A vortex street, seen in the Meteosat image of the Canary Islands below.
The ocean and atmosphere are fluids in constant motion and we are aware of such motion when we feel the wind or a strong current. But we rarely have the possibility to observe large scale fluid motion in the ocean and atmosphere. An exception is this feature is, in effect, lines or streets of spiral eddies. They are called von Kármán vortices after Theodore von Kármán (1881–1963, co-founder of NASA's Jet Propulsion Laboratory) who first described the phenomenon in the atmosphere.
More information and detailed analysis of the feature can be found in the In Depth section.
Von Kármán 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 eight 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.
In Figure 2, it can be seen that the vortices diverge as they become larger further downstream in the wake.
Von Kármán 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 ast). Some sample images are shown below (Figures 3–5).
Figure 1: Meteosat-7 (visible channel)
4 April 2001, 10:00 UTC, Animated GIF (1 MB)
Figure 2: Schematic diagram
Schematic diagram of Von Kármán vortices to the lee of an isolated island.
Figure 3: Von Kármán vortices, GOES-9 (visible channel)
25 July 1996 (18:45 UTC). Source: Co-operative Institute for Meteorological Satellite Studies (CIMSS), University of Wisconsin-Madison.
Figure 4: Von Kármán vortices over Heard Island, Antarctica
Image seen from the Space Shuttle, 14 November 1994. Source: NASA, Goddard Space Flight Center (GSFC).
Figure 5: MISR on the Terra platform
23 April and 9 May 2000. Source: NASA, Jet Propulsion Laboratory.
Von Kármán vortex streets in 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 km equatorward of the southern tip of Baja California.