Friday, January 27, 2012

Arctic Ocean freshwater bulge detected

ESA satellites show that a large dome of fresh water has been building up in the Arctic Ocean over the last 15 years.
A change in wind direction could cause the water to spill into the north Atlantic, cooling Europe.

From BBC

UK scientists have detected a huge dome of fresh water that is developing in the western Arctic Ocean.

The bulge is some 8,000 cubic km in size and has risen by about 15cm since 2002.
The team thinks it may be the result of strong winds whipping up a great clockwise current in the northern polar region called the Beaufort Gyre.
This would force the water together, raising sea surface height, the group tells the journal Nature Geoscience.
"In the western Arctic, the Beaufort Gyre is driven by a permanent anti-cyclonic wind circulation. It drives the water, forcing it to pile up in the centre of gyre, and this domes the sea surface," explained lead author Dr Katharine Giles from the Centre for Polar Observation and Modelling (CPOM) at University College London.

This is a mosaic of Envisat radar images acquired between 9 and 11 September 2011 over the Arctic Ocean.
The sea ice extent highlighted in blue corresponds to the areas where more than 80% of the sea surface is covered by ice (from an analysis performed by the US National Ice Center).

"In our data, we see the trend being biggest in the centre of the gyre and less around the edges," she told BBC News.
Dr Giles and colleagues made their discovery using radar satellites belonging to the European Space Agency (Esa).
These spacecraft can measure sea-surface height even when there is widespread ice cover because they are adept at picking out the cracks, or leads, that frequently appear in the frozen floes.
The data (1995-2010) indicates a significant swelling of water in the Beaufort Gyre, particularly since the early part of the 2000s.
The rising trend has been running at 2cm per year.

Model prediction

A lot of research from buoys and other in-situ sampling had already indicated that water in this region of the Arctic had been freshening.
This fresh water is coming in large part from the rivers running off the Eurasian (Russian) side of the Arctic basin.
Winds and currents have transported this fresh water around the ocean until it has been pulled into the gyre.
The volume currently held in the circulation probably represents about 10% of all the fresh water in the Arctic.

Of interest to future observations is what might happen if the anticyclonic winds, which have been whipping up the bulge, change behaviour.
"What we seen occurring is precisely what the climate models had predicted," said Dr Giles.
"When you have clockwise rotation - the fresh water is stored. If the wind goes the other way - and that has happened in the past - then the fresh water can be pushed to the margins of the Arctic Ocean.
"If the spin-up starts to spin down, the fresh water could be released. It could go to the rest of the Arctic Ocean or even leave the Arctic Ocean."
If the fresh water were to enter the North Atlantic in large volumes, the concern would be that it might disturb the currents that have such a great influence on European weather patterns.
These currents draw warm waters up from the tropics, maintaining milder temperatures in winter than would ordinarily be expected at northern European latitudes.

The USS Annapolis rests on the Arctic Ocean after breaking through three feet of ice.

The creation of the Beaufort Gyre bulge is not a continuous development throughout the 15-year data-set, and only becomes a dominant feature in the latter half of the study period.
This may indicate a change in the relationship between the wind and the ocean in the Arctic brought about by the recent rapid decline in sea-ice cover, the CPOM team argues in its Nature Geoscience paper.
It is possible that the wind is now imparting momentum to the water in ways that were not possible when the sea-ice was thicker and more extensive.
"The ice is now much freer to move around," said Dr Giles.

Arctic Ocean mean sea-surface with respect to the geoid for the past 15 years of satellite radar altimetry data.
The Beaufort Gyre is the yellow/orange dome in the Western Arctic.
Credits: CPOM/UCL/ESA/Planetary Visions

"So, as the wind acts on the ice, it's able to pull the water around with it.
Depending on how ridged the surface of ice is or how smooth the bottom of the ice is - this will all affect the drag on the water.
If you have more leads, this also might provide more vertical ice surfaces for the wind to blow against."
One consequence of less sea-ice in the region is the possibility that winds could now initiate greater mixing of the different layers in the Arctic Ocean.
Scientists are aware that there is a lot of warm water at depth.
At present, this deep water's energy is unable to influence the sea-ice because of a buffer of colder, less dense water lying between it and the floes above.
But if this warm water were made to well up because of wind-driven changes at the surface, it could further accelerate the loss of seasonal ice cover.
The CPOM team is now investigating the likelihood of this happening with Cryosat-2, Esa's first radar satellite dedicated to the study of the polar regions.
"We now have the means to measure not only the ice thickness but also to monitor how the ocean under the ice is changing," says Dr Seymour Laxon, director of CPOM and co-author of the study, "and with CryoSat-2, we can now do so over the entire Arctic Ocean."

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