Satellite imaging has revealed the deep understanding of ocean currents possessed by medieval cartographers. Comparison of a pioneering 16th-century chart of the North Atlantic with the latest temperature maps of the region has identified hitherto unrecognised representations of giant eddy currents.
Alongside the sea monsters, serpents and sinking galleons used to decorate the map, a group of swirls depicted off the east coast of Iceland in the Carta Marina has been found by scientists to correspond to the meandering Iceland-Faroes front, where the warm Gulf Stream meets cold water from the Arctic.
The chart was produced by the Swedish cartographer Olaus Magnus in 1539. It was the most comprehensive and accurate map of its time.
Thomas Rossby, professor of oceanography at Rhode Island University, and Peter Miller, manager of Plymouth Marine Laboratory's remote sensing data analysis service, matched satellite images of water temperature to swirls that stretch east from Iceland around to the Faroe Islands.
Professor Rossby said the lines appeared to have been drawn to aid navigation. "Their location, size and spacing seem too deliberate to be purely artistic expression," he said.
The eddies are slowly turning bodies of water that can reach 100km in diameter. They form along a front where water from the south runs into colder water from the north.
Dr Miller said satellite water temperature images of the region were collected over a month.
"Things got exciting when I was able to provide Tim with an image of the eddy field that confirmed his theory," he said.
The oceanographers believe Magnus collected his information from German mariners operating from the Hanseatic League.
They would have made frequent trips across the region and would have noticed striking differences along the Iceland-Faroes front such as colour changes - warm water appears a deeper blue - and drops in air temperature.
The eddies were powerful enough to knock a ship off course, but if approached correctly could speed a vessel on its way.
The research, part funded by the Natural Environment Research Council, will appear in the journal Oceanography .