SNAP-DRAGON: Subpolar North Atlantic Processes - Dynamics and pRedictability of vAriability in Gyre and OverturNing

SNAP-DRAGON will produce a step change in our understanding of the processes that link atmospheric changes to subpolar ocean variability, their implications for ocean and climate predictability in this region, and the degree to which we can trust their representation in climate models.

The subpolar North Atlantic Ocean, stretching between the UK, Greenland and Canada, plays a crucial role in local and global climate. This is the critical region where much of the warm water flowing northward in the upper North Atlantic releases its heat to the atmosphere and is converted to cold, dense water, before flowing southward again at depth in what is known as the Atlantic overturning circulation. The huge amount of heat this circulation carries northward and releases to the atmosphere impacts the storm track that determines the weather over western Europe. The overturning circulation also has profound implications for African rainfall and hurricane statistics via its effect on sea surface temperatures at lower latitudes. In addition, the sinking of water in the subpolar region ventilates the deep ocean, transferring heat and carbon away from the surface and moderating the impact of anthropogenic greenhouse gases on surface temperature. Any warm water which does not sink in the subpolar region recirculates or carries its heat further north towards the Arctic, influencing sea-ice conditions and polar marine ecosystems before it too sinks and flows south.

Recently, the first ever observations of the overturning circulation in the subpolar North Atlantic have been made by the Overturning in the Subpolar North Atlantic Programme (OSNAP, www.o-snap.org). These have revealed large amplitude variations in the overturning, but raised questions about the locations and processes that give rise to this variability, and its likely impact on surface ocean conditions and climate.

Representing this region properly in climate models is essential if we are to make useful climate predictions on seasonal, interannual, decadal and longer timescales. However, the current generation of models struggle to represent the processes we know to be important here, and disagree with the observations on the locations in which warm water is transformed into dense water. The disagreements limit our confidence in model predictions. We cannot assess model performance properly because we do not understand all the links between atmospheric conditions and ocean circulation variability.

In SNAP-DRAGON we will combine OSNAP and other observations with numerical models that can represent small-scale processes to work out what causes variations in subpolar ocean circulation. Once we know which processes are most important and how they work, we will be able to establish what climate models are getting wrong, and suggest improvements. This will improve predictions of ocean and climate variability in the subpolar North Atlantic and beyond.

We will investigate how cold, dense waters find their way into the boundary currents that export them to the south. We will establish the role that winds play, which is likely more complicated than we have assumed in the past. And we will determine the impact on overturning variability of changes in freshwater export from the Arctic and Greenland. To characterize and quantify these key processes, in addition to using ocean observations, we will perform "What if?" experiments in ocean models, asking questions such as: what happens to the subpolar ocean circulation if the atmospheric jet stream over the Atlantic shifts or strengthens? We will use statistical methods more common in weather forecasting to figure out how subpolar ocean properties and overturning connect to potentially predictable larger-scale atmospheric circulation patterns. And we will employ innovative ways of combining models with observations to determine a best estimate of the evolution of the subpolar North Atlantic over the OSNAP observation period.

We target two broad communities and one specific community as beneficiaries of SNAP-DRAGON. The broad communities are (i) those who require seasonal to decadal and longer-term climate forecasts for risk management and planning purposes, and (ii) those who are interested in knowing more about the North Atlantic Ocean and about climate research. The specific community is DSTL and the Royal Navy.

Seasonal, decadal and longer-term climate forecasts provide information to planners and policymakers to assess the risk of hazards and to understand the level of vulnerability of people and infrastructure. They are part of the suite of information needed to plan how to respond to risk and reduce exposure to hazards. Planners and policy-makers need to have an understanding of the skill of predictions in order to decide how to respond: uncertainty is a key challenge for decision-making. These stakeholders will directly benefit in the long-term from SNAP-DRAGON through improved skill in seasonal, decadal and longer-term climate predictions which will come about as we identify processes in models that work well, and those that require improvement.

There is growing interest in ocean literacy as people are increasingly aware of climate change, species extinctions and plastic and other pollution, through the efforts of popular figures such as David Attenborough and Greta Thurnberg. Politicians are responding to that interest too, and attendees at OceanObs19 heard that political staffers and researchers predominantly rely on google for instantaneous information about latest research. Thus, a growing audience is looking for easily-digestible information in a range of formats delivered online in a timely manner. This broad community will benefit by having access to our background knowledge and new information presented and made available in a range of easily accessible ways. The information will contribute to the UN Decade of Ocean Science for Sustainable Development (2021-2030) through a focus on providing material to promote ocean literacy.

The Defence Science and Technology Laboratory (DSTL) and the Royal Navy are also looking for information on the latest research about the North Atlantic. They will benefit from having close interaction with SNAP-DRAGON researchers throughout the project, and thereby having access to research as it is developing.