Summary Of Chapter

1 The major components of equatorial current systems are westward-flowing North and South Equatorial Currents, one or more eastward-flowing Counter-Currents (surface and subsurface), and an eastward-flowing Equatorial Undercurrent, which is generally centred on the Equator. Flow in the North and South Equatorial Currents is partly directly driven by the Trade Winds and is partly geostrophic flow.

2 The equatorial current system is best developed in the Pacific Ocean, where the surface waters are under the cumulative influence of the prevailing Trade Winds over the greatest distances. In the Atlantic, the equatorial circulation is affected by the shape of the ocean basin and, indirectly, by the effect of the continental masses on the ITCZ. In the Indian Ocean, the circulation is monsoonal, most resembling that in the other tropical oceans in the northern winter.

3 The Intertropical Convergence Zone is generally displaced north of the Equator so that the South-East Trade Winds blow across it. As a result, divergence of surface waters, and upwelling, occur just south of the Equator. There is a convergence of surface waters at about 4° N.

4 The prevailing easterly winds over the tropical ocean cause the sea-surface to slope up (and the thermocline to slope down) towards the west. As a result, there is an eastward horizontal pressure gradient force and the Equatorial Counter-Current(s) flow(s) down this gradient towards the east in zones of small westward wind stress (the Doldrums).

5 This eastward horizontal pressure gradient force also drives the Equatorial Undercurrent, which flows in the thermocline below the mixed surface layer. The Equatorial Undercurrent is a ribbon of fast-flowing water, many hundred times wider than it is thick. It is generally aligned along the Equator, although it may have long-wavelength undulations; if it is diverted away from the Equator, the Coriolis force turns it equatorwards again. The Equatorial Undercurrent has a significant volume transport, particularly in the Pacific.

6 In the Pacific and the Atlantic, extensive areas of upwelling occur just south of the Equator, in association with the South Equatorial Current. There is also coastal upwelling along the eastern boundaries - either year-round or seasonal - as a result of the Trade Winds blowing along the shore.

7 Surface divergence and upwelling may occur below the ITCZ because it is a region of low pressure and cyclonic winds. When the ITCZ is over certain regions of doming isotherms (apparently associated with flow in subsurface counter-currents), the doming intensifies and 'protrudes' into the thermocline. These thermal domes seem to be a feature of the eastern sides of oceans. In the Pacific and the Atlantic, all types of upwelling occur most readily on the eastern side of the ocean, because there the thermocline is at its shallowest, and the mixed layer at its thinnest.

8 The winds over the Indian Ocean change seasonally as a result of the differential heating of the ocean and the Asian landmass. During the NorthEast Monsoon (northern winter), the winds are from Asia and are dry and cool; during the stronger South-West Monsoon, the winds carry moisture from the Arabian Sea to the Indian subcontinent. Because the winds over the equatorial zone change over the course of the year, so does the direction of the sea-surface slope along the Equator. As a result, in the Indian Ocean the Equatorial Undercurrent is only a seasonal feature of the circulation.

9 The most dramatic seasonal change in the surface circulation of the Indian Ocean is the reversal of the Somali Current which flows south-westwards during the North-East Monsoon but is a major western boundary current during the South-West Monsoon. At that time of the year, the North Equatorial Current reverses and becomes the South-West Monsoon Current. During the South-West Monsoon, there are regions of intense upwelling on the western side of the ocean, off Somalia and Oman.

10 The Agulhas Current is the next most powerful western boundary current, second only to the Gulf Stream. Its retroflection off the tip of southern Africa is a source of eddies, many of which are carried into the Atlantic.

11 The ocean can respond to the winds in distant places by means of large-scale disturbances that travel as waves. These waves may propagate along the surface (barotropic waves) or along a region of sharp density gradient such as the thermocline (baroclinic waves); surface waves, in particular, travel very fast. Two of the most important types of waves are Kelvin waves and Rossby (or planetary) waves. Rossby waves result from the need for potential vorticity to be conserved and, relative to the flow, only travel westwards. Kelvin waves may travel eastwards along the Equator (as a double wave) or along coasts (with the coast to the right in the Northern Hemisphere and to the left in the Southern Hemisphere). In these cases, the Equator and the coast, respectively, are acting as wave guides. Because of the equatorial wave guide, the ocean in low latitudes can respond much more rapidly to changes in the overlying wind than can the ocean at higher latitudes.

12 El Niño or ENSO events are climatic fluctuations centred in the tropical Pacific, in which the east-west slopes in the sea-surface and thermocline collapse, and warm water spreads across the tropical Pacific, along with areas of vigorous convection and heavy rainfall. During El Niño events, the difference in pressure between the South Pacific High and the Indonesian Low is less than usual (i.e. the Southern Oscillation Index is large and negative), and the Trade Winds are weaker than usual. When the Southern Oscillation Index is large and positive (i.e. conditions are an extreme version of the 'normal' situation), there is said to be a La Niña.

