Question

lai Win JtH-'s coastal upwelling occur ail year round lo the north <>1 Cape Blanc and to the south of Cape Frio, hut only seasonally between these latitudes?

(hi By reference lo Figure 5.1, explain why there is up welling in the region indicated by the hand of pale blue shading aligned along the Equator.

Mid-ocean equatorial upwelling. like coastal upwelling. is subject to seasonal variations. The main period of mid-ocean upwelling in the eastern equatorial Atlantic is from July to September. This upwelling maximum is partly the result of increased divergence of surface water in response to the seasonal increase in the strength of the South-East Trade Winds (Figure 5.1(a)). However, the main reason is probably that this increase in wind strength increases the flow in the South Equatorial Current, so that the north-south slope in the thermocline becomes so steep that it intersects the surface (cf. Figure 5.1(b)). allowing deeper water to come to the surface.

Remember also that in low latitudes the thermocline slopes down towards the western coastal boundary, in response to the generally westward wind stress of the Trades (Figures 5.3 and 5.4). Because the mixed surface layer is therefore thinner in the eastern ocean, it is more easily affected by the wind, and sub-thermocline water may more readily be brought up to the surface. This is the main reason why mid-ocean upwelling is a feature of the eastern equatorial oceans in particular.

In certain localities of the eastern tropical Atlantic and Pacific, the isotherms bow up into a dome-like shape. Figure 5.9 shows the position of the Guinea Dome (also visible on Figure 5.8) and the Angola Dome; a similar dome in the tropical Pacific is named the Costa Rica Dome. These thermal domes seem to have a number of causes. Their existence is certainly linked to strong subsurface cyclonic flow patterns: Figure 5.9 shows how the positions of the Guinea Dome and the Angola Dome relate to subsurface flow in the North and South Equatorial Counter-Currents, while Figure 5.8 shows how the Guinea Dome corresponds to the northern edge of the region of sloping isotherms associated with the North Equatorial Counter-Current.

The degree of 'doming' varies seasonally. During the northern summer when the ITCZ is in its most northerly position, the Guinea Dome protrudes up into the thermocline. but the Angola Dome is weak. In the southern summer, the situation is reversed: the Guinea Dome no longer distorts the thermocline. and the Angola Dome is at its most prominent (although it is never as strong a feature as the Guinea Dome).

QUESTION 5.5 Winds are generally light and variable in the vicinity of (he ITCZ. In some circumstances, how ever, they do significantly affect surface waters, particularly in regions where the atmospheric pressure is especially low.

taj Will winds he cyclonic or anticyclonic around such regions?

ihi How does this help to explain the fact that the Guinea Dome 'protrudes"

into (he surface waters when the ITCZ is in its most northerly position?

As in other areas of tipwelling. productivity of phytoplankton and hence o! other organisms is enhanced in the surface waters above thermal domes.

To round off ibis Section, look at Figure 5.10 which shows the distribution of sea-surface temperature in the tropical Atlantic and. lor comparison, the variation in the depth of the 2! QC isotherm (which corresponds roughly to the middle of the thermoeline) at the same time of year.

Does Figure 5,10 show the situation in the northern summer ( July to Septemberi or the southern summer (January to March f'

Figure 5.10(a) shows that the surface ocean is generally warmer to the north of the Equator than to the south. Furthermore, Figure 5.10(b) shows that the thermoeline is very shallow along the Equator in the eastern Atlantic ; more importantly, it shows thai the thermoeline is also shallow along the coast of ihe Gulf of Guinea and that the Guinea Dome (at ~10a N) is well developed. These upwelling-related features may also be seen to some extent in Figure 5.10(a). The maps must therefore show the situation in Julv to September, rather than January to March.

Figure 510 (a) The mean sea-surface temperature (°C) in the tropical Atlantic at une season of the year lb) The average depth (in metres) of the 21 "C isotherm (which corresponds approximately to the middle of the thermoeline) in the tropical Atlantic at the same season of the year as (a).

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