Thursday, November 27, 2014

Goce gravity map traces ocean circulation

Floating buoys were used to validate the map of ocean circulation and speed
The ocean currents and their speeds (in cm/s) derived from GOCE data.
During the mission’s final year, its super-low orbit was lowered even further to obtain improved measurements of Earth’s gravity field, from which information on ocean currents was derived.
Buoys floating in the oceans were used to validate the above map, proving that this GOCE-based model is more accurate than any other model based on space-based data to date.

From BBC by Jonathan Amos

Scientists have produced what they say is the most accurate space view yet of global ocean currents and the speed at which they move.

The information has been drawn from a range of satellites, but in particular from the European Space Agency's Goce mission.
This platform, which operated from 2009 to 2013, made ultra-precise measurements of Earth's gravity.
It has detailed the role this force plays in driving ocean circulation.

The new model - presented at a Goce conference at the Unesco HQ in Paris, France - will be of fundamental importance to climate modellers, because it is the mass movement of water that helps to transport heat around the globe.
Goce carried instrumentation capable of sensing very subtle changes in Earth's gravitational tug.
This pull varies ever so slightly from place to place because of the uneven distribution of mass inside the planet.

Data on sea-surface height combined with gravity information tells scientists
where the water is piled up :
The mean dynamic topography (MDT, in cm) of the world’s oceans in the highest resolution ever achieved from space-based data.
MDT is calculated by taking the mean sea-surface height measured by satellites like Envisat, and subtracting the gravity model from GOCE.
Red areas show where water levels are above the surface of the gravity model, while blue depicts areas where the water is below.
From this, scientists calculated the speed of ocean currents.

Scientists used these observations to construct what is called a "geoid", which essentially describes the "level surface" on an idealised world.
It is the shape the oceans would adopt if there were no winds, no currents and no tides to disturb them.
By comparing this geoid with measurements of sea-surface height made by other spacecraft, researchers can see where water has become piled up.

2011 GOCE geoid
In 2011, GOCE delivered a model of the 'geoid' pictured here.
At the time, it was the most accurate ever produced.
The colours in the image represent deviations in height (–100 m to +100 m) from an ideal geoid.
The blue shades represent low values and the reds/yellows represent high values.
A precise model of Earth's geoid is essential for deriving accurate measurements of ocean circulation, sea-level change and terrestrial ice dynamics.
The geoid is also used as a reference surface from which to map the topographical features on the planet.
In addition, a better understanding of variations in the gravity field will lead to a deeper understanding of Earth's interior, such as the physics and dynamics associated with volcanic activity and earthquakes.
By the middle of 2014, no less than five gravity field models and corresponding geoids had been generated from GOCE's data.
Each version more accurate than the last.
The fifth gravity model and geoid, includes all of the gravity data collected throughout the lifetime of the mission – right up until November 2013 when the satellite finally stopped working and succumbed to the force it was designed to measure.

And it is water's desire always to "run downhill" that is a major influence on the direction and speed of currents - although atmospheric winds and the Earth's rotation are of course critical partners in the overall picture.

Clearly visible in the map at the top of this page are the Agulhas Current flowing down the African coast; the Gulf Stream running across the Atlantic; the Kuroshio Current that sweeps south of Japan and out into the North Pacific; as well as the Antarctic Circumpolar Current, and the system of currents that hug the Equator. In places, these great trains of water move in excess of 1m per second.

The new Goce model of ocean circulation has been checked and integrated with the point measurements from drifting buoys.
This has helped capture some of the smaller-scale features in the currents that lie beyond the capabilities of satellites, even one that made as highly resolved observations as the Esa mission.
"Goce has really made a breakthrough for the estimation of ocean currents," said Marie-Helene Rio from the Italian National Research Council's Institute of Atmospheric Sciences and Climate.
"The mission objective in terms of geoid [measurement] has been achieved at 1-2cm accuracy at 100km resolution, and in terms of ocean currents this translates into an error that is less than 4cm/s."

Scientists can now add in data collected about sea temperature to calculate the amount of energy the oceans move around the Earth.
Computer models that try to forecast future climate behaviour have to incorporate such details if they are to run more realistic simulations.

Goce's sleek looks led to it being nicknamed the "Ferrari of space"

The 5th International Goce User Workshop this week will be looking at the many other applications that came out of the satellite's mission.
Mapping gravity variations can yield information about ice mass loss in the Antarctic, and the deep-Earth movements that give rise to great quakes.
Goce was dubbed the "Ferrari of space" because of its sleek looks and the fact that it was assembled in Italy.

When operational, it was the lowest flying scientific satellite in the sky, making observations at an altitude of just 224km during its late phases.
This allowed the spacecraft to better sense the tiny gravity variations, but meant it had to constantly thrust an electric engine to stay aloft.
When the xenon fuel for this engine was exhausted in November 2013, Goce succumbed to the force it had been sent up to study and fell back to Earth.
Eyewitnesses saw surviving debris fall into the South Atlantic, just off the tip of South America, south of the Falkland Islands.

 Photo of GOCE reentering the atmosphere taken by Bill Chater in the Falklands at 21:20 local time on 11 November.
After more than four years mapping Earth’s gravity with unrivalled precision, the GOCE mission came to a natural end when it ran out of fuel on 21 October.

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