Gerard McCarthy, National Oceanographic Centre
The Atlantic Meridional Overturning Circulation (AMOC) plays a crucial role in the climate system, in particular in maintaining the mild climate in northwestern Europe relative to similar maritime climates. Fluctuations in AMOC strength have been hypothesised as the main driver of Atlantic Multidecadal Variability (AMV) of SSTs. A cool period in the 1970s and 1980s was linked to droughts in the Sahel region of Africa and the recent warm period associated with accelerating temperature rise in the northern hemisphere and an active hurricane period in the Caribbean. There is a body of evidence that the Atlantic is currently entering a cool period.
We examine a number of proxies for the AMOC in the 20th century, including the sea level gradient along the US east coast and Labrador Sea density, and place this recent cooling in the context of long term atmospheric variability, viewed through the North Atlantic Oscillation, and changing patterns of ocean circulation.
We analyse direct observations of the AMOC made by the RAPID programme since 2004 to look at emerging long term changes. These observations support a cooling Atlantic driven by a declining AMOC. We discuss this change in the context of predicted slowdowns of the AMOC due to anthropogenic climate change relative and consider what is needed to detect and attribute changes to natural and man-made sources.
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