The mighty currents in the oceans have a major effect on the Earth's climate, and perhaps none more so than the Antarctic Circumpolar Current (ACC). Now, even as we worry that rising global temperatures are causing one great current to slow down, we have proof the ACC is getting faster, just as most climate models predicted.
The winds around Antarctica blow forever to the east with so much force they are known as the roaring forties, and the less famous but even faster furious fifties and screaming sixties. Years of observations have shown these are picking up, driven by a combination of rising global temperatures and stratospheric ozone depletion, but our data on the waters beneath is less comprehensive.
Now, however, Dr Jia-Rui Shi of the Woods Hole Oceanographic Institute has helped combine satellite data with evidence from ocean floats to not only establish what is happening to these currents, but why. In Nature Climate Change, Shi and co-authors report the current is gaining strength, and warmer ocean temperatures are responsible.
The ACC exists because the wind pushes on water, particularly the tops of waves. Intuitively we would expect stronger winds to equal stronger currents. However, climate models predict above existing speeds the effects will be marginal because most of the additional energy supplied by the wind is lost in eddies, where water spins off the main current, sometimes going against the primary direction.
“From both observations and models, we find that the ocean heat change is causing the significant ocean current acceleration detected during recent decades,” Shi said in a statement.
While the weakening of the Gulf Stream has potentially disastrous consequences for northern Europe. The effects of a stronger ACC aren't as obvious, but Shi said; “This speedup of the ACC, especially its jet centered on the Subantarctic Front, facilitates property exchange, such as of heat or carbon, between ocean basins and creates the opportunity for these properties to increase in subsurface subtropical regions.”
Healing of the ozone hole is expected to partially balance the effects of increased global warming on wind speeds around the Antarctic. For the ACC however, where ozone is only a minor factor, the increased warmth is expected to cause further strengthening for some time to come.
When South America and Australia were attached to Antarctica there was no way for waters to get around, and ocean currents brought warm water from the equator to lap at the edges of Antarctica. This heat made Antarctic summers temperate enough for forests to grow and prevented the build-up of permanent ice sheets.
Even as the continents separated, enough was initially left behind to prevent a build up of the force we see today, which has a flow of more than a hundred million cubic meters per second. It was only around 30 million years ago, when the Drake Passage opened up, and Tasmania moved far enough north to stop interrupting the eastward flow that the ACC formed, isolating Antarctica. The effects changed the temperature of the whole planet, as permanent ice reflects more light.