Лекция: Circulation changes
Tile immediate cause of the recent climatic fluctuations appears to be the strength of the global wind circulation. The first thirty years of last century saw a pronounced increase in the vigour of the westerlies over the North Atlantic, the north-east trades, the summer monsoon of South Asia and the southern hemisphere westerlies (in summer). Over the North Atlantic, these changes consisted of an increased pressure gradient between the Azores high and the Icelandic low, as the latter deepened, and also between the Icelandic low and the Siberian high, which spread westwards. These changes were accompanied by more northerly depression tracks, and this resulted in a significant increase in the frequency of mild south-westerly airflow over the British Isles between about 1900 and 1930, as reflected by the average annual frequency of Lamb's westerly airflow type. For 1873-97, 1898-1937, 1938-61 and 1962-95 the figures are 27, 38, 30 and 21 per cent, respectively. Coinciding with the westerly decline, cyclonic and anticyclonic types increased substantially. The decrease in westerly airflow during the last thirty-year interval, especially in winter, is linked with greater continentality in Europe. These regional indicators reflect a general decline in the overall strength of the mid-latitude circumpolar westerlies, accompanying an apparent expansion of the polar vortex.
Global climate is closely related to the position and strength of the subtropical high-pressure cells. It has been estimated that a warming of the Arctic tropopause (winter +10°C; summer +3°C; annual +7°C), without changing equatorial or Antarctic temperatures, would cause an annual shift of the subtropical high-pressure belt from its present average position of 37°N to 41- 43°N (i.e. some 100-200 km in summer but as much as 800 km in winter). This would bring drought to the Mediterranean, California, the Middle East, Turkestan and the Punjab, as well as displacing the thermal equator from 6°N to 9-10°N, increasing the desertification in the belt 0-20°.
Упражнение 9.
Ответьте на следующие вопросы, исходя из информации
в тексте:
1. Что является непосредственной причиной современных колебаний климата?
2. Как изменились траектории циклонов?
3. Какие черты приобрел климат в Европе?
4. Какие изменения произошли в циркумполярном западном переносе средних широт и полярном вихре?
Упражнение 10.
Выпишите 10 ключевых слов из текста в упражнении 8.
Упражнение 11.
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