Ocean currents are important drivers of planetary climate. Often there is little direct evidence of their presence at the sea surface, but that changes when observing from orbit. ESA’s GOCE gravity mapper is providing more accurate maps of global currents than ever before.
Imagine magically snapping your fingers to bring the sea to a state of perfect rest. Before you would lie a vast still body of water without currents or even ripples, no longer subject to wind or tide or the force of Earth’s rotation – influenced now only by gravity.
Pinpointing the details of such a theoretical scenario has been the goal of ESA’s Gravity Field and Steady-State Ocean Circulation Explorer (GOCE). In March last year, GOCE produced the most accurate ‘geoid’ yet produced, representing a worldwide surface of equal gravitational potential, equivalent to the shape the global ocean would follow if unaffected by winds, currents or other perturbing forces.
An improved model of this inert, theoretical ocean represented the geoid represents has important applications when it comes to studying the actual circulation of the actual, dynamic ocean – it was one of the main reasons the mission was approved in 1999, and accounts for the middle initials of GOCE’s name.
Differences between the geoid and the actual sea surface highlight the presence of ocean currents. Such knowledge has many applications, including guiding offshore activities and fishing fleets, optimising shipping routes, pollution monitoring and marine safety.
Planetary air conditioning
Most important of all, marine currents are an important driver of the climate system, absorbing radiative heating from the Sun to transport it around the world, like a planetary air conditioning system. Notably, the warm waters of the Gulf Stream moderate Europe’s climate, keeping us 4°C warmer than comparable latitudes worldwide.
“The ocean transports around 30% of the heat on this planet, with the rest going through the atmosphere,” explains Rune Floberghagen, GOCE Mission Manager. “So this is one of the two really major regulating mechanisms for planetary climate.
“What we’re now able to do is provide a worldwide determination of the dynamic topography of ocean current circulation. This is something that would otherwise take thousands of buoys and drifters to obtain.”
GOCE’s third-generation geoid has been created more than 50 million measurements of slight variations in gravitational attraction as it orbits the globe at just 255 km up, much closer than any other satellite. It is an Earth Observation mission that doesn’t observe, instead recording the changing tug of gravity on its advanced gradiometer instrument, which is accurate down to a scale of one in a billion.
“The geoid has a dynamic range up and down of about 200 m around the world, and we aim to map it to a precision of a centimetre or two, at a spatial scale of hundred kilometres or less,” adds Rune. “We’ve already taken it down to about 4 cm, much better than has ever been achieved before. The longer the mission goes on the more we can improve its accuracy.”
Combining satellite data to trace ocean currents
As the next step in mapping ocean currents, GOCE’s geoid is combined with data from radar altimeters, generally regarded as the single most useful class of satellite instrument for oceanography. Employed on satellites such as Envisat, Jason-1 or CryoSat-2, radar altimeters bounce microwave pulses off the ocean to record sea surface height. Zones of warmer water stand a few centimetres higher than surrounding colder water, enabling surface currents to be mapped on an ongoing basis.
Applying GOCE results to altimeter data amplifies the signal from such currents. In essence, it comes down to a process of subtraction, Rune explains: “Think of the geoid as an ideal height reference, representing the flat water as it would lie with no disturbance perpendicular to local gravity at rest – like water in a bath.
“Then we take the actual water surface height, as measured with radar altimetry. We then subtract the second value from the first and the difference we’re left with is the steady state dynamic topography of the ocean. Regions are revealed where the ocean stands higher than the equal energy surface of the geoid – just like someone pulling their hand through the water of their bath-tub. When this happens the water above their hand rises, to fall somewhere else.
“The water rise – with an amplitude on the order of 2 m – we actually observe is instead due to the steady state circulation pattern, in the form of a massive conveyor belt system that snakes around the world.
“So we can then say something about how much water is transported in the ocean, and then, by comparing it against satellite measurements of water temperature gathered by instruments such as Envisat’s AATSR, we gain insight into the amount of energy being carried through the ocean along with this water.
“The option is also there to combine it with the ocean salinity measurements coming back from ESA’s SMOS mission. In addition, the geoid value does not vary over time, so it can be used to look back at archived satellite data, at which point we begin to see how these currents have changed over time.
“Accordingly we get to refine the accuracy of the currents as an input for climate models, and it is at this point that a fairly abstract measurement of accelerations in space starts having an influence on the real political and societal issues of the day, like the way our climate is going to go.”
Fine-tuning current maps
The specialist algorithms needed to apply the mean dynamic topography values delivered by GOCE results were developed in advance of the mission reaching space, in dedicated activities such as the Geoid and Ocean Circulation in the North Atlantic (GOCINA) project, funded by the European Union. The surface velocity of currents can be tracked along with the mean topography.
At the moment the GOCE team is attempting to fine-tune its maps of ocean circulation to get a better visualisation of what is happening not just in the major oceanic circulation system – such as the Atlantic Gulf Stream, Japan’s Kuroshio and Africa’s Agulhas currents – but much small currents in comparatively shallow seas such as the Mediterranean.
“Crudely put, the higher the water level in the steady-state dynamic topography, the bigger the pressure gradient of the current,” Rune explains. “Around the top of the scale, the Gulf Stream moves at the rate of a couple of metres per second, with large tankers often positioning themselves in the middle of its current to gain a nearly free ride to Europe.
GOCE was originally planned to gather just one year’s worth of data, so its operational lifetime has already more than doubled. This has been partially due to an unusually tranquil solar cycle, meaning the top of the atmosphere has proved thinner and less turbulent than anticipated, meaning less of GOCE’s finite xenon fuel supply has been needed to overcome air drag.
In addition to fuel, the mission’s funding will enable it to continue data gathering until at least the end of 2012, Rune concludes: “Our drag-free propulsion system has turned out to be a marvel: you have to zoom in to the margins of our results to see any impact from air drag whatsoever.
“So the GOCE satellite has proved more than capable of performing its assigned task, and as extra data go on being gathered, the accuracy of our geoid is getting better all the time.”