Few recall 17th century accounts of the Gulf Stream. Benjamin Franklin sketched its course at sea as he sailed back and forth from Philadelphia to London on the eve of the American Revolution and, being Postmaster, publicized his results in hope of speeding the Transatlantic mails.
Stefan Rahmsdorf writes at RealClimate:
In 1987, the famous US oceanographer Wally Broecker titled an article in the scientific journal Nature “Unpleasant surprises in the greenhouse”. Even Hollywood took up the subject in 2004 in the film “The Day After Tomorrow”by the German director Roland Emmerich. However, there were no measurement data that could prove an ongoing slowdown.
Only since 2004 has there been continuous monitoring at 26°N in the Atlantic (RAPID project)...
Two new studies now provide further independent evidence of this weakening. In August a paper by Christopher Piecuch of the Woods Hole Oceanographic Institution on the Florida Current – the part of the Gulf Stream system along the Florida coast – was published. Although continuous measurements of the current have only been available since 1982, Piecuch was able to reconstruct the strength of the Florida Current over the last 110 years from measurements of the sea level difference between the two sides of the current. To do so, he used 46 tide gauge stations in Florida and the Caribbean as well as a simple physical principle: the Coriolis force deflects currents in the northern hemisphere to the right, so that the water on the right side of a current stands higher than on the left. The stronger the current, the greater the difference in sea level. Comparison with measurements since 1982 shows that the method works reliably.
The result: the Florida current has weakened significantly since 1909 and in the last twenty years has probably been as weak as never before. Piecuch’s calculations also show that the resulting reduction of heat transport is sufficient to explain the ‘cold blob’ in the northern Atlantic.
This Monday, in Nature Climate Change a further study appeared, of researchers of Peking University and Ohio State University (Chenyu Zhu and Zhengyu Liu). For the first time, their paper provides evidence for an AMOC slowdown based on data from outside the North Atlantic. Model simulations show that a weakening of the AMOC leads to an accumulation of salt in the subtropical South Atlantic. This is due to the fact that strong evaporation in this region constantly increases the salinity, while the upper branch of the ocean circulation drains the salty water northwards, continually bringing in less salty water from the south. When this current weakens, the water in this region becomes saltier. This is exactly what the measured data show, in accordance with computer simulations. The authors speak of a “salinity fingerprint” of the weakening Atlantic circulation.
Video animation of ocean currents in the CM2.6 climate model of the Geophysical Fluid Dynamics Lab in Princeton:
In addition to these oceanographic measurements, a number of studies with sediment data indicate that the Gulf Stream circulation is now weaker than it has been for at least a millennium.
These current changes also affect Europe, because the ‘cold blob’ out in the Atlantic also influences the weather. It sounds paradoxical when you think of the shock frost scenario of the Hollywood blockbuster The Day After Tomorrow: but British researchers found that in summer the jet stream in the atmosphere likes to take a route around the south side of the cold blob – this then brings warm winds from the southwest into Europe, leading to heat waves there, as in the summer of 2015. Another study found a decrease in summer precipitation in northern Europe and stronger winter storms. What exactly the further consequences will be is the subject of current research.
However, the latest generation (CMIP6) of climate models shows one thing: if we continue to heat up our planet, the AMOC will weaken further – by 34 to 45% by 2100. This could bring us dangerously close to the tipping point at which the flow becomes unstable.
This article appeared originally in German in Der Spiegel: Das Golfstromsystem macht schlapp
