Historical Trends in Ocean Heat, Carbon, Salinity, and Oxygen Simulations: Impact of a Changing Ocean Transport

Examination of historical simulations from CMIP6 models shows substantial pre-industrial to present-day changes in ocean heat (ΔH), salinity (ΔS), oxygen (ΔO₂), dissolved inorganic carbon (ΔDIC), chlorofluorocarbon-12 (ΔCFC12), and sulfur hexafluoride (ΔSF₆). The spatial structure of the changes and the consistency among models differ among tracers: ΔDIC, ΔCFC12, and ΔSF₆ all are largest near the surface, are positive throughout the thermocline with weak changes below, and there is good agreement among the models. In contrast, the largest ΔH, ΔS, and ΔO₂ are not necessarily at the surface, their sign varies within the thermocline, and there are large differences among models. These differences between the two groups of tracers are linked to climate-driven changes in the ocean transport, with this tracer “redistribution” playing a significant role in changes in ΔH, ΔS, and ΔO₂ but not the other tracers. The spatial structure, and differences between models, of changes in age tracers are consistent with ΔH, ΔS, and ΔO₂, supporting the hypothesis that redistribution plays a major role for these tracers. Further, the impact of the vertical displacement of isopycnals (heave) plays a major role in the differing impact of redistribution between the two groups, with this process causing insignificant changes to ΔDIC, ΔCFC12, and ΔSF₆ due to their weak spatial gradients. A similar multi-tracer analysis of observations could provide insights into the relative role of the addition and redistribution of tracers in the ocean.