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Ekman transport

current because of, resultant surface gradient in tranzón sea-surface, '

level ol zero current upward-sloping isobars and northerly Dow

Figure 4.36 Diagrams (not to scale) to illustrate the essentials of coastal upwelling (here shown for the Northern Hemisphere).

(a) Initial stage: wind stress along the shore causes surface transport 45° to the right of the wind, and Ekman transport (average motion in the wind-driven layer) 90° to the right of the wind (cf. Figure 3.6(h)). (Note: this shows the idealized situation which is never observed in reality.)

(b) Cross-section to illustrate the effect of conditions in (a): the divergence of surface waters away from the land leads to their replacement by upwelled subsurface water, and to a lowering of sea-level towards the coast.

(c) As a result of the sloping sea-surface, there is a horizontal pressure gradient directed towards the land (black arrows in (d)) and a geostrophic current develops 90° to the right of this pressure gradient. This 'slope' current flows along the coast and towards the Equator. The resultant surface transport, I.e. the transport caused by the combination of the surface transport at 45° to the wind stress and the slope current, still has an offshore component so upwelling continues.

(d) Cross-section to Illustrate the variation with depth of density (the blue lines are Isopycnals) and pressure (the dashed black lines are isobars and the horizontal arrows represent the direction and relative strength of the horizontal pressure gradient force). Isopycnals slope up towards the shore as cooler, denser water wells up to replace warmer, less dense surface waters. The shoreward slope of the isobars decreases progressively with depth until they become horizontal; at this depth the horizontal pressure gradient force is zero, and so the velocity of the geostrophic current is also zero. At greater depths, isobars slope up towards the coast indicating the existence of a northerly flow; a deep counter-current Is a common feature of upwelling systems.

level ol zero current upward-sloping isobars and northerly Dow

Figure 4.36 Diagrams (not to scale) to illustrate the essentials of coastal upwelling (here shown for the Northern Hemisphere).

(a) Initial stage: wind stress along the shore causes surface transport 45° to the right of the wind, and Ekman transport (average motion in the wind-driven layer) 90° to the right of the wind (cf. Figure 3.6(h)). (Note: this shows the idealized situation which is never observed in reality.)

(b) Cross-section to illustrate the effect of conditions in (a): the divergence of surface waters away from the land leads to their replacement by upwelled subsurface water, and to a lowering of sea-level towards the coast.

(c) As a result of the sloping sea-surface, there is a horizontal pressure gradient directed towards the land (black arrows in (d)) and a geostrophic current develops 90° to the right of this pressure gradient. This 'slope' current flows along the coast and towards the Equator. The resultant surface transport, I.e. the transport caused by the combination of the surface transport at 45° to the wind stress and the slope current, still has an offshore component so upwelling continues.

(d) Cross-section to Illustrate the variation with depth of density (the blue lines are Isopycnals) and pressure (the dashed black lines are isobars and the horizontal arrows represent the direction and relative strength of the horizontal pressure gradient force). Isopycnals slope up towards the shore as cooler, denser water wells up to replace warmer, less dense surface waters. The shoreward slope of the isobars decreases progressively with depth until they become horizontal; at this depth the horizontal pressure gradient force is zero, and so the velocity of the geostrophic current is also zero. At greater depths, isobars slope up towards the coast indicating the existence of a northerly flow; a deep counter-current Is a common feature of upwelling systems.

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Iront

current direction O flow 'out o1 paper' ® 'into paper'

current direction O flow 'out o1 paper' ® 'into paper'

weak front weak front

Note that the current arrows on Figure 4.36(a) and (c) are idealized representations of a steady-state situation, assuming a fully developed Ekman spiral, resulting in Ekman transport at right-angles to the wind (Section 3.1.2). Even if the wind was steady for some time, a fully developed Ekman spiral would not be possible in shallow coastal waters, and in reality, upwelling occurs in response to particular wind events, which might be quite shortlived. Thus, the actual pattern of isopycnals and along-shore current flow varies from time to time, depending on the direction and strength of the wind, and is also affected by local factors like the topography of the sea-bed and the shape of the coastline. Three examples of upwelling regimes are shown in Figure 4.37 - note that they all include a poleward-flowing counter-current, which is found in most upwelling regions in eastern boundary currents.

