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Figure 3.24 The effect of a cyclonic wind in the Northern Hemisphere (a) on surface waters, (b) on the shape of the sea-surface and thermocline. Diagrams (c) and (d) show the effects of an anticyclonic wind in the Northern Hemisphere. (Remember that in the Southern Hemisphere, cyclonic = clockwise and anticyclonic = anticlockwise.)

surface convergence surface divergence ^¡ijifeilifig*'

sea-surtace lb)

If down welling ii

^^^ Ekman transport thermocline wind _surface current thermocline

The convergence of water as a result di atHicy clonic winds thus causes ihe sea-Surface to slope upwards towards the middle of the gyre. As a result, the circulating water will be acted upon by a horizontal pressure gradient force.

It will act outwards from the centre, from the region of higher pressure to the region of lower pressure (see Figure 3.2M. Under steady conditions the horizontal pressure gradient force will he balanced by the Coriolis force, and a geostrophic ('slope') current will flow in the same direction as the wind. The gyral current systems of the Atlantic and Pacific Oceans between about IU and 40 degrees of latitude - the subtropical gyres, which tie beneath the subtropical highs (cf. Figures 2.2(a) and 2.3}- are large-scale gyres of the t\ pe we have been discussing; (Figure 3.11, raised sea-siirlace raised sea-siirlace

Coriolis force '

geostroptw; current -J—horizontal pressure gradient force

Coriolis force '

geostroptw; current -J—horizontal pressure gradient force

Figure 3.25 The generation ot geostrnpint current ¡low iri a gyre driven by ariltdyctoinrc winds in the Northern Hemisphere. This current is driven by the wind only indirectly and persists below the wind-driven (Ekman) layer.

IIjmiii! idemilied the subtropical gyres on 1 igure 3.1, identify, at higher latitudes, areas characterized by in lonii gyres, within which outward likman transport would lead (o upwelling as shown in Figure 3.24(a) and ibi According to figures 2.21 a I and 2.3, whai wind systems characteristic-all) at feet these regions?

We hope you identified (he cyclonic gyres in the northern parts of the North Pacific and ¡he North Atlantic, and in (he Norwegian and Greenland Sea. These arc I he subpolar gyres, driven by the subpolar low pressure systems (Figure 2.2(a)). especially in winter (cf Figure 2.3). Yon w ill have noticed ¡hat subpolar gyres do not form in the uninterrupted expanse of the Southern Ocean, but further south, off Antarctica (cf. Figure 3.11, are the cyclonic Weddell Sea Gyre and Ross Sea Gyre

Figure 3.26 Global distribution of the potential for primary (plant) production in surface ocean waters and on land, as indicated by chlorophyll concentration, determined by satellite-borne sensors. In surface waters, regions of highest productivity are bright red, followed by yellow, green and blue, with least productive surface waters shown purplish red. In the Northern Hemisphere, the red areas around coasts correspond to high primary productivity supported by fertilizer run-off and sewage from the land. On land, darkest green = areas with the greatest potential for primary production, yellow = least productive areas.

Figure 3.26 Global distribution of the potential for primary (plant) production in surface ocean waters and on land, as indicated by chlorophyll concentration, determined by satellite-borne sensors. In surface waters, regions of highest productivity are bright red, followed by yellow, green and blue, with least productive surface waters shown purplish red. In the Northern Hemisphere, the red areas around coasts correspond to high primary productivity supported by fertilizer run-off and sewage from the land. On land, darkest green = areas with the greatest potential for primary production, yellow = least productive areas.

The vertical motion of water that occurs within wind-driven gyres - whether upwelling or downwelling - has a profound effect on the biological productivity of the areas concerned. In cyclonic gyres, upwelling of nutrient-rich water from below the thermocline can support high primary (phytoplankton) productivity. By contrast, in the subtropical gyres, the sinking of surface water and the depressed thermocline tend to suppress upward mixing of nutrient-rich water. The satellite-derived data in Figure 3.26 show clearly the difference between the productive cyclonic subpolar gyres (green/light blue) and the 'oceanic deserts' (purplish-red) of the subtropical gyres.

The surface waters of the ocean move in complex patterns, and divergences and convergences occur on small scales as well as on the scale of the subtropical and subpolar gyres. Figure 3.27 illustrates schematically several types of flow that would lead to vertical movement of water. Such divergences and convergences may be seen in rivers, estuaries, lakes and shelf seas, as well as in the open ocean.

The position of a divergence may sometimes be inferred from the colour of the surface water: because it is usually richer in nutrients, and therefore able to support larger populations of phytoplankton, upwelled water often becomes greener than the surrounding water. It is also generally colder than surface water and so divergences are sometimes marked out by fog banks. Linear convergences are often known as fronts, especially when water properties (e.g. temperature, salinity and productivity) are markedly different on either side of the convergence.

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