Modelling of Southern Ocean decadal variability arising from eddy-mean interactions

The Southern Ocean is an important component of the Earth climate system through its role in regulating and impacting the global ocean circulation. The Southern Ocean is known to be strongly turbulent, and that eddies play a role in regulating the mean and vice-versa. It is of interest to understand and model the resulting internal variability arising from such eddy-mean interactions, from a theoretical point of view because it provides further understanding to strongly interacting fluid systems, but also in practical terms because such internal variability is present in eddy-present/rich models but not so in coarse resolution parameterised models, which has consequences for example for anthropogenic carbon uptake. Here a low-order dynamical systems model of the eddy-mean interaction is constructed/derived, bearing resemblance to nonlinear oscillator and/or predator-prey type models of storm-tracks in the atmosphere and those in plasma physics for zonal-flow/drift-wave turbulence. Oscillatory time-scales for the model are derived, and testing is done on whether the derived time-scales are present in a hierarchy of numerical ocean models ranging from layered models to a primitive equation sector model. Evidence is presented that the GEOMETRIC parameterisation for geostrophic mesoscale eddies may improve the representation of decadal variability in the Southern Ocean, potentially leading to impacts in the modelled ventilation of oxygen and anthropogenic carbon in the Southern Ocean.