Tuesday, December 21, 2010

Cryosat-2 data reveals Arctic ocean circulation

21 December 2010, by Tamera Jones
Scientists have used data from the European Space Agency's (ESA) ice mission, Cryosat-2 to create the first complete map of ocean circulation in the Arctic.
Cryosat above the Earth
CryoSat's 'roof' is formed from solar panels rigidly fixed to the satellite body, designed to provide adequate power under all orbital conditions for this non-Sun synchronous satellite.
Although the map is based on just two months' worth of data, it's uncannily similar to a theoretical map predicted by an ocean circulation model, and it shows the satellite is working well.
'We can see for the first time that Cryosat is providing scientifically useful data,' says Dr Seymour Laxon from the Centre for Polar Observation and Modelling at University College London, and a member of the team that conceived the idea for Cryosat-2 over ten years ago.
Cryosat-2 is designed to take precise measurements of the thickness of ice in the Arctic and Antarctica, helping scientists better understand how melting polar ice could affect ocean circulation patterns, sea-level rise and the global climate.
The satellite measures the shape and thickness of polar ice using an instrument called an altimeter, which fires pulses of microwave energy at the ice and records how long it takes for those pulses to return to the satellite.
'What's important is that as ice retreats, ocean circulation may change and Cryosat-2 will be the only tool that will be able to pick up those changes.'
Dr Seymour Laxon, University College London
Scientists will be able to calculate the thickness of the ice by comparing how long it takes for the echoes to return from the top of ice floes and from the water in cracks in the ice, called leads. Essentially the aim is to measure the freeboard – the part of the ice that sits above the waterline.
But the satellite will also tell scientists how winds affect the Arctic Ocean by measuring differences in the height of the sea surface exposed between ice floes.
Echoes returning from leads have a much sharper signature than echoes from the ice. 'When Cryosat-2 goes over a lead, the water's flat, so it acts like a mirror, producing a narrow, sharp echo. But over the ice, the echo broadens,' explains Laxon.
It's this data, as well as data from another ESA satellite called Envisat, that has allowed Laxon and his colleagues to create the map of ocean circulation in the Arctic. 'Envisat leaves a huge hole over the North Pole that Cryosat-2 plugs. It goes right up to 88 degrees north,' says Laxon.
Ocean topography
Ocean dynamic topography and currents. Scale equals metres above gravitational level. The image on the left shows a map created from Cryosat-2 and Envisat data. The image on the right is a theoretical model.
The map shows how much higher or lower the water in the Arctic Ocean is compared with a theoretical map of the ocean with no tides, waves or currents. This means the map, which scientists call ocean dynamic topography, shows up ocean circulation patterns.
The map also validates currents predicted by a global circulation model produced by scientists at the National Oceanography Centre, showing that the satellite is delivering useful data. 'Here we've got an estimate of ocean surface height,' explains Laxon. 'The map isn't perfect, but will sharpen up as we get more data from the satellite.'
The different colours on the map show up currents in the Arctic. The most striking is the patch of red just north of Alaska and Canada. It shows a well-known ocean current called the Beaufort Gyre. It's red, because the sea surface is higher here than it would be if the Arctic Ocean was completely at rest. This current rotates clockwise, creating a 'bump' in the ocean surface, where water literally piles up.
In contrast, the map shows a blue patch between Greenland and Spitzbergen, depicting a trough in the sea surface, called the Transpolar Drift, which flows from north to south.
The concern is that as sea ice in the Arctic retreats, it leaves the ocean much more exposed to winds that may 'spin up' the ocean, potentially changing ocean circulation in the region.
'What's important is that as ice retreats, ocean circulation may change and Cryosat-2 will be the only tool that will be able to pick up those changes. Envisat doesn't have the coverage above 81.5 degrees north,' adds Laxon.
Cryosat-2 could be in orbit and delivering data for up to five years.
Laxon presented the results in San Francisco at the American Geophysical Union Fall Meeting.
Cryosat-2 was launched onboard a Dnepr rocket – a converted intercontinental ballistic missile – from the Baikonur cosmodrome in Kazakhstan on 8 April this year.

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