Since the 1990s, scientists have observed a slowdown in the Atlantic meridional overturning circulation (AMOC)—the massive system of ocean currents that transports heat and salt between the tropics and the North Atlantic, and plays a crucial role in the Earth's climate.
However, more recently several years of near-record AMOC strength and ocean heat storage have occurred. This research, published today in Nature, explains this apparent paradox.
The team led by the UEA, with the National Oceanography Centre (NOC), the University of Washington and the Met Office Hadley Centre, investigated ocean observations from the past 30 years and found that the transport of heat and salt associated with the AMOC indeed weakened over the past two decades.
The strength of the AMOC is directly related to the difference in ocean density between the north and south—as this difference decreases the AMOC weakens. The difference in density is affected by ocean temperature and salt content—the colder and saltier the water is, the denser it is.
To the scientists' surprise, they also found that the density difference between the ocean surface and the deep ocean increased during the same period, despite the weakening of the AMOC.
Lead author Dr. Tianze Zhou, from UEA's School of Environmental Sciences and NOC, said: "We know the weakening of the AMOC will cool the North Atlantic and eventually cause less melting of the Greenland Ice Sheet.
"Given that the AMOC carries large amounts of heat from the tropics to the North Atlantic, we initially expected the amount of heat stored in the North Atlantic to decrease, too, as the AMOC weakened.
"However, our research suggests that since 2005 the ocean heat transport increased during an exceptionally strong state of the AMOC.
"So we looked into why ocean density has changed in ways seemingly unrelated to the AMOC."
The researchers found that the surface waters in the North Atlantic have become lighter, due to the decrease in surface salt content.
On the other hand, some very salty water from the Mediterranean Sea has been circulating farther into the subpolar North Atlantic, making the deep ocean denser and compensating for the freshwater input that comes from melting ice.
The combination of these two effects has maintained a large density difference between the surface and the deep ocean, helping the ocean absorb more heat.
The scientists also find that the speed at which the AMOC transports heat and salt from the south to the north has increased over the past decade.
"So the ocean circulation is carrying denser and warmer water northward more quickly, effectively taking up more heat," Dr. Zhou said.
"What we are observing now is a combination of the impacts of human-caused climate change and natural variability. Natural variability will likely reverse at some point, but in the meantime, the warming due to increasing greenhouse gas concentrations will add on top of this, increasing the likelihood that the warming will persist," he added.
"This will result in higher sea levels, accelerated ice-sheet melt and increasing heatwaves over Europe."
The study 'Accelerated Meridional Overturning Circulation drives observed North Atlantic warming' is published in Nature.
