Von Karman Vortex Streets, Canary Islands, 4 April 2001
by: J. Kerkmann (EUMETSAT)
The ocean and atmosphere are fluids in constant motion and we are aware of such motion when we feel
the wind blow, or when we encounter a strong current running along a river or the sea shore. However
we rarely have the possibility to observe large scale fluid motion in the ocean and atmosphere.
An exception is a phenomenon that is shown in the Meteosat image of the Canary Islands below called
a vortex street. This feature is, in effect, lines or streets of spiral eddies, called von Karman vortices,
and named after Theodore von Karman (1881-1963, co-founder of NASA's Jet Propulsion Laboratory)
who first described the phenomenon in the atmosphere.
Meteosat-7, visible channel, 4 April 2001, 10:00 UTC
Von Karman vortex streets form at all scales of fluid motion. They consist of two roughly parallel rows
of vortices positioned such that a vortex in one row is situated opposite the mid-point of two adjacent
vortices in the other row (see figure below). Initially, the vortices are comparable in size to the island
creating the disturbance. As the vortices move downstream they tend to increase in diameter and they
may persist to form a wake 100 km wide and several hundred of kilometres in length. A pair of vortices
is created by an island roughly once every 8 hours and may have a duration as long as 30 hours. All the
vortices in one row have similar circulation, but opposite to that in the other row.
Schematic diagram of von Karman vortices to the lee of an isolated island.
The vortices diverge, and increase in diameter, as they drift further
downstream.
Von Karman vortices will normally form in any region where fluid flow is disturbed by an object. In
the atmosphere, the resulting vortex streets are easily identified by the characteristic formations in
stratocumulus cloud sheets. Such clouds are often associated with a well marked temperature inversion
in the lower levels of the atmosphere. The height of the temperature inversion is particularly important
because it determines whether the air flows more easily around the island rather than over it. In
addition, the low level wind flow should be steady, at around 10 m/s, and the surface isobars (lines of
equal atmospheric pressure) should be generally straight over a large area.
The areas in the world where these conditions are most frequently encountered are the regions of the
Trade Winds. Thus places to look-out for vortex streets would be the Canary Islands, Madeira Island,
Cape Verde Islands, Guadalupe and Socorro Islands (west of Baja California in the Pacific Ocean) and
the Juan Fernandez Islands (off the Chilean coast). Some sample images are shown below.
Von Karman vortices as seen in a GOES-9, visible image, 25 July 1996, 18:45 UTC
(courtesy Co-operative Institute for Meteorological Satellite Studies (CIMSS),
University of Wisconsin-Madison)
Von Karman vortices over Heard Island, Antarctica, from the Space Shuttle, 14 November
1994 (courtesy NASA, Goddard Space Flight Center (GSFC))
MISR (Multi-angle Imaging Spectro-Radiometer on the Terra satellite) images from 23 April and 9
May 2000, showing von Karman vortex streets to the lee of the island of Socorro, Mexico (1050
m). The island is part of a group known as the Revillagigedo Archipelago, located about 400
kilometres equatorward of the southern tip of Baja California. (courtesy NASA, Jet Propulsion
Laboratory).