Landerer, F. W., D. N. Wiese, K. Bentel, C. Boening, and M. M. Watkins (2015), North Atlantic meridional overturning circulation variations from GRACE ocean bottom pressure anomalies, Geophysical Research Letters, 42(19), 8114–8121, doi:10.1002/2015GL065730.
A global system of surface and deep water currents carries water through all the ocean basins, redistributing heat from the equator to the poles. The Atlantic Meridional Overturning Circulation (AMOC) is the part of this system that includes the Gulf Stream and a deep bottom flow. On the western side of the Atlantic, surface waters carry warm tropical water northward. As the water cools in higher latitudes it sinks several kilometers down and makes its way back south. The whole process is central to regulating global climate.
It has been hypothesized for some time that this circulation is slowing down due to melting of the Greenland ice sheet. The new fresh water at the surface of the ocean is too light to sink and slows down the entire circulation. Now a team of researchers has found more support for this idea by using satellites to measure ocean bottom currents.
Yes, they have figured out a way to track currents at the bottom of the ocean from space!
Well, it’s actually a little more complicated than that. The satellites aren’t directly measuring the current; they’re measuring a change in the water pressure at the bottom of the ocean on either side of the Atlantic. This change in pressure leads to a change in the southward transport of water in the deep Atlantic. Before getting into the results and their implications for the climate, let’s back up for a moment and understand 1) how measuring bottom pressure leads to knowing the strength of the bottom currents, and 2) how the satellites are able to measure bottom pressure.
How gravity relates to ocean currents
The surface of the Earth does not have a uniform gravitational field. Any geographical feature that has an unusual amount of mass, like a mountain, or an ore deposit, has a stronger gravitational pull because of its mass. Seawater is also not uniform in mass – saltier or colder water is denser which means that there are differences in mass at different locations in the ocean. A location on the ocean floor that has more water mass above it will feel more pressure than a neighboring location with less mass in the water column. Differences in pressure across the ocean basin determine the direction in which the large ocean circulation currents flow. Water flows in the direction from high to low pressure. But because the Earth is rotating, the water is deflected to the right of its path in the northern hemisphere (and effect known as the Coriolis force) (Fig. 1).
For large-scale ocean currents, knowing the pressure difference across a line of latitude from east to west is all you need to know if the current is flowing northward or southward and how fast it’s going. Pressure is directly related to the amount of mass lying above it. This means that a satellite measuring mass anomalies is also measuring bottom pressure anomalies and therefore changes in the deep ocean currents. Landerer et al. used this concept and a technique that isolates just the ocean bottom pressure in the 3000-5000 km range and tracked changes in the pressure to see how the bottom current has been changing.
How it works
The satellite measurements are made from GRACE, the “Gravity Recovery and Climate Experiment.” GRACE is, not one, but a pair of satellites orbiting Earth together. Neither one would be able to measure gravity without the other because the measurement relies on the distance between the satellites themselves (Fig. 2). The first satellite orbits the planet trailed by the second, about 220 km behind. The distance between the two is continuously measured by microwave signals and their geographic location on Earth is tracked by GPS. When the first satellite passes over some massive feature like a mountain, it speeds up a little, accelerated by the extra gravitational force that comes along with the extra mass. This increases the distance between the two satellites. As the trailing satellite passes over the same feature, it speeds up too, closing the gap, but then falls behind again as the first satellite escapes the gravitational field of the mass while it still feels the effects.
All these expansions and contractions of the distance between the satellites can be worked out to form a picture of the variable gravitational field below. The measurement is not absolute: only relative differences in gravity are measured, but this has been useful enough to detect things like changes in the groundwater supply of drought-ridden California and the loss of ice sheet mass in Antarctica and Greenland.
What’s going on with the currents
The measurements showed a decrease in the deep southward transport of AMOC since 2002 when GRACE was put into orbit. In other words, the rate at which cold water is sinking in the North Atlantic and returning southward miles beneath the surface is slowing down. A line of oceanographic instruments along the 26.5°N latitude line called RAPID has been measuring transport across that latitude for the last several years. This project noted a particularly strong decrease in transport across this latitude in 2009-2010. The GRACE satellites measured the same magnitude of decrease during this event, and measured very similar anomalies throughout the entire time-series, showing credibility for the new satellite measurements (Fig. 3).
This change in circulation may have important effects on the climate, especially in Europe where the upper limb of the AMOC circulation (the Gulf Stream) keeps conditions relatively warm. A slowdown in AMOC could lead to much cooler air temperatures. Landerer et al. showed that we can now track these changes on a basin-wide scale from space, which will help us make better predictions about what’s to come.