OSNAP

OSNAP. The Overturning in the Subpolar North Atlantic Program is a major international observational initiative designed to provide a continuous, long-term record of the strength and variability of the Atlantic Meridional Overturning Circulation (AMOC) in the subpolar region. Launched in 2014, it employs a trans-basin array of moored instruments, Argo floats, and glider surveys to measure ocean currents, temperature, and salinity. The data collected is critical for understanding the role of the ocean in Earth's climate system and for improving predictions of future climate change.

Overview

OSNAP was established to address a critical gap in understanding the full Atlantic Meridional Overturning Circulation, a key component of global thermohaline circulation. Prior to its deployment, continuous measurements of the AMOC were primarily confined to latitudes near 26.5°N from the RAPID array. The program's observational network stretches from Labrador to Scotland and from Scotland to Greenland, crossing the major pathways of the subpolar gyre. This strategic design allows scientists to monitor the integrated overturning flow and its constituent heat and freshwater transports. The initiative represents a collaboration between numerous research institutions across North America and Europe.

Scientific objectives and design

The primary scientific objective is to quantify the mean and variability of the AMOC in the subpolar North Atlantic Ocean and to understand what controls its changes. A key design principle is distinguishing between the western and eastern boundary contributions to the overturning, which involves monitoring deep-water formation in basins like the Labrador Sea and the Irminger Sea. The array consists of moored instruments measuring current velocity, temperature, and salinity across critical choke points, supplemented by autonomous platforms. This data is integrated with satellite observations and models, such as those from the National Centers for Environmental Prediction, to create a comprehensive picture. The design specifically aims to resolve the transformation of warm, salty water flowing northward into cold, dense deep water that returns southward.

Key findings and discoveries

Data from OSNAP has led to paradigm-shifting discoveries about the Atlantic Meridional Overturning Circulation. A landmark 2018 study published in Science (journal) revealed that between 2014 and 2018, the majority of deep-water formation driving the overturning occurred east of Greenland, in the Irminger Sea and Iceland Basin, rather than in the traditionally emphasized Labrador Sea. This challenged previous assumptions held since the World Ocean Circulation Experiment. Subsequent analyses have shown significant variability in overturning strength on timescales from weeks to years, often linked to intense atmospheric forcing from events like the North Atlantic Oscillation. The program has also provided crucial insights into the transport of heat toward the Arctic and the interplay between the AMOC and the Greenland ice sheet's meltwater.

International collaboration and operations

OSNAP is a model of multinational cooperation in big science. The leadership team includes scientists from the Woods Hole Oceanographic Institution, the National Oceanography Centre in the United Kingdom, and the University of Miami. Funding and support come from agencies like the National Science Foundation, the Natural Environment Research Council, and the European Union's research frameworks. Field operations involve research vessels from multiple nations, including the RRS Discovery and the RV Neil Armstrong, to service moorings and conduct complementary hydrographic surveys. Data is managed and made publicly available through designated data assembly centers, fostering transparency and wider use within the global scientific community, including groups like the Intergovernmental Panel on Climate Change.

Significance and impact

The significance of OSNAP lies in its provision of the first continuous, direct measure of the subpolar Atlantic Meridional Overturning Circulation, a climate system component often dubbed the planet's "Achilles' heel." Its findings are vital for evaluating and improving the realism of climate models used by institutions like the Met Office and the National Center for Atmospheric Research. By clarifying the sensitivity and drivers of the AMOC, the program directly informs assessments of future climate risks, including potential shifts in European climate patterns and North American sea level. The long-term time series it generates is an indispensable tool for detecting early-warning signals of possible circulation slowdown, thereby contributing to global climate resilience and policy planning.

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