Historical Trends in Ocean Heat, Carbon, Salinity, and Oxygen
Simulations: Impact of a Changing Ocean Circulation
Abstract
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 amongst 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. Tracer redistribution is prominent in the
southern subtropics, in a region where apparent oxygen utilization and
ideal age indicate increased ventilation time scales. The tracer changes
are linked to a poleward shift of the peak Southern Hemisphere westerly
winds, which causes a similar shift of the subtropical gyres, and
anomalous upwelling in the subtropics. This wind - tracer connection is
also suggested to be a factor in the large model spread in some tracers,
as there is also a large model spread in wind trends. A similar
multi-tracer analysis of observations could provide insights into the
relative role of the addition and redistribution of tracers in the
ocean.