Scientists have been surprised to discover that the strength of the world’s largest ocean current has remained similar over tens of thousands of years, despite vast differences in the world’s climate during that time.
The current circumnavigates the Antarctic and, when analysed, it showed that its strength in comparatively recent times matched its strength during the last ice age.
This was unexpected because ocean circulation – a network of interconnected currents – is affected by climate, winds, the amount of heat entering the ocean and the size of the sea ice field, some of which factors were considerably different between the times studied.
This new information has given researchers a better insight as to how the current has responded to changes in its driving forces – and in to how it might change in the future.
Known as the Antarctic Circumpolar Current (ACC), this connects the Atlantic, Pacific and Indian Oceans. It flows from west to east between about 45° and 70° south and is driven by westerly winds and exchanges of heat and freshwater at the ocean surface. With no landmasses to impede its flow, it circulates continuously around Antarctic, connecting back to itself.
UHI Professor Mike Meredith of the Scottish Association of Marine Science’s [SAMS] Physics, Sea Ice and Technology explains the importance of the ACC: ‘The Antarctic Circumpolar Current is the biggest current system in the world, and plays a huge role in controlling planetary-scale climate.
‘These new results show that its strength during the last ice age was surprisingly similar to more recent times, with the exception of where more extensive sea ice blanketed the ocean.
‘As well as helping us understand how much the winds that drive the current might have changed over thousands of years, this gives us new insight into how the current actually works and therefore how it might change in future.’
Because of its very strong flow and its huge reach, the ACC plays a major role in moving heat, salt and climatically-important chemicals around the globe, so any changes in its flow could have planetary-scale implications.
The ACC is also an important site for ocean mixing – a key process in the conversion of dense water to light water and vice versa, and hence the vertical flow of the water [see previous information from SAMS on this matter]. This process is termed ‘overturning circulation’, and has implications for the drawdown of carbon from the atmosphere and, consequently, for the global climate.
Although the overall picture was of minimal change in the ACC, the researchers did find significant change in ACC flow in the region where sea ice has changed since the last ice age. This hints at changes in the overturning circulation over this time scale. But, as ever, the research team see there is more work needed to verify and to understand this better.
The paper ‘Minimal change in Antarctic Circumpolar Current flow speed between the last glacial and Holocene” is published on Nature Geoscience’s website as linked – available form 18.00, 22nd December, at which point the news embargo is lifted.
Mike Meredith, a physical oceanographer, is one of the authors of this paper, working both for SAMS and for the British Antarctic Survey and inevitably spending a lot of time in the Southern Ocean.
Note: SAMS – near Oban in Argyll, a partner of UHI and founded in 1884, is Scotland’s oldest oceanographic research institution.
The image above of the Antarctic Circumpolar Current is in the public domain. The other two images of Antarctica – of the research ship, RRS James Clark Ross, in the Antarctic and the generic shot of Antarctica at the top, are © Professor Mike Meredith.