2010 NASA Study Finds Atlantic ‘Conveyor Belt’ Not Slowing
Yesterday’s “The Day After Tomorrow” climate explainer’s excuse for cold winters is back – research suggests that the North Atlantic current is weaker than anytime for the last 1000 years
Climate change dials down Atlantic Ocean heating system
By Victoria Gill
Science correspondent, BBC News
11 April 2018
A significant shift in the system of ocean currents that helps keep parts of Europe warm could send temperatures in the UK lower, scientists have found.
They say the Atlantic Ocean circulation system is weaker now than it has been for more than 1,000 years – and has changed significantly in the past 150.
The study, in the journal Nature, says it may be a response to increased melting ice and is likely to continue.
Researchers say that could have an impact on Atlantic ecosystems.
Scientists involved in the Atlas project – the largest study of deep Atlantic ecosystems ever undertaken – say the impact will not be of the order played out in the 2004 Hollywood blockbuster The Day After Tomorrow.
But they say changes to the conveyor-belt-like system – also known as the Atlantic Meridional Overturning Circulation (Amoc) – could cool the North Atlantic and north-west Europe and transform some deep-ocean ecosystems.
That could also affect temperature-sensitive species like coral, and even Atlantic cod.…
The abstract of the paper;
Observed fingerprint of a weakening Atlantic Ocean overturning circulation
L. Caesar, S. Rahmstorf, A. Robinson, G. Feulner & V. Saba
The Atlantic meridional overturning circulation (AMOC)—a system of ocean currents in the North Atlantic—has a major impact on climate, yet its evolution during the industrial era is poorly known owing to a lack of direct current measurements. Here we provide evidence for a weakening of the AMOC by about 3 ± 1 sverdrups (around 15 per cent) since the mid-twentieth century. This weakening is revealed by a characteristic spatial and seasonal sea-surface temperature ‘fingerprint’ — consisting of a pattern of cooling in the subpolar Atlantic Ocean and warming in the Gulf Stream region—and is calibrated through an ensemble of model simulations from the CMIP5 project. We find this fingerprint both in a high-resolution climate model in response to increasing atmospheric carbon dioxide concentrations, and in the temperature trends observed since the late nineteenth century. The pattern can be explained by a slowdown in the AMOC and reduced northward heat transport, as well as an associated northward shift of the Gulf Stream. Comparisons with recent direct measurements from the RAPID project and several other studies provide a consistent depiction of record-low AMOC values in recent years.
Read more (paywalled): https://www.nature.com/articles/s41586-018-0006-5
A NASA study in 2010 using direct satellite measurement rather than models suggested there is no evidence the North Atlantic Current is slowing.
NASA Study Finds Atlantic ‘Conveyor Belt’ Not Slowing
PASADENA, Calif. – New NASA measurements of the Atlantic Meridional Overturning Circulation, part of the global ocean conveyor belt that helps regulate climate around the North Atlantic, show no significant slowingover the past 15 years. The data suggest the circulation may have even sped up slightly in the recent past.…
Until recently, the only direct measurements of the circulation’s strength have been from ship-based surveys and a set of moorings anchored to the ocean floor in the mid-latitudes. Willis’ new technique is based on data from NASA satellite altimeters, which measure changes in the height of the sea surface, as well as data from Argo profiling floats. The international Argo array, supported in part by the National Oceanic and Atmospheric Administration, includes approximately 3,000 robotic floats that measure temperature, salinity and velocity across the world’s ocean.
With this new technique, Willis was able to calculate changes in the northward-flowing part of the circulation at about 41 degrees latitude, roughly between New York and northern Portugal. Combining satellite and float measurements, he found no change in the strength of the circulation overturning from 2002 to 2009. Looking further back with satellite altimeter data alone before the float data were available, [Josh] Willis found evidence that the circulation had sped up about 20 percent from 1993 to 2009. This is the longest direct record of variability in the Atlantic overturning to date and the only one at high latitudes.
The latest climate models predict the overturning circulation will slow down as greenhouse gases warm the planet and melting ice adds freshwater to the ocean. “Warm, freshwater is lighter and sinks less readily than cold, salty water,” Willis explained.
For now, however, there are no signs of a slowdown in the circulation. “The changes we’re seeing in overturning strength are probably part of a natural cycle,” said Willis. “The slight increase in overturning since 1993 coincides with a decades-long natural pattern of Atlantic heating and cooling.”…
On the long-term stability of Gulf Stream transport based on 20 years of direct measurements
T. Rossby1,*, C. N. Flagg2, K. Donohue1, A. Sanchez-Franks2, J. Lillibridge3
In contrast to recent claims of a Gulf Stream slowdown, two decades of directly measured velocity across the current show no evidence of a decrease. Using a well-constrained definition of Gulf Stream width, the linear least square fit yields a mean surface layer transport of 1.35 × 105 m2 s−1 with a 0.13% negative trend per year. Assuming geostrophy, this corresponds to a mean cross-stream sea level difference of 1.17 m, with sea level decreasing 0.03 m over the 20 year period. This is not significant at the 95% confidence level, and it is a factor of 2–4 less than that alleged from accelerated sea level rise along the U.S. Coast north of Cape Hatteras. Part of the disparity can be traced to the spatial complexity of altimetric sea level trends over the same period.
Ocean array alters view of Atlantic ‘conveyor belt’
PORTLAND, OREGON—Oceanographers have put a stethoscope on the coursing circulatory system of the Atlantic Ocean, and they have found a skittish pulse that’s surprisingly strong in the waters east of Greenland—discoveries that should improve climate models.
The powerful currents known as the Atlantic meridional overturning circulation (AMOC) are an engine in Earth’s climate. The AMOC’s shallower limbs—which include the Gulf Stream—move warm water from the tropics northward, warming Western Europe. In the north, the waters cool and sink, forming deeper limbs that transport the cold water back south—and sequester anthropogenic carbon in the process. This overturning is why the AMOC is sometimes called the Atlantic conveyor belt.
Last week, at the American Geophysical Union’s Ocean Sciences meeting here, scientists presented the first data from an array of instruments moored in the subpolar North Atlantic. The observations reveal unexpected eddies and strong variability in the AMOC currents. They also show that the currents east of Greenland contribute the most to the total AMOC flow. Climate models, on the other hand, have emphasized the currents west of Greenland in the Labrador Sea. “We’re showing the shortcomings of climate models,” says Susan Lozier, a physical oceanographer at Duke University in Durham, North Carolina, who leads the $35 million, seven-nation project known as the Overturning in the Subpolar North Atlantic Program (OSNAP). […]
At the meeting, researchers working with the 21 moorings of the 26°N array also released their latest findings, which include measurements through February 2017. They show that the AMOC has weakened by about 15% compared with its 2004–08 level. Some climate models have raised the specter of a sudden shutdown of the AMOC—the apocalyptic scenario, leading to a frozen Europe, depicted in the 2004 movie The Day After Tomorrow—and the possibility is also supported by evidence from the geological past. But the decline in the AMOC hasn’t persisted long enough yet to be a cause for concern, says David Smeed, a physical oceanographer at the National Oceanography Centre in Southampton, U.K.
The overall trend of the AMOC will become clearer with time. This summer, researchers on the R/V Neil Armstrong will pull up OSNAP moorings and retrieve readings recorded from 2016–18.