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AMOC | Climate change is slowing Atlantic currents that help keep Europe warm

AMOC Time Series Dr. Peter T. Spooner

AMOC Time Series – Modeled evolution of the maximum AMOC streamfunction at 44 8 Nand deeper than 400m. The time series are plotted from 1871 to 2100 for all 12 models considered in this study.


AMOC – Atlantic Meridional Overturning Circulation
The Atlantic Meridional Overturning Circulation (AMOC) is defined as the constant, northward flow of warm, salty water in the upper layers of the Atlantic Ocean coupled with a circulating southward flow of colder water in the deep Atlantic. It is driven by temperature differences and salinity causing thermohaline circulation known as the THC), and is credited with helping to maintain a more moderate clime in the coastal, land regions surrounding the North Atlantic ocean. The AMOC is part of the global ocean conveyor belt, a circulating system that constantly moves ocean currents around the globe.


Original Article By Peter T. Spooner, Research Associate in Paleoceanography, University College London.

“The ocean currents that help warm the Atlantic coasts of Europe and North America have significantly slowed since the 1800s and are at their weakest in 1600 years,” according to new research conducted by Dr. Spooner and colleagues and presented in the scientific journal, Nature.

Dr. Spooner states, “the weakening of this ocean circulation system may have begun naturally but is probably being continued by climate change related to greenhouse gas emissions. This circulation is a key player in the Earth’s climate system and a large or abrupt slowdown could have global repercussions. It could cause sea levels on the US east coast to rise, alter European weather patterns or rain patterns more globally, and hurt marine wildlife.”


PDF ABSTRACT
Coupled climate models predict density-driven weakening of the Atlantic meridional overturning circulation (AMOC) under greenhouse gas forcing, with considerable spread in the response between models. There is also a large spread in the predicted increase of the southern annular mode (SAM) index across these models. Regression analysis across model space using 11 non-eddy-resolving models suggests that up to 35% of the intermodel spread in the AMOC response may be associated with uncertainty in the magnitude of the increase in the SAM. Models with a large, positive SAM index response generally display a smaller weakening of the AMOC under greenhouse gas forcing. The initial AMOC strength is also a major cause of intermodel spread in its response to climate change. The increase in the SAM acts to reduce the weakening of the AMOC over the next century by around 1 / 3 , through increases in wind stress over the Southern Ocean, northward Ekman transport, and upwelling around Antarctica. The SAM response is also related to an increase in the northward salt flux across 30 8 S and to salinity anomalies in the high-latitude North Atlantic. These provide a positive feedback by further reinforcement of the AMOC. The results suggest that, compared with the real ocean where eddies oppose wind-driven changes in Southern Ocean circulation, climate models un- derestimate the effects of anthropogenic climate change on the AMOC.


PDF Extracts

“The Atlantic meridional overturning circulation (AMOC) consists of a northward flux of warm water in the Atlantic basin, which cools and sinks at high latitudes, returning southward as dense water in the deep ocean (Wunsch 2002). Because it transports a large amount of heat northward, it plays an important role in Northern Hemisphere climate (Vellinga and Wood 2002; Knight et al. 2005). It is generally predicted that the AMOC will weaken in response to anthropogenic climate change (e.g., Thorpe et al. 2001; Gregory et al. 2005; Cheng et al. 2013) with the potential for both regional and global climate im- pacts, such as moderation of global warming in Europe (Christensen et al. 2007; Meehl et al. 2007).”

“Similar but larger changes in climate have been linked to the AMOC ‘‘bipolar seesaw’’ during glacial periods (Broecker 1998). AMOC strength, estimated using proxies such as 231 Pa/ 230 Th and 14 C (McManus et al. 2004; Robinson et al. 2005), is correlated with Arctic temperature as well as the intensity of Asian monsoons and climate over the Americas; it is thought to be the driver of such changes, although modeling has proved inconclusive (Wang et al. 2001; Alley 2007; Seager and Battisti 2007; Broecker et al. 2010). A weakening AMOC may also reduce the oceanic capacity for uptake of anthropogenic CO 2 via increases in North Atlantic stratification and the associated weakening of the biological pump and decreased transportofCO 2 to depth (Schmittner 2005; Obata 2007; Zickfeld et al. 2008). The paleoclimate record also hints that changes in AMOC strength are related to the capacity for terrestrial storage of methane and nitrous oxide, two potentially potent greenhouse gases (Fl € uckiger et al. 2004; Sowers 2006; Wolff et al. 2010).”


Dr. Spooner’s Full Statement Article
theconversation.com/climate-change…help-keep-europe-warm…

Original Research Paper (PDF)
earth.ox.ac.uk…SpoonerJohnsonWoollings_JClim2013.pdf

NATURE Magazine
nature.com/articles/s41586-018-0006-5

UCL Dept. of Geology
blog.geog.ucl.ac.uk…climate-change…currents-that-help-keep-europe-warm

Scientific American
scientificamerican.com/article…weakest-in-1-600-years

Smithsonian Magazine
smithsonianmag.com…ocean-current-keeps-europe-warm-weakening

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