Climate change causing deep ocean churning

The oceans around Hawai`i are changing in many ways—and the latest to be detected is how fast the great currents flow.

Certainly the seas are warming, are acidifying, are rising, but now thereʻs evidence they are churning in ways that had not been predicted.

The evidence has been building. Five years ago, a paper in Science by Scripps researchers Dean Roemmich and John Gilson reported the great South Pacific Gyre had been increasing in speed, driven by increased surface winds. 

Those winds drive currents, and the currents have been speeding up for the past quarter-century, says a new report in Science Advances. 

"We have found a strong acceleration in the global mean ocean circulation over the past two decades. The acceleration is deep-reaching and particularly prominent in the global tropical oceans and can be attributed to the planetary intensification of surface winds since the 1990s," the authors wrote.

The currents not only are increasing in energy by 15 percent a decade, but they are also driving ocean mixing between shallow and deep waters.

"The increasing trend in kinetic energy is particularly prominent in the global tropical oceans, reaching depths of thousands of meters," say the authors, Chinese, American and Australian researchers Shijian Hu, Janet Sprintall, Cong Guan, Michael J. McPhaden, Fan Wang, Dunxin Hu and Wenju Cai. The paper is entitled "Deep-reaching acceleration of global mean ocean circulation over the past two decades."

What that means is complicated. It can mean that more atmospheric heating can be trapped and delivered into the deep oceans, reducing some of the immediate surface impacts of global warming, but also changing conditions for marine life in the deep oceans. It can change weather patterns on land and over the seas.

There is still a lot to know. Most of this paper is based on observations that go down 2000 meters (a little more than a mile), and it is still uncertain whatʻs happening in the very deep oceans.

"The data-void abyssal ocean is likely to be important. Thus, intensive observations that monitor the deep global ocean circulation are urgently needed not only for understanding past conditions but also for reducing uncertainty in future projections of the global ocean circulation," the authors say.

Wind speed is driving the increased water speed, and wind speeds are expected to continue to increase.

As little as 10 years ago, scientists were concerned that climate change was quieting the worldʻs winds, but even as they were writing those papers, the winds were picking up, dramatically.

©Jan TenBruggencate 2020