Decomposing oceanic temperature and salinity change using ocean carbon change

Abstract. As the planet warms due to the accumulation of anthropogenic CO2 in the atmosphere, the global ocean uptake of heat can largely be described as a linear function of anthropogenic CO2 uptake. This relates the oceans mitigation of atmospheric warming and carbon sequestration, as well as its increasing heat content. Patterns of ocean salinity also change as the earth system warms due to hydrological cycle intensification and perturbations to air-sea freshwater fluxes. Local temperature and salinity change in the ocean may result from perturbed air-sea fluxes of heat and freshwater (excess temperature, salinity), or from variability resulting from reorganisation of the preindustrial temperature and salinity fields (redistributed temperature, salinity), which are largely due to circulation changes. Here, we present a novel method in which, by tracking the redistribution of preindustrial carbon, we may estimate the redistribution of temperature and salinity using only local spatial information. We demonstrate this technique by estimating the redistribution of heat and salinity in the NEMO OGCM coupled to the MEDUSA-2 Biogeochemistry model under a RCP8.5 scenario over 1860–2099. We find on the longest timescales, the patterns of excess heat and salinity storage are dominated by increases in excess heat and salinity in the Atlantic, and that excess salinity is generally negative in other basins, compensating for strong atmospheric transport of excess salinity to the Atlantic. We also find significant redistribution of heat away from the North Atlantic, and of salinity to the South Atlantic, consistent with AMOC slowdown. Temperature change at depth is accounted for predominately by redistributed, rather than excess heat, but the opposite is true for salinity, where the excess component accounts for the majority of changes at depth. Though by the end of the simulation excess heat is the largest contribution to density change and steric sea level rise, the storage of excess salinity greatly reduces variability in excess density, particularly in the Atlantic. Here, redistribution of the preindustrial heat and salinity fields also produce generally opposing changes in sea level, though patterns are less clear elsewhere. As expected, the regional strength of excess heat and salinity signal grows through the model run. In addition, the regional strength of the redistributed temperature and salinity signals also grow, indicating increasing circulation variability or systematic circulation change on at least the time scale of the model run.