Energy budgets

Circulation changes

Tile immediate cause of the recent climatic fluctua­tions1 appears to be the strength of the global wind circulation. The first thirty years of this century saw a pronounced increase in the vigour of rhe westerli es 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 41^3°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.

Письменно переведите текст. (Контрольное время – 35 минут)

The key to these atmospheric variations must be linked to the heat balance of the earth-atmosphere system and this forces us to return to the funda­mental energy considerations with which we began this book. The evidence for fluctuations greater than 0.1 per cent in the 'solar constant' is inconclusive, although significant variations apparently do occur in the emission of high-energy particles and ultra­violet radiation during brief solar flares. All solar activity follows the well-known cycle of approxi­mately eleven years, which is usually measured with reference to the period between sunspot maxi­mum and minimum, but numerous attempts to establish secure correlations between sunspot activity and terrestrial climates have produced mostly negative results. Nevertheless, a statistical relationship has been found between the occurrenceof drought in the western United States over the list 300 years and the approximately 22-year double(Hale) cycle of the reversal of the solar magnetic polarity. Drought areas are most extensive in the two to five years following a Halespot minimum (i.e. alternate eleven-year sunspot minima).

Changes in atmosphericcomposition may also have modifiedthe atmospheric heat budget. The presence of increased amounts of volcanicdust and sulphate aerosols in the stratosphere is one suggested e cause of the 'Little IceAge'. Major eruptions can result in a surface cooling of perhaps 0.2°C for a years after the event. Hence, frequent volcanic activity would be required for persistently cooler conditions. Conversely, it is suggested thatreduced volcanic activity after 1914 may have contributed in part to the early twentieth-century warming. New interest in this question has been aroused by erup­tions of El Chichon (March 1982) and Mount Pinatabo (June 1991). It has been estimated that huge volcanic eruptions such as these, can, during a given decade, produce a forcing effect onglobal temperature about one-third as great as that exerted by greenhouse gases - but in the oppo­site direction(i.e. to produce surface cooling). The role of low-level aerosols is also complex. These originate naturally, from wind-blown soil and silt for example, as well as from atmospheric pollution due to human activities (industry, domestic heating and modern transportation).

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