Ocean Currents

The winds affect the oceans mostly by influencing surface currents. Let us now consider the pattern of these currents, then an explanation of the pattern and finally the effects on flows beneath the surface.

Gyres and Eddies

The first maps of the main ocean currents were compiled by Matthew Maury in 1855, using data from the ships' logs of ten countries. A modern map (Figure 11.15) shows the huge swirls called 'gyres', which turn anti-clockwise in the southern hemisphere. A complete rotation within each ocean basin takes years. Along the north and south edges of the gyres there are currents about 150 m deep with a speed of 3-5 km/day, though 'boundary currents' on the east and (especially) the west edges of the oceans are appreciably faster and deeper. For instance, the part of the South Pacific gyre against Australia (called the East Australian Current) flows south and constantly impeded the northward progress of James Cook on his voyage of discovery in 1770.

Embedded within the boundary currents are transient eddies, especially at the western edge of an ocean. These can be discerned on a snapshot map of sea-surface heights over a short period (Figure 11.16). The eddies form when a boundary current meanders so widely that a loop becomes short-circuited, and they are about 30-300 km in diameter, drifting polewards. They stir a warm current into the surrounding colder water, and so contribute to the transport of heat (and momentum) towards the pole in the same way that midlatitude frontal systems in the atmosphere mix warm and cold air masses (Chapter 13). Clockwise eddies in the southern hemisphere have a cold centre and a slightly depressed ocean surface, while the anticlockwise eddies have a warm core and a slightly elevated ocean surface, as explained later.

If we follow the South Pacific gyre onwards, we see that the East Australian Current eventually mixes into the eastwards flow of cold water through Bass Strait, between Tasmania and the mainland, to become part of the relatively slow West-Wind Drift (or Antarctic Circumpolar Current). This takes months to cross the Pacific ocean at about 50°S, becoming colder to match the latitude. (There is a weak westwards flow further south, close to Antarctica, driven by east winds there—Figure 11.15.) Having reached South America, most of the Circumpolar Current turns north along the coast of Chile, where it is called the Humboldt Current. This and its extension as the 'Peru current' further north are more shallow than western boundary currents, and the thermocline is nearer the surface. The current turns westward at low latitudes, flowing near the equator towards northern Australia, becoming warmer all the time. A substantial part of the equatorial flow continues westward between the southern islands of Indonesia (the Indonesian Throughflow), especially just after mid-year, and some of that forms the Leeuwin Current about 30 km wide down the west coast of Australia in autumn and winter. (This current is unusual in flowing towards the pole on the eastern side of an ocean, opposing the much larger anti-clockwise South Indian gyre.) Part o°

Antarctic Convergence Zone

Figure 11.15 Surface currents in the world's southern oceans. The names of the currents indicated by numbers on the map are as follows: 1. South equatorial current. 2. Equatorial counter-current. 3. East Australian current. 4. Indonesian throughflow. 5. Leeuwin current. 6. South Indian ocean current. 7. West wind drift. 8. Humboldt current. 9. Peru current. 10. Falkland current. 11. Brazil current. 12. Agulhas current. 13. Benguela current. 14. Antarctic circumpolar current. 15. Antarctic counter-current. The circumpolar line labelled A denotes the Subtropical Convergence Zone, and that labelled B is the Antarctic Convergence Zone.

Figure 11.15 Surface currents in the world's southern oceans. The names of the currents indicated by numbers on the map are as follows: 1. South equatorial current. 2. Equatorial counter-current. 3. East Australian current. 4. Indonesian throughflow. 5. Leeuwin current. 6. South Indian ocean current. 7. West wind drift. 8. Humboldt current. 9. Peru current. 10. Falkland current. 11. Brazil current. 12. Agulhas current. 13. Benguela current. 14. Antarctic circumpolar current. 15. Antarctic counter-current. The circumpolar line labelled A denotes the Subtropical Convergence Zone, and that labelled B is the Antarctic Convergence Zone.

of the low-latitude current across the Pacific that does not provide the Indonesian Throughflow turns south down the eastern side of Australia.

