Question

(a i Use Figure 5.5(a) to estimate the approximate vertical extent of the Cromwell Current, and its width, given thai 1 of latitude is aboul 110 km. How many times wider is this 'ribbon of water' than it is deep'.' tbl Now estimate (i \ the cross-sectional area of the Cromwell Current and tiii ils av erage velocity, and use this irformation to calculate an approximate volume transport in m's '. How does your estimated volume transport compare with the values given in Figure 5.2'.'

N latitude S

Figure 5.5(c) and (d) show transverse and longitudinal sections across the Cromwell Current, obtained by a shipborne Acoustic Doppler Profiler (cf. Section 4.3.7). A comparison of Figure 5.5(d) with the part of Figure 5.4(c) between 135° W and 108° W illustrates vividly how, as the resolution of observational techniques improves, so the flow patterns in the ocean are seen to be more and more complex.

N latitude S

Equatorial Counter-

How well do the areas of easterly and westerly current How shown in Figure 5.5(a) and (c) lie up with those shown schematically in Figure 5.2'?

They tie up pretty well. Westward-flowing water is shown in blue in Figure 5.5(a) (as it is in Figure 5.2) and in various shades of blue in the ADCP section. Both the sections in Figure 5.5(a) and (c) show not only an Equatorial Undercurrent and North Equatorial Counter-Current, but also at greater depths, a South Subsurface Counter-Current and (merging with the North Equatorial Counter-Current) a North Subsurface Counter-Current. Both also suggest an Equatorial Intermediate Current below the Equatorial Undercurrent. The main differences seem to be in the extent to which the different eastward flows have merged or are clearly separate. (Rest assured that you will not be expected to remember all the details of the equatorial current system.)

Perhaps surprisingly, almost all the water in the Cromwell Current originates in the Southern Hemisphere. Much of it comes from the South Equatorial Current (which extends to significant depths - Figure 5.2) via a current that flows at depth along the northern part of the Great Barrier Reef, and then along the northern coast of New Guinea. In the eastern Pacific, some of the water leaving the Cromwell Current flows north, and some flows south, so contributing to both the North Equatorial Current and the South Equatorial Current. These flows are shown schematically in Figure 5.6.

Figure 5.6 Schematic map to show how the Equatorial Undercurrent in the Pacific (the Cromwell Current) is fed at its western end by the New Guinea Coastal Undercurrent (which contains water from the South Equatorial Current) and itself feeds both the South Equatorial Current and the North Equatorial Current at its eastern end. Apart from the Peru Current, all the flows are subsurface (UC = Undercurrent). The dashed arrow represents a current that only flows from August to December.

In the Atlantic, the transport in the Equatorial Undercurrent is about one-third of that in the Cromwell Current. The Atlantic Equatorial Undercurrent was first observed in 1886 by John Buchanan, who had earlier participated in the Challenger Expedition. The Undercurrent was then forgotten about until it was rediscovered in 1959 - it is sometimes called the Lomonosov Current, after the vessel from which it was observed. This rediscovery was not accidental. Oceanographers were keen to know whether the Pacific Equatorial Undercurrent had a counterpart in the Atlantic: if none could be found, that would indicate that the Cromwell Current was caused by some peculiarity of the Pacific; if one was found, it would seem much more likely that an Undercurrent was a consequence of some fundamental aspect of equatorial circulation. It is now known that an Equatorial Undercurrent is indeed an intrinsic component of the equatorial circulation pattern but the explanation of its generation given above (Figure 5.3) probably represents

(to northern flank of SEC)

(to northern flank of SEC)

Figure 5.6 Schematic map to show how the Equatorial Undercurrent in the Pacific (the Cromwell Current) is fed at its western end by the New Guinea Coastal Undercurrent (which contains water from the South Equatorial Current) and itself feeds both the South Equatorial Current and the North Equatorial Current at its eastern end. Apart from the Peru Current, all the flows are subsurface (UC = Undercurrent). The dashed arrow represents a current that only flows from August to December.

Figure 5.7 The vertical temperature distribution (in JC) across the Equator in the Atlantic at 2Q"W. plotted from measurements made in August-September 1%3 from the research vessel John Pillsbury onl> part nf the story, As in the Pacific, the system of eastward equatorial currents in the Atlantic has two narrow, swift flow s. eilher side of the main undercurrent in the thermocline and somewhat deeper - a current pattern too complex to he explained by the mechanism illustrated in Figure 5.3.

