NSFGEO-NERC: Collaborative Research: Subpolar North Atlantic Processes - Dynamics and pRedictability of vAriability in Gyre and OverturNing (SNAP-DRAGON)

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This is a project that is jointly funded by the National Science Foundation's Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with their own country. 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 large amount of heat this circulation carries northward and releases to the atmosphere impacts the track of storms and determines the weather over western Europe. The overturning circulation also has profound implications for African rainfall and hurricane development via its effect on sea surface temperature 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. This study aims to 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. 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 Program (OSNAP, www.o-snap.org). These have revealed large amplitude variations in the overturning and 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 for making useful climate predictions on seasonal, interannual, decadal and longer timescales. However, the current generation of models struggle to represent the processes known 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. The scientific community cannot assess model performance properly because there is limited understanding of all the links between atmospheric conditions and ocean circulation variability. This project will combine OSNAP and other observations with numerical models that can represent small-scale processes to reveal the cause of the variations in subpolar ocean circulation. Once it is clear which processes are most important and how they work, the team 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. The team will investigate how cold, dense waters find their way into the boundary currents that export them to the south. It will establish the role that winds play, which is likely more complicated than we have assumed in the past and it 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, the investigators 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? They 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. Innovative ways of combining models with observations will be used to determine a best estimate of the evolution of the subpolar North Atlantic over the OSNAP observation period.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.