NOAA.

There has been growing concern, of late, that one predicted consequence of a changing climate — the slowing of the great “overturning” circulation in the Atlantic Ocean — is already starting to happen.

Some scientists have already suggested that the odd cold “blob” pattern on the map above, featuring record cold North Atlantic temperatures on an otherwise quite hot planet, may be attributable to this development. The gigantic circulation, technically termed the Atlantic Meridional Overturning Circulation or AMOC, carries warm water northward even as it also sends cold salty water back south at depth. Thus, changes here can reverberate around the globe — one recent study even found that a full AMOC shutdown could trigger a temporary period of global cooling.

Now, in a new paper in Geophysical Research Letters, researchers from NASA’s Jet Propulsion Laboratory and the University of Texas at Austin use a new and sophisticated tool — the “Gravity Recovery and Climate Experiment” satellites, or GRACE — to confirm some pretty odd behavior in the circulation in 2009 and 2010 that has also been linked to a sudden and extreme 4-inch sea-level rise on the East Coast.

The new satellite technique, the researchers say, holds great promise to determine if — as feared — the circulation is indeed slowing down. “A lot of the evidence that has been amassed is indirect, and what we’re trying to do is provide a way to really observe the AMOC across all latitudes,” says Felix Landerer, a researcher with NASA’s Jet Propulsion Laboratory who is lead author of the study.

To understand how the researchers achieved this, it’s first important to describe — and profusely praise — the GRACE satellites that made it all possible. These satellites are not only a fascinating tool but, frankly, a testament to the stunning power of science — and they’re already responsible for a huge number of new insights into the workings of our planet.

The GRACE satellites are a pair of twin observing devices that orbit the Earth 137 miles from one another. The Earth’s gravitational pull on the satellites varies depending upon the mass of what is below them at a particular time — a mountain, an ocean — and so by measuring slight perturbations in the distance between the two satellites, scientists detect these mass changes.

But when does the Earth’s mass change in a significant way, one that would suggest an important anomaly? Mostly, this does not happen with the ground, rocks, mountains — at least not on human time scales. But water at the Earth’s surface moves around a great deal — California’s drought has been picked up by GRACE, as has dramatic melting of the glaciers of Alaska.

The new research now shows that the same can be done with changes in the oceans. Specifically, the researchers used GRACE to detect what they call ocean “bottom pressure anomalies” between 3,000 and 5,000 meters deep in the Western part of the North Atlantic in 2009 and 2010. The anomalies were so big that the researchers estimate that they translate into a decline of 5.5 sverdrups of ocean flow — and a sverdrup is a gigantic number. It refers to a water flow of one million cubic meters of water per second.


NASA’s GRACE satellites (artist’s concept) measured Atlantic Ocean bottom pressure as an indicator of deep ocean current speed. In 2009, this pattern of above-average (blue) and below-average (red) seafloor pressure revealed a temporary slowing of the deep currents. (NASA/JPL-Caltech.)

“The Mississippi is a fraction of a sverdrup,” says study co-author Michael Watkins at UT-Austin. “You’re talking about huge flows in the ocean, compared to what we think of as typical river flux.”

Some figures provided by Landerer give another way of looking at how big a change this is. A sverdrup, he explains, translates into the transport of 84.6 billion tons of water per day, or 84.6 gigatons. 5.5 sverdrups is therefore more than 450 gigatons per day, and this seems to have happened over an extended period of time.

“The time-mean AMOC flow (16-18 Sv) moves about 350 times as much (salt)water every day as CA is short in the 4-year drought,” adds Landerer by email.

What GRACE picks up on, specifically, is regions where the ocean contains more or less mass because more water is collected in that area, due to the actions of waves, winds or currents. “If you imagine putting a bathroom scale under that water it would weigh more,” says Watkins. “So that’s really what GRACE measures. As ocean circulation piles up water somewhere, we see it, and the AMOC has a particular geographic distribution of that piling up of water, and that’s the thing that we’re grabbing.”

Some of the key measurements were taken for the latitude line at 26.5 degrees North, which cuts across Florida, but the GRACE technique is not limited to any particular line of latitude. It just so happens that 26.5 North is also where ocean based measurements have detected changes in the circulation in the last decade or so — and the new satellite observations are in general agreement with those findings. Which means that if the ocean keeps changing in the way that it has apparently been changing so far, we’ll going to have a new and impressive way of monitoring it that doesn’t simply rely on major ship voyages.

Granted, the current study does not claim to have detected a long-term, downward trend in the strength of the AMOC. The point was more to prove that the technique works. And now that it does, the researchers say they are preparing to do more long-term analyses.

Thus, GRACE can now, going forward, complement ocean missions like the U.S.-U.K. RAPID program and the O-SNAP program, led by Duke University, the Woods Hole Oceanographic Institute, and many other institutions. These programs monitor the circulation at 26.5 North and the far North Atlantic using buoys and ship measurements.

“It’s the first time we’ve been able to see this from satellites,” says Watkins. “It means that we can actually take a closer look at it and keep watching it over the years, and start looking at different geographical locations as well.”

In other words, if we really are moving into a troubling world in which global warming changes the circulation of the oceans, at least the good news is that we’ll be able to detect that from space.

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