EGUsphere EGUsphere EGUsphere EGUsphere Copernicus Publications Göttingen, Germany 10.5194/egusphere-2025-5406 Simulated and Observed Transport Estimates Across the Overturning in the Subpolar North Atlantic Program (OSNAP) Section Danabasoglu Gokhan https://orcid.org/0000-0003-4676-2732 1 Castruccio Frederic S. 1 Boza Burcu https://orcid.org/0000-0002-1305-1240 2 Barthel Alice M. https://orcid.org/0000-0002-2481-8646 3 Biastoch Arne https://orcid.org/0000-0003-3946-4390 4 5 Blaker Adam https://orcid.org/0000-0001-5454-0131 6 Bozec Alexandra 7 Bruciaferri Diego https://orcid.org/0000-0001-5114-693X 8 Bryan Frank O. 1 Chassignet Eric P. https://orcid.org/0000-0003-4710-7502 7 Fu Yao https://orcid.org/0000-0003-2227-3694 9 10 Grooms Ian https://orcid.org/0000-0002-4678-7203 11 Guiavarc'h Catherine 8 Hayashida Hakase https://orcid.org/0000-0002-6349-4947 12 Hogg Andrew McC. https://orcid.org/0000-0001-5898-7635 13 Holmes Ryan M. https://orcid.org/0000-0002-6799-9109 14 Iovino Doroteaciro https://orcid.org/0000-0001-5132-7255 15 Kiss Andrew E. https://orcid.org/0000-0001-8960-9557 13 Lozier M. Susan https://orcid.org/0000-0003-4129-6349 9 Marques Gustavo https://orcid.org/0000-0001-7238-0290 1 Megann Alex https://orcid.org/0000-0003-0975-6317 6 Schwarzkopf Franziska U. https://orcid.org/0000-0002-2747-8456 4 Storkey Dave https://orcid.org/0000-0001-8000-5335 8 van Roekel Luke https://orcid.org/0000-0003-1418-5686 3 Wolfe Jon 3 Xu Xiaobiao https://orcid.org/0000-0001-8902-7645 7 Zhang Rong 16 17 US National Science Foundation (NSF) National Center for Atmospheric Research (NCAR), Boulder, CO, USA Eurasia Institute of Earth Sciences, Istanbul Technical University, Istanbul, Turkiye US Department of Energy (DOE), Los Alamos National Laboratory (LANL), Los Alamos, NM, USA GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany Kiel University, Kiel, Germany National Oceanography Centre, Southampton, UK Center for Ocean-Atmospheric Prediction Studies (COAPS), Florida State University, Tallahassee, FL, USA UK Met Office, Exeter, UK School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA College of Marine Science, University of South Florida, St. Petersburg, FL, USA Applied Mathematics Department, University of Colorado Boulder, Boulder, CO, USA Application Laboratory, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan Research School of Earth Sciences and ARC Centre of Excellence for Climate Extremes, Australian National University, Canberra, Australia Australian Bureau of Meteorology, Sydney, Australia Fondazione Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC), Bologna, Italy Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ, USA National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory (GFDL), Princeton, NJ, USA 12 11 2025 2025 1 89 Copyright: © 2025 Gokhan Danabasoglu et al. 2025 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5406/ The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5406/egusphere-2025-5406.pdf

A comparison of simulated and observed overturning transports and related properties across the Overturning in the Subpolar North Atlantic Program (OSNAP) sections for the 2014–2022 period is presented, considering both depth and density space transports. The effort was motivated by the observational transport estimates at both OSNAP-West (OW) and OSNAP-East (OE) sections which show a minor role for the Labrador Sea (LS) in setting the mean and variability of the overturning in the subpolar North Atlantic. There are 9 participating groups from around the world, contributing a total of 18 ocean – sea simulations with 6 different ocean models. The simulations use a common set of interannually-varying atmospheric forcing datasets. The horizontal resolutions of the simulations range from nominal 1° to eddy-resolving resolutions of 0.1°–0.05°. While there are many differences between the simulations and observations as well as among the individual simulations in terms of transport properties, the simulations show significantly larger transports at OE than at OW in general agreement with the observations. Analyzing overturning circulations in both depth and density space together provides a more complete picture of the overturning properties and features. This analysis also reveals that, in both the simulations and observations, northward and southward flows substantially cancel each other, producing much smaller residual (total) transports. Such cancellations tend to be much more prominent in depth space than in density space. In general, the observed transport features are captured better at OE than OW. The simulations generally show larger (smaller) transports with positive (negative) temperature and salinity biases in the upper ocean near the OSNAP sections, but with no such relationship with density biases. In high-resolution simulations, the transport profiles agree better with the observations in general, but challenges remain in some other metrics considered in our analysis. When transports are calculated using a density referenced to 2000-m depth, rather than the ocean surface, the relative contributions of transports at OW increase modestly.