Oceans are changing: New understanding of acidification in Southern Ocean

Credit: Photos courtesy of Liang Xue/ University of Delaware

Antarctica has a current that circles the landmass as part of the Southern Ocean. This current is called the Antarctic Circumpolar Current. When the westerly winds strengthen during the Southern Hemisphere’s summer, waters south of the current acidify faster than can be accounted for in carbon dioxide from the atmosphere alone. The opposite pattern was observed north of the current.

Why does this happen and why does it matter?

University of Delaware Professor Wei-Jun Cai co-wrote a paper that appeared in Nature Communications and addresses those questions.

Researchers involved in the study say these effects are due to a combination of processes driven by these westerly winds — a theory that was borne out by two decades’ worth of observational data from south of Tasmania.

Understanding the factors that control ocean acidification is important for predicting the impact that the changing chemistry of the ocean will have on marine organisms and ecosystems in the future. The Southern Ocean, also known as the Antarctic Ocean, is a critical place to study these mechanisms because of its vast capacity to store carbon dioxide from the atmosphere, a crucial component of climate change.

Data-driven results benefit future prediction models

The fourth largest ocean, the Southern Ocean has a naturally low pH and saturation state for aragonite, a carbonate mineral that marine organisms need to build their shells. This is considered to be due to the Southern Ocean’s cold temperatures, which average -2 to 7 degrees Celsius (approximately 28 to 45 degrees Fahrenheit), and strong vertical mixing throughout the water column.

Because of these cold temperatures and deep mixing, the carbon dioxide absorbed at the water’s surface can be quickly transferred to and stored in the deep regions of the Southern Ocean, unlike most lower latitude oceans where huge temperature differences prevent the surface water and the deep ocean from mixing.

As atmospheric carbon dioxide levels continue to increase, however, surface waters in the Southern Ocean are expected to become increasingly vulnerable to ocean acidification.

“The Southern Ocean is a window to the deep ocean,” said Cai, an expert in inorganic carbon chemistry and the Mary A.S. Lighthipe Chair of Earth, Ocean and Environment at UD. ” Better understanding the mechanisms of ocean acidification here can help improve prediction models for how much atmospheric carbon dioxide the ocean can take up here and elsewhere.”

In the southern hemisphere, the main way that the atmosphere varies is through what is known as the Southern Annual Mode (SAM). As this mode changes from one extreme to another, the pressure difference causes the belt of westerly winds (or jet stream) around Antarctica to move north or south. When the jet stream of air strengthens (a positive SAM trend), it contracts toward Antarctica. When the jet stream weakens (a negative SAM trend), it expands north toward the equator.
In their study, the researchers explored how westerly winds regulate the rates of ocean acidification, using continuous data measurements of carbon dioxide from south of Tasmania recorded over two contrasting decades, 1991-2000 and 2001-2011. The researchers attributed the enhanced acidification to the westerly winds transporting more acidic waters horizontally from higher latitude locations toward the equator and vertically from the subsurface to the surface.

University of Delaware (via ScienceDaily), 9 October 2018. Full article.

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