Atlantic Meridional Heat Transports from Ocean Reanalyses, Monitored Sections and Heat Budget Constraints

Abstract. Meridional ocean heat transport (MHT) plays a central role in the climate system by redistributing energy across latitudes and influencing global and regional climate variability. While sustained observing systems in the Atlantic, subpolar North Atlantic and Arctic gateways provide valuable observational constraints on Atlantic heat transport, their spatial coverage remains limited and assessing their uncertainties remains challenging. Ocean reanalyses offer a complementary, spatially complete view of the ocean, but their ability to represent Atlantic heat transport across those gateways has so far not been consistently assessed.

Here we evaluate MHT estimates from 17 global ocean reanalyses using in-situ observational transport estimates and complementary heat-budget-derived transport estimates. The assessment covers major observing sections across the Atlantic and Arctic Oceans, including the South Atlantic Meridional Overturning Circulation Basin-wide Array (SAMBA), the RAPID array at 26.5°N, the Overturning in the Subpolar North Atlantic Program (OSNAP), the Greenland-Scotland Ridge (GSR), Davis Strait, Fram Strait and the Barents Sea Opening.

The two sets of estimates generally agree within two sigmas, despite the fact that biases and errors cannot be fully accounted for (whether in mooring-based estimates or in reanalyses). We demonstrate this level of agreement both for the meridional structure and for temporal variability in MHT. Temporal correlations are as high as 0.75–0.8 at RAPID and 0.65–0.75 at the GSR, but significant differences exist in both mean state and variability. Increasing resolution in reanalyses tends to improve agreement with observational estimates when computed from monthly temperature and velocity output. However, for one degree reanalyses it is essential to include the parameterized eddy terms (advective and diffusive) rather than using temperature and velocity output. Accounting for these parameterized contributions removes significant but artificial biases in MHT at this resolution. 

The reanalysis ensemble mean shows lower mean heat transport than the mooring based estimates at RAPID and Fram Strait, while showing higher transport through the Barents Sea Opening. Hence there is significant uncertainty in terms of the partitioning of Arctic heat inflow pathways. Heat-budget-derived transport estimates show generally good agreement with the available transport observations across the evaluated sections. The representation of variability depends strongly on region and timescale. At RAPID and the GSR, the considered reanalyses show the highest consistency with observational estimates across seasonal to interannual timescales, whereas OSNAP is the most challenging section. Trend analyses from the reanalysis ensemble indicate a broad reduction of northward Atlantic heat transport across most gateways since the early 1990s, although substantial uncertainties remain prior to the Argo era. At the RAPID latitude (26.5°N), the full reanalysis ensemble indicates 1993–2023 declines of -0.058 ± 0.039 PW decade^-1 in MHT.