Ocean 'pump' turns back on, possibly balancing greenhouse gases

WHOI research team finds answers to reengaged circulation 'pump'
Scientists study movement of denser seawater near Greenland

By Samantha Pearsall

For about a year now, a research team from Woods Hole Oceanographic Institution, led by Kjetil Våge and Robert Pickart, has been studying the North Atlantic circulation "pump" near Greenland, right, that could naturally help offset global warming by rotating seawater.

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In doing so, greenhouse gases, like carbon dioxide, are pulled from the air and sent deeper into the ocean where they're then stored for centuries.

As air temperatures over the past two decades have generally warmed, there were few signs this process was still taking place.

Last winter, however, Våge and his team discovered the circulation "pump" had turned on again for the first time since the winter of 1999-2000.

In the early 1990s, there were consecutive harsh winters that contributed to deep convection in the North Atlantic.

"The winters since have been milder, and deep convection did not happen until last winter, when it suddenly happened again," he said.

"The process of convection is not really like a ‘pump,'" explained Våge, former student at the Massachusetts Institute of Technology and a current graduate student at WHOI.

The ocean organizes itself, he explained, by having dense seawater reside under less dense seawater. The colder and saltier the water, the denser it is, and therefore further below the surface it will be. As very cold, dry air blows over the water's surface it cools the surface water and causes it to sink lower.

Once water sinks below about .62 miles, it can be carried and dispersed around the world on what is known as the Great Ocean Conveyor, below. This is the process of convection that some refer to as the ocean's circulation "pump."

The sinking of surface water last winter in the North Atlantic was largely due to colder air temperatures than previous years, averaging between 9° and 11°F. Also, nearby storms and wind patterns were tracked and might have played a role in engaging the "pump."

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For the first time in eight years and only the second time since the mid-1990s, cold surface water sank significantly below .62 miles.

This particular circulation "pump" in the North Atlantic significantly impacts the regional climate. The depth and volume of the "convected" area are both important tools for studying the climate.

Våge explained what happened last winter: "Heat was released from the ocean to the atmosphere and carbon dioxide that had been dissolved in the ocean near the surface was brought down and stored in the deep ocean."

In this way, the sub-polar region stays warmer during the winter months than it would be otherwise, while the ocean works to remove harmful greenhouse gases form the atmosphere.

Greenhouse gases, like carbon dioxide, are a suspected contributor to global warming, which is expected to make the process of convection difficult to take place.

However, Våge said, "Based on our set of observations it is impossible to tell whether there is a connection between global warming and the last seven or eight years in which deep convection did not take place."

Although no concrete correlations were made to global warming, the research project did indicate several factors that together helped create the appropriate conditions for deep convection to occur.

"We think that it has illuminated the complexity of the system and shown that convection is not simple to predict, and hopefully also increased our understanding of convection in this region," Våge said.

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