Global air-sea heat and freshwater fluxes constrained by ocean observations

Abstract. A substantial portion of the transport of heat and freshwater in the climate system occurs through the ocean. Heat and fresh water enter the ocean as air-sea fluxes, which are typically estimated based on observationally constrained ‘reanalyses’ of the atmosphere. However, these estimates are uncertain, with broad differences in space and time between different products, and do not align with estimates of integrated ocean temperature and salinity. As a result, inferred global heat and freshwater transport, including transport trends, remain unclear. In this work, we use a novel water mass-based inverse modelling method, the Optimal Transformation Method (OTM), to reconcile reanalysis-based changes in surface heat and freshwater fluxes with changes in observed ocean temperature and salinity. We present estimates of air-sea surface fluxes since the 1970s, derived from reanalysis products ERA5, JRA-55 and COREv2, which are adjusted to be physically consistent with observation-based 3D ocean temperature and salinity products EN4 and IAP. OTM adjusts air-sea surface fluxes in a consistent manner, thereby also enabling estimation of meridional heat and freshwater transports. Inferred mean meridional heat transport is relatively consistent, and aligns with independent ocean transport observations and the spread of estimates from historical reconstructions from the 6th Climate Model Intercomparison Project. Mean freshwater transport and trends in both heat and freshwater transport are less consistent. Either a narrowing of the range of estimates provided by reanalyses and/or additional constraints to OTM (in addition to the thermodynamic constraints currently provided) are therefore still needed.