Using Lagrangian Transit Time Distributions to investigate eddy effects on Carbon uptake in the Southern Ocean

Manita Chouksey1, Alexa Griesel2, Carsten Eden1 and Reiner Steinfeldt3, (1)University of Hamburg, Institute of Oceanography, Hamburg, Germany, (2)University of Hamburg, Institute of Oceanography, Germany, (3)University of Bremen, Bremen, Germany
The oceanic interior transports and pathways and the rate at which anthropogenic carbon (Cant) is sequestered in the ocean interior, as well as the processes that control them, remain uncertain. Subantarctic Mode Waters and Antarctic Intermediate Waters make the largest contribution to the uptake and storage of Cant in the Southern Ocean. However, the role played by eddies in these water mass transformations as well as Cant uptake remains less understood. We aim to assess the storage potential and possible changes in intermediate, deep, and bottom water masses of the global ocean, in particular the Southern Ocean, and the effect on the oceanic anthropogenic Carbon uptake.

To predict the time scales of tracer storage, a distribution of the different possible transit times from the surface to the interior is useful.
Transit Time Distributions (TTDs) from tracer distributions, such as CFCs, can provide information on the distribution of such time scales at a single location. However, the accuracy of such TTD estimates and in particular the role of the isopycnal eddy mixing in the process remains poorly known. A better understanding can be furnished by Lagrangian particles which provide a unique way to track water masses, to characterize water mass ages, and transport pathways, including the effects of multiple paths between the surface mixed layer and the interior ocean.
Therefore, in addition to estimating the TTDs from tracer distributions, such as CFCs, we also include estimates from Lagrangian particle trajectories in a high-resolution eddying model and compare it to the observations.

Our results show that the mean ages obtained from CFC based TTDs differ from the Lagrangian based TTDs.
Further, Lagrangian backtracking is used to obtain a distribution of Lagrangian particles in latitude and longitude, which show upto tri-modal structures suggesting multiple sources of origin and hence multiple oceanic pathways. These estimates provide further information on the CFC inventories and water mass formation rates. Finally, using Lagrangian advection with seasonally averaged velocities, as compared to advection with the total velocities that include the eddying component, or with additional diffusive transports, reveal the impact of eddies and mixing.