El Nino

Upwelling

In contrast with the currents on the west sides of the oceans, equatorward-flowing eastern currents acquire cyclonic vorticity, which is in opposition to the anticyclonic wind tendency, leading to relatively broad flows of low velocity. In addition, the deflec­tion due to the Ekman effect causes the surface water to move westwards away from the coasts, leading to replacement by the upwelling of cold water from depths of 100-300 m. Average rates of upwelling are low (l-2m/day), being about the same as the offshore surface current velocities, with which they are balanced. The rate of upwelling therefore varies with the surface wind stress. As the latter is proportional to the square of the wind speed, small changes in wind velocity can lead to marked variations in rates of upwelling. Although the band of upwelling may be quite narrow (about 200 km wide for the Benguela Current), the Ekman effect spreads this cold water westwards. On the poleward margins of these cold-water coasts, the meridional swing of the wind belts imparts a strong seasonality to the upwelling, the California Current upwelling, being particularly well marked during the period March-July.

A major region of deep-water upwelling is along the west coast of South America, which is normally assisted by the offshore airflow associated with the large-scale convective Walker cell linking South-east Asia with the eastern South Pacific. Every 2-10 years or so this pressure difference is disturbed, producing an El Nino event with weakening trade winds and a pulse of warm surface water spreading eastwards over the South Pacific, raising average ocean-surface temperatures from about 24 to 30 °C. Coastal upwelting can also he caused by other less important mechanisms such as surface current divergence or the effect of the ocean bottom config­uration.

Упражнение 9.

Выберите из текста Upwelling 10–15 основных, с точки зрения смысловой нагрузки, слов (ключевые слова). Определите, к каким частям речи они относятся.

Упражнение 10.

Переведите текст письменно. (Контрольное время – 30 минут)

The ocean is a thin, spherical shell of fluid on a rotating sphere and, in addition to the waves seen on the ocean surface when the wind is blowing, there are other waves that have a large amplitude, not at the surface, but along the subsurface thermocline. Some are confined to the neighborhood of the equator where they travel exceptionally fast. Because of those waves, warm surface waters can be redistributed along the equator far more rap­idly than is possible at higher latitudes, which is why large-scale changes in sea surface temperature patterns, associated with a hori­zontal redistribution of upper ocean water, occur more readily in the tropics than in higher latitudes. Such an occurrence that leads to high sea surface temperatures in the eastern tropical Pacific is known as El Nino. The interval between El Nino episodes, three years approx­imately, depends on the time it takes the waves to propagate across the basin and hence depends on the width of the Pacific.

EL NINO affects everyone, either directly because of its influence on climate and weather, or indirectly because of its influence on the global economy. The impact of this phenomenon can be: devastating floods in Ecuador and Peru, where a warming of the surface waters of the eastern tropical Pacific – the signature of El Nino – is associated with the disappearance of the usually abundant fish; disastrous droughts in the "maritime" continent of southeastern Asia and northern Australia; unusual weather patterns over North and South America; poor mon­soons over India; and low rainfall over southeastern Africa.

Упражнение 11.

Составьте 5 общих вопросов к тексту El Nino.

Упражнение 12.

(Парная работа) Ответьте на вопросы, составленные в упражнении 11.

Дата добавления: 2014-12-27