13 Unlike the tropical Pacific and Atlantic, the Indian Ocean does not have well defined climatic oscillations, but it does have an anomalous mode in which conditions along the Equator become more like those in the other two oceans (warm in the west, cool in the east).

14 As a result of the contrasting distributions of land and sea in northern and southern high latitudes, both the type of ice cover and the current pattern of the two regions are very different. A large proportion of Arctic pack ice is several years old, while most Antarctic ice is renewed yearly. The main circulatory pattern in the Arctic Sea is an anticyclonic gyre with cross-basin flow between the Bering Straits and the Fram Strait, where the outflow becomes the East Greenland Current.

15 The Great Salinity Anomaly was a pulse of low salinity water which, between 1968 and 1981-82, travelled westwards round Greenland, around the Labrador Sea, and the subpolar gyre, and then back to the north-east Atlantic and the Norwegian and Greenland Seas.

16 The major current feature of the Southern Ocean is the Antarctic Circumpolar Current (ACC) which, by virtue of its great depth, has an enormous volume transport. The cumulative influence of the westerly wind stress acting on the ACC is balanced mainly by frictional forces generated by the interaction of the ACC with the sea-floor topography. The strongest currents in the ACC flow along fronts in the Antarctic Polar Frontal Zone, and these current jets often form meanders and eddies. The Antarctic Polar Frontal Zone is a region where surface water converges and sinks; the Antarctic Divergence, between the Antarctic Circumpolar Current and the Antarctic Polar Current, is a region of upwelling.

17 The Antarctic Circumpolar Wave is a wave-like progression of maxima and minima of atmospheric pressure, meridional wind stress, sea-surface temperature and sea-ice extent, which travels eastwards around the Antarctic continent. The wave has two wavelengths end-to-end and, as it has a period of 4-5 years, it takes 8-10 years to travel all the way round the Antarctic continent. Its importance for the global climate is not yet understood.

Now try the following questions to consolidate your understanding of this Chapter.

QUESTION 5.8 A number of A LACE floats (Section 4.3.41 were deployed in the South Atlantic in 1990, Figure 5,34 (opposite) shows their paths up until mid-1997. Study the paths and on the basis of your study so far (mainly Chapter 5. but also Figure 3.1) make a list of the current features that you can recognize. Which floats had been travelling the fastest?

QUESTION 5.9 Monsoon winds are sometimes described as land breezes and >ea breezes, on a very large scale. Would you say that this is a fair description of them?

QUESTION 5.10 AI (he beginning ol Section 5.2.2. we say: 'As you might expect, the surface circulation of the northern Indian Ocean .. most resembles that of the other two oceans in the northern w inter". However, there is a significant feature of the circulation in the northern summer that is shared by the other two oceans. What is it?

QUESTION 5.11 The Rossby radius of an equatorial Kelvin wave is defined mathematically as the distance from the Equator by which the wave's amplitude has fallen to Me of its maximum value (<\ the base of natural logarithms is -2.72. so Me is about 0,4). Use tins information to estimate the Rossby radius of deformation for the (computer-generated) equatorial Kelvin wave shown in Figure 5.20. Use diagram (a), and note that the contours are 5 tn (or cm) apart: I of latitude is about I If) km.

Figure 5.34 The tracks of ALACE floats deployed in the South Atlantic in January 1990. Each colour corresponds to a different float; this map shows their paths up until June 1997. (For use with Question 5.8.)

QUESTION 5 12 As you saw in Section 5.3.2, Rossby waves result from the need for potential vorticiu (/ + t)!D to be conserved. Bearing that in mind, can you suggest why similar waves (known as 'shelf waves') occur over regions of relatively sharp depth change between the continental shelf and the deep sea-floor? Begin by imagining a current flowing above (say) the continental slope, parallel to a particular depth contour What happens if, for some reason, the flow is displaced either towards or away from the coast (assume that distances involved are sufficiently small for changes in latitude to be neglectedt? (You do nut need to go into details.)

QUESTION 5.13 The vessel From took three years to drif t in the ice from north of the Bering Straits to Spitsbergen l Figure 5.26). (a) With which current did the From travel?

tbi Given that the distance involved is roughly 4000 km, approximately what is the implied average speed of this current?

Figure 5.34 The tracks of ALACE floats deployed in the South Atlantic in January 1990. Each colour corresponds to a different float; this map shows their paths up until June 1997. (For use with Question 5.8.)

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