Southern Ocean overturning across streamlines in an eddying simulation of the Antarctic Circumpolar Current

Abstract

Abstract. An eddying global model is used to study the characteristics of the Antarctic Circumpolar Current (ACC) in a streamline-following framework. Previous model-based estimates of the meridional circulation were calculated using zonal averages: this method leads to a counter-intuitive poleward circulation of the less dense waters, and underestimates the eddy effects. We show that on the contrary, the upper ocean circulation across streamlines agrees with the theoretical view: an equatorward mean flow partially cancelled by a poleward eddy mass flux. Two model simulations, in which the buoyancy forcing above the ACC changes from positive to negative, suggest that the relationship between the residual meridional circulation and the surface buoyancy flux is not as straightforward as assumed by the simplest theoretical models: the sign of the residual circulation cannot be inferred from the surface buoyancy forcing only. Among the other processes that likely play a part in setting the meridional circulation, our model results emphasize the complex three-dimensional structure of the ACC (probably not well accounted for in streamline-averaged, two-dimensional models) and the distinct role of temperature and salinity in the definition of the density field. Heat and salt transports by the time-mean flow are important even across time-mean streamlines. Heat and salt are balanced in the ACC, the model drift being small, but the nonlinearity of the equation of state cannot be ignored in the density balance.

Understanding the Regional Variability of Eddy Diffusivity in the Pacific Sector of the Southern Ocean

“…The subtle relationship between the eddy field and mean flow, which varies from region to region, provides a challenge to eddy parameterization schemes if they are to accurately reproduce the eddy influence on the overturning circulation and ACC transport (Hallberg and Gnandesikan 2006;Treguier et al 2007). Certainly the results presented here and in Shuckburgh et al (2009b) cast doubt on the ability of parameterizations based solely on the eddy kinetic energy (Eden and Greatbatch 2008) to accurately represent the eddy diffusivity in regions of strongly varying mean flow such as the Southern Ocean.…”

“…In addition there is strong variability in the strength of the eddy activity along these currents. Treguier et al (2007) note that in both observations and modeling studies the EKE varies by a factor of more than 10 along the path of the ACC, with particularly large values in the regions of the East Australia Current, the Southwest Indian Ridge, the Brazil-Malvinas confluence, and the Agulhas Retroflection (see also, e.g., Gille 1997).…”

“…The results of the modeling study of Treguier et al (2007) highlight the need to take into account alongstream variations to understand the Southern Ocean overturning circulation and its relation to surface forcing. Sallée et al (2008) considers subantarctic mode water, which is formed in deep winter mixed layers directly north of the Subantarctic Front.…”

Modelling the seasonal variability of the Antarctic Slope Current

“…Smedsrud et al, 2006;Rintoul, 2007;Pauly et al, 2000;Heywood et al, 2004). Unfortunately, shipboard field work in this area is expensive and measurements are rather difficult to make because of weather conditions.…”

Observations of watermass modification by mesoscale eddies in the subtropical frontal region of the Indian ocean sector of southern ocean

“…The air-sea fluxes of the SO are therefore very sensitive to oceanic eddies in this region (Tulloch et al, 2014). Recent modelling studies have also shown the role of eddy fluxes in maintaining the strong meridional gradients of the ACC and their contribution to the net transport of the MOC (Treguier et al, 2007). Variations in eddy transport thus have implications for the MOC and hence the global climate system.…”