The vortex generators themselves are pins or vanes that protrude from the surface and set up microscopic eddies in the boundary layer.

ECONOMIST: The turbulent world of swimming

They say there is particular concern over the very long-lived northern low pressure zone of circumpolar winds, called a vortex, found at between 13 and 34 km (8 and 21 miles) above the Earth's surface.

BBC: By Environment Correspondent Alex Kirby