An earth system model of intermediate complexity: Simulation of the role of ocean mixing parameterizations and climate change in estimated uptake for natural and bomb radiocarbon and anthropogenic CO₂

We examine the sensitivity of simultaneous simulation of climate, natural ¹⁴C, bomb ¹⁴C, and anthropogenic CO₂ uptake to the choice of three different ocean mixing schemes: horizontal/vertical mixing (HV), isopycnal mixing (ISO), and Gent-McWilliams mixing (GM) using an Earth system model of intermediate complexity, Integrated Science Assessment Model-2.5D (ISAM-2.5D). Our modeling results suggest that the HV scheme greatly underestimates the observed values of natural ¹⁴C in the deep ocean, while the ISO and GM schemes yield more realistic results by simulating increased amounts of natural ¹⁴C values through enhanced vertical diffusion and deep water formation. The GM scheme further improves the ISO-based natural ¹⁴C distribution in the Southern Ocean and the deep Pacific and Indian oceans through a more realistic simulation of the Southern Ocean circulation. The model simulated global uptake of anthropogenic CO₂ for the 1980s ranges between 1.8 and 2.3 PgC/yr, largely consistent with data-based estimates and OGCM results. The ISAM-2.5D simulated oceanic uptake of ¹⁴C and CO₂ is highest for the ISO scheme and lowest for the HV scheme, with the largest discrepancies occurring among different mixing schemes found in the Southern Ocean. However, no single mixing scheme is more successful than the others in simulating GEOSECS-measured uptake of bomb ¹⁴C and anthropogenic CO₂ for various ocean basins. Climate change is found to reduce CO₂ uptake by 7-9% and 6-8% for the 1980s and over the period 1765-1990, mainly as a result of decreased CO₂ solubility associated with increased sea surface temperatures. However, the effect of climate change on bomb ¹⁴C uptake is negligible.