The world's major wind-driven ocean currents are moving toward the poles at a rate of about a mile every two years, potentially depriving important coastal fishing waters of important nutrients and raising the risk of sea level rise, extreme storms and heatwaves for some adjacent land areas.
The shift was identified in a new study by researchers with the Alfred Wegener Institute at the Helmholtz Centre for Polar and Marine Research (AWI) in Bremerhaven, Germany, and published Feb. 25 in the journal Geophysical Research Letters.
The poleward shift is bad news for the East Coast of the U.S., because it makes sea level rise even worse, the researchers said. At about 40 degrees latitude north and south, where the effects of the shifting currents are most evident, sea level rise is already 8 to 12 inches more than in other regions, said lead author Hu Yang, a climate researcher with AWI.
On the West Coast, salmon are being pushed out of traditional fishing waters. In densely populated coastal Asia, the changes could unleash more intense rainstorms, and the shift also makes heat waves more likely in subtropical areas.
Eight major wind-driven ocean currents, known as gyres, circulate around vast areas of ocean: three in the Atlantic, three in the Pacific, and one each in the Indian and Antarctic Oceans. The rotating currents shape the weather and ocean ecosystems in coastal regions, where parts of the currents have regional names, like the Gulf Stream along the East Coast of the U.S.
More than 40 years of satellite measurements of sea levels and surface temperatures show how the gyres are changing because the poleward shift changes the distribution of water across the oceans.
Direct ocean current measurements are hard to come by, so satellite data is the best way to get a global picture of ocean circulation changes. But satellites don't directly measure ocean currents, so "you have to find some other way to relate the data to the large scale ocean gyres," Yang said.
He added that the satellites "can accurately measure areas of high and low sea level and sea surface temperatures. By tracking the position of these patterns, we are able to identify the movement of the center of the ocean gyres."
The satellites clearly show how the sharp boundary between warmer and cooler ocean gyres has shifted toward the poles. The movement of those ocean fronts helps identify the position of the ocean gyres.
Some of the currents run close to densely populated areas, including coastal China and Japan, Argentina and eastern Australia, and the impacts of the changes will be felt strongly in those areas, Yang said.
At their western edge, the gyres move warmth and moisture from the tropics to higher latitudes, which affects air temperature and rainfall. The shift is likely to drive more extreme heatwaves in many subtropical regions, as warmer water and air from the tropics surge poleward.
The study suggests ocean current shifts will squeeze commercially important fisheries especially in the Pacific Ocean.
"If we move the temperature gradient toward a higher latitude, the coldwater species don't have a lot of room to escape," Yang said. "The North Pacific is blocked by continents. There is no space for fish to retreat." He said this partly explains a trend of declining and rapidly shifting fisheries documented elsewhere.
A fisheries decline in the South Atlantic off the coast of Argentina documented as long as 10 years ago may also be linked with southward movement of the South Atlantic Gyre, as the shift rapidly warms the ocean in that region.
Geologic clues from the last ice age, about 20,000 years ago, include ocean-bottom sediments. Such paleoclimate evidence can show the paths of the boundary currents shifting as global temperatures change. Sediment deposits on the ocean floor show that, during that period, the Agulhas Current, flowing south along the east coast of Africa, was 800 kilometers from its modern position, a monumental shift of seven degrees latitude.
Ocean currents also distribute the eggs and larvae of marine organisms over wide areas, so the shift of the gyres is likely to affect the distribution of many species.
There are signs of similar changes all over the world. In Antarctica, for example, a 2019 study tracked a rapid poleward shift of krill as the region warms. At the same time, research shows, Southern Hemisphere subpolar westerly winds are intensifying and also moving poleward. And in the Northern Hemisphere, the Gulf Stream has shifted northward significantly since it's position was first noted by mariners, a change that has chased cod out of the Gulf of Maine.
Previous research by Yang and other scientists also has shown how some coastal currents associated with the major gyres are in synch with global warming, transporting more heat and pushing more intense storms toward the coast of Asia.
Some of the movement detected in the study is partly due to natural fluctuations. But climate models that include greenhouse gas concentrations in the equation suggest the observed changes are "most likely to be a response to global warming," the scientists wrote in the study.
The climate models with high atmospheric CO2 levels "produced the same trends we saw in the satellite data," AWI climate modeler and co-author Garrit Lohman said. Simulating climates with different CO2 levels enabled the researchers to separate the influence of greenhouse gas warming from natural variations, he said.
Yang said there was no reason to think the changes will slow down or stop anytime soon.
"As long as the global temperature keeps increasing, this movement of the currents cannot really stop, because the climate is not in equilibrium with CO2 levels. In our lifetime, I don't think it will stop," he said.
An earlier version of this article incorrectly identified the ocean current that drives nutrient-rich water from the depths to the surface. It is the Benguala Current.