Reduced Southern Ocean Convection Raises Climate Regulation Concerns

Polynyas may sound to you like a new kind of plastic, or something you slept through in geometry class. They are really openings in sea ice occurring near Antarctica, and the lack of them in recent times is both a byproduct of climate change and the potential harbinger of further problems in absorbing the planet's heat and carbon dioxide output. So says a team of researchers from the University of Pennsylvania and McGill University, in a study published in Nature Climate Change.

The concern is the interaction between the Antarctic Bottom Waters and the Southern Ocean that surrounds Antarctica.

The Atlantic Bottom Waters is a powerful, deep ocean current originating from near the coast of Antarctica and heading north toward the equator. This current filled with dense salty water is thought to effectively hide heat and carbon from the atmosphere-which is an important factor in regulating the climate, given that the Southern Ocean is thought to absorb around 60% of the earth's man-made heat and between 40% and 50% of the man-made carbon dioxide.

The Atlantic Bottom Waters have been shrinking over the last few decades, and the research team suspected that man-made climate change could be a least part of the reason why. Polynyas enter into the equation because they are part of an effective mixing mechanism between the Atlantic Bottom Waters and the Southern Ocean, and their absence may be related to climate change.

Polynyas were last observed in 1976 in the Weddell Sea. They are a byproduct of warm water that is forced up toward the surface of the Southern Ocean meeting very salty and cold water at the Weddell Sea's surface. The heavier salty water sinks as the lighter warm water rises, creating a convective mixing process that is believed to feed the Atlantic Bottom Waters.

The inability to observe polynyas in recent times led scientists to assume that both polynyas and that type of open-sea convection was not common. However, modeling performed by the research team indicated that convection was a much more common process in pre-industrial times.

This suggests that the Atlantic Bottom Waters are likely to continue to shrink, and that with less overall mixing the oceans will continue to stratify, thus reducing the ability of the Southern Ocean to absorb and disperse heat and carbon dioxide.

How is climate change to blame? The team suggests that climate change has increased the precipitation in the Antarctic, providing more fresh water at the surface. This effectively dilutes the salt content of the surface water, making it less likely to sink and mix. Heat that is already in the deeper ocean may be partially trapped there with less of a dispersing current, effectively reducing the ability of the ocean to absorb more heat.

Data has shown a freshening of the Southern Ocean surface waters, and more intense gradients of salinity and corresponding density are appearing in the ocean depths, lending credence to this theory.

It remains to be seen how climate change will continue to affect these currents and the corresponding dissipation of heat and carbon dioxide, but it certainly bears further watching as we struggle to control the variables that produce man-made climate change.