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NASA taps ESA satellite Swarm for salty ocean temperature tales

Magnetic method explored, NASA tells The Reg

Ocean from above photo via Shutterstock

Research scientists working at NASA have hit upon a potentially revolutionary way of measuring the heat hidden deep in Earth's oceans: track the subtle shifts in our planet's magnetic field caused by tides, swells, eddies, and even tsunamis.

Put simply, the salt in ocean water makes it conductive, and as it ebbs and flows it drags Earth's magnetic field to and fro. Highly sensitive magnetic-field sensors such as those aboard the European Space Agency's three-satellite Swarm mission can detect those magnetic shifts, and research scientist Robert Tyler of the University of Maryland and his NASA Goddard Space Flight Center colleague Terence Sabaka have demonstrated – at least at the proof-of-concept level – that such data can be used to determine the ocean's hidden heat content.

"The higher the conductivity of the ocean, the better the dragging," said Tyler, speaking at last month's American Geophysical Union Fall Meeting in San Francisco, California, "and the higher the temperature of the water, the better the conductivity."

To fully understand this effect, we need to take a bit of a dive into terminology to understand the difference between conductance and conductivity. If you'd prefer to skip this geekery, scroll down to "So what?"

Conductance, measured in siemens, is the degree to which an actual object – or area of ocean – conducts electricity. Conductivity, measured in siemens per meter, is the degree to which the substance that comprises an object conducts electricity, irrespective of the specific amount, shape, or configuration.

As Tyler told The Reg: "Conductance (units of S) is the electrical conductivity (units of S/m) integrated through the ocean's depth."

Still not clear enough? This might help.

What's important is that Tyler, working with colleagues at the National Oceanic and Atmospheric Administration (NOAA), examined the global data set of ocean temperature and salinity (the concentration of salt and other inorganic compounds).

"From this," he told The Reg, "we calculated the conductivity and then found that while conductivity varies with salinity and temperature in a regional way, conductance really varies primarily with only the depth-integral of temperature (heat content). The upshot is that if we infer conductance from magnetic data this can be converted into heat content."

Bingo. A new way to measure fully depth-integrated ocean heat content.

So what?

As has been proven repeatedly in multiple peer-reviewed studies, and accepted as reality by the vast majority of the world's climate scientists, the Earth is increasingly experiencing an imbalance its incoming and outgoing solar energy.

This imbalance is due to an increase in atmospheric concentrations of certain gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and others, the molecular structures of which absorb and re-radiate infrared energy at specific wavelengths, thus returning some of that energy to warm the Earth's land, oceans, ice sheets, and lower atmosphere (troposphere).

Although the amount of this heat imbalance is currently tiny – about 0.6±0.4 watts per meter squared (W/m2) out of the full incoming 340 W/m2 – that's enough to throw our planet's heat balance out of whack, causing changes in the climate.

What's more, according to Tim Boyer of NOAA's National Centers for Environmental Information, also speaking at AGU's Fall Meeting: "More than 90 per cent of that imbalance in the Earth's heat balance is going into the oceans."

But there are still uncertainties about how that heat content is distributed in the ocean, and that's a problem when it comes to developing models to accurately predict ocean currents, short and long-term heat-content variability, ice-sheet robustness, future phytoplankton distribution, fishery migrations, and the like.

What's more, even though the ocean as a whole is warming, certain parts are either cooling or warming at a slower rate. Since heat-content distribution effects change the dynamics of the oceans, better data is needed to be able to better understand and predict ocean behaviour.

Today, our best data for ocean heat content comes from Argo, a global collection of sensor-equipped floats that sink to 2,000 metres every 10 days, then rise to the surface and report temperature and salinity from different depths, as well as info on the direction and velocity of currents. There are more than 3,700 Argo floats operational in the world's oceans.

Bad news: most of the ocean is deeper than 2,000 meters.

Sorta good news: there is a new generation [PDF] of deep-diving systems being developed, tested, and deployed. These include two that can dive to 4,000 metres: Deep-Arvor, designed by the French oceanographic institute Ifremer and built by NKE Instrumentation, and Deep NINJA, designed by the Japanese research group JAMSTEC and built by Tsurumi-Seiki. Diving to 6,000 metres are two Deep Argo systems: Deep SOLO [PDF], designed by and built at the Scripps Institution of Oceanography, and Deep APEX, designed by the University of Washington [PDF] and built by Teledyne Webb Research.

The reason for the "sorta" qualifier is that these new systems are still in their early stages, and it remains unclear if they'll ever be deployed – and funded – anywhere near as effectively and robustly as the current near-global 2,000m Argo system.

Enter Tyler and his magnetic field heat-content assessments, which theoretically could determine heat content throughout the oceans' depths. His work is still in its early stages, however; his current computational proof-of-concept model relies on theoretically generated ocean-tide magnetic fields, and not real-world, somewhat-noisy magnetic data – although his model's results are highly correlated with real-world magnetic-field data from land observatories such as those participating in INTERMAGNET, and from Germany's CHAMP minisatellite.

