A Comparison of OSNAP Observations and Ocean Reanalyses in the Subpolar North Atlantic

The Overturning in the Subpolar North Atlantic Program (OSNAP) provides key insights into the dynamics of the North Atlantic by delivering observational estimates of the Meridional Overturning Circulation (MOC) and Oceanic Heat Transport (OHT) along a section extending from southern Greenland to Scotland (OSNAP East). Despite its valuable contributions, OSNAP observations are subject to limitations, particularly in spatial resolution, potential measurement and post-processing errors, and a still relatively short but growing observational record of just over six years. This study compares OSNAP-derived transport estimates with those from state-of-the-art ocean reanalyses (ORAs), identifying key discrepancies and investigating their underlying causes.
While mean transport values for 2015–2020 agree well across datasets, notable differences emerge in spatially and temporally resolved analyses. Discrepancies in temporal variabilities are found to be linked to the limited spatial coverage of observations. When mimicking OSNAP’s observational coverage in ORAs by prescribing the mean annual temperature cycle in unobserved regions, the agreement in both mean values and variability of OHT improves, highlighting the role of observational sampling limitations in these differences.
This intercomparison highlights the mutual benefits of integrating observational and modeling approaches. It not only validates ORA performance but also exposes potential limitations in observational products. Ultimately, this work emphasizes the importance of synergistic efforts to capture the variability and dynamics of the North Atlantic, advancing our understanding of its role in climate regulation and improving predictions on regional to global scales.