Asymmetries in the thermal and carbon response in climate projections due to effects of air-sea exchange and dynamic redistribution for the Atlantic Ocean

The ocean is providing an important role in climate change by sequestering extra carbon and heat supplied to the climate system from carbon emissions and the resulting increase in radiative forcing. The anthropogenic warming and carbon uptake in the upper ocean of the Atlantic differs regionally in climate projections from a suite of Earth system models and in an idealised slab atmosphere and ocean model, both forced by an annual rise in atmospheric CO₂. Differences in the thermal and carbon response are explained in terms of (i) a passive uptake involving air-sea exchange and the pre-industrial circulation, and (ii) a dynamical effect from the redistribution of the pre-industrial temperature and carbon distributions. The passive uptake is affected by the transport of the anthropogenic heat and carbon away from the sites of strong air-sea exchange that are affected by surface property distributions, such as with relatively less atmospheric carbon uptake in upwelling zones in the tropics and Southern Ocean. For the dynamical exchange, there are opposing heat and carbon contributions whenever the gradients in the pre-industrial heat and carbon distributions oppose each other. There are inter-model differences in these thermal and carbon responses due to the strength of overturning changes and the strength of intermediate water signals. In particular, a weakening in the ocean overturning in the North Atlantic leads to relatively less heat uptake versus carbon uptake, while a strengthening in the ocean overturning in the Southern Ocean leads to relatively more heat uptake versus carbon uptake. These regional characteristics of anthropogenic heat and carbon are important measures of ongoing climate change and potentially affect the likelihood of marine hazards.