A quick aside for you Reg readers who enjoy maths. Describing his model's development, Tyler told us: "I developed a hybrid finite-difference/spherical-harmonic approach for solving the electromagnetic induction equation near the global surface. It happens to be written in the MATLAB language, though using somewhat generic Krylov-subspace solution methods and spherical-harmonic transforms accessible in most other languages (i.e. there's nothing really "MATLAB" about the solution method)."

Tyler's magnetic-field method of determining ocean heat content has a powerful ally in the Swarm satellite trio. This mission, building on the earlier work of Denmark's Ørsted and Germany's CHAMP missions, consists of three satellites, Swarm Alpha, Bravo, and Charlie; Alpha and Charlie fly essentially side by side in a polar orbit at an altitude of 462km, and Bravo circles the planet above them at 510km in a polar orbit offset from those of its brethren. Launched in 2013, they've already made discoveries about the subtle interactions of the oceans and the Earth's mantle, and thus its magnetic field.

Swarm provides a broad range of high-resolution, quality-assured data, publicly available both through direct FTP download and through a data visualisation, manipulation, and retrieval interface called VirES (explanatory video here).

If – when – Tyler and his team succeed in precisely discerning signal from noise in the cavalcade of Swarm data and effectively transforming those data into ocean heat-content information, the goal of satellite-based, whole-globe ocean-heat monitoring may be realised.

So what else?

Let's say that Tyler and his team successfully partner with Swarm and its voluminous data. Who'd then benefit?

Well, Catherine Walker of NASA's Jet Propulsion Laboratory, for one.

Walker studies the interaction of the warming Southern Ocean with the receding glaciers and melting ice sheets of Antarctica. Unfortunately, Argo data is of little use to her – not only are Argo floats sparse in the Southern Ocean, but around Antarctica that ocean is covered in sea ice half the year.

With Argo data being sparse, Walker and her fellow Antarctic oceanographers have turned to imaginative data sources such as MEOP (Marine Mammals Exploring the Oceans Pole to Pole), a programme in which marine mammals such as seals have satellite-communicating CTD sensors affixed to their heads. CTD stands for "conductivity, temperature, and depth" and since conductivity is directly related to salinity, when combined with temperature they produce density data that oceanographers can use to determine critical information about ocean currents.

The global MEOP programme, started in 2004 and expanded to Antarctica in 2005, has as already this year collected 517,429 vertical CTD profiles from what are currently 1,197 tagged marine mammals, many of them being southern elephant seals roaming around the Antarctic. This data is useful, but not ideal – seals obviously can't be told where to go, so their data is randomly distributed. As Walker puts it: "They go wherever they want, and we get data wherever they go."

But Walker needs specific and replicable data from, for example, the West Antarctic peninsula, where the seals have told her that the ocean above the continental shelf is warming and thus destabilising the floating ice shelf and the grounded ice sheet behind it. "Thanks, seals," we can imagine Walker thinking, "but I'd prefer the regularity, timeliness, and comprehensiveness of satellite-based, full-depth heat content data."

Stephanie Schollaert Uz of NASA's Goddard Space Flight Center could use some help, too. Since 1997 NASA has been monitoring phytoplankton in the ocean by tracking the concentrations of the pigment chlorophyll. Since 2002, those observations have been made using the MODIS sensor (Moderate Resolution Imaging Spectroradiometer) on NASA's Aqua satellite. Schollaert Uz said at last month's AGU meeting that these observations have "revolutionised" our understanding of phytoplankton distribution.

Since it's at the base of many a food chain, knowing the health and distribution of phytoplankton concentrations – and knowing quickly and accurately – can help planners develop strategies for, among other things, fisheries management. And that's where ocean heat content comes in. As Schollaert Uz and her team have discovered in their study of how phytoplankton fared under 50 years of warm El Niño and cooler La Niña conditions in the equatorial Pacific, the little green guys prefer it cool. Tyler's ocean heat-content monitoring could provide fast and accurate information on such conditions and their extents.

If Tyler and his crew get their Swarm-based ocean-monitoring system up and running, does that mean the end of the line for expensive, cumbersome, somewhat spotty Argo? "In principle, no other observational method would be needed," Tyler told The Reg.

Principles, however, thrive best in an ideal world, and not in the messy, noisy, real world in which science is done. "In practice," he continued, "the accuracy will likely benefit and probably even depend on combining with other data. This method does not at this point obviate any other measurements."

But it would certainly be useful to Boyer, Walker, Schollaert Uz, and the rest of the oceanographic community – and, by extension, to you and me.

Bootnote

Since phytoplankton concentrations dip during the warm El Niño cycle, we asked Schollaert Uz the obvious question: in an overall warming world will there be an overall decrease in that essential nutrient? Her answer was instructive of the cautious and data-centric manner of a reputable scientist. "That's a big open question, and a lot of people are wondering that," she said. ®

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