Guest essay by Eric Worrall
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 slowing over 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.”
One model based study suggests a 15% slowdown, a direct measurement suggests a 20% acceleration. Settled science anyone?