Ocean Gyres Moving Towards the Poles
Over the past 40 years, the world's eight major ocean gyres (wind-driven current systems) have steadily shifted toward the poles, according to new research published in an American Geophysical Union (AGU) journal.
There are three gyres in the Atlantic Ocean, three in the Pacific Ocean and one each in the Indian and Antarctic oceans. These rotating current systems largely determine the weather and marine productivity in coastal regions.
In the new study, experts at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), analyzed long-term global satellite data of ocean surface temperature and sea levels. Both datasets offer insights into the evolution of large-scale surface currents and indicate that the borders of the ocean gyres and their boundary currents are moving closer to the poles, at a rate of more than 800 meters (2,600 feet) per year.
To study the influence of climate change on the ocean gyres, the researchers simulated the evolution of the current systems with a new AWI climate model. In the first simulations, the starting conditions were equivalent to those for a world with the same level of atmospheric carbon dioxide content as in 1850, the dawn of industrialization. The researchers then gradually raised the amount of carbon dioxide in the Earth’s atmosphere until it was twice the 1850 level and calculated the potential current development for a range of initial climatic conditions.
The modeling allowed the team to discriminate between changes caused by global warming, and those produced by natural variations. “Our calculations for a world with high carbon dioxide values produced the same trends that we saw in the satellite data. And we’re seeing similar changes in the analyses we run with other available model runs from around the globe. In this way, we can show that global warming is a major motor for these shifting currents,” said co-author and AWI climate modeller Gerrit Lohmann.
According to the researchers, the consequences of the shift can already be felt. In affected regions, sea levels are rising, indigenous species are migrating, and storms are now following new courses.
“As the western boundary currents continue to shift, the courses of winter storms and of the jet stream are following suit,” said AWI oceanographer Hu Yang, first author of the study. “At the edges of the eastern boundary currents, we’re now seeing the rich ecosystems begin to shrink, because the shifting currents are changing the living conditions too quickly for marine organisms to adapt.”
Dramatic temperature changes have been observed in the Gulf of Maine, due to the shifting Gulf Stream, resulting in a migration of cod stocks. Researchers have observed similar changes off the Atlantic coasts of Uruguay and Argentina, where the Brazil Current is gradually moving south.
In addition, when boundary currents penetrate higher latitudes, the local sea level rises disproportionately – a problem that communities on the northeast coast of North America are now confronted with.
The displacement of the major subtropical gyres is causing the nutrient-poor regions to expand, reducing the productivity of the ocean as a whole, so the shift in the gyres could represent the beginning of a fundamental change in the ocean, according to the study’s authors.