This article is part of the 4sea Project.
The thermohaline circulation, thermo referring to heat and haline referring to salt, has an important role for the state of the global climate. The ocean currents, driven by wind systems and density differences in the oceans, transport heat from tropical latitudes near the equator to higher latitudes close to the poles, which stabilises the uneven distribution of solar energy around the globe. This large-scale circulation, consisting of numerous ocean currents, connects all the ocean basins to each other and has therefore a large impact on the global climate system as a whole. In the Northern hemisphere, the Gulf Stream system makes up a large part of the thermohaline circulation, and is one of the most studied ocean current systems in the world because of its big impact on local weather and climate.1
Situated in the Atlantic Ocean, the Gulf Stream system is one of the largest and fastest ocean current systems in the world and plays a significant role for the European and the eastern North American climate. It is around 90 km wide, has a maximum speed of around 2,5 meters per second and transports up to 150 million cubic meters of water per second, which is more water than that carried by all the world’s rivers combined.1,2 The Gulf Stream system is primarily wind-driven, rather than dictated by density differences, and is fed by the westward flowing North Equatorial Current that drifts from the eastern North African continent to the northwestern South America. A part of the warm tropical waters then flow to the Gulf of Mexico from where the Gulf Stream system begins. It then transports warm equatorial water northward along the coastline of North America before taking an eastward shift across the Atlantic Ocean, from where it splits into different branches of which the North Atlantic Current is the largest.2 It is this North Atlantic Current that then flows northward along the western coastline of Northern Europe and affects Europe’s climate.
The warm North Atlantic Current brings relatively warm water to the coastline of Western Europe and is thought to be the reason why Europe’s climate, especially in the north, is up to 10 °C warmer than it would normally be on the same latitude.3 If you compare areas around the same latitude on the North American continent, the climate is a lot cooler. The warm water as well as the eastward blowing westerlies largely affect weather patterns in Europe. This is the reason for why especially Ireland, the UK and coastal Norway in Northwestern Europe have milder winters and receive more precipitation compared to the latitudinal mean.
As the North Atlantic Current flows northwards towards the Nordic seas of Scandinavia, it cools down as the temperatures drop. The combination of the colder water temperatures and the relatively high salinity increase the density of the water, which makes it sink. This results in deepwater formation in the North Atlantic, where a large quantity of water sinks down in a vertical motion and becomes deepwater. There are five large deepwater formation regions in the world; the Greenland-Norwegian Sea, the Labrador Sea, the Mediterranean Sea, the Weddell Sea and the Ross Sea, of which the first one in the North Atlantic is the largest one.3 These deepwater regions are extremely important for the functioning of the thermohaline circulation and ultimately the whole global climate, as they act as engines for the whole system and store absorbed carbon and heat from the atmosphere in the bottom layers of the ocean. This storage process is extremely important for lowering the carbon and heat concentration in the atmosphere.
Paleoclimatic records demonstrate that changes in the climate system have resulted in abrupt changes in the thermohaline circulation before.3 It is therefore well known that there is a tipping point for the system, but difficult to know how far from it we are. A slow-down, or even a stop, of the Gulf stream system is feared by many scientists as anthropogenic climate change is accelerating.2 The fear lies in the fact that the Greenland ice sheet is melting at an accelerating rate and contributes therefore to meltwater runoff into the North Atlantic. This addition of freshwater decreases the salinity in the ocean and therefore decreases its density. As the density decreases, so does deep water formation, as less water is heavy enough to sink. The deep water formation regions are, as mentioned earlier, the driving forces of the thermohaline circulation, and a rupture in the system could have catastrophic consequences. In case of a slowdown in the Gulf stream system, the European climate would most likely experience more storm activity as well as extreme temperatures and a drop in average temperatures especially in Northern Europe.2 More generally a weakening of the circulation would have an impact on the ocean current patterns, the sea level, marine ecosystems, heat distribution and ultimately the whole global climate.
The problem with the future projections of the thermohaline circulation, and therefore the Gulf Stream system, is the lack of direct and continuous measurement from earlier periods.4 Scientists have to rely on indirect observations and measurements for the analysis of long term development of the ocean circulation. Measuring the ocean currents and anticipating their reaction to changes is therefore very challenging, and different predictions on the future of the Gulf Stream system have emerged. Nevertheless, most climate models predict a significant weakening (between 20 to 50 %) of the North Atlantic deepwater formation and the Gulf Stream system in the 21st century because of anthropogenic climate change.3 How fast and when we will start witnessing the consequences of this possible slowdown in the North Atlantic, or if it in fact already has started, remains to be seen.
About the author: Venni Arra is currently completing her bachelor’s degree in physical geography at Stockholm University and is doing her internship at CliMates in Paris for the Ocean and Climate change project 4sea.
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!
Joanna Gyory, Arthur J. Mariano, Edward H. Ryan. The Gulf Stream – Ocean Surface Currents. [website], 2013, http://oceancurrents.rsmas.miami.edu/atlantic/gulf-stream.html (accessed 25 July 2017)
Briney Amanda. The Gulf Stream – The Warm Ocean Current Flows from The Gulf of Mexico into the Atlantic Ocean. [website], 2017, https://www.thoughtco.com/what-is-the-gulf-stream-1435328
Rahmstorf. Thermohaline Ocean Circulation. In: Encyclopedia of Quaternary Sciences, Edited by S. A. Elias. Elsevier, Amsterdam 2006. [website], http://pik-potsdam.de/~stefan/Publications/Book_chapters/rahmstorf_eqs_2006.pdf (accessed 25 July 2017)
Rahmstorf. What’s going on in the North Atlantic. [website], 2015, http://www.realclimate.org/index.php/archives/2015/03/whats-going-on-in-the-north-atlantic/ (accessed 26 July 2017)