Southern Ocean dredges up more surprises
...and more questions
22 Dec 2013, Oban -- 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 were considerably different between the times studied.
This new information has given researchers a better insight in to how the current has responded to changes in its driving forces, and how it might change in the future.
Known as the Antarctic Circumpolar Current (ACC), this current 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 SAMS Physics, Sea Ice and Technology explained 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”, said Professor Meredith.
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, which is 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 article). This process is termed overturning circulation, and has implications for the drawdown of carbon from the atmosphere and consequently 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, hinting 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 better understand this.
The paper “Minimal change in Antarctic Circumpolar Current flow speed between the last glacial and Holocene” is published on Nature Geoscience's website http://dx.doi.org/10.1038/ngeo2037
Mike also works at the British Antarctic Survey.
Photos courtesy of Mike Meredith.