3 December 2012

Polar vortex moves deepest current

Slight changes in the northern ‘polar vortex’ affects everything from weather systems to ocean currents, atmospheric scientists said.
Polar vortex affects thermohaline circulation

Thermohaline circulation: red lines indicate movement of warm, surface water while blue lines indicate the cold, salty water that moves deeper in the ocean. In certain places, the currents rise or sink, including near Greenland. Credit: NASA; Miraceti/Wikimedia

SAN FRANCISCO: Slight changes in the northern ‘polar vortex’ affects everything from weather systems to ocean currents, atmospheric scientists said.

Findings presented at the meeting of the American Geophysical Union in San Francisco showed that changes in the northern ‘polar vortex’ – a giant air circulation that emerges each winter in the stratosphere over the North Pole – can influence everything from weather systems and the ocean surface to the ‘thermohaline circulation’ – the large-scale ocean circulations driven by the temperature and saltiness of the water.

The research, by atmospheric scientist Thomas Reichler of the University of Utah and his student Junsu Kim, may have practical implications for medium-range climate prediction.

Cold, salty water sinks near Greenland, sensitive to change

One of the drivers of the thermohaline circulation occurs when the Gulf Stream approaches Greenland. Having seen a lot of sunshine and evaporation along the way, the Gulf Stream is saltier than the melt water from the Greenland glaciers it encounters. As it cools, it becomes denser than its surroundings and sinks to the bottom of the ocean.

As every next cubic kilometre of water arrives, the previous one is pushed in the only direction available: south. Eventually, it replaces warm water at the equator that will flow north and bring Northern Europe its temperate climate.

The system is very sensitive to the weather over the seas east and west of Greenland, said Reichler. A few tenths of degrees cooling more or less will make the difference between sinking or not. And that climatological Achilles’ heel is one of the factors that make it possible that events about 30 kilometers higher have an influence on it.

Reversing the polar vortex affects currents

“Another reason is that changes in the stratosphere seem to happen on very long time scales, there’s a decadal rhythm to it”, Reichler says. “The same goes for any changes in the oceans. That’s why the weather down below doesn’t have much influence – storms come and go in a matter of days, for instance. The ocean couldn’t care less, it can never respond and go along. But if you force it with a slow enough signal, that’s more likely to drive changes, to have ‘resonance’.”

The slow effect in the stratosphere that does the trick is the number of times that during a given winter the polar vortex suddenly weakens or even reverses the direction it turns in. This happened often in the last decade, and in the 80s, but it didn’t happen at all in the 90s.

Polar vortex affects cold air over North America

Those changes reduce the strength of the prevailing westerly winds over Greenland, and with that the transport of cold air from North America. That, in turn, will decrease the rate of cooling of the ocean water.

Temperature records and measurements of the Atlantic Meridional Overturning Circulation, or AMOC, as the sinking of cold and salt water is called, seem to confirm the hypothesis, Reichler said. But records for the area only go back about 30 years, and measuring the AMOC is not very precise.

For further confirmation, he turned to existing computer models of the atmosphere-and-ocean system, and even though these didn’t have the level of geographical detail, in runs simulating 4,000 years of the present climate he could reproduce the effect.

Carbon dioxide cools the stratosphere

It means, he said, that future climate models should be written so as to allow the stratospheric influence to be fully taken into account, thus improving their power to predict the world’s climate on time scales of tens of years.

Whether the addition of carbon dioxide to the atmosphere – which warms the lower layers, but cools the stratosphere – will have any influence on the polar vortex, and thus on the well-being of the Gulf Stream, is not known, Reichler says.

The vortex is caused by temperature differences in the stratosphere, so if it cools uniformly, there probably won’t be important changes.


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