Guest essay by Eric Worrall

Climate scientists like Michael Mann have long predicted the slowdown of ocean currents, including the North Atlantic current which keeps Europe warm in winter, but actual measurements suggest that global wind speed and ocean currents are actually accelerating.

Climate change may be speeding up ocean circulation

Since the 1990s, wind speeds have picked up, making surface waters swirl faster

By Carolyn Gramling

FEBRUARY 5, 2020 AT 4:29 PM

Winds are picking up worldwide, and that is making the surface waters of the oceans swirl a bit faster, researchers report. A new analysis of the ocean’s kinetic energy, measured by thousands of floats around the world, suggests that surface ocean circulation has been accelerating since the early 1990s. 

Some of that sped-up circulation may be due to naturally recurring ocean-atmosphere patterns, such as the Pacific Decadal Oscillation, researchers report February 5 in Science Advances. But the acceleration is greater than can be attributed to natural variability alone — suggesting that global warming may also be playing a role, says a team led by oceanographer Shijian Hu of the Chinese Academy of Sciences in Qingdao. 

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Global warming has long been predicted to slow global wind speeds, called “global stilling.” That’s because the poles are warming faster than the equatorial region, and a smaller temperature gradient between the two zones might be expected to result in weaker winds (SN: 3/16/18). But recent studies, such as a report published November 2019 in Nature Climate Change, suggest that wind speeds around the world have actually been speeding up, at least since about 2010. 

The new study suggests that winds have actually been picking up over the oceans for several decades, leading to the faster-swirling surface waters especially in the tropics. The study used data collected by over 3,000 Argo floats, which measure temperature, salinity and speeds of currents down to about 2,000 meters, in oceans around the world. Then, the team combined these data with a variety of climate simulations to calculate the change in kinetic energy —energy from the wind motion that gets transferred to the water — in that upper part of the ocean.

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Read more: https://www.sciencenews.org/article/climate-change-speeding-up-ocean-circulation

The abstract of the study;

Deep-reaching acceleration of global mean ocean circulation over the past two decades

Shijian Hu, Janet Sprintall, Cong Guan, Michael J. McPhaden, Fan Wang, Dunxin Hu and Wenju Cai

Ocean circulation redistributes Earth’s energy and water masses and influences global climate. Under historical greenhouse warming, regional ocean currents show diverse tendencies, but whether there is an emerging trend of the global mean ocean circulation system is not yet clear. Here, 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 increasing trend in kinetic energy is particularly prominent in the global tropical oceans, reaching depths of thousands of meters. The deep-reaching acceleration of the ocean circulation is mainly induced by a planetary intensification of surface winds since the early 1990s. 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.

Read more: https://advances.sciencemag.org/content/6/6/eaax7727

What does Michael Mann have to say about the impact of global warming on ocean currents? This article is from five years ago, but the science is settled, right?

Global Warming Is Slowing Ocean Currents Causing Dire Consequences, Warns Climate Expert Michael Mann

 Cole Mellino Mar. 25, 2015 09:49AM EST

Climate scientists Michael Mann and Stefan Rahmstorf announced the findings of their new study yesterday, which shows that the rapid melting of the polar ice has slowed down currents in the Atlantic Ocean, particularly since 1970. The scientists say “the slowdown in ocean currents will result in sea level rise in cities like New York and Boston, and temperature changes on both sides of the Atlantic,” reports NPR’s Jeremy Hobson. Mann, who is a professor and the director of the Earth System Science Center at Penn State University, joined Hobson yesterday on Here and Now to discuss the study and the implications of its findings.

Not only would North America and Europe experience colder temperatures, but “If those current systems shut down, then suddenly the North Atlantic [fisheries] would no longer be productive,” says Michael Mann. 

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Read more: https://www.ecowatch.com/global-warming-is-slowing-ocean-currents-causing-dire-consequences-war-1882023145.html

Obviously Shijian Hu and colleagues may have gotten their calculations wrong, who can dispute the word of scientists like Mann and Rahmstorf?

On the other hand, if ocean currents are actually accelerating, and we apply Mann’s theory that global warming causes ocean currents to slow, does this mean ocean currents are accelerating because world is actually cooling? Or perhaps Mann’s theory is not reversible, warming causes a slowdown but cooling does not cause an acceleration? Or will ocean currents turn out to be an uncertain indicator subject to significant natural variation? Inquiring minds would like to know.