Contradicting the prevailing view on what causes major changes to a climate-regulating ocean circulation pattern in the northern hemisphere, the first results from the “OSNAP” project, an internationally collaborative effort designed to monitor large-scale ocean dynamics, report that deep water formation in the Labrador Sea does not drive Atlantic Meridional Overturning Circulation (AMOC) variability.
According to results from the first 21-month record of observations by OSNAP (or, the Overturning in the Subpolar North Atlantic Program), the Labrador Sea plays only a minor role – instead, deep waters that move south through the abyssal basins east of Greenland dominate AMOC variability. Largely responsible for regulating climate in the northern hemisphere by redistributing substantial amounts of heat energy, the large-scale Atlantic Ocean circulation pattern brings warm and salty tropical surface water to colder northern latitudes.
When this water cools, it becomes more dense, sinks and flows south, returning to lower latitudes through the deep ocean – a process also known as deep water formation. Current climate models predict severe consequences should the AMOC slow or stop due to disruptions in North Atlantic deep water formation, however, and the most recent report of the Intergovernmental Panel on Climate Change (IPCC) includes projections that a slowdown of the AMOC will occur during the next century, according to the authors.
Using data obtained from the OSNAP array during 2014 and 2016, Susan Lozier and colleagues created a time series that showed striking variability in overturning circulation across the Atlantic, particularly in areas east of Greenland; the variability they observed in the Atlantic was nearly seven times greater than in the Labrador Sea.
According to Lozier et al., while the initial results have invited a reevaluation of long-held beliefs about the AMOC and provide a much-needed baseline for assessing numerical models, longer-term observations are needed to fully understand the phenomenon.
In a related Perspective, Monika Rhein writes: “The promising results from the OSNAP array, its proximity to the key region Labrador Sea and the questions raised about the processes causing AMOC variability provide excellent incentives to continue the OSNAP array for the next decades.”
An array of overturning data
The Atlantic meridional overturning circulation (AMOC) has a strong influence on climate, so it is important to understand how global warming may affect it. Lozier et al. report initial results from the Overturning in the Subpolar North Atlantic Program (OSNAP) (see the Perspective by Rhein). OSNAP has been measuring the flux of water transported by overturning in the high latitudes in the North Atlantic. The measurements reveal the strong variability of transport in the region and show that deep water formation in the Labrador Sea may not, as previously believed, be the major determinant of AMOC variability.
(Release by: American Association for the Advancement of Science and add. Science)