Initial findings of an international project designed to study the link between water mass transformation at high latitudes and the meridional overturning circulation in the North Atlantic (AMOC) were published on 1 February in Science (1). The new information has challenged long-held views that the AMOC is primarily driven by processes in the Labrador Sea, which is in the North West Atlantic. But may instead be influenced by currents in the North East Atlantic. The observations could help predict Arctic ice melt and the ocean’s future capacity to store atmospheric carbon and mitigate climate change.
The so-called Overturning in the Subpolar North Atlantic Program (OSNAP) ― launched in 2014 ― was set up to provide a continuous record of the trans-basin fluxes of heat, mass, and freshwater and therefore, a better understanding of convective activity and water mass transformation at high latitudes in the North Atlantic. The project is an international effort involving scientists from the UK, US, Canada, China, France, Germany, and the Netherlands. As the authors write, the new records provide an “observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century.”
The AMOC is a large system of ocean currents that carry warm water from the tropics northwards into the North Atlantic, and is sometimes referred to as the “Atlantic conveyor belt.” These large-scale ocean currents carry warm, salty water from the tropics to more northern regions like western Europe, and have a strong influence on climate; therefore, understanding how global warming impacts the AMOC may be crucial to guiding future efforts for mitigating climate change. Two studies published last year in Nature suggest the AMOC had slowed down by 15 per cent since the mid-20th century, which may be contributing to increased risk of storms in Europe and faster sea level rise in the US (2,3).
The first 21-month record has revealed strong transport variability in the region, which is responsible for the majority of heat and freshwater transport across Subpolar North Atlantic. However, deep water formation in the Labrador Sea ― previously believed to be a dominant factor ― may not be the major determinant of AMOC variability. Instead, AMOC variability has now been strongly linked to processes in waters between Greenland and Scotland, farther away in the North East Atlantic.
According to a perspective accompanying the paper written by Prof Monika Rhein, an oceanographer from the University of Bremen in Germany, the Intergovernmental Panel on Climate Change (IPCC) regularly assesses the fundamental influence of the AMOC on future climates, including the most recent special report on global warming of 1.5°C (4). However, observational data was previously sparse and previous beliefs were mainly based on model simulations. Thus, continuous observations of the OSNAP represent a huge achievement.
The new findings are important as they provide a deeper understanding of how climate change may affect the AMOC. While researchers recognise it may be too soon to draw conclusions, the new data could help refocus studies and highlights the importance of long-term observations to fully elucidate the impact of climate change on the AMOC.
(1) Lozier, M.S. et al. A Sea Change in Our View of Overturning – First Results from the Overturning in the Subpolar North Atlantic Program. Science (2019). DOI: 10.1126/science.aau6592
(2) Caesar, L. et al. Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature (2018). DOI: 10.1038/s41586-018-0006-5
(3) Thornalley, D.J.R. et al. Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years. Nature (2018). DOI: 10.1038/s41586-018-0007-4
(4) Rhein, M. Taking a close look at ocean circulation. Science (2019). DOI: 10.1126/science.aaw3111