Rahmstorf, S., J.E. Box, G. Feulner, M.E. Mann, A. Robinson, S. Rutherford, and E.J. Schaffernicht (2015), Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation, Nature Clim. Change, 23 March 2015, doi: 10.1038/NCLIMATE2554
Cover Photo by R. Curry, Woods Hole Oceanographic Institution. Northern limb of the Atlantic Meridional Overturning Circulation.
In 2014, global mean sea surface temperatures reached an all time high and broke a 135-year record according to the National Oceanic and Atmospheric Administration. Although global warming is largely dominated by warm water anomalies, a region south of Greenland and Iceland has ironically been getting colder over the past century (Figure 1). A new study, led by Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, shows that the co-location of this defiant region of cold water with circulation pathways in the North Atlantic may be far from coincidental.
It is well understood that ocean circulation plays a major role in regulating climate by redistributing heat absorbed at low latitudes to high latitudes where it is then released. In the North Atlantic, a fast moving Gulf Stream delivers warm, salty water poleward at the surface, where it merges with a cooler North Atlantic Current near Greenland. These waters eventually become dense and sink to form North Atlantic Deep Water, which returns cold arctic water towards the equator. This system is referred to as the Atlantic Meridional Overturning Circulation (AMOC) and helps to regulate Northeastern U.S. and European climate through atmospheric heating via warm surface currents (Figure 2a). The AMOC is part of a global-scale ocean conveyor belt, called thermohaline circulation, which redistributes temperature and salinity anomalies throughout the world’s oceans. Variations in the flow and delivery of heat by these current systems over time can have significant impacts on both climate and extreme weather events.
This study finds that a twentieth-century weakening in the AMOC may be responsible for the unusual cooling in North Atlantic (Figure 2b). Consistent evidence from other studies supports these findings and shows a rapid slowdown in the AMOC especially after 1970. This study, however, is the first to point out that this reduction has been unprecedented over the past millennium, and suggests that recent changes in the AMOC are not due to natural climate variability, but instead driven by ocean-freshening effects of climate change.
The authors make the connection between subpolar cooling and the strength of the AMOC by comparing model simulations. They identify geographic regions that are most sensitive to a slowdown in circulation caused by adding freshwater anomalies in the North Atlantic, thereby reducing the amount of deep water formation and slowing the continuation of circulation pathways. These types of model studies are often called “hosing experiments” and are useful in studying the effects of localized freshening in the ocean.
Plausible real-word explanations for this freshening are anomalous sea-ice export from the Arctic Ocean, increased river discharge and increased meltwater from the Greenland Ice Sheet. A freshening trend in the North Atlantic has been observed since the 1970s (Figure 3). The accumulation of fresh, buoyant seawater near Greenland can de-stabilize the AMOC by preventing cold, dense waters to sink and complete deep ocean circulation back to the tropics.
To determine what effect density variations have on the intensity of the AMOC, the authors develop an index using sea surface temperature from instrumental data and coral growth proxies through artful reconstruction dating back to AD 900. This index describes the uniqueness of subpolar cooling compared to Northern Hemisphere mean temperatures and theoretically removes the effect of other sources of climate variability in this region, such as large-scale temperature fluctuations known as the Atlantic Multidecadal Oscillation. The authors test their index in state-of-the-art climate models to see if their metric realistically captures the overall shape and intensity of the AMOC, and indeed it does. The AMOC index and models have 90% agreement and they show the recent and projected slowdown in North Atlantic circulation (Figure 4).
Increased meltwater from glaciers due to global climate change increases the amount of low salinity seawater in the North Atlantic, thereby causing a slowdown in the AMOC. Although a permanent AMOC shutdown in the future remains very unlikely, short-term consequences from sluggish circulation are expected. Likely impacts include a rise in sea level along the US eastern seaboard that would impact cities like New York and Boston, more severe winter storms over Europe, and negative impacts on marine ecosystems and fisheries. Although these findings are significant, there is still much uncertainty about the natural variability and evolution of the AMOC over time due to unattainable continuous and direct measurements. This study is unique because it uses inferred evidence based on coral proxies to make hypotheses about past changes in ocean circulation. To what extent will global climate change affect the strength of the AMOC in the future? We may just have to wait and see. If the Greenland Ice Sheet continues to melt and the Arctic to freshen, the AMOC may continue to weaken over the next century, leading to more severe weather and extreme events.
- Animation of the Thermohaline Circulation animation by NASA
- Opinion piece by Judith Curry, “What’s up with the Atlantic?”