G02_01

Recent changes in ocean DI¹⁴C and implications for ocean circulation

Lester J¹,  Graven H¹, Khatiwala S², Key R³, McNichol A⁴

¹Imperial College London, London, United Kingdom, ²University of Oxford, Oxford, United Kingdom, ³Princeton University, , USA, ⁴Woods Hole Oceanographic Institution, , USA

Anthropogenic perturbations from fossil fuel burning and nuclear bomb testing have created a useful transient tracer of ocean circulation from measurements and modelling of dissolved ¹⁴C. The atmospheric ¹⁴C/C ratio (∆¹⁴C) peaked in the early 1960s and has decreased now to nearly pre-bomb levels. We present the first analysis of a new decade of observations from 2007 to 2016 which gives a comprehensive overview of the changes in ocean ∆¹⁴C since the 1990s. Surface ocean ∆¹⁴C decreased from the 2000s to 2010s at a similar rate as from the 1990s to 2000s (20‰/decade). In contrast to the period from the 1990s to the 2000s when denser waters gained ¹⁴C from the continued downward ventilation of bomb ¹⁴C, the extent of positive ∆¹⁴C between the 2000s to 2010s is much reduced. Comparison to two ocean models, the Community Earth System Model v2 (CESM2) and the Estimating the Climate and Circulation of the Ocean v4 (ECCOv4), shows evidence from ∆¹⁴C of decadal variability in the ventilation of Southern Ocean intermediate waters. The decrease in surface tracers from the 2000s to the 2010s is consistently stronger in observations than in these models, which may result from a reduction in vertical transport and mixing due to stratification.