Eddy-driven transports in the Antarctic Circumpolar Current
Final Report Abstract
Transports in the Southern Ocean are strongly affected or even controlled by the mesoscale eddies field, since mean transports can be here as large as eddy-driven transport. Westerly winds over the Southern Ocean drive strong equatorward Ekman transports and thus lead to strong upwelling on the poleward flank of the Antarctic Circumpolar Current (ACC). This upwelling connects the deep carbon-rich water masses of the worlds ocean with the surface. The associated meridional overturning circulation (MOC) is of fundamental importance for the global meridional heat, freshwater and tracer transports. In particular changes in wind forcing of the Southern Ocean might significantly change the MOC and for instance the carbon uptake or release of the Southern Ocean with corresponding effects on climate. The MOC in the Southern Ocean is in principle given by a component related to the northward Ekman transport, the so-called Deacon Cell, and a meso-scale eddy-driven compensation, which acts to redirect the Deacon Cell such that the so-called residual flow, i.e. the sum of the mean, wind-driven and the eddy-driven circulation, is along the mean isopycnals in agreement with our physical intuition. It was the aim of this project to understand and to better parameterize this effect in non-eddy-resolving ocean models. • Different averaging frameworks, i.e. the Residual Mean and the Isopycnal Mean, have been discussed to construct a physically consistent streamfunction for the MOC. Results of an idealized meso-scale eddy-permitting model of the Southern Ocean indicate that the often used low order approximation of both averaging frameworks can be of little use in regions of large topographic variations, or near boundary layers. An objective measure to indicate an initially increasing or decreasing series expansion was proposed for practical use. • Topographically induced meanders of the mean flow in the Southern Ocean are related to standing eddies in the MOC, which are related to the mean circulation in contrast to transient eddies, which we aim to parameterize in coarse models. Exact and approximate methods have been developed and tested to eliminate the standing eddy and to isolate the transient eddy component. • It was shown how trends in surface winds are related to changes in the residual MOC in an idealized model of the Southern Ocean. The eddy-driven component increases with increasing winds, but does not completely compensate the wind-driven MOC increase. Model versions with diffusive parameterizations of the eddy effect are compared to the eddying model, and demonstrate that the sensitivity of the residual MOC towards changing winds crucially depends on the corresponding sensitivity of the eddy diffusivity in use. It is therefore important to implement the correct sensitivity of the eddy diffusivity in order to reproduce the sensitivity of the residual MOC towards changing winds. A new closure for the diffusivity based on linear stability analysis yields promising first results and will be tested in the future to improve climate predictions of the Southern Ocean.
Publications
- (2008): Towards a mesoscale eddy closure. Ocean Modelling. 20 (3) 223-239
Eden, C. and R. J. Greatbatch
- (2009): Effects of different closures for thickness diffusivity. Ocean Modeling. 26 (1-2) 47-59
Eden, C., M. Jochum and G. Danabasoglu
- (2010): Towards the impact of eddies on the response of the Southern Ocean to climate change. Ocean Modelling. 34 (3-4) 150-165
Viebahn, J. and C. Eden
- (2011): Lagrangian and Eulerian lateral diffusivities in zonal jets. Ocean Modelling. 39 (1-2) 114-124
Riha, S. and C. Eden