The Equatorial Undercurrent is generally aligned along the Equator even though the Trade Winds that drive it blow over a fairly wide latitude hand. One of the reasons for this is ihat, on both side.s of the Equator, the geos trophic How resulting from ihe wind-induced eastward horizontal pressure gradient (Figure 5.3) is ¡ow'unls the Equator: once on the Equator the water can flow directly 'down" the pressure gradient

Nevertheless. Equatorial Undercurrents do not always flow along the Equator. Their cores have been detected as much as 150 km away, and in all three oceans there ha\e been observations indicating that the current as a whole may oscillate about the Equator, with wavelengths of the order of 10110 km The initial displacement of the current may be caused by northerly or southerly winds. or by I he presence of islands, but because the current is flowing cast, when it strays from the Equator it is always deflected back by the Coriolis force: if it strays to the north, the Corioil's force acts to turn it to lhe south: tl ii strays to the south, the Coriolis force acts to mm it to (he north. This is in contrast to the action of the Coriolis force on westward-flowing Currents, which are always dellecied polewards Icf. Figure 1.2(b)).

Before leaving this discussion of the main equatorial currents, look at Figure 5.7. a temperature section across the eastern Atlantic made during August and September !LX>3. For convenience, the ! 7 T isotherm may be taken to correspond to the bottom of the theimocline.

Equatorial convergence Divergence convergence

Equatorial convergence Divergence convergence

Figure 5.7 The vertical temperature distribution (in JC) across the Equator in the Atlantic at 2Q"W. plotted from measurements made in August-September 1%3 from the research vessel John Pillsbury

latitude

Compare Figure 5.7 with Figure 5.lib], and use the slopes in the thermocline (which are in the opposite sense to the slopes in the sea-surface) to try to identify ( 1 (the North Equatorial Current. (2i the North Equatorial Counter-Current and (3) the South Equatorial Current (ignoring for the moment their precise positions relative to the Equator).

The two regions of westward flow that make up the South Equatorial Current may be seen in the region above the thermocline between about 8° S and the Equatorial Divergence (thermocline sloping up towards the north), and between the Equatorial Divergence and about 3° N (thermocline sloping down towards the north). The change in the direction of slope of the thermocline occurs a few degrees to the south of the Equator - this probably means that when the observations were made, the South Equatorial Current had recently shifted with respect to the Equator and geostrophic equilibrium had yet to be re-established. The region of westward flow corresponding to the North Equatorial Current may be seen between about 12° and 18° N (thermocline sloping downwards to the north).

Note that two counter-currents are visible: the North Equatorial Counter-Current flows between the North and South Equatorial Currents in roughly the position shown in Figure 5.1(b) (thermocline sloping down towards the south), and the South Equatorial Counter-Current (upper part of the thermocline sloping up to the south) may be seen south of about 8° S.

is it possible to distinguish ihe Equatorial Undercurrent '

No obvious spreading of the isotherms can be seen in the upper part of the thermocline in the region of the Equator, but the spreading of the 13-17 °C isotherms is probably related to vigorous mixing associated with the Equatorial Undercurrent. Such well-mixed regions where temperature and density vary little with depth are sometimes referred to as thermostads or pycnostads. A similar thermostad/pycnostad may be seen on Figure 5.5(b) in the region of the Equator, between about 200 m and 400 m depth.

Another feature visible in Figure 5.7 is the region of 'doming' isotherms at about 12° N. This is known as the 'Guinea Dome', and will be discussed in the next Section.

latitude

Guinea Dome

Equatorial convergence Divergence convergence

Figure 5.8 As for Figure 5.7, but with the regions corresponding to the various currents identified. (The doming of isotherms at about 12° N is known as the Guinea Dome, and will be discussed in Section 5.1.2.)

Figure 5.9 The positions of the various upwelling aieas in the tropical Atlantic, in relation to the eastward subsurface flow m the equatorial current system (the westward-(towing North and South Equatotial Currents have been omitted for the sake uf clarity). The upper part ot the map may be compared with Figure 4.38. Note that to the north of the Cape Blanc zone of year-round upwelling there is another zone ol seasonal upwelling (off the map!

Figure 5.8 repeats Figure 5.7. hut here the various regions of current How have been labelled. Do not worry if you found it hard to interpret Figure 5.7 - ¡is rather ~mcssy' contour pattern illustrates the extent to which diagrams such as Figures 5.1(b) and 5.2 are simplifications of the complex distributions observed in reality.

By now it will be clear lhat although the equatorial current systems of the Atlantic and Pacific Oceans share certain recognizable features, there are also differences between them. Furthermore, at any one time and place the actual patterns may look \ery different from the cross-sec lions shown here. Changes in the e Me til. speed and even the direction of ihe currents occur in response to changes in the overlying w itid pattern, especially changes in the position of the ITCZ. This is particularly true in the western Pacific Ocean and (as you will see in Section 5.2) the Indian Ocean, where seasonally rev ersing winds known as Monsoons have dramatic effects on current speeds and directions.

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