Antarctic Circumpolar Current Dynamics


In old work, I estimated transport in a wind-driven beta plane channel using eddy saturation ideas; this led to an estimate of transport determined by the necessary condition for baroclinic instability.  Today, this estimate is thought to apply in the weakly forced limit and the work can be seen as modifying Johnson and Bryden's prediction that channel transport should go like the square root of the wind stress in that limit.  Note that the total transport might also include a contribution stemming from a basin-like circulation to the north of Drake Passage latitudes.

Straub, D.N. 1993, On the transport and angular momentum balance of the Antarctic Circumpolar Current, J. Phys. Oceanogr., Vol. 23, # 4, pp. 776-782.

In more recent work, former PhD student L-P Nadeau and I have revisited an old idea (dating to Stommel) that the basin-like region to the north of Drake Passage might influence ACC transport.  In an idealized quasigeostrophic setting, we find that transport can saturate (become nearly independent of forcing strength) in a strongly forced regime.  This saturation is explained by appealing to basin circulation theories.  We also find that in this limit transport depends on the bottom drag coefficient and on the form of the wind.  Specifically, i) stronger drag implies more transport, ii) addition of a curl-free wind stress increases the transport, and iii) addition of a wind stress curl (chosen such that the Drake Passage wind stress is minimally effected) decreases the transport.  Also of interest is that, even though the transport is not simply determined by the wind stress curl, all time mean circumpolar streamlines in our model stem from a Sverdrup-like interior circulation.

Nadeau, L-P. and D. N. Straub, 2011, Influence of wind stress, wind stress curl, and bottom friction on the transport of a model Antarctic Circumpolar Current, in press, J. Phys. Oceanogr.

Nadeau, Louis-Philippe and David N. Straub, 2009, Basin and channel contributions to a model Antarctic Circumpolar Current, J. of Phys. Oceanogr., Vol 39, pp. 986-1002.

The eastward transport of the ACC is thought to be linked to its stratification and to the winds.  With its zonally reconnecting geometry, it is the most atmospheric-like of ocean currents, and ideas normally invoked to understand ocean currents in basin geometries are often dismissed as irrelevant to the Southern Ocean.  On the other hand, the sharp northward turns taken by the ACC just east of Drake Passage and of the Campbell Plateau are suggestive of western boundary current dynamics.  With this in mind, we have been exploring the possibility that basin dynamics are relevant to the ACC.

From Mazloff et al., JPO, 2010:  barotropic streamfunction.  Note that the current takes on western boundary current like characteristics east of S America and of the Campbell Plateau (southeast of New Zealand).