A fraction of the Circumpolar Current passes south of Cape Horn at the tip of South America and then northwards as the Falkland Current up the Atlantic coast. This cold water from the direction of the Pole stabilises onshore easterly winds, which, in conjunction with shelter from the westerlies provided by the Andes, leads to little rain and thus the aridity in Patagonia in southern Argentina (Figure 10.3). Also the Falkland Current carries icebergs to lower latitudes (Figure 11.7).

The gyre in the south Atlantic is complicated by the bulge of north-eastern Brazil, which planes off some of the current, diverting it into the north Atlantic. This promotes the Gulf Stream, which is responsible for the relative warmth of Europe.

The pattern of currents alters during the year.

Sch Circulation Sanguine

sea-surface height

Figure 11.16 Contours of the sea surface (i.e. the direction of surface-slope currents) off New South Wales measured during a 24-day voyage in summer 1964. The arrows indicate the direction of surface-slope currents. A typical eddy of warm water (with anticyclonic flow) can be seen off Jervis Bay, where the surface is more than 0.6m higher than the cold pool off Sydney.

sea-surface height

Figure 11.16 Contours of the sea surface (i.e. the direction of surface-slope currents) off New South Wales measured during a 24-day voyage in summer 1964. The arrows indicate the direction of surface-slope currents. A typical eddy of warm water (with anticyclonic flow) can be seen off Jervis Bay, where the surface is more than 0.6m higher than the cold pool off Sydney.

For instance, the Peru current reaches almost to the equator in winter, while the occasional El Niño in summer deflects it westwards at about 15°S instead (Note 11.C). Also, westerly winds along the coast of New South Wales in winter (Chapter 12) cause a narrow ocean current northward, against the prevailing East Australian current. Similarly, strong westerly winds in winter enhance the Falkland current, bringing cool waters as far north as Buenos Aires. In addition, ocean currents near the equator are influenced by the annual reversal of low-latitude winds called the 'monsoons' (Chapter 12).

Effects

The oceanic gyres explain why east coasts (where the gyres come from the equator) are usually warm and wet (Figure 10.3b), while west coasts are cool and dry because of (i) the advection of coldness from the poles, and (ii) upwelling (Section 11.2). For instance, places in subtropical latitudes (i.e. 20-35°S) along the east coast of South Africa are 3-8 K warmer than those on the west coast. An exception is Australia's west coast, where the southwards Leeuwin current along the coast (Figure 11.15) brings warmth towards Perth and suppresses any upwelling. However, there is no exception to the rule that continental east coasts in the subtropics are humid, and west coasts arid (Figure 10.3, Table 11.2). This rule is due mainly to the predominant easterly winds around the Tropic (Chapter 12).

Gyres transport heat polewards (Figure 5.4), in amounts comparable to those in warm winds, though oceanic advection is less notable in the southern hemisphere than in the northern, which contains the Gulf Stream in the Atlantic and the Kuroshio current past Japan.

Explanation

All these surface currents are governed by four factors: (i) wind drag, (ii) the slope of the ocean surface, (iii) differences of water density, and then (iv) the Coriolis force. The 'absolute current' (with respect to the land) is the outcome of all these factors together, which we will consider in turn.

Ocean currents dragged by the prevailing winds (Chapter 12) are called 'drifts', and some are named in Figure 11.15. These currents are usually less than 50 m deep, or 100 m if winds are strong. They are deflected by the Coriolis force, though this is often weakened by an opposing slope of the ocean surface, discussed below. A drift typically moves at less than 1 km/h, carrying buoys and icebergs at about 2 per cent of the speed of the local wind.

Currents induced by tilting of the ocean surface are called 'slope currents'. The slope is imperceptible, e.g. a metre in 500 km (Figure 11.16), but it is important. Ways in which it arises include these: (i) tides, (ii) differences of mean sea-level pressure, (iii) Ekman transport, (iv) the convergence of currents, (v) drifts piling against a coast, and (vi) ocean-density differences. The first of these, tides, cause changes of level twice daily, by up to 8 m in the inlets of north-west Australia, for instance, but only a metre around most of the continent and less than a metre in the open sea. Such temporary variations of level do not affect large-scale currents. The second factor, surface pressure, explains why the sea is elevated in the centre of tropical cyclones (Chapter 13).

