Meridional overturning circulation (MOC)
Meridional Overturning Circulation (MOC) refers to a significant oceanic system that involves the flow of water in both vertical and north-south directions across ocean basins. This system plays a critical role in the thermohaline circulation, which facilitates the large-scale exchange of water masses, enhances oxygen distribution in the deep ocean, and regulates global climate patterns. The MOC is primarily driven by the formation of dense water in high-latitude regions, particularly in the North Atlantic and Southern Oceans, where cold and salty conditions cause surface waters to sink and create deepwater currents.
The MOC is essential for transporting heat from tropical regions to higher latitudes, particularly benefiting the climate of Europe. However, climate change poses a risk to this circulation, as the melting of ice sheets and increased freshwater from precipitation could disrupt the salinity and density of surface waters, hindering their ability to sink. Historical evidence suggests that such disruptions have previously led to rapid climate changes, as seen during the Younger Dryas period. Researchers are increasingly concerned that a weakening or collapse of the MOC due to current warming trends could lead to severe climate impacts, including extreme weather events in various regions, notably in Australia and the southwestern United States. Understanding the MOC is crucial for predicting future climate scenarios and their potential effects on ecosystems and human societies.
Meridional overturning circulation (MOC)
Definition
Meridional overturning circulation (MOC) is an oceanographic term for water that flows in the plane defined by the vertical and meridional (or north-south) axes. It is calculated by averaging those north-south, up-down flows from east to west across the width of an ocean basin.
Most discussions of the MOC focus on the deep, overturning circulations that connect the ocean abyss to the surface. Deepwater formation (the “sinking branches” of the deep MOC) occurs in two broad regions of the global ocean: the high-latitude North Atlantic Ocean, predominantly in the Labrador Sea and the Nordic Sea, and the Southern Ocean, near Antarctica. Water’s density increases when it is cold and salty. Thus, the densest surface waters occur in the polar regions. Temperatures there are low, and brine-rejection during ice-formation increases the of surface ocean water.
Dense polar water sinks to form deepwater masses that spread out horizontally along the meridional axis to fill most of the global deep oceans. The return branch of this deep MOC is more diffusely distributed. Deep water becomes more buoyant, and it will return back toward the surface if it is heated or made less saline. Vertical mixing with less dense water higher in the water column will decrease the density of deep water, producing a return flow toward the surface. The deep MOC is sometimes called the “thermohaline circulation,” although that term refers to such movement in all oceans, not just the Atlantic. The term "thermohaline circulation" is meant to evoke the idea that vertical motions are caused by changes in the temperature and salinity of seawater.
Shallow, wind-driven overturning circulations exist closer to the ocean’s surface, the most prominent such feature being the subtropical cells in the Atlantic and Pacific Oceans. The winds blowing over the subtropical oceans force a convergence of surface waters, which pushes surface water downward in a process known as “Ekman pumping.” This water travels at depth toward the equator, where the pattern of winds forces surface waters to diverge, bringing the water that was pumped downward in the back to the surface. Surface winds then force the surface waters back toward the subtropics, completing the subtropical cell.
Significance for Climate Change
The MOC plays a crucial role in maintaining Earth’s climate. The earth is heated primarily in the tropics. Warm ocean currents move a large amount of tropical heat to higher latitudes in the subtropical cell and the deep MOC. This transfer of heat helps keep the high latitudes warm. The poleward heat is particularly strong in the North Atlantic Ocean. Heat brought poleward in the North Atlantic is then advected by large-scale winds eastward, where it warms Europe. Scientists have suggested that the North Atlantic MOC may slow down or shut down completely as a result of global warming; if this happens, the heat flux associated with the MOC would decrease, and Europe would become colder.
A climatic change that warms the high latitudes will introduce freshwater into the high-latitude North Atlantic Ocean as ice sheets melt and precipitation increases. This added freshwater would decrease the salinity of surface waters, reducing their tendency to sink. If polar water stops sinking, the warm surface current that brings lighter water to replace the sinking water would be disrupted, so the flow of heat to the North Atlantic and Europe would diminish or cease entirely.
Geologic evidence supports the argument that the introduction of freshwater into the North Atlantic can weaken heat transport by the Atlantic MOC. During the last ice age, ice sheets several kilometers thick covered a large portion of North America and northern Europe. Around 14,000 years ago, glaciers in the Northern Hemisphere retreated as a result of astronomically forced changes in Earth’s orbit. Temperatures rose, and the ice sheets began to melt. Then, approximately 12,800 years ago, temperatures dropped rapidly back into the glacial range and ice sheets returned for another 1,300 years. This rapid drop in temperatures, known as the Younger Dryas, is believed to have been caused by the rapid input of freshwater into the North Atlantic from the melting North American ice sheet, which dramatically decreased the Atlantic MOC. While there is no large on North America today, scientists are concerned that global warming could cause freshwater melt from Greenland to trigger analogous processes. Furthermore, research in 2022 conducted by the University of New South Wales in Sydney, Australia, looked at the consequences of the weakening Atlantic MOC and concluded that, if the MOC collapsed completely, areas such as Australia and the southwest United States would be significantly impacted and might experience increased flooding rains, worse droughts, and more intense wildfire seasons.
Bibliography
Boers, Niklas. "Observation-Based Early-Warning Signals for a Collapse of the Atlantic Meridional Overturning Circulation." Nature Climate Change, vol. 11, 2021, pp. 680-688.
England, Matthew et al. "A Huge Atlantic Ocean Current Is Slowing Down. If It Collapses, La Nina Could Become the Norm for Australia." UNSW Sydney Newroom, 7 June 2022, newsroom.unsw.edu.au/news/science-tech/huge-atlantic-ocean-current-slowing-down-if-it-collapses-la-niña-could-become-norm. Accessed 3 Feb. 2023.
Kuhlbrodt, T., et al. “On the Driving Processes of the Atlantic Meridional Overturning Circulation.” Reviews in Geophysics 45 (April 24, 2007).
"The Ocean Conveyor." Woods Hole Oceanographic Institution. Woods Hole Oceanographic Inst., 2014. Web. 23 Mar. 2015.
Rahmstorf, Stefan. "Is the Atlantic Overturning Circulation Approaching a Tipping Point?" Oceanography, vol. 37, no. 3, 10 Apr. 2024, pp. 16-29, doi.org/10.5670/oceanog.2024.501. Accessed 12 Dec. 2024.
Richardson, P. L. “On the History of Meridional Overturning Circulation Schematic Diagrams.” Progress In Oceanography 76 (2008): 466–486.
Schmittner, A., John C. H. Chiang, and Sidney R. Hemming, eds. Ocean Circulation: Mechanisms and Impacts. Geophysical Monograph 173. Washington, D.C.: American Geophysical Union, 2007.
Siedler, Gerold, John Church, and John Gould, eds. Ocean Circulation and Climate. London: Academic Press, 2001.