The Equatorial Undercurrent

The Equatorial Undercurrent (Figures 5.1(b) and 5.2) is a major feature of equatorial circulation. Such an undercurrent occurs in all three oceans, although it is only a seasonal feature in the Indian Ocean.

Equatorial Undercurrents flow from west to east, below the direct influence of the wind, yet they arc w ind-driven. How can this be?

The effect of the wind is transmitted downwards to deeper layers via turbulence (eddy viscosity) and is mainly confined to the mixed surface layer above the thermocline/pycnocline (Section 3.1.1). At high latitudes, winter cooling of surface waters causes them to become denser, destabilizing the upper part of the water column, so that it may more easily be mixed by wind and waves. At low latitudes, there is no winter cooling and the mixed surface layer is thin - as shown in Figure 5. Kb), in the vicinity of the Equator it may only be 50-100 m thick. In addition, as mentioned earlier, the pycnocline is a sharper boundary in low latitudes than in other regions; this is partly because surface heating is most intense there.

Remembering ml urinal ion given in Figure 2.2lbl and Seel ion 2.3.1, can you give the o! he i reason lor I he low densilv ol surlace \\ aler* in tow latitudes'.'

The cumulonimbus activity associated with the ITCZ means that the equatorial zone is characterized by heavy rainfall, which significantly lowers the density of surface waters. Indeed, salinity has a greater control over the density distribution in the equatorial zone than in most other regions of the ocean.

Now study Figure 5.3 which shows a schematic east-west section across the equatorial ocean. The Trade Winds blow from east to west and drive a westward flow in a fairly well-defined mixed surface layer. As a result of this westward transport (and despite some return flow in the Equatorial Counter-Current(s)). water piles up against the western boundary of the ocean; there is therefore a sea-surface slope up towards the west, and a concomitant adjustment in the thermocline so that it slopes down to the west. Because the mixed layer is thin, the horizontal pressure gradient which results from the sea-surface slope extends to greater depths than the effect of the wind. Hence, although current flow 'down' the horizontal pressure gradient is opposed by the wind-driven current in the mixed surface layer, this is not the case below the mixed layer. As a result, in the thermocline there is an eastward jet-like current, which accelerates until mixing between it and surrounding water causes enough friction (i.e. eddy viscosity) for a balance, and hence a steady speed, to be attained. This jetlike current is the Equatorial Undercurrent (EUC).

The most powerful Equatorial Undercurrent is the one in the Pacific (see Figures 5.4 and 5.5). Its existence had been suspected as early as 1886, but it was not investigated properly until 1951 when it was fortuitously rediscovered by Townsend Cromwell and Ray Montgomery, who were on an expedition to study tuna. It is often referred to as the Cromwell Current.

wind

West

mixed surface layei

Figure 5.3 Schematic east-west section across the upper few hundred metres of the equatorial ocean, showing how the slope in the sea-surface caused by the Trade Winds, and the resulting horizontal pressure gradient, leads to the generation of the Equatorial Undercurrent. The lengths of the black arrows indicate the relative magnitude of the zonal horizontal pressure gradient. The vertical scale is greatly exaggerated.

thermocline

Figure 5.4 shows the relationship between the mean wind stress and the resulting east-west slopes in the sea-surface and thermocline along the Equator in the Pacific (cf. the schematic diagram in Figure 5.3). In Figure 5.4(c), the position of the core of the Undercurrent (the region of highest velocity) is indicated by the blue crosses; note that in the eastern Pacific, where the thermocline is especially shallow, and upwelling brings cooler water to the surface, the flow in the Undercurrent may extend to the surface.

According to Figure 5.4, is the depth of the EUC shown in Figure 5.2 consistent with the location of the section?

Figure 5.4 (a) Mean westward wind stress along the Equator in the Pacific (negative values correspond to eastward wind stress), and (b) the dynamic height of the sea-surface, assuming no horizontal pressure gradient at 1000 m; remember that dynamic metres are numerically very similar to geometric metres, (c) Vertical distribution of temperature (°C) along the Equator between ~ 160° E and 94° E. The blue crosses indicate the position of the core of the Cromwell Current (cf. Figure 5.5(d)).

Yes, it is. Figure 5.4 shows that at 170° W the thermocline is still fairly deep, so the core of the EUC is at about 200 m.

The Equatorial Undercurrent is very fast, especially in the eastern Pacific; velocities in the core are typically of the order of 1.5 m s"'. Indeed, the Equatorial Undercurrent is the fastest component of the equatorial current system. In the context of its contribution to ocean circulation, however, it is not the speed of a current so much as its volume transport that is important. We can make an estimate of the volume transport of the Equatorial Undercurrent in the Pacific by reference to Figure 5.5(a), a meridional section showing the distribution of velocity in the vicinity of the Equator. Note that the vertical scale is greatly exaggerated so that the actual shape of the Undercurrent -that of a horizontal 'ribbon' of water - is not immediately apparent.

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