von Karman Vortex Street - Alejandro Selkirk Island

Landsat 7 image, 180 km on-a-side, 15 September 1999, latitude 33.2 South, longitude 80 West

A Landsat 7 image taken off the Chilean coast near the Juan Fernandez Islands (also known as the Robinson Crusoe Islands) on 15 Sep 1999, a portion of which is on cover of Bull. Amer. Met. Soc. May 2000, with article by T. deFelice, D. Meyer, and G. Xian of USGS, J. Christopherson of Raytheon, and R. Cahalan of Goddard (BAMS, 81, 1047-1049). The island rises 1640 m into a layer of marine stratocumulus clouds, much studied due to their strong cooling of Earth global average surface temperature, partially counteracting Greenhouse warming. This scene shows a rare situation in which an extended steady equatorward wind creates vortex shedding with clockwise flow off the eastern edge and counterclockwise off the western edge of the island. The vortices grow as they advect hundreds of km downwind. Such vortex streets are created on much smaller scales, for example in laboratory smoke and soap film experiments. Observing the same phenomenon extended over a scale 10000 times larger, as here, dramatizes the fractal nature of atmospheric convection and clouds. The island's namesake, Alejandro Selkirk, was stranded there from 1704 to 1710, and survived by catching wild goats with his bare hands, and domesticating wild cats to fend off rats that knawed on him as he slept. Plan your island vacation today!

von Karman Vortex Street from Landsat 7

For a ZOOMED view, select the above Landsat 7 image, or the following image of QuikSCAT winds:

The QuikSCAT winds are generally equatorward, but also veering counter-clockwise along the vortex street. This same veering can be seen in the vortex street itself. Though both clockwise and counter-clockwise vortices are generated by flow around the island, the larger-scale flow is generally anti-cyclonic, due to the influence of the coriolis force around the large-scale oceanic subtropical high pressure, driving counter-clockwise flow in the Southern Hemisphere, similar to the clockwise flow around the Bermuda High in the northern hemisphere. This same anti-cyclonic large-scale circulation drives oceanic currents along the Peruvian coast, creating the Eckman upwelling that normally leads to abundant seafood in those coastal waters, except during el Nino. This same flow continues into the South Equatorial Current, that flows westward along the equator. (The intense "equatorial counter-current" and the "equatorial undercurrent" flow eastward along the equator, but these have no obvious connection to near-surface winds.)

The coriolis force that causes the leftward veering of this train of vortices is generally important only at scales larger than 100 km in the atmosphere. The rumor that toilets flush in opposite directions in the two hemispheres is quite incorrect! Tornadoes and waterspouts can spin either direction, though their spin (angular momentum) initially comes from the large-scale, with its Coriolis-favored spin, with direction determined by Earth's rotation, along with wind velocity. These von Karman vortices, however, garner their spin from the island obstacle, and the direction depends which way they flow around the obstacle. As the flow separates from the island's leeward side, the vortices "swallow" some of the clear air over the island. (Air of the island is clear due to local subsidence over the land, which has been losing heat overnight faster than the surrounding ocean, due to water's high heat capacity. The thermal gradient sets up a local "land breeze" circulation.) The "swallowed" gulps of clear island air get advected along within the vortices, but are soon entrained into the surrounding clouds. It is only the vortex train as a whole that gets pulled to the left by the Coriolis force.

Cloud Vortex Animations