Figure 3.27 Schematic representations (as seen from above) of (a) divergent surface flow patterns which would lead to upwelling of subsurface water and (b) convergent flow patterns that would lead to sinking of surface water.
Small-scale convergences are often marked by a collecting together of surface debris, seaweed or foam (Figure 3.28(a)). In certain circumstances, linear convergences form parallel to the wind (Figure 3.28(b)). These 'windrows' were first studied by Langmuir, who in 1938 noticed large amounts of seaweed arranged in lines parallel to the wind, in the Sargasso Sea. He proposed that the wind had somehow given rise to a series of helical vortices, with axes parallel to the wind. This circulation system - now called Langmuir circulation - is illustrated schematically in Figure 3.28(c). It is thought to result from instability in the well-mixed, and therefore fairly homogeneously dense, surface water. Such 'longitudinal roll vortices' are now known to be common in both the upper ocean and the lower atmosphere, where they give rise to the linear arrangements of clouds, known as 'cloud streets', that are often seen from aircraft (Figure 3.28(d)).
The spatial scales of the circulatory systems of windrows and of basin-wide features like the subtropical gyres are very different. The latter involve horizontal distances of thousands of kilometres; the former extend only for a kilometre or so. It is characteristic of dynamic systems that phenomena with small length-scales also have short time-scales, while phenomena with long length-scales also have long time-scales. This aspect of ocean circulation will be developed further in the next Section.
Figure 3.28 (a) Foam-line observed oft the north-west coast ol Spain at about 42° 20' N and 8U 54' W. (As In (b) and (c), this is an example ot a smalt-scale Iron! caused by convergence ot surlace water rather than by lateral variations in density.)
(b) Windrows on a Welsh lake. The streaks, which were 5-10 m apart, were observed to respond quickly to changes in the wind direction (interred from the orientation of the wave crests).
(c) Schematic diagram to show Langmuir circulation in the upper ocean. The distance between the surface streaks may be as much as a lew hundred metres, but is typically a few tens gf meltes
(d) Cloud streets over the reels of the Maldive Islands in the Indian Ocean.
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