Giant “whirlpools” in the ocean, up to 500 kilometres across, are driving the world’s climate on a scale previously unimagined. We just don’t know exactly how yet.
The bodies of swirling water, called mesoscale eddies, are 100 km to 500 km in diameter. They form when patches of water are destabilised by obstacles like islands. The eddies carry huge volumes of water and heat across the oceans, until they slowly stop spinning over days or months and reintegrate with the surrounding water.
The assumption was that they gradually diffused the heat they carried in all directions as they travelled, which would hardly do anything to the climate. Now, for the first time, the amount of water and heat they carry has been measured and it turns out the eddies have a big effect after all.
Bo Qiu at the University of Hawaii in Honolulu and colleagues used satellite data from 1992 to 2010 to spot eddies, and floating sensors to map their shapes, volumes and temperatures.
The team found the eddies move as much water as the biggest ocean currents. They mostly move west, driven by the spinning of the Earth. As a result, over 30 million tonnes of water arrive on the east coasts of continents every second.
“The amount of water they can carry westward was a huge surprise,” says Qiu.
It’s not clear what this means for the weather, but it is likely to be significant. Some of the world’s biggest sources of climate variability, such as the El Niño Southern Oscillation, are powered by heat moving around the oceans, driven by wind and ocean currents. The eddies could have similar effects, says Qiu, and once we understand them it should help us create more accurate predictions of the regional effects of climate change.
For instance, eddy-driven currents are probably exacerbating extreme weather around Japan, says Wenju Cai from CSIRO in Melbourne, Australia. Warm water carried by the giant Kuroshio current drives extreme weather, and the eddies carry even more warm water, making the weather worse.
It’s also unclear how the eddies will affect weather in the future. It will depend on how climate change affects them, which Qiu says they haven’t looked at yet.
It may be that the eddies get bigger and more common in a warmer world. They are the ocean equivalent of storms, and since storms and hurricanes are predicted to become more powerful due to the extra heat energy, the eddies might too.
Journal reference: Science, DOI: 10.1126/science.1252418
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.