Ocean radiocarbon response to circulation changes

This study investigates the impact of circulation changes on the distribution of radiocarbon in the ocean interior using simulations of the NEMO model forced by JRA-55 atmospheric reanalysis data. We performed four simulations from 1850 to 2017 with transient or fixed boundary conditions for atmospheric ∆¹⁴C and CO₂, forced with inter-annually varying or fixed neutral year JRA-55 reanalysis data. The difference between variable and steady-state ocean dynamics revealed the areas in the world ocean where the radiocarbon distribution is most affected by variable ocean circulation: the midlatitude North Pacific and North Atlantic, the midlatitude South Pacific and South Indian, and the Weddell Sea. The difference between fixed and transient atmospheric boundary conditions over the past few decades shows the impact on natural vs bomb ¹⁴C. We investigate the potential drivers of variability from gas exchange, mixing in the thermocline, and interior transport. In general, the impact of circulation changes on bomb and natural ¹⁴C shows similar patterns but the effect is larger for bomb ¹⁴C. Compared to observed decadal changes in ∆¹⁴C between the 1990s and 2010s, the model underestimates the changes in ∆¹⁴C and potential density suggesting that the model response to circulation change is rapid.