The third cause of ocean-surface slope, Ekman transport, occurs near coasts with parallel winds (Section 11.4) and within a gyre. The winds which drive a gyre also lead to flows towards the centre, heaping the water there (Figure 11.17). The heap consists of surface water and it is relatively warm, i.e. less dense, so that a greater depth of it is needed to create a sea-bed pressure equal to (or very slighly higher than) that created by the cooler water around. Therefore the gyre's centre is elevated, perhaps by a metre or so. The resulting slope makes surface water flow downhill away from the centre, and then this flow is deflected by the Coriolis effect to become an anti-clockwise turning in the southern hemisphere. Such a flow is called a 'geostrophic current', as with 'geostrophic winds' (Chapter 12). In both cases, the direction and speed of the flow results in a Coriolis force which just offsets the slope, and the flow is parallel to contours of the surface's elevation, as in the case of much smaller eddies (Figure 11.16). For example, the warm Coral Sea (east of Queensland) is around 0.5 m higher than the Tasman Sea to the south, causing a geostrophic flow eastwards, away from the Australian coast. Geostrophic currents occur at any depth, wherever horizontally adjacent masses of water differ in density. The only exception occurs near the equator, where the Coriolis effect is too weak (Note 11.D).

A convergence of currents (the fourth factor affecting the surface slope) also causes heaping and then subsidence. It happens, for instance, at the Antarctic Convergence (Figure 11.18), where the Antarctic Circumpolar Current (which is towards the south-east) flows next and opposite to the northwestward current around

Ocean Gyre Ekman

Figure 11.17 Vertical circulations induced in a south-hemisphere ocean gyre. The low-latitude easterly winds and midlatitude westerlies, and the associated Ekman transport cause a heaping of water at A, which maintains the anticlockwise slope currents. The heaping at A leads to a pressure at B higher than that at C, so there is a flow from B to C and subsidence from A to B. Upwelling from C to D completes the circuit. The entire circulation may take several hundreds of years.

Figure 11.17 Vertical circulations induced in a south-hemisphere ocean gyre. The low-latitude easterly winds and midlatitude westerlies, and the associated Ekman transport cause a heaping of water at A, which maintains the anticlockwise slope currents. The heaping at A leads to a pressure at B higher than that at C, so there is a flow from B to C and subsidence from A to B. Upwelling from C to D completes the circuit. The entire circulation may take several hundreds of years.

Antarctic convergence

Antarctic convergence

Antarctica Coriolis Effect

Figure 11.18 Layers and conditions within the Antarctic ocean and the longitude of the Atlantic; SASW is subAntarctic surface water, AIW is Antarctic intermediate water, UDW is upper deep water.

Figure 11.18 Layers and conditions within the Antarctic ocean and the longitude of the Atlantic; SASW is subAntarctic surface water, AIW is Antarctic intermediate water, UDW is upper deep water.

the Antarctic coast. The latitudes of such convergences fluctuate considerably. Sea-surface temperatures on opposite sides of the boundary may differ in temperature by about 4 K, as cold water from the south meets warmer water from lower latitudes.

There is a compensating divergence of surface waters between regions of convergence. The divergence between the Antarctic and the Subtropical Convergence Zones (Figure 11.15) leads to upwelling there and therefore nutrient-rich surface waters, which attract whales and other marine life.

The barriers between different bodies of water created by continents, by convergence zones, by the self-contained nature of gyres, and by the stratification of the oceans (Section 11.2) all inhibit the mixing together of seas with different temperatures and degrees of saltiness. Consequently there are distinct bodies of water (or water masses) in various parts of the oceans, each with its own characteristic temperature and salinity. The boundary between adjacent water masses is called a 'front', and may be discernible for weeks.

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Responses

  • bisrat
    What is the ocean current off of buenos aires?
    8 years ago
  • lewis anderson
    Do ocean currents off the coast of Peru affect the the weather in australia?
    6 months ago

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