On the reliability of reanalysis-derived heat transport in the North Atlantic

The North Atlantic is a key region of the climate system, where ocean circulation redistributes heat across latitudes and drives pronounced variability on interannual to multidecadal time scales. Observational programs have provided valuable insights into Atlantic circulation and variability, but their spatial and temporal coverage remains limited. Global ocean reanalyses offer a complementary, spatially complete view of the ocean and therefore provide potentially valuable tools to investigate heat transports and their variability in the North Atlantic.

This contribution aims to provide a clear and quantitative assessment of the usefulness and reliability of ocean reanalyses for diagnosing ocean heat transport and its variability in the North Atlantic. It is carried out within the framework of the Marine Environment Reanalyses Evaluation Project (MER-EP), which aims to systematically evaluate and intercompare global marine reanalyses. By systematically comparing multiple reanalyses with observational and budget-based constraints, we aim to identify where and under which conditions reanalysis-derived transport estimates are robust and where important limitations remain. This assessment is essential for the appropriate use of ocean reanalyses in studies of North Atlantic variability and its role in climate change.

We evaluate ocean heat transport and related diagnostics in the North Atlantic using a large ensemble of global ocean reanalyses from different modelling centers. Transport calculations are performed using the newly developed StraitFlux (Winkelbauer et al. 2024) diagnostic framework, which enables consistent transport estimates across different model grids and vertical coordinate systems.

Our analysis focuses on the North Atlantic sector and its variability, with particular attention to major observing systems such as RAPID, SAMBA, OSNAP (Winkelbauer et al., preprint) and transports across the Greenland-Scotland Ridge, Fram Strait and the Barents Sea Opening. In addition to transports obtained from ocean reanalyses and insitu observations, we estimate meridional ocean heat transport indirectly from the ocean heat budget. These inferred transports are obtained by combining surface heat fluxes inferred from the atmospheric energy budget, ocean heat content tendencies and contributions from sea ice melt, and by imposing appropriate boundary conditions at basin chokepoints. This approach provides complementary ocean heat transport estimates that are largely independent of both the reanalysis circulation fields and the insitu observations. It allows to assess ocean reanalysis performance consistently across the entire North Atlantic, including regions and latitude bands where no insitu transport measurements are available.

Winkelbauer, S., Mayer, M., and Haimberger, L.: StraitFlux – precise computations of water strait fluxes on various modeling grids, Geosci. Model Dev., 17, 4603–4620, https://doi.org/10.5194/gmd-17-4603-2024, 2024.

Winkelbauer, S., Winterer, I., Mayer, M., Fu, Y., and Haimberger, L.: Subpolar Atlantic meridional heat transports from OSNAP and ocean reanalyses – a comparison, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-4093, 2025.