Chemical fingerprints of decadal changes in Southern Ocean circulation

The westerly winds that in part drive the circulation of the Southern Ocean have intensified and shifted poleward over the past few decades, due to global warming and a loss of stratospheric ozone. Several modeling studies suggest that this has caused large-scale changes in the circulation of the Southern Ocean, as the wind-driven overturning in these models exhibits a large positive trend over this period. Here, we aim to determine the chemical fingerprint of recent Southern Ocean circulation trends using estimates of dissolved oxygen changes from hindcast simulations of two different ocean biogeochemical models (BEC-CCSM and MITgcm). We also conduct several perturbation simulations with an idealized version of MITgcm to establish the sensitivity of thermocline oxygen concentration to changes in wind stress. Intensification of westerly wind leads to a drop in thermocline oxygen south of the Antarctic Polar Front (APF), and an increase in thermocline oxygen north of the APF. This pattern of oxygen depletion at high latitudes is consistent with the spin-up of the mean meridional overturning circulation that enhances upwelling of oxygen-depleted waters south of the APF and enhances subduction of oxygen-rich waters to the north. We further investigate whether other processes, such as changes in oxygen solubility and biological production and remineralization, may be responsible for this trend pattern. As dissolved oxygen is an observable quantity in the open ocean, and oxygen sensors are currently being tested for ARGO float deployment in the Southern Ocean, these model results may help to guide the interpretation of observed variability and trends.