How the oceans affect the weather

Swathes of the northern hemisphere are smashing temperature records. Could it be because we've broken the ocean currents that stabilise our weather?

THE Atlantic Ocean conveyor belt, also known as the Atlantic meridional overturning circulation, or AMOC, is part of a global network of currents that push all the water in the oceans up and down the length, breadth and depth of the various interconnected basins. From the tropical Atlantic off the coast of South America, warm surface water flows north towards Greenland and western Europe, bringing with it an uncharacteristically warm climate, carried by the Gulf Stream.

The water becomes saltier as it evaporates and cools as it moves north. Both factors make it denser, so that by the time it reaches the Norwegian and Greenland seas, it has sunk by 2 or 3 kilometres. From there, it makes its way back south at depth (see map, above). Changes in salinity and temperature in the north Atlantic drive the entire set-up, which has caused concerns that chaos might ensue if anything changes in that region.

In 1961, oceanographer Henry Stommel showed that, in theory, these currents could exist in one of two states, with water flowing in opposite directions depending on the balance of temperature and density. At the time, this was just a curiosity. In the 1980s, growing evidence that greenhouse gas emissions were heating up the planet caused concern that much of the Arctic’s ice would melt, including Greenland’s ice sheet. Climatologists warned that fresh water pouring into the north Atlantic would slow the natural sinking of AMOC waters and put a brake on one end of the conveyor belt.

Then came the finding that the Atlantic conveyor belt had stalled during the last glacial period, between 110,000 and 12,000 years ago, when much of northern Eurasia and North America were covered in ice. Battalions of icebergs periodically broke off and went marauding around the Atlantic. Spiked by fresh water, the AMOC weakened. The 2004 disaster movie The Day After Tomorrow took the concept to the wider public by imagining that the AMOC shut down in a matter of days, triggering a snap “ice age” across Eurasia and North America and prompting ocean-spanning megastorms.

In reality, a total collapse would probably take decades or a century. The most likely effect would be severe sea level rise on the US eastern seaboard, extreme heat in Europe and chaotic monsoons in Africa and Asia.

In the same year that the film came out, instruments were deployed to monitor the AMOC. One of these, the RAPID array, relies on an undersea cable running beneath the current, from Florida to the Bahamas. Because the AMOC carries lots of salt, it contains charged ions, and their movement sets up a voltage in the cable, which can be used to estimate the current’s strength. Later, a second array was installed from Labrador in Canada to Scotland.

Thanks to RAPID, we now have nearly 20 years of continuous data that shows the AMOC is subject to huge variability. The current is strongest in the northern hemisphere’s autumn and weakest in its spring.

In the winter of 2009 to 2010, the AMOC weakened by 30 per cent, but it recovered the following year. The belief is that strong surface winds blowing against the current might have put the brakes on. To get around this short-term variability and look for long-term changes, researchers sought data spanning even longer timescales. Stefan Rahmstorf at the Potsdam Institute for Climate Impact Research in Germany and his colleagues looked at how sea surface temperatures varied worldwide from 1901 to 2013. Mostly, they found a warming trend, but in the north Atlantic, a blob-shaped region had cooled, particularly since 1970. Tellingly, climate models suggest that such a cool spot is a sign of a weak AMOC.

Just months after Rahmstorf’s study was published in March 2015, Europe was hit by a scorching summer that broke a number of temperature records. The following year, Aurélie Duchez at the National Oceanography Centre in Southampton, UK, showed it was linked to the cold spot in the north Atlantic and that, since the 1980s, similar heatwaves were more likely if the cold spot was more intense. There is also evidence that this mechanism has operated for millennia. All roads lead to the fact that the AMOC moderates Europe’s weather, reducing both winter storms and summer heatwaves. Losing it unleashes both.

A related possibility is that the recent spate of extremely cold winters and snowstorms in the eastern US might be linked to the current’s weakening. The idea is that the cold patch in the north Atlantic affects the jet streams over North America, unleashing blizzard after blizzard.

Other studies have also contributed evidence that the AMOC is slowing down. The first was led by Marilena Oltmanns at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany. She and her colleagues focused on the Irminger Sea south of Greenland from 2002 to 2014. They found it to be unusually warm and low in salt for several summers, particularly 2010 – exactly the conditions that would weaken the AMOC. What’s more, the winters that followed were so mild that the water never cooled enough to sink properly. More often than not, a quarter of the fresh water was still there as spring broke, suggesting the convection current wasn’t working as it should.

Then Rahmstorf returned with better evidence that the north Atlantic cool blob really was a signature of a weak current. His team also reconstructed how the current had changed from 1870 to 2016, showing that it had weakened by 15 per cent since the middle of the 20th century and, after a brief recovery in the 1990s, had been declining steadily throughout the 21st century.

Finally, David Thornalley at University College London and his colleagues examined Rahmstorf’s claim that the weakening was bigger than anything in the past 1000 years. They focused on one part of the current: the deep western boundary current, or DWBC, which carries the cold waters back south.

To find out how it had changed over centuries, they used sediment cores that had been drilled out of layers of mud and sand on the bottom of the Labrador Sea. The researchers measured the sediment grains – bigger grains meant a faster current. In this way, the sediments offered direct evidence of what was happening in the deep-sea currents. They found that the DWBC began weakening around 1750. By 1870, it was significantly weaker than at any point in the previous 1500 years. It has slowed ever since.

Put all the evidence together and the case that the AMOC is getting weaker starts to look quite strong, rather than there just being internal variability. This weakened AMOC could already be affecting weather patterns, but what about the more extreme possibility: a collapse, with the more violent climatic impacts that would follow?

The core problem with predicting that eventuality is that we don’t know how slow the flow has to get before it collapses. As the AMOC slows down, it must be coming closer and closer to the tipping point that would lead to its collapse, but even after decades of study, science doesn’t know where that boundary lies, and therefore how close we are to it.