How does the Southern Ocean respond to changes
in wind forcing?

Andrew McC. Hogg 1 , Michael P. Meredith 2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET , UK

The Southern Ocean is characterised by turbulent flow associated with the world's strongest ocean current, the Antarctic Circumpolar Current (ACC). This turbulence is dominated by mesoscale eddies – vortices which are small (~50km) compared with the size of the Southern Ocean. Mesoscale eddies play a variety of roles in the dynamics of the Southern Ocean: they help to balance the forces contributing circulation in this region, thereby controlling the zonal momentum balance of the ACC, and are responsible for carrying heat across the Southern Ocean.

We can measure the energy contained in the ocean eddy field using satellite observations. These measurements show that there was a strong peak in the eddy energy between 2000 and 2002 (Meredith & Hogg, 2006). The spatial distribution of the excess eddy kinetic energy, as shown in Figure 9, are distributed around the Southern Ocean, and are not confined to a single region. These observations imply that variations in the eddy field are due to a spatially distributed source; the primary candidate for these variations is the strong westerly wind field that drives the ACC.

Analysis of the westerly winds in the Southern Ocean reveal a peak in the wind stress in 1998, 2-3 years before the peak in the eddy field. This observation is compared with numerical simulations using an eddy resolving ocean model. The model indicates that a 2-3 year lag in the eddy field is to be expected, and also demonstrates the dynamics which cause this lag – the energy resides in the potential energy field before being converted into eddy energy through baroclinic instability (Meredith & Hogg, 2006). It is notable that standard ocean-climate models, which do not resolve small scale eddies, predict a completely different response to wind variability. This is of particular concern, given that the response of the circulation to changes in wind forcing has a number of other consequences, including the poleward heat transport, which may have feedbacks on the climate system.

Figure 9. The eddy kinetic energy anomalies in the Southern Ocean for three different periods: 1993-1999, 2000-2002 and 2003-2004.

References: Meredith M. P., and Hogg A. McC. (2006) Circumpolar response of Southern Ocean eddy activity to a change in the Southern Annular Mode. Geophys. Res. Lett. 33 , L16608, doi:10.1029/2006GL026499.