Earth Sciences / 61Ordinarily, the South Equatorial Current, driven by the trade winds, carries surface water
westward, away from South America and towards Indonesia. Beneath this fairly shallow layer
of surface water there is a boundary, the thermocline, below which the temperature drops sharply.
The westward movement of surface water is compensated by an easterly flow along the
thermocline, known as the Equatorial Undercurrent, or Cromwell Current. This system moves
warm surface water westward, so the surface layer, above the thermocline, is about 200 m deep
around Indonesia but very shallow off the American coast, where the thermocline almost reaches
the surface. The sea-surface temperature is between 27.5°C and 30°C; this is close to the
maximum temperature sea water can reach, because it is cooled by evaporation and the higher
its temperature the more water evaporates from it.
The change begins with the distribution of atmospheric pressure over the Indian and Pacific Oceans.
This causes the intertropical convergence zone (ITCZ) to move further south than it usually does in
the midwinter months of December to February. This is the ‘Southern Oscillation’ and it causes the
southern-hemisphere trade winds to weaken or even change direction. The wind driving the South
Equatorial Current weakens or reverses, and so warm surface water begins to accumulate off South
America, sometimes reinforced by water being driven by the wind from the west. Air moving across
the warm water becomes moist and loses its moisture when it reaches the coast. The resulting rains
brings abundant grazing for livestock along the arid coastal region, heralding an año de abundancia,
and because it usually commences around Christmas the phenomenon is known as El Niño, ‘the
(boy) child’. In some years the opposite happens. The ITCZ stays far to the north, the trade winds
strengthen, and the water off South America is colder than usual. This is known as ‘La Niña’ (HIDORE
AND OLIVER, 1993, pp. 169–178).
Although El Niño brings abundance to the farmers of the coastal strip, most other people suffer,
especially the fishermen. The Humboldt or Peru Current, flowing northward along the western coast
of South America, carries nutrients collected on its long journey from Antarctica, and off the coast of
Peru and Ecuador its rich, cold waters well up to the surface through the thin warm-water layer.
These upwellings sustain abundant populations of marine plants and animals and support an important
anchovy fishery. During an El Niño, however, the nutrient-rich water ceases to reach the surface and
the fishery fails.
ENSO events happen roughly every seven years. They are not new: the first was recorded in
1541 and more recently they happened in 1891, 1925, 1953, and 1972–83. There was a particularly
strong one in 1982–3, the effects of which were still being detected ten years later. Severe
ENSO events also occurred in 1986–87, 1995–96, and 1998–99 (www.elnino.noaa.gov/
lanina_new-faq.html). The change in the flow of equatorial water caused long-period waves
(Rossby waves) that crossed the North Pacific and shifted the flow of the Kuroshio Current
northwards, bringing warmer surface waters to the mid-latitude Pacific that were still being
detected in 1993 (JACOBS ET AL., 1994).
Ocean currents play an important part in transporting heat from the equator to high latitudes. The
currents themselves are mainly driven by the prevailing winds, but as water moves away from the
equator or towards it the motion is influenced by the Coriolis effect. This results in the generally
circular currents, or ‘gyres’, of the North and South Pacific, North and South Atlantic, and Indian
Oceans. Figure 2.21 shows the principal gyres and currents and their directions of flow.
With centres about 30° north and south of the equator, the gyres flow in a clockwise direction
in the northern hemisphere and an anticlockwise direction in the southern hemisphere. Generally,
the climatic effect is similar in both hemispheres. Water travelling from the poles towards
the equator passes close to the western coasts of the continents, cooling them, and