That tug of gravity

Climate Science

The great sweep of water around Planet Earth has been captured from space in greater detail than ever before.

New observations from Europe’s Goce gravity mapping satellite have allowed scientists to plot ocean currents with unprecedented precision.

GOCE is an acronym for Gravity field and steady-state Ocean Circulation Explorer.

It is part of a series of missions that aim to do innovative science in obtaining data on issues of pressing environmental concern.

Understanding gravity is fundamental to being able to track the direction and speed of water across the globe.  The data should improve climate models which need to represent better how oceans move heat around the planet.

Very strongly represented in the new map is the famous Gulf Stream, the most intense of all the currents where water zips along at velocities greater than one metre per second in places.

“The Gulf stream takes warm water from the tropics and transports it to higher latitudes, and that warmth is released to the atmosphere and keeps the British Isles, for instance, much warmer than they would otherwise be,” said Rory Bingham from Newcastle University, UK.

“When this water has reached higher latitudes, still, because it is by then cold, salty and dense, it will sink; and you get this overturning circulation that helps regulate Earth’s climate,” he said.

Bingham presented his ocean circulation data using the latest information from Goce at the American Geophysical Union (AGU) Fall Meeting held in December, the largest annual gathering of Earth and planetary scientists. The European Space Agency (Esa) satellite was launched in March 2009, and is delivering a step change in our vision of how gravity varies across the globe.

Pull of gravity is not uniform

Contrary to popular perception, the pull of gravity is not the same everywhere. There are actually very subtle differences in the tug exerted by the mass of the planet from one place to the next. In the oceans, this has the effect of making water bulge over great submarine mountain ranges and to dip over the deepest ocean trenches.

Goce, which circles the Earth from pole to pole, carries a state-of-the-art gradiometer to sense the variations on a scale better than 100km. The information is critical to oceanographers attempting to trace the currents.

Wealth of information from GOCE

If they compare the gravity information with measurements of sea surface height made by other spacecraft, they can establish a much better picture of where water is piled up and where it is likely to flow and at what speed. Very accurate data on currents is already obtained from drifting sensors thrown into the water, but these are necessarily just point measurements.

Oceanographers would hope therefore to combine the truly global perspective they can only get from Goce with the “ground truth” they can retrieve from drifters. Add in further data collected about sea temperature and it becomes possible to calculate the amount of energy the oceans are moving around Earth’s climate system. Computer models that try to forecast future climate behaviour have to incorporate these details if they are to improve their simulation performance. “The new information coming from Goce is amazing,” said Bingham.

“We’re getting down to very fine scales now. It’s incredible to think for example that we can sense from space very small circulation features like the Mann Eddy, a persistent pocket of water in the Atlantic that just goes around and around.” The ocean circulation information presented at AGU was built using just two months of Goce gravity data.

Scientists expect to construct improved maps when they understand better how the satellite’s sophisticated instrument behaves and the observations accumulate. Goce is not expected to be a long-lived mission. Flying at an altitude of just 255 km – the lowest orbit of any research satellite in operation today – it experiences significant drag from the atmosphere.

This has to be counteracted by constantly throttling an ion thruster on the back of the satellite. When the fuel for the thruster runs out, however, Goce will fall from the sky. “We have the funding and resources on board to go until at least the end of 2012,” said Rune Floberghagen, Esa’s Goce mission manager. If European governments then provide additional money and Goce can be frugal with its fuel reserves, the end date could move out to 2014.

Other satellites to study water, ice

The European Space Agency points out that the SMOS satellite was launched to improve our knowledge of the water cycle. It is observed on the website that, “we are now not only closer to understanding more about the earth, but the novel technology employed by SMOS is clearly demonstrating a new way of monitoring Earth from space.”

The Earth Explorer Soil Moisture and Ocean Salinity (SMOS) satellite was lofted into space in the early hours of November 2, 2009 from the Plesetsk Cosmodrome in northern Russia. The mission formally began operations at the end of May and is now delivering a wealth of data on soil moisture and sea-surface salinity.

CryoSat-2 carries sophisticated technologies to measure changes at the margins of the vast ice sheets that overlay Greenland and Antarctica and marine ice floating in the polar oceans.

CryoSat-2 will provide information to complete the picture and lead to a better understanding of the role ice plays in the earth system. Its main payload is an instrument called Synthetic Aperture Interferometric Radar Altimeter (SIRAL). Previous radar altimeters have been optimised for operations over the ocean and land, but SIRAL is the first sensor of its kind designed for ice.