Monsoonal Winds

A feature of winds near the equator is their annual reversal. This was pointed out by George Hadley in 1735, who explained the reversal as due to the movements of the ITCZ. For instance, the south-east Trades from the southern hemisphere cross the equator when the ITCZ is in the north in July, and thereafter the Coriolis effect influences them to the right (Section 11.4). So southeasterly winds from the south of the equator become south-westerlies. And conversely for north winds in January, again producing a narrow band of westerlies near the equator within the limits of the seasonal fluctuations of the ITCZ. The resultant winds can be seen in Figure 12.6. Air movements across northern Australia, for example, are from the south-east in July (i.e. from the arid inland), but from the warm oceans to the north-west in January. So they are alternately dry and wet winds. Similarly, the Trades in Papua and New Guinea prevail from the south-east in May to October, and then there are winds from the north-west during December to April.

This seasonal switching of direction we call monsoonal. (The term 'monsoon' comes from a word meaning 'season'.) Nowadays, it signifies either a wind or the rainy season of the summer monsoon. To qualify as a monsoon wind, the seasonal change of direction has to be at least 120 degrees. South of the equator they are found only on the east coast of Africa, down to northern Madagascar, and over south-east Asia, including

Figure 12.5 Annual and zonal average winds at various latitudes.

Dec - March


I ^^

—surface wind \ —> 5.6 km

S/s n ^

V ^

Figure 12.6 Upper and surface winds near the monsoonal equator.

Figure 12.6 Upper and surface winds near the monsoonal equator.

than the ocean at year's end, generating an offshore land breeze at the surface (Figure 12.6 and Chapter 14). Another factor affecting the monsoons is the Plateau's obstruction of the strong winds in the upper atmosphere, so that they flow round, either to the north or the south, with a consequent deflection of surface winds.

The monsoons are ill-defined or non-existent in the Americas because north-south mountain ranges obstruct the Trade winds, and because sea-surface temperatures to the west of South America stay permanently lower than land temperatures. Monsoons do affect northern Australia, but more modestly and briefly than in India, for instance, for lack of mountains or of a land mass as big as Asia to generate the equivalent of a vast sea breeze. Also, there is some removal of moisture by prior rainfall over Indonesia. Nevertheless, north-west winds from the equatorial Indian ocean bring heavy rains to Australia north of 25°S, during the Wet at year's end. The Wet is sometimes interrupted by a dry spell lasting a few days or weeks, when surface winds are mostly easterly and the upper-level winds more westerly. But particularly wet spells can arise from surges of cold air into south-east Asia from Siberia, where it is winter.

Papua New Guinea and the northern coast of Australia. They are induced by Asia, the Earth's largest continent, which drives strong Trades south across the equator in the northern hemisphere winter and pulls in the southern Trades during its summer.

The explanation of monsoons given above is incomplete, as the Coriolis force is only slight near the equator—especially with the light winds that prevail there. At least two more factors are involved in the case of the main (Indian) monsoon. One is the reversal of temperature difference between the Indian ocean and Asia. The continent becomes much hotter than the ocean in mid-year, drawing air inland as a great sea breeze (Chapter 14). Conversely, the Tibetan Plateau at about 4,500 m becomes much colder

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