Geography · Ch 15 — Atmospheric Circulation and Weather Systems
Seasonal Wind
Seasonal Wind
The general circulation of the atmosphere is not a fixed, permanent machine. It responds to the changing position of the Sun, which shifts the belts of maximum heating, pressure, and wind throughout the year. As these belts move, the wind circulation pattern gets modified. The most dramatic and well-known result of this seasonal shift is the monsoon, especially over southeast Asia, where the reversal of winds between summer and winter is most pronounced. The detailed mechanics of the monsoon are studied separately in the context of India's physical environment.
Apart from the grand monsoon system, the seasonal shift of pressure and wind belts also produces several other local deviations from the general circulation. These are smaller in scale but still important features of the wind system.
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Land and sea breezes: Along coastlines, the land heats up and cools down much faster than the adjacent sea. During the day, the land becomes warmer than the sea, creating a low-pressure area over the land. Air flows from the higher pressure over the sea towards the land, producing a cool sea breeze. At night, the land cools rapidly while the sea remains relatively warm, reversing the pressure gradient. Air now flows from the land towards the sea, producing a land breeze.
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Mountain and valley winds: In mountainous regions, the slopes and valleys experience differential heating. During the day, the mountain slopes facing the sun heat up quickly, warming the air above them. This warm air rises, and cooler air from the valley below moves up the slope to replace it, creating a valley breeze. At night, the slopes cool rapidly, and the dense, cold air drains down the slopes into the valley, creating a mountain breeze.
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Katabatic winds: These are cold winds that flow down a slope under the influence of gravity. They occur when a highland area, such as a plateau or a mountain range, is cooled by radiation at night or is covered with ice and snow. The dense, cold air then flows downhill, sometimes with considerable force. …
The pattern of wind circulation is modified in different seasons due to the shifting of regions of maximum heating, pressure and wind belts. The most pronounced effect of such a shift is noticed in the monsoons, especially over southeast Asia.
The general circulation of the atmosphere also affects the oceans -- the large-scale winds initiate large and slow moving currents of the ocean, and oceans in turn provide input of energy and water vapour into the air. Warming and cooling of the Pacific Ocean is most important in terms of general atmospheric circulation. The warm water of the central Pacific Ocean slowly drifts towards the South American coast and replaces the cool Peruvian current -- such appearance of warm water off the coast of Peru is known as El Nino. The El Nino event is closely associated with the pressure changes in the Central Pacific and Australia; this change in pressure condition over the Pacific is known as the southern oscillation. The combined phenomenon of southern oscillation and El Nino is known as ENSO. In years when ENSO is strong, large-scale variations in weather occur over the world -- the arid west coast of South Amer …