
The force of the wind pushes the surface layer of the ocean to the south (in the northern hemisphere, and vice versa) however, due to the anticlockwise rotation of the earth the moving water masses are deflected to the right in the northern hemisphere and to the left in the southern hemisphere, so away from the west coasts. The missing surface waters are replaced by water masses from the depth by upwelling of former mentioned. Usually, these rising waters are colder, nutrient and oxygen rich waters, leading to increasing numbers of phytoplankton when reaching the surface. Phytoplankton on the other hand forms the base of the food chain making upwelling regions the most productive fisheries of the world. About half the world’s total fish catch comes from upwelling zones.
Where are Upwelling zones?
The major coastal upwelling zones are situated along the edge of the eastern boundary current of the pacific and atlantic ocean and are part of the Eastern Boundary Upwelling System (EBUS). They can be found in front of the peruvian and canarian coast as well as in front of california and western africa (benguela, Angola) .
All those regions are hence dependent on the abundance of fish, making not only human population living along these coasts vulnerable to any changes in the upwelling system but also all marine and coastal life in these regions.
The connection between Climate and upwelling
Furthermore, upwelling determines the climate as well. Cold water at the surface cools down air temperatures and with that decrease the water saturation point of air, leading to lower precipitation rates. This can be seen in Peru, where the coastal areas are dry and in california where the air-sea temperature exchange leads to summer fogs . How sensible these upwelling systems are can be seen again in peru, where every few years the deep water masses cannot reach the sea surface, leading to increased sea surface temperature in front of the peruvian coast. The air can suddenly take up more moisture which results in heavy rainfalls and floods. This phenomenon is called El Nino.
So what will happen with the ongoing climate change?
Studies show that due to an increased land-ocean temperature gradient the coastal upwelling will actually increase. Well that’s good, right? More fish for everyone? Well, yes and no. Yes, increased upwelling will lead to more fresh nutrient, rich water masses reaching the surface and maybe even an increase in fish production. However, the balance between nutrient supply, fish production and retention processes is delicate and global fish production could actually decline, furthermore, changes in the Northeast Pacific, supporting this hypothesis are already evident , . Due to increasing wind stress between the sea surface and the ocean surface, the wind induced mixing of the water column will also reach further, mixing nutrient rich waters with warmer depleted waters working against the freshening and phytoplankton production .
And their remains still one problem. When deep water forms in the Arctic regions it entrains gas from the atmosphere in the water parcels and takes these gases with it into the deep ocean. Gases such as CO2. The ocean actually works as a CO2 sink and regulates our pollution, however, with enhanced upwelling these water masses and with them the CO2 might come back faster than anticipated. When those deep water masses come back up to the surface they release the entrained CO2 back into the atmosphere. So increased upwelling might also lead to positive feedback in putting the stored CO2 back into the atmosphere .
The feedbacks that enhanced ocean upwelling might induce are still not entirely understood and need to be look into further, however, studies and models show that the ocean and upwelling zones will react to anthropogenic climate change and it becomes more and more evident that they will do so not to our benefit.
About the author: Marie Harbott is studying Marine Geoscience in the North of Germany and writing her Master thesis about deep water formation in the Labrador Sea. She founded 4Sea to draw attention to the Ocean and its importance to the climate system.
This article has been written in the context of 4sea. 4sea, a project about the importance of the world oceans, addresses the interdependence between the oceans and climate change, entraining everyone to become ocean lovers – for now through articles and videos on this blog and in November on our own platform. 4sea is a joint project between the youth organisations CliMates, Youth for Ocean and Vitamin Sea. Love it? Stay tuned for our platform!