Variability and controls of the interior density structure of the Southern Ocean

Raquel Somavilla, Spanish Institute of Oceanography (IEO, CSIC), Physical Oceanography, Santander, Spain, Alberto Naveira Garabato, University of Southampton, Ocean and Earth Science, Southampton, United Kingdom, Cesar González-Pola, Spanish Institute of Oceanography, Physical Oceanography, Gijon, Spain and Julio Manuel Fernandez, University of Oviedo, Applied Physics, Oviedo, Spain
The Southern Ocean (SO) exerts a powerful influence on Earth’s climate. Substantial progress in understanding the dynamics of the SO overturning circulation and Antarctic Circumpolar Current (ACC) has been made by considering a time-mean zonally averaged view of the circulation. It is now well established that this zonally averaged approach has the limitation of neglecting the important effects of local and regional dynamics on the circulation. Significantly, this averaging also affects the SO’s interior vertical structure (e.g: blurring the sharp decrease in density below the mixed layer, or changing the slope of isopycnals in the permanent pycnocline by averaging density profiles with different vertical structure). Since steeply sloping isopycnals provide the reservoir of potential energy sustaining the generation of the SO’s eddy field by baroclinic instability, preservation of localised density changes is likely to be key to determining variations in the overturning. In this work, we investigate the spatial and temporal variability of the ocean interior density structure from surface to depth (from 0 to 2000 m) across the ACC. To do this, we apply an algorithm characterising the ocean’s vertical structure, and calculate the potential energy perturbation to each density profile from Argo floats in the SO from 2003 to 2018. We find that the slope of the permanent pycnocline and potential energy show similar non-uniform distribution in the SO associated with the position of ACC fronts, resembling the asymmetrical distribution of eddy kinetic energy. Moreover, we find that both the slope of the permanent pycnocline and potential energy also exhibit remarkable seasonal and interannual variability. These results indicate that the limit of invariable tilting of isopycnals often assumed in eddy compensation-based descriptions of the ACC does not apply strictly in the SO, and suggest that fingerprints of overturning change may occur in the density structure of the SO.