In yesterday's post, we focused on phytoplankton growth in the Southern Ocean, the role of iron in limiting or favoring that growth and the importance of those Southern Ocean biological processes in the global carbon cycle. Today we explore the role of winds in the same region - the Southern Ocean - and again the importance of processes in that region for the global carbon cycle.
In a paper published 14 March 2010, oceanographers from Australia and the USA have examined processes that mix the upper ocean in the regions around Antarctica. Shallow stable mixed layers act as barriers between the atmosphere and the deep ocean, but deep mixed layers can act as gateways between the atmosphere and the interior ocean. Through strong winds and deep mixing in the Southern Oceans (oceanographers use the term 'ventilation'), heat and carbon can move from the atmosphere into the global oceans. The image below shows how deep mixing occurs in a band around Antarctica during the winter - red values indicate mixing to depths of 300 to 500 meters.
Deep mixing can have a complicated impact on Southern Ocean biological systems, and particularly on the phytoplankton. Mixing can bring up more nutrients from below, but it can also mix the plants down below the zone where they have enough light to grow. Near Antarctica, the mixing can have very different effects in summer than in winter. This paper identifies stronger winds and deeper mixed layers as a recent trend, and the authors say: "Our results suggest that changes in the [Southern Ocean wind regime], including recent and projected trends attributed to human activity, drive variations in Southern Ocean mixed-layer depth, with consequences for air–sea exchange, ocean sequestration of heat and carbon, and biological productivity." What happens at the poles affects us all!
For more information, read the original paper in Nature Geoscience or read a very nice guide and summary by Sarah Gille.