A warming climate appears to be altering global currents, reconstructed here from satellite and ship readings. NASA/GODDARD SPACE FLIGHT CENTER SCIENTIFIC VISUALIZATION STUDIO
The oceans' great continent-wrapping currents, each one moving as much water as all the world's rivers combined, can rightly be considered the planet's circulatory system. And this circulation, it appears, has started to thump faster: For nearly 25 years the currents have been rapidly speeding up, partly because of global warming.
That's the conclusion of a new paper today in Science Advances. Based on observations combined with models, the authors claim that from 1990 to 2013, the energy of the currents increased by some 15% per decade. "This is a really huge increase," says Susan Wijffels, an oceanographer at the Woods Hole Oceanographic Institution. "It's going to stimulate a lot of other work." If the acceleration is real, it could affect jet streams, weather patterns, and the amount of heat stored in the ocean's depths.
Oceanographers have suspected that climate warming is affecting ocean circulation, but so far, observations haven’t shown a trend, says Hu Shijian, an oceanographer at the Chinese Academy of Sciences's Institute of Oceanology and lead author of the study. The Kuroshio Current, running up Eastern Asia, has seemed stable, whereas the Agulhas, flowing along Africa’s eastern coast, has broadened, fracturing into meandering eddies. The Atlantic Ocean’s Gulf Stream may be weakening as Arctic melt slows its driver, the sinking of salty water in the North Atlantic, whereas currents in the Pacific Ocean have seen a strong uptick. Hu decided only a global view could reveal any overall trend.
No sustained, direct measurements of currents around the world are available, however. Instead, Hu's team turned to so-called reanalyses,which combine observations of the ocean and atmosphere with computer models to fill in the gaps and produce a global picture. The approach is tricky to use for time spans of decades: Changes in observations, for example when new satellites come online, can cause unknown biases.
So Hu's team combined five different reanalyses of ocean circulation, hoping their differing methods could reveal a true trend. From each one they extracted the ocean’s kinetic energy month by month, at a coarse scale that would ignore the turbulence of eddies and storms. And each showed a distinct rise starting around 1990.
Was it real? A look at data from the Argo array, a fleet of nearly 4000 robotic floats deployed around the world, provided the best test. The floats have been bobbing up and down in the ocean’s uppermost 2000 meters for the past 15 years, measuring temperature and salinity. They don't track velocity through the water column. But their data do indicate where winds have piled up water, helping create differences in pressure that drive large-scale flows. By combining those data with the floats' own current-borne trajectories, investigators can reconstruct overall currents and their speed.
The data set, compiled by oceanographer Alison Gray of the University of Washington, Seattle, covers only 6 years, from 2005 to 2010, but Hu found that it reveals an even clearer global speedup than the reanalysis models. "The evidence in the Argo data is absolutely astonishing," says Eleanor Frajka-Williams, an oceanographer at the United Kingdom's National Oceanography Centre, who was not part of the study.
Gray says she was startled by the magnitude of the acceleration. But she notes that ocean winds, which drive most currents, have steadily increased over the past 3 decades. And Hu says there’s good evidence that human activity has contributed to that strengthening. For example, in the Southern Hemisphere, ozone depletion and greenhouse warming have altered atmospheric circulation to push the Southern Ocean’s famed westerly winds to the south, perhaps causing a slight strengthening and spreading of the Antarctic Circumpolar Current. Meanwhile, heat from the warming tropical Atlantic has goosed the Walker Circulation, an equatorial pattern that drives the Pacific trade winds.
Still, natural fluctuations can’t be ruled out, says Gerrit Lohmann, a climate scientist at the Alfred Wegener Institute. Over the past few decades, long-term cooling off western North America has caused Pacific winds to pick up—and that cooling may reflect natural oscillations in the ocean’s state. Other researchers doubt these cycles exist. Either way, Hu thinks the oscillations could be responsible for at most one-third of the wind speedup.
The ocean acceleration could have globe-spanning effects. Stronger tropical currents could carry more warm water to higher latitudes, for example. Because carbon dioxide (CO2) is less soluble in warm water, that could slow the ocean's uptake of CO2 from the atmosphere. The high-latitude warming may also be shifting weather patterns. At the same time, Hu adds that by reaching deep into the ocean, the acceleration could boost the storage of heat in the depths, helping slow the warming on land. "This is the first global study," says Janet Sprintall, a co-author and oceanographer at the Scripps Institution of Oceanography. "There's a lot of uncertainty."
Oceanographers will likely fan out to test the study’s findings. Perhaps the strongest confirmation could come from updated data from the Argo floats, due out later this year. Still, it will probably take another decade of observations to be sure the trend is real and driven by global warming, Wijffels says. "This paper does highlight how ill prepared we are to truly diagnose what’s going on." (Science)
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