A US-led international effort, Overturning in the Subpolar North Atlantic Program (OSNAP), has provided a continuous record of the full-water column, trans-basin fluxes of heat, mass, and freshwater in the subpolar North Atlantic since 2014, in partnership with the UK, Netherlands, Canada and Germany. Results from the first four years of OSNAP observations have challenged the current understanding of overturning circulation in the subpolar North Atlantic. This grant provides funding for an additional four years of observations in order to deliver to the community a ten-year time series of the meridional overturning, heat and freshwater fluxes in the subpolar basin. Variations in these quantities have been invoked to explain changes in a wide range of physical, chemical and biological parameters in the North Atlantic, Nordic Seas, and Arctic Ocean. Thus, by quantifying Atlantic meridional overturning variability and understanding its drivers, OSNAP is providing a critical first step towards addressing societally-relevant, interdisciplinary questions concerning the melting of Greenland ice and Arctic sea-ice, heat content in the Arctic Ocean, climate of the Nordic Seas, and anthropogenic carbon storage. The researchers will actively engage the broader international communities through four workshops, each targeting a societally relevant theme: 1) ocean biogeochemistry and carbon sequestration; 2) overturning in ocean models; 3) Arctic cryosphere; and 4) Nordic Seas variability. Two new postdocs and one graduate student will be supported in this phase of OSNAP and will benefit from the diversity of methodologies and exposure to the large number of OSNAP international scientists. In addition, approximately 20 graduate students from different US and international institutions will receive field training through participation in OSNAP cruises through 2024.<br/><br/>The first four years of data from the OSNAP observing system have shown that the eastern subpolar region, from Greenland to Scotland, dominates the mean meridional mass and heat transport in the subpolar North Atlantic, while more than half of the total meridional freshwater transport occurs across the Labrador basin. Based on the success of the previous work, this project extends the time series and address the following critical questions: 1. What governs overturning variability in the North Atlantic subpolar gyre on intra-seasonal to interannual time scales? 2. What are the sources of freshwater across the OSNAP section and what governs their variability? 3. What are the impacts of the meridional heat and freshwater fluxes in the subpolar gyre? Additional motivation for this next phase of OSNAP is provided by the fundamental advancements in our understanding of subpolar North Atlantic dynamics and variability, that will result from the four main U.S. OSNAP mooring arrays, individually and in combination with other OSNAP observations. Further observations are expected to provide a strong observational basis for a new paradigm of overturning in this region, which will include an understanding of the linkage between dense water formation and overturning – a connection present in climate models, yet unobserved to date. Additionally, this this work will further quantify the structure and transport of the upper and deep ocean boundary currents off the east and west coasts of Greenland and within the Iceland Basin, as well as determine their variability and forcing mechanisms.<br/><br/>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.