The ecological importance of coastal upwelling lies in the fact that - like upwelling in cyclonic gyres (Figure 3.24) - it usually replenishes surface waters with nutrient-rich sub-thermocline water, stimulating greater productivity of phytoplankton (and hence supporting higher trophic levels). Figure 4.38 illustrates the marked effect that wind has on upwelling, and hence primary productivity, in the surface waters of the Canary Current off north-west Africa. Upwelling off the coast of north-west Africa occurs in response to the North-East Trade Winds. To the north of about 20° N. there is a region where the Trade Winds blow along the coast all year, but to the north and south of this, the wind direction changes seasonally (cf. Figure 2.3). As a result, although upwelling occurs all year round to the north of Cape Blanc, further to the north and to the south of Cape Blanc it varies seasonally. Similar seasonal variations are observed in all eastern boundary currents, and there are also marked differences between one year and another - compare Figure 4.38(b) and (c) for the month of November in 1982 and 1983.

Coastal upwelling is most marked in the Trade Wind zones, but it can occur wherever and whenever winds cause offshore movement of water. Nevertheless, it is a difficult process to investigate directly because it occurs episodically, and because the average speeds of upward motion are very low - generally of the order of 1-2 metres per day though sometimes approaching 10 metres per day. Indirect methods must be used and. as with indirect methods of measuring currents, these may be based on either the causes or the effects of upwelling.

The cause of coastal upwelling is the offshore movement of water in response to wind stress. Since the upwelled water rises to replace that moved offshore by the wind, the rate at which water upwells is the same as that with which it moves offshore. Hence, the rate of upwelling may be calculated using Equation 3.4. which tells us that it must be directly proportional to the wind stress and inversely proportional to the sine of the latitude. This method of calculating the rate of upwelling gives reasonable results only if the assumption that a steady state has been attained is valid, and there is adequate information about local winds. However, as mentioned earlier, the average speed of the wind is not the best indicator of the amount of upwelling it will induce, because the wind stress is proportional to something like the square of the wind speed (Equation 3.1). Thus, occasional strong winds have a disproportionately large influence on water movement, and upwelling rates fluctuate greatly in response to fairly small changes in wind speed.

Figure 4.37 Three examples of upwelling regimes over different continental margins. In each case, the wind is equatorward and out of the page. The darker the blue-green tone, the denser the water. The diagrams show the uppermost 200 m or so of the water column; the vertical scale is greatly exaggerated. Note the fronts; these tend to develop wave-like instabilities, eddies and filaments (cf. Figure 4.38).

Cape Blanc

Figure 4.38 CZCS images showing the concentration of chlorophyii-a pigment in surface waters off the coast of north-west Africa, in each case averaged over about a month: (a) March-April 1983; (b) November 1982; (c) November 1983. In these images, clouds and land are shown as white and turbid coastal waters are black. The colours represent pigment concentration according to a logarithmic scale: dark blue is smallest concentration; dark green largest. Note the wave-like undulations, eddies and filaments, made visible by their higher chlorophyll content.

Figure 4.38 CZCS images showing the concentration of chlorophyii-a pigment in surface waters off the coast of north-west Africa, in each case averaged over about a month: (a) March-April 1983; (b) November 1982; (c) November 1983. In these images, clouds and land are shown as white and turbid coastal waters are black. The colours represent pigment concentration according to a logarithmic scale: dark blue is smallest concentration; dark green largest. Note the wave-like undulations, eddies and filaments, made visible by their higher chlorophyll content.

Figure 4.39 Mean anomaly in the sea-surface temperature off north-west Africa for April. The large area with a negative anomaly (i.e. region of significant difference between actual surface temperatures and average surface temperatures for these latitudes) is mainly attributable to upwelling (cf. Figure 4.38).

Figure 4.39 Mean anomaly in the sea-surface temperature off north-west Africa for April. The large area with a negative anomaly (i.e. region of significant difference between actual surface temperatures and average surface temperatures for these latitudes) is mainly attributable to upwelling (cf. Figure 4.38).

Although there are chemical and biological indicators of upwelling (Figure 4.38), m practice it is the physical characteristic of temperature that is most often used to identify and investigate regions of upwelling. Water that up wells to the surface conies from only -100-200 m depth, but it is nevertheless significantly colder than surface water (Figure 4.39). Relatively cold surface water does not always imply upwelling. however: it may simply result Irom advectton of water from colder regions, by currents or in mesoscale eddies. When subsurface measurements are available, the clearest indication of coastal upwelling is the upward slope of isotherms land associated isopycnalsi towards the coast (Figure 4.36(d)).

Before leaving the topic of upwelling. we should emphasize that upwelling does not only occur in response to longshore or cyclonic winds. 11 may also occur, on a local scale, as a result of subsurface currents being deflected by bottom topography. The most extensive areas of upwelling occur in mid-ocean, in response to w ind-driven divergence of surface waters. These areas of upwelling. which are at high southern latitudes and along the Equator; will be discussed in Chapters 5 and 6

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