Overturning in the Subpolar North Atlantic Program

Overturning in the Subpolar North Atlantic Program is a major international oceanographic research initiative focused on observing and understanding the dynamics of the Atlantic Meridional Overturning Circulation in a critical high-latitude region. Launched in 2014, the program employs a comprehensive array of moored instruments, ship-based surveys, and autonomous platforms to measure the strength, variability, and drivers of this climatically crucial ocean circulation system. Its findings are vital for improving predictions of future climate change impacts on regional and global climate systems.

Background and Scientific Motivation

The scientific impetus for the program stemmed from recognized limitations in existing observations of the Atlantic Meridional Overturning Circulation, particularly in the subpolar North Atlantic where key water mass transformations occur. Prior programs like the RAPID array at 26.5°N provided invaluable data but left a critical knowledge gap further north. Research from institutions like the National Oceanography Centre and Woods Hole Oceanographic Institution indicated that the subpolar gyre is where warm, salty waters transported northward by the Gulf Stream and North Atlantic Current cool and sink, forming deep waters that return southward. Understanding this process is essential because the Atlantic Meridional Overturning Circulation is a primary mechanism for redistributing heat globally, influencing weather patterns from Europe to Africa, and sequestering carbon dioxide. Concerns about potential slowdowns due to Greenland ice melt and changing freshwater fluxes, highlighted in reports by the Intergovernmental Panel on Climate Change, underscored the urgent need for sustained observations in this sensitive region.

Program Design and Implementation

The program was designed as a multi-national collaboration, primarily funded by the United States National Science Foundation and the United Kingdom's Natural Environment Research Council, with significant contributions from Canada, Germany, and the Netherlands. The observational network was strategically planned to capture the full overturning loop in the subpolar basin. Implementation involved the coordinated deployment of a large array of moored instruments across key choke points and deep basins, requiring extensive ship time from vessels like the RRS Discovery and the RV Neil Armstrong. The field campaign officially began in 2014 with initial deployments, reaching full design strength by 2016. Governance is led by co-chairs from the international community, ensuring data from partners like GEOMAR Helmholtz Centre for Ocean Research Kiel and the Scottish Association for Marine Science are integrated into a unified dataset.

Key Observational Components

The observational system is built around several complementary elements. A central component is an array of moored instruments spanning the entire subpolar region, from the eastern boundary near the Rockall Trough to the western boundary off Labrador and Greenland. This includes critical sections across the Greenland-Scotland Ridge to measure overflow waters, and in the Irminger Sea and Labrador Sea to monitor deep convection sites. The moored arrays are supplemented by regular hydrographic surveys using CTD casts and water sampling to provide high-resolution spatial context. Furthermore, the program incorporates data from autonomous platforms like Argo floats and gliders, and leverages satellite observations from agencies like NASA and the European Space Agency to measure surface parameters such as sea surface height and temperature.

Major Findings and Scientific Impact

Research from the program has yielded transformative insights into the Atlantic Meridional Overturning Circulation's behavior. A landmark finding published in journals like *Science* and *Nature* was the documentation of exceptional variability in the overturning strength, including a significant weakening period around 2014-2015 linked to a persistent atmospheric pattern known as the North Atlantic Oscillation. The data have clarified the relative importance of different deep-water formation regions, showing the Irminger Sea as a more consistent convection site than the Labrador Sea in recent years. These observations have been critical for evaluating and improving the realism of climate models used by the Intergovernmental Panel on Climate Change and centers like the Met Office. The program's unique dataset has also advanced understanding of how the ocean transports heat and carbon, directly impacting predictions for sea level rise along the U.S. East Coast and Western Europe.

Future Directions and Legacy

The initial phase of the program was successfully completed in 2020, but its legacy continues through an extended monitoring effort and deep analysis of the collected dataset. Future work aims to extend the time series to distinguish multi-decadal trends from shorter-term variability, which is essential for detecting anthropogenic climate signals. The program's infrastructure and international collaboration framework serve as a model for other observing systems, influencing the design of the broader Atlantic Meridional Overturning Circulation observing network. Its data archive, managed by entities like the British Oceanographic Data Centre, remains a foundational resource for the global climate science community, informing policy discussions at forums like the United Nations Framework Convention on Climate Change and guiding future missions by agencies like NASA and the European Space Agency.

Category:Oceanography Category:Climate change assessment and attribution Category:Scientific organizations