OSNAP (ocean observing system)

OSNAP (ocean observing system) is a major international oceanographic research program designed to provide a continuous, trans-basin record of the Atlantic Meridional Overturning Circulation (AMOC) in the subpolar gyre. Launched in 2014, its primary objective is to understand the dynamics, variability, and stability of this critical component of the global climate system. The program maintains a fixed array of moored instruments and conducts regular hydrographic surveys across key gateways from Labrador to Scotland.

Overview and Purpose

The OSNAP observing system was established to address critical gaps in understanding the Atlantic Meridional Overturning Circulation, a vast system of ocean currents that transports warm water northward and cold, dense water southward, profoundly influencing regional and global climate. Prior to OSNAP, sustained measurements of the full AMOC were primarily confined to latitudes near 26.5°N, monitored by the RAPID array. Scientists from institutions like the Woods Hole Oceanographic Institution and the National Oceanography Centre recognized the need to directly measure the overturning in the subpolar North Atlantic, where a major portion of the dense water formation that drives the AMOC occurs. The program's core purpose is to quantify the strength and variability of these overturning flows and their connection to atmospheric forcing, freshwater fluxes, and changing conditions in the Arctic Ocean.

Observational Array and Design

The OSNAP array is a strategically designed network of moored instruments spanning two primary sections: the western section from Labrador to southern Greenland, and the eastern section from southeastern Greenland to Scotland. This configuration captures the major inflow and outflow pathways of the subpolar gyre. The moorings are equipped with a suite of sensors, including current meters, CTD profilers, and ADCPs, to measure water velocity, temperature, and salinity throughout the water column. Complementary to the moored array, the program conducts regular ship-based hydrographic and geochemical tracer surveys along the lines to calibrate the continuous measurements and study water mass properties. Key partners in maintaining the array include the Scottish Association for Marine Science, Dalhousie University, and the University of Miami.

Key Scientific Findings

Data from the OSNAP array has led to several transformative discoveries about the North Atlantic circulation. A landmark 2018 study published in Science (journal) revealed that the eastern subpolar North Atlantic, particularly the Irminger Sea and the Labrador Sea, is the dominant region for the densification of waters that contribute to the lower limb of the AMOC, challenging previous paradigms that emphasized the Labrador Sea alone. The observations have shown that the overturning circulation in the subpolar region exhibits significant intra-annual and interannual variability, influenced by factors such as the North Atlantic Oscillation. Furthermore, OSNAP measurements have been crucial for validating and improving the representation of these processes in major climate models used by the Intergovernmental Panel on Climate Change.

International Collaboration and Management

OSNAP is a model of large-scale international cooperation in ocean science, involving scientists and funding agencies from the United States, the United Kingdom, Canada, Germany, the Netherlands, and China. The program is jointly led by a team of principal investigators from leading oceanographic institutions. Project coordination and data management are handled through a central executive committee, which ensures standardized data collection, timely processing, and open access to the dataset through repositories. This collaborative framework, supported by agencies like the National Science Foundation and the Natural Environment Research Council, enables the sustained operation of a logistically complex and costly observing system in a harsh oceanic environment.

Future Developments and Challenges

The future of OSNAP involves extending the time series to capture decadal-scale variability and trends, which is essential for detecting climate-change-driven shifts in the AMOC. A key challenge is securing long-term funding to maintain the expensive moored array and continue regular research cruises. Technological advancements, such as the integration of autonomous platforms like gliders and Argo floats, are being explored to enhance spatial coverage and reduce costs. Future scientific priorities include better quantifying the impacts of Greenland ice sheet meltwater on circulation and improving the synthesis of OSNAP data with satellite observations from missions like ESA's Sentinel satellites to provide a more holistic view of the changing North Atlantic.

Category:Oceanography Category:Climate change Category:Scientific organizations