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Mixing in the deep

18 Sept 2013 -- In the seas off Antarctica, scientists have unravelled more of the mystery of ocean mixing.

The researchers have solved a longstanding puzzle of how mid-depth and deep waters in the Southern Ocean mix, which is one of the most important questions taxing oceanographers.

Surveying the sea at Drake Passage – the channel between South America and Antarctica where the Atlantic and Pacific Oceans meet — the team found that seawater mixes dramatically as it rushes over undersea mountains there.

Published in the journal Nature, this research -Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release- provides detailed information needed by climate modellers to produce improved long-term climate projections. Until now this level of detail on ocean mixing has been lacking.

Ocean mixing moves heat, salt, chemicals and pollutants between different vertical layers in the oceans, and is crucial for regulating ocean currents and Earth’s climate and ecosystem. But turbulence can be weak in the deep ocean, which has raised questions of how mixing actually works. Learning how that mixing happens and at what rate is furthering understanding of how our oceans function.

Researchers from the universities of Exeter, East Anglia and Southampton, the Woods Hole Oceanographic Institution, the British Antarctic Survey and the Scottish Association for Marine Science (SAMS) worked together in some of the wildest waters on the planet. They measured mixing by releasing tiny quantities of an inert chemical tracer into the Southeast Pacific, tracking it for several years in the Southern Ocean to see how quickly the ocean mixed.

The tracer showed very little vertical mixing in the Pacific but as the water passed over the mountainous ocean floor in the relatively narrow, continental gap that forms Drake Passage it began to mix dramatically.

UHI Professor Mike Meredith of SAMS Physics, Sea Ice and Technology group explained how this research had thrown light on ocean mixing and how this new understanding can be used in climate prediction models.

"Mixing of Southern Ocean waters is one of the key processes that control ocean circulation, with impacts on the carbon cycle and global climate. This work has unravelled some important aspects of what causes that mixing, and how it varies from region to region, and will be extremely useful in guiding the improvement of climate prediction models,” said Professor Meredith.

Mixing is a key process in ocean circulation, which transfers carbon dioxide from the atmosphere to the deep sea, and ultimately controls the rate at which the ocean takes up carbon dioxide. Over several hundred years this process will remove much of the carbon dioxide that we release into the atmosphere and store it in the deep ocean. Ocean mixing also affects climate because an increase in the rate of deep sea mixing would change the rate that heat can move towards the poles.

Scientists believe that lower concentrations of atmospheric carbon dioxide present during ice ages may have been the result of slower ocean mixing between the surface and the deep sea. Although the reasons for this are not yet clear, this further emphasises the link between ocean mixing and climate.

The paper "Rapid cross-density ocean mixing at mid-depths in the Drake Passage measured by tracer release" is published by Nature here

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