Ocean currents have long been a phenomenon understood by early explorers and sailors. More recently, we have come to appreciate the role ocean currents play in the nutrient cycles and climate of our planet. Among other benefits, currents bring warmth and nutrients from the ocean to different regions.
What Impacts Ocean Currents
Ocean currents are driven by different factors, including temperature, earth rotation, water density, land masses and their positioning, and wind. However, these factors are prone to change, which can alter local and global current cycles.
For surface currents, among the most important factors are winds driving them. Prevailing winds, such as the Westerlies that move at between 30 and 60 degrees in the Northern Hemisphere and the Southern Hemisphere, moves these surface currents such as in the Atlantic Ocean.
The Coriolis effect is a natural phenomena that causes ocean currents to veer to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Another important current type occurs from upwelling, where water from the deep rises to the surface, which brings nutrients vital for sea life. Finally, tidal currents are the last major category of currents. These are primarily driven by the interactions of the Earth, Moon, and Sun.[1]
Five Major Global Ocean Currents
There are five major global currents, called gyres, which rotate around the globe. These five major ocean-wide gyres are: —the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean gyres.
The North Pacific is made up of four California current, the North Equatorial current, the Kuroshio current, and the North Pacific current. The movement of these four currents brings in a large amount of plastic and fishing trash into what is known as the “Great Pacific Garbage Patch.”
Trash from river runoffs, debris from cargo ships, fishing lines and nets, and other sources of garbage all make their way into this oceanic mass of garbage.
To understand changes to currents, and given their importance to many different Earth cycles and ecosystems, scientist monitor currents globally.
Two Main Ways to Map Ocean Currents
There are two primary ways scientists can map ocean currents.
One is using sea buoys that send radio data back to receiving stations.
The second way is using satellite data, particularly from the Joint Altimetry Satellite Oceanography Network (Jason) series satellites that sent altimeter data, with the current Jason-3 being active since 2016. Less commonly used, although likely to increase importance in coming years, deep sea robots are also deployed to track deep ocean currents.
Current information on global wind and surface ocean current patterns can be seen from published maps by the Earth Null School.[2] Additionally, simulation modeling, particularly global circulation models, such as one developed by MIT (MITgcm model), that are used for global weather forecasts, are also applied to forecasting current flows, including the Gulf Stream stretching from Mexico to Western Europe.[3]
Ocean Currents and Climate Change
With climate change, subtle changes to the ocean currents may exacerbate or even lead to more extreme weather.
Changes to Ocean Circulation May Fuel More Extreme Winter Weather
For instance, in a recent study, the authors found that changes to the Atlantic Meridional Overturning Circulation may likely be fuelling more extreme winter weather, particularly colder weather. During the winter, a lot of heat is transferred to northern latitudes from this current system.
However, warming of waters and freshening of water that occurs as ice melts in northern latitudes leads to reduced surface water density. This then prevents water from sinking in the current, minimizing heat release in northern latitudes.
As the current slows, then weather becomes more extreme as it is not regulated by the warming waters, having a greater impact during the winter months.[4]
Ocean Water Currents May Change with Climate Change
Similar freshening of the eastern subpolar Atlantic was found from data between 2012-2016, suggesting water is become less saline, which has a density effect on water and contributes to current change and slowing.[5]
In contrast, in Antarctica, winds are speeding up as the climate warms. Here, currents are moving faster as winds are moving more quickly.
The change here is likely to have some major effects on climate, although scientists are not sure what that might be exactly. Data collected from robots and satellites demonstrate that this speeding up is also likely to continue.[6]
Ocean Currents are Important to the Health of the Planet
Ocean currents are incredibly important to our planet. Scientists are improving the way we monitor currents, but understanding the consequences of the rapid change to currents we are observing is not always clear.
Overall, we know changes to our currents are likely to cause major change to life and our climate, but what that change is needs further study.
Read next: Understanding Ocean Wave Patterns from Satellite Imagery of Sun Glitter
References
[1] For more on the types of currents and mapping them, see: Joseph, A. Measuring Ocean Currents: Tools, Technologies, and Data; 1st ed.; Elsevier: Waltham, MA, 2014; ISBN 978-0-12-415990-7
[2] Map of ocean currents from the Earth Null School: https://earth.nullschool.net/#current/ocean/surface/currents/orthographic.
[3] For more on outputs on the Gulf Stream as derived from MIT’s global circulation model, see: https://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=3913.
[4] For more on the article discussing the changing Atlantic currents, see: Yin, J.; Zhao, M. Influence of the Atlantic Meridional Overturning Circulation on the U.S. Extreme Cold Weather. Commun Earth Environ 2021, 2, 218, doi:10.1038/s43247-021-00290-9.
[5] For more on changes to water salinity in the eastern subpolar Atlantic, see: Holliday, N.P.; Bersch, M.; Berx, B.; Chafik, L.; Cunningham, S.; Florindo-López, C.; Hátún, H.; Johns, W.; Josey, S.A.; Larsen, K.M.H.; et al. Ocean Circulation Causes the Largest Freshening Event for 120 Years in Eastern Subpolar North Atlantic. Nat Commun 2020, 11, 585, doi:10.1038/s41467-020-14474-y.
[6] For more on changes to ocean currents in the Antarctic, see: Shi, J.-R.; Talley, L.D.; Xie, S.-P.; Peng, Q.; Liu, W. Ocean Warming and Accelerating Southern Ocean Zonal Flow. Nat. Clim. Chang. 2021, 11, 1090–1097, doi:10.1038/s41558-021-01212-5.
