Global ocean circulation of water is speeding up contrary to earlier climate change models, a study published in Science Advances on February 5 shows. Within the ocean basins, the water movements have been increasing in speed over the last 20 years.
The implications are yet unknown for this trend, but the findings will be critical for informing future climate change models, researchers say.
“This is quite an exciting paper,” says Joellen Russell, a geoscientist at the University of Arizona who was not involved in the study. “I think the results are robust, I think they’re important, and I think they are a little shocking,” she says.
The water of the oceans is continuously circulating the planet via currents, gyres, and eddies. Ocean circulation is a fundamental process for life on Earth. These movements by dispersion of heat regulate the Earth’s climate. They help to transport warm water from the tropics to the Polar Regions and bringing back cooler water through the oceans’ depths. Similarly, atmospheric gases, like oxygen and carbon dioxide dissolved in water, get transported by them, and they churn up nutrients from below.
Specific models, based on water temperature measurements and other factors, predicted global warming to weaken these currents. “Most people are expecting the global ocean circulation to slow,” says Russell, and thus seas to become more stratified in temperature gradient along the surface to their depths and more stable with less mixing.
But, in contrast to these predictions, the new study that examines circulation on a global scale shows “a clear increase over the last twenty years . . . [in] the strengths of the currents,” says the University of Reading’s David Ferreira, who studies the dynamics of the oceans but was not involved in the research. And that’s “pretty interesting,” he says.
There have been “numerous studies on various important but regional or basin-scale ocean circulations,” writes coauthor Shijian Hu of the Chinese Academy of Sciences’ Institute of Oceanology in an email to The Scientist. “However, it has not been well understood what the trend of the Earth’s large-scale ocean circulation is under the background of climate warming, partly because of a lack of systematic and continuous direct observations of the Earth’s oceans.”
Hu’s team has examined the historical observations and assimilated datasets from a wide range of sources. It included satellite measurements, acoustic Doppler current profilers, and different temperature and salinity measurements obtained by the global Argo system that included a fleet of thousands of free-drifting floats deployed in the world’s oceans since the early 2000s and provide continuous monitoring of the temperature, salinity, and velocity of the upper ocean layer.
While a decelerating trend is found in ocean circulation in some regions, for example, in the North Atlantic, across the planet as a whole, the team discovered from these data that there was a “surprising significant acceleration of global mean circulation during the past two decades,” Hu writes. Particularly prominent acceleration was apparent in the tropical oceans, the team notes.
According to the team, the likely reason behind the acceleration is increasing surface wind speeds. Indeed, “All the [data assimilation] products show a remarkable increase in the global mean sea surface wind speed over the past two decades,” Hu writes.
Now, what are the likely impacts of accelerated ocean movements? “If the large-scale planetary transports are increasing, that could play a role in modulating climate heat uptake,” says oceanographer and climatologist Gael Forget of MIT, who was not involved in the study. “As a general trend,” he explains, more turbulence would “probably increase the uptake of anthropogenic heat.”
“The most important influence of changes in ocean circulation probably is on the biology,” says marine systems modeler George Nurser of the National Oceanography Centre in the UK who also was not part of the research team. Some fish species only spawn at particular temperatures. The accelerated movement of water masses might cause alteration of spawning grounds, or it may impact the marine ecosystems’ connectivity.
Food sources may be affected besides altered geographical distributions, says Forget. “Marine ecosystems rely on nutrients that in many ways come from below, so if you increase the mixing near the surface where marine ecosystems reside, you might expose them to more nutrients.”
According to Russell, the acceleration of the ocean’s circulation is not necessarily good or bad news regarding climate change. “There are always winners and losers in these situations.” And there are likely to be on-going debates on effects, the primary drivers of the trend, and its longevity, she says. “But to have this paper as the launching spot for those debates is marvelous.”
