Research gives new knowledge about the Atlantic overturning circulation
Scientist Laura de Steur of the Norwegian Polar Institute has, together with her colleague Femke de Jong, done research for the NIOZ Royal Netherlands Institute for Sea Research and Duke University that contributes to the understanding of the Atlantic Meridional Overturning Circulation (AMOC).
The scientists found that the recent temperature changes in the Irminger Sea between Iceland and Greenland can be explained through regional ocean-atmosphere interaction during the cold winter of 2014–2015. This rejects a hypothesis posing that increased meltwater from Greenland weakened deep water formation and caused the cold blob.
Deep water formation is an important process in the global ocean circulation. When high latitude winters are cold enough, the salty surface water of the North Atlantic cools enough to increase its density and mix with underlying deeper waters. This mixing is called deep convection. It forms the vertical link between the warm northward flow near the surface and the cold southward flow along the bottom, which is generally referred to as the Atlantic Meridional Overturning Circulation. The transport of warm water in the overturning circulation is partly responsible for the mild climate in northwestern Europe compared to similar latitudes in North America.
In climate model simulations the strength of convection south of Greenland is related to the strength of the overturning circulation. Some models predict that convection will weaken due to the input of freshwater released from the melting ice on Greenland. Because of the much lower density of freshwater compared to seawater it forms a barrier that isolates the deep ocean from the cold atmosphere. While the majority of the Earth warms as a result of climate change the region around southern Greenland would cool.
In temperature observations of the earth’s surface in 2015 a similar pattern seems to appear. The Earth warmed while the ocean southeast of Greenland cooled. This led to speculation that convection had already weakened as a result of increased melting of Greenland’s icecap. It would mean that the overturning circulation would be affected faster than expected.
Instruments moored in the Irminger Sea, southeast of Greenland, shows that this is not (yet) the case. This mooring, deployed by the NIOZ Royal Netherlands Institute for Sea Research at 3 km depth in 2003, shows that convection strengthened in recent years. Record depths of convection were observed in the Irminger Sea in the winter of 2014–2015. This strong mixing was caused by an extremely cold and long winter. Two NIOZ scientists, Femke de Jong and Laura de Steur, have shown that the temperature evolution in the Irminger Sea (including the strong decrease in 2015) can be explained through regional interaction between the ocean and atmosphere. The manuscript that describes the convection and explains the temperature changes is accepted in Geophysical Research Letters. This coincides with a publication by a German group in Nature Geoscience last week, in which they use a model to show that it will take some time before freshwater from Greenland enters the deep water formation regions in large enough amount to weaken convection.
The measurements in the Irminger Sea were partly funded by the European North Atlantic CLIMate (NACLIM) project and are part of the international Overturning in the Subpolar North Atlantic Program (OSNAP). The goal of OSNAP is to quantify the overturning circulation and its heat transport at high latitudes and to establish a relation to convection and wind forcing. On 26 July a research expedition will return to the Irminger Sea to recover the NIOZ and other OSNAP moorings and do a hydrographic survey.