To forecast regional and global climate decades into the future, scientists rely on the assumption that the slow ocean processes helping drive our planet’s climate operate in predictable patterns. One of those slow processes, the Atlantic Meridional Overturning Circulation (AMOC), is a major circulation pattern in the small Atlantic basin—with an outsized effect on climate.
Warm waters flow northward along the ocean surface toward the pole, where the surface waters cool, sink, and return southward at greater depths. Upwelling in the Southern Ocean around Antarctica brings the dense waters back to the surface. But a lack of observational studies and understanding of the more nuanced processes driving the AMOC has hindered attempts to predict decadal climate variability on the basis of AMOC behavior.
Here Buckley and Marshall review our current knowledge of the AMOC—the variability in the circulation patterns, the mechanisms that control it, and the role the AMOC plays in global climate—over various time scales.
The researchers report that the AMOC is relatively stable over decadal time scales but varies considerably on seasonal time scales or within a single year. Winds are a major contributor to that short-term variability. But the team concludes that buoyancy anomalies just east of the Grand Banks off of Newfoundland (where the Gulf Stream, the North Atlantic Current, and the Labrador Current intermingle) govern decadal AMOC variability.
The researchers report several pathways by which the AMOC might influence climate: On time scales ranging from decades to centuries, changes in the AMOC may be linked to changes in sea surface temperatures known as the Atlantic Multidecadal Variability—a warming and cooling of the North Atlantic over decadal time frames. Such a pathway extends the AMOC’s reach beyond the ocean, as sea surface temperatures tend to influence the climate of nearby landmasses.
Additionally, ocean heat transport by the AMOC is responsible for the relative warmth of the Northern Hemisphere compared to the Southern Hemisphere and likely determines the mean position of the Intertropical Convergence Zone—the region encircling the globe where the trade winds from each hemisphere meet—north of the equator. Finally, the authors note, the AMOC can influence the trajectory of human-induced climate change, as the circulating cell helps pull heat and carbon from the surface deep into the ocean.
Predicting the AMOC on decadal time scales—and its subsequent effects on climate—is a “tantalizing possibility,” according to the authors. But before that possibility becomes a reality, more observational studies will be necessary to fill in the remaining gaps in our knowledge of the processes underlying ocean circulation. In the near future, arrays deployed in the South Atlantic and the subpolar North Atlantic will no doubt provide valuable insights into the AMOC’s stability and its connections to other ocean basins. (Reviews of Geophysics, doi:10.1002/2015RG000493, 2015)
—Kate Wheeling, Freelance Writer
Citation: Wheeling, K. (2016), A big climate driver in a small ocean basin, Eos, 97, doi:10.1029/2016EO044075. Published on 25 January 2016.