Subpolar Atlantic meridional heat transports from OSNAP and ocean reanalyses – a comparison

Abstract. Ocean reanalyses are potentially useful tools to study ocean heat transport (OHT) and its role in climate variability, but their ability to accurately reproduce observed transports remains uncertain, particularly in dynamically complex regions like the subpolar North Atlantic. Here, we evaluate currents, temperatures, and resulting OHT at the OSNAP (Overturning in the Subpolar North Atlantic Program) section by comparing OSNAP observations with outputs from a suite of global ocean reanalyses. While the reanalyses broadly reproduce the spatial structure of currents and heat transport across OSNAP West and East, systematic regional biases persist, especially in the representation of key boundary currents and inflow pathways.

Temporal variability is well captured at OSNAP West, but none of the reanalyses reproduce the observed OHT variability at OSNAP East, especially a pronounced peak in 2015. This discrepancy in 2015 is traced to the glider region over the eastern Iceland Basin and Hatton Bank, where OSNAP data show a strong, localized inflow anomaly associated with the North Atlantic Current (NAC). This signal is absent from all reanalyses as well as from independent, indirect heat transport estimates based on surface heat fluxes and heat content. Investigation of sea level anomalies and implied geostrophic currents further confirm that this mismatch is mainly driven by differences in flow structure rather than temperature anomalies alone.

Our results highlight both the value and limitations of reanalyses in capturing subpolar heat transport variability. While higher-resolution products such as GLORYS12V1 better represent circulation features, significant mismatches remain, especially in regions with sparse observational coverage. The findings underscore the need for improved observational networks and higher-resolution modeling to more accurately constrain subpolar OHT.