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Warming Speeds Up Ocean Currents Far Sooner Than Climate Models Predicted

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Just over three-quarters of the world’s oceans have sped up in the last decade, in what the Washington Post calls a “massive development that was not expected to occur until climate warming became much more advanced”.

The research result, reported last Wednesday in the journal Science Advances, is “the latest dramatic finding about the stark transformation of the global ocean—joining revelations about massive coral die-offs, upheaval to fisheries, ocean-driven melting of the Greenland and Antarctic ice sheets, increasingly intense ocean heat waves, and accelerating sea level rise,” the Post writes [1]. “The change is being driven by faster winds, which are adding more energy to the surface of the ocean. That, in turn, produces faster currents and an acceleration of ocean circulation.”

“The Earth is our patient, and you look for symptoms of how it is reacting to anthropogenic greenhouse gas forcing,” said U.S. National Oceanic and Atmospheric Administration (NOAA) researcher and study co-author Michael McPhaden. “This is another symptom.”

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The acceleration is most pronounced in tropical oceans, particularly the Pacific, the Post says. But the study found that 76% of the global ocean is speeding up, when the top 2,000 metres of the ocean are taken into account.

Scientists are still assessing the consequences of the speedup, including ocean hotspots that have “upended marine life”, and impacts along the east coasts of continents that have seen stronger currents, the paper adds.

The Post says the study team led by researcher Shijian Hu of the Chinese Academy of Sciences identified a global increase in ocean wind speeds of about 2% per decade since the 1990s, producing a 5% per decade increase in currents. Science magazine reports [3] the increase at 15%. The published paper puts [4] the rate of change at 15 ±12% per decade.

“We show a statistically significant increasing trend in the globally integrated oceanic kinetic energy since the early 1990s, indicating a substantial acceleration of global mean ocean circulation,” the scientists write in their abstract. “Although possibly influenced by wind changes associated with the onset of a negative Pacific decadal oscillation since the late 1990s, the recent acceleration is far larger than that associated with natural variability, suggesting that it is principally part of a long-term trend.”

“Since these currents do not move very fast to begin with, the change would not be noticeable from, say, the bow of a ship,” the Post says. “Still, taken across the entire planet, this represents an enormous change and a tremendous input of wind energy. And it was not expected to happen yet,” with extreme warming scenarios calling for the speedup to peak toward the end of the century.

“This suggests the Earth might actually be more sensitive to climate change than our simulations can currently show, McPhaden said.”

“It’s analogous to the changes in sea level in terms of the accelerated rise over the last 25 years,” he told the Post. “And these may be connected, and likely are.”

Citing ocean and climate experts who were not involved with the study, The Post says further research will be needed to translate the global findings into an assessment of local impacts.

“Perhaps the most important consequence is the increased redistribution of heat around the planet that stronger circulation would bring,” said Alex Sen Gupta of the University of New South Wales. “This would affect temperature distributions and could affect weather patterns—but more work would be needed to make these links.”

“I’m sure our ocean observations have improved in the early 2000s, so I wonder how much of the change in the ocean reanalyses is a reflection of the inclusion of this information,” added Edward Vizy of the University of Texas at Austin.

“In related research, McPhaden and his colleagues have found that around the globe, a key set of ocean currents, which are located on the western side of ocean basins, have been shifting their movements and in some cases, intensifying. As they’ve done so, these currents have often left behind zones of extreme warming as they transport warm waters to new places,” the Post writes. “These changes, too, are being driven by shifting ocean winds, so they could be connected.”