Latitudinal Distribution of Atmospheric Radiocarbon over the Southern Ocean Driven by Ocean Upwelling

The Southern Ocean is a key anthropogenic carbon dioxide (CO2) sink, yet the processes governing the rate of uptake remain only partly understood. We use observations of radiocarbon (∆14C) in CO2 from Southern Ocean shipboard transects to develop latitudinal gradients of ∆14C in the surface atmosphere. We present eight years of austral summer observations (2016-2023) from ships of opportunity travelling between New Zealand and Antarctica, along with time series measurements from Baring Head New Zealand, Macquarie Island Australia and Arrival Heights Antarctica. We observe lower ∆14C in the 50°S to 70°S latitude band, consistent with upwelling of 14C-depleted deep waters in this region. We then combine ocean model simulations of CO2 and 14C with atmospheric dispersion model simulations to predict surface atmosphere ∆14C and compare with the observations. Our model simulation reasonably matches the observations, capturing the spatial patterns and day-to-day variability with considerable accuracy. However, the model simulation somewhat underestimates the magnitude of the observed ∆14C gradient, particularly between 50°S-60°S. When we artificially increase the strength of the strongest winds over the Southern Ocean in the ocean model, we find an improved match with the ∆14C observations, suggesting that the Southern Ocean windiness may be underestimated, resulting in a modeled underestimate of deep water upwelling and CO2 outgassing. Our dataset provides ∆14C measurements with dense spatial resolution across the Southern Ocean that will be useful for testing ocean model and CO2 exchange parameters.