How Ocean Currents Affect Climate

Did you ever really thought about how the ocean is floating into its depths? The wind controls only a small part of ocean currents on the surface. What about the rest? How do huge currents like the gulf stream come into existence? In which way do they connect to our worldwide current system? In this blog post you will learn how ocean currents affect climate, taking the golf stream as an example.  

The thermohaline circulation 

The course about ocean circulation at my guest university in Guadeloupe excited me. It fascinated me that changes in salt concentration and temperature are driving the movement of water. I´ve learned that differences in seawater density are mainly responsible for deep ocean currents. Cold water is denser than warm water. Density also increases with the salt content. As a result, water traveling into colder regions sinks through cooling in the ocean depths. Warmer surface water fills its place, causing vertical circulation, known as thermohaline circulation. ¹

The so-called AMOC (Atlantic Meridional Overturning Circulation) is a significant part of the thermohaline circulation. ² The AMOC influences the climate in Western Europe considerably since it carries warm tropical water to the North Atlantic. ³ An important part of the AMOC is the gulf stream. 

The gulf stream 

The gulf stream transports over 100 million cubic meters of hot equatorial water to the North Atlantic per second. It originates in the Gulf of Mexico, and passes the east coast of the United States to end up in the North Atlantic. ⁴ Through the atmospheric exchange, it heats the climate of western Europe. To make it a little clearer how much it impacts the climate in Europe: If the gulf stream stopped, we would need to compensate for the loss of this energy through 1 million nuclear power plants. The temperature in Western Europe would be 5-10 degrees colder. ⁵

When the gulf stream arrives in the North Atlantic it gets significantly colder there, causing the water to sink close to Greenland. In the ocean depths, it joins the water floating back south. The reason why we speak of overturning circulation. ³

Human impact on thermohaline circulation 

Climate projections show that climate change could alternate thermohaline circulation. Although models can´t provide a clear prognosis, researchers estimate that climate change weakens the AMOC, followed by a diminution of heat transport. ² But how does this happen exactly? How can a rise in temperature change ocean currents?

Due to climate change, the water in the North Atlantic could become warmer and less dense. Therefore, the water sinks less into the ocean depths reducing the maelstrom pulling in new surface water. The vertical circulation is reduced. Consequently, the current becomes slower. ⁵

Furthermore, climate change could provoke increasing precipitation and the melting of ice shields. Both phenomena reduce the salt concentration in the water, making the ocean water less heavy and dense, also reducing thermohaline circulation. ³

Conclusion

Climate change could lead to an increased water temperature in the ocean, causing the water to sink less deep. The melting of ice shields could also enforce this phenomenon because the water becomes less dense due to a sinking salt concentration. A slowdown of the golf stream could have a significant impact on the Western European climate.

Whether climate change compensates for the heat loss in Western Europe remains unclear. Research in this department is still in its infancy. Do you think models can ever calculate all the ways in which climate change might impact us?  

References:

1. Jennifer Verduin., 2019. How do ocean currents work? Youtube. Available at https://www.youtube.com/watch?v=p4pWafuvdrY [1.10.2022].
2. Jungclaus, J.H., Haak, H., Esch, M., Roeckner, E., Marotzke, J., 2006. Will Greenland melting halt the thermohaline circulation? Geophysical Research Letters, Volume 33, Issue 17.
3. Cherchi, Annalisa., 2019. Connecting AMOC changes. Nature Climate Change, volume 9, issue 10, 729-730.
4. Lumni., 2021. Ralentissement du Gulf Stream, un danger ? Youtube. Available at https://www.youtube.com/watch?v=QSflUD0623M [1.10.2022].
5. Kurzgesagt., 2014. The golf stream explained. Youtube. Available at https://www.youtube.com/watch?v=UuGrBhK2c7U&t=152s [3.10.2022].