News in Science

Ozone hole changes ocean flow

CFC legacy The hole in the Antarctic ozone layer has caused changes in the way that waters in the southern oceans mix, an international study shows.

A team of scientists led by Professor Darryn Waugh of Johns Hopkins University, has found that waters originating at the surface at sub-tropical latitudes is mixing into the deeper ocean at a much higher rate than it did 20 years ago, and the reverse is true for waters closer to Antarctica.

The study shows these changes are part of the oceans' response to strengthened winds caused by thinning of the ozone layer by the now-banned chlorofluorocarbons (CFCs).

With these oceans playing a major role in absorbing anthropogenic carbon dioxide and heat, the changes have the potential to impact the global climate, says study co-author Associate Professor Mark Holzer, of the University of NSW.

The team analysed measurements of chlorofluorocarbon-12 or CFC-12 concentrations in the southern oceans.

Their results are published today in the journal Science.

CFC-12 was used commercially in aerosols, refrigerators and air-conditioning units around the world until it was phased out by the 1989 Montreal Protocol that was created in response to the depletion of the ozone layer.

"Because production of the compound was ramped up from the 1930s until the Montreal Protocol kicked in, its presence in the atmosphere and in the surface waters of the ocean increased rapidly," Holzer says.

"The measured increases at depth in the ocean allow us to determine how surface waters are mixed over time into the ocean interior."

By analysing changes in the concentrations of CFC-12 at depths from 200 to 1500 metres between the early 1990s to late 2000s, the team was able to determine changes in the rate with which surface waters are moving into the interior of the ocean, Holzer says.

The higher the concentration of CFC-12 deeper in the ocean, the more recently those waters were at the surface.

The work shows waters in the subtropics (about 25 to 45 degrees latitude south) have on average had more recent contact with the atmosphere ("become younger"), while closer to Antarctica (50 to 60 degrees south) the average time since waters were at the surface has become longer ("the waters have become older").

Stronger westerlies

Holzer says the study shows the pattern of these changes is consistent with the fact that westerly winds near the ocean surface have strengthened as the ozone layer has thinned.

The researchers say any changes in the southern oceans are important because over the past few decades, the southern oceans have warmed at roughly twice the rate of the global mean ocean, and around 40 per cent of the anthropogenic carbon in the oceans enters below latitude 40 degrees south.

As the ozone recovers over the next 50 years, the researchers say, the strengthening of summer winds may reverse, but increasing greenhouse gases will likely counter this in other seasons.

Holzer says efforts are now under way to quantify how these changes will affect the amount of anthropogenic carbon dioxide (CO2) and other trace gases taken up by the southern oceans. Changes in the uptake of CO2 affect global warming and ocean acidification.