Лекция: 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)