Ocean carbon and heat uptake and redistribution diagnosed from observations using a water mass inverse model

The ocean absorbs a quarter of the anthropogenic carbon and 90% of the anthropogenic heat in the Earth system, significantly impacting the climate. On decadal timescales most relevant for climate prediction, the ocean circulation plays a central role in modulating the ocean heat and carbon sinks. It is therefore crucial to understand how these sinks interact with changes in the circulation. We have applied a novel water mass based inverse model, the optimal transformation method (OTM), to study the uptake of heat and carbon by the ocean and its redistribution in the interior by the ocean circulation. The OTM simultaneously calculates budgets of heat, freshwater, and carbon from a combination of observational data products, solving for the air-sea flux and transport and mixing of these tracers in a manner consistent with the available observational data. We apply OTM to a combination of data products: the EN4 objective analysis of temperature and salinity; the ECCO ocean state estimate; our own machine learning reconstruction of ocean interior carbon based on the GLODAP dataset; ERA5 and JRA55 reanalyses of air-sea heat and freshwater fluxes; and air-sea CO₂ fluxes from the SeaFlux product. We analyse two decades, estimating global carbon uptake of 2.02 ± 0.22 PgC yr^-1 for 1993-2002 and 2.86 ± 0.25 PgC yr^-1 for 2003-2012. We find that changes in the carbon uptake between the two decades are dominated by the Southern Ocean (>35°S) and North Pacific (>10°N) basins, and our results also suggest a southwards redistribution of carbon in the Atlantic linked to changes in ocean circulation. Meanwhile, a redistribution of carbon northwards in the Pacific is accompanied by a southwards redistribution of heat.