Recent changes in ocean tracers and their implications for ocean circulation

• Main Author: Lester, Joanna Gorham
• Format: Text
• Language: unknown
• Published: Imperial College London 2020
• Subjects: Antarctic; Pacific; Southern Ocean; Antarc*; North Atlantic Deep Water; North Atlantic
• Online Access: https://dx.doi.org/10.25560/88446; http://spiral.imperial.ac.uk/handle/10044/1/88446

The oceans play a critical role in the climate system, so it is important to understand how ocean circulation is changing with time in order to evaluate the ongoing uptake of anthropogenic heat and carbon from the atmosphere. Ocean tracers can provide valuable insights into such large- scale ocean circulation changes. In this study, I review a new decade of ocean radiocarbon (∆14C) and chlorofluorocarbon (CFC) observations, and describe their recent changes in the context of the last three decades. I compare the observations to a selection of seven ocean models, to evaluate model biases and potentially detect changes in ocean circulation. I also investigate whether the observed changes can be attributed to external forcing, or internal variability. The most recent decade of observations show that average surface ocean ∆14C and CFCs are decreasing, corresponding to their decrease in the atmosphere. Tracers accumulated in the shallow ocean continue to disperse into deeper waters. By comparing the observed tracer changes to modelled tracer changes, I identify regions of enhanced and reduced tracer uptake which could be due to ocean circulation changes, including the Northwest Pacific, the Southern Ocean, the subtropical gyres, the North Atlantic Deep Water and Antarctic Bottom Water. I investigate changes in Southern Ocean circulation in a modelled large ensemble using CFCs, the idealised model tracer Ideal Age (IAGE) and observed changes in CFCs from 1991 to 2005. The ensemble mean change in IAGE is small, suggesting that there has been very little externally forced change in Southern Ocean circulation over this period, in contrast to strong internal variability. My analysis implies that real-world observations of changes in CFCs may not be a robust way to characterize externally driven changes in Southern Ocean circulation because of the large internal variability in CFCs exhibited by the individual ensemble members.