On the pattern, magnitude, and timing of anthropogenic heat uptake in the Southern Ocean

Nathaniel Tarshish, Eli Tziperman

Most of the anthropogenic heat absorbed by the ocean enters through the Southern Ocean, yet the processes that set the pattern, magnitude, and timing of this uptake remain unclear. Using a climate model (CESM1) under abrupt 4xCO2, we show that uptake by the preindustrial ocean circulation alone accounts for more than half of the Southern Ocean uptake and reproduces its slow decline. We isolate this uptake by piggybacking a second, 4xCO2 atmosphere onto a preindustrial control simulation. The 4xCO2 atmosphere exchanges heat and freshwater with passive copies of temperature and salinity carried by the control ocean. This uptake is strongest where upwelling renews the mixed layer with water not yet exposed to the warmed atmosphere, as shown by a tracer we introduce that measures the cumulative time water spends exposed to the warmed atmosphere in the mixed layer. Uptake in these regions declines toward a quasi-equilibrium whose magnitude is set by the rate at which the residual overturning renews the mixed layer and is nearly insensitive to the strength of the bulk air--sea exchange coefficients. Prior to this quasi-equilibrium, the multidecadal evolution is set by heat diffusion along the upwelling isopycnals in the Southern Ocean. We introduce a minimal model of these dynamics: a well-mixed surface layer coupled to a one-dimensional advective--diffusive equation along the upwelling isopycnals. With all parameters measured in the control run, this minimal model captures both the magnitude and timing of the uptake simulated by the climate model.