This week sees the release of a new special issue of the Royal Society's prestigious journal, 'Philosophical Transactions A' focused on the Atlantic Meridional Overturning Circulation (AMOC), a key component of our planet’s climate system. The new issue, called ‘Atlantic overturning: new observations and challenges', was compiled and edited by NOC scientists Prof. Meric Srokosz and Prof. Penny Holliday, with University of Southampton's Prof. Harry Bryden.
Prof. Penny Holliday, NOC's Associate Director of National Capability Science explains current research into the AMOC, how recent discoveries have challenged our previous understanding, and key priorities for future work to provide better actionable evidence for climate mitigation and adaptation:
The articles in the new Royal Society special issue provide a timely summary of the important new understanding that has grown through nearly 20 years of intensive study of the AMOC.
The AMOC is a complex system of ocean currents that has a surprisingly direct influence on the weather and climate on land. The secret of the importance of the AMOC lies in its ability to move heat over thousands of kilometres from one end of the Atlantic Ocean to another.
Even small changes in the strength of the AMOC, and especially the rate at which it transports warm water to the North Atlantic, can bring about large changes in the air temperature and rainfall that we receive in the UK. It can change rainfall in the Amazon Basin and over central Africa, the strength of hurricanes in the Caribbean, and even the severity of monsoons in southeast Asia.
Modelling the AMOC
Climate scientists have long known that getting the AMOC right in climate models is essential for us to have clear and reliable projections of how the climate will change under greenhouse gas emissions. Achieving this is harder than it sounds, because the AMOC is complex and it is very difficult to measure.
The consensus that emerges from climate models is that the AMOC will slow down over the coming century under anthropogenic climate change. However the rate of slowdown varies so much between models that we cannot be sure by how much things will change. We cannot rule out the possibility of the AMOC undergoing very fast change or even coming to a halt, but the evidence points to that doom-laden scenario being very unlikely.
Nearly two decades of observing and modelling the varying strength of the AMOC has helped to improve the way it is represented in our ocean and climate models. Collaborative international research programmes have taught us a lot about the many small physical processes that combine to form a global-scale ocean current system that extends thousands of kilometres around the Earth, and that carries ocean water from the sunlit surface to the cold, dark deep layers over 4000m below.
Challenging the state-of-the-art
The collection of papers published in the special issue marks a milestone in the history of AMOC research. Our discoveries have challenged the previous state of the art, most notably about the defining aspect of the AMOC: the phenomena by which warm surface water becomes dense, sinks and flows southward (the 'overturning' itself).
At the start of the 21st century our understanding was that densification and sinking was caused by cold wintery air blowing over the Labrador Sea (between the Labrador Peninsula and Greenland). and that the cold dense water flowed south in a current that hugged the seafloor close to the western edge of the Atlantic Ocean. We now know that the densification happens all along the pathway of the warm North Atlantic Current, and that it peaks in the Irminger Sea (between Greenland and Iceland). We have learned that when the cold water sinks it vigorously mixes with the water already in the deep layers, and that the rate of mixing far below the surface is as important as the rate of cooling in setting the strength of the AMOC.
One surprising result was the realisation that the southward flow is not confined a tight river of cold dense water leaned up against the western edges of the Atlantic Ocean basins, but that it also takes winding, eddying and diffuse paths across the deep abyssal plains. The importance of this knowledge is that it means measuring the AMOC in just one place it does not give us a clear view of the strength of the AMOC everywhere - so we need to find new ways to measure the AMOC in more places to know whether it is slowing.
With every precious extra day of observations of the AMOC, a clearer picture comes into focus of just how variable it is. We can now detect regular patterns of AMOC change due to the passing of every season. We witnessed an extraordinary event in 2009/10 where the AMOC suddenly slowed and then just as suddenly sped up again a few weeks later. We have just started to be able to see in the real ocean a pattern first found in ocean models - the decadal-scale variability of the North Atlantic Ocean. This means that the AMOC has periods of stronger and weaker states that fluctuate on timescales of around 10 years. The size and timescale of those natural fluctuations mean that we will need to keep measuring the AMOC for at least another 20 years before we can say with confidence whether the AMOC has already started to slow down.
Priorities for future research
The long timescale of a possible climate-change signal in the AMOC, and the confusing 'noise' of variability on shorter timescales that obscures it, lead us to the priorities for future research that have emerged. First and foremost we must keep measuring the strength of the AMOC because models cannot yet reliably reproduce the present-day state of it. At the same time, NOC researchers are working hard to find new ways to observe the AMOC at lower cost and in more places at once.
There is an important and urgent challenge in knowing how melting ice from the Arctic and Greenland might act to disrupt the AMOC. The extra freshwater entering the Atlantic Ocean is thought to be one major reason why the AMOC might slow down, but where and how that happens is yet to be understood. We need the knowledge to help us have early warning of AMOC changes that will impact societies around the world.
Both of those research topics will together address the third and equally important priority; improving the physics in climate models so that we have more confidence in climate projections. Plans and policy for acting on climate change requires good evidence in the form of global and regional projections. Without better representation of the AMOC in the models the projections will have more uncertainty than we want. NOC researchers are working with international partners to develop new models to provide better actionable evidence for climate mitigation and adaptation.