Page 198 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

162 part I The energy–atmosphere System
reversed from the winter pattern. As a result, hot sub­ tropical air sweeps over the warm ocean toward India, producing extremely high evaporation rates.
By the time this air reaches India, the air is laden with moisture and clouds, which produce the monsoonal rains from about June to September (Figure 6.15b). These rains are welcome relief from the dust, heat, and parched land of Asia’s springtime. World­record rainfalls occur in this region: Cherrapunji, India, holds the record for both the second highest average annual rainfall (1143 cm) and the highest single­year rainfall (2647 cm) on Earth. In the Himalayas, the monsoon brings snowfall.
Human Influences on the Asian Monsoon In Chapter 4, we discussed the effects of increased aerosols from air pollution over the Indian Ocean, which appear to be causing a reduction in monsoon precipitation. Both air pollution and the warming of atmospheric and oceanic temperatures associated with climate change affect mon­ soon circulation. Research shows complicated interac­ tions between these factors. New studies indicate that warmer temperatures caused by rising greenhouse gas concentrations have increased monsoon precipitation in the Northern Hemisphere over the past few decades. However, other research suggests that rising concentra­ tions of aerosols—principally sulfur compounds and black carbon—cause an overall drop in monsoon precipi­ tation. Air pollution reduces surface heating and there­ fore decreases the pressure differences at the heart of monsoonal flows.
A further complicating factor is that these influences are occurring at a time of some unusually heavy precipi­ tation events, such as the monsoonal deluge on July 27, 2005, in Mumbai, India, that produced 94.2 cm of rain in only a few hours, causing widespread flooding. Again in August 2007, India experienced extensive flooding from an intense monsoon, and in 2010 Pakistan was devas­ tated by record­breaking monsoon rains.
Further study is critical to understand these complex relationships, as well as the role of natural oscillations in global circulation (such as El Niño, discussed later in the chapter) in affecting monsoonal flows. Considering
that 70% of the annual precipitation for the entire south Asian region comes during the wet monsoon, alterations in precipitation would have important impacts on water resources.
Local Winds
Local winds, which occur on a smaller scale than the global and regional patterns just discussed, belong to the tertiary category of atmospheric circulation. Land and sea breezes are local winds produced along most coast­ lines (Figure 6.16). The different heating characteristics of land and water surfaces create these breezes. Land gains heat energy and warms faster than the water offshore during the day. Because warm air is less dense, it rises, creating a lower­pressure area that triggers an onshore flow of cooler marine air to replace the rising warm air— the flow is usually strongest in the afternoon, forming a sea breeze. At night, land cools, by radiating heat energy, faster than offshore waters do. As a result, the cooler air over the land subsides (sinks) and flows offshore toward the lower­pressure area over the warmer water, where the air is lifted. This nighttime land­breeze pattern reverses the process that developed during the day.
Mountain and valley breezes are local winds result­ ing, respectively, when mountain air cools rapidly at night and when valley air gains heat energy rapidly dur­ ing the day (Figure 6.17). Valley slopes are heated sooner during the day than valley floors. As the slopes heat up and warm the air above, this warm, less­dense air rises and creates an area of low pressure. By the afternoon, winds blow out of the valley in an upslope direction along this slight pressure gradient, forming a valley breeze. At night, heat is lost from the slopes, and the cooler air then subsides downslope in a mountain breeze.
Katabatic winds, or gravity drainage winds, are of larger regional scale and are usually stronger than local winds, under certain conditions. An elevated plateau or highland is essential to their formation, where lay­ ers of air at the surface cool, become denser, and flow downslope. Such gravity winds are not specifically re­ lated to the pressure gradient. The ferocious winds that can blow off the ice sheets of Antarctica and Greenland are katabatic in nature.
Worldwide, various terrains produce distinct types of local winds that are known by local names. The mis- tral of the Rhône Valley in southern France is a cold north wind that can cause frost damage to vineyards as it moves over the region on its way to the Gulf of Lion and the Mediterranean Sea. The frequently stronger bora, driven by the cold air of winter high­pressure systems occurring inland over the Balkans and southeastern Europe, flows across the Adriatic Coast to the west and south. In Alaska, such winds are called the taku. In west­ ern Canada and the U.S. West, chinook winds are dry, warm downslope winds occurring on the leeward side of mountain ranges such as the Rockies in Alberta and Montana or the Cascades in Washington. These winds
CRITICALthinking 6.2 What Causes the North Australian Monsoon?
Using your knowledge about global pressure and wind pat- terns, and the maps provided in this chapter and on the back inside cover of the book, sketch a map of the seasonal changes that cause the monsoonal winds over northern australia. Begin by examining the pressure patterns and associated winds over this continent. How do they change throughout the year? Sketch the patterns for January and July on your map. Where is the position of the iTCZ? Finally, during which months do you expect a rainy season related to monsoonal activity to occur in this region? (Find the an- swers at the end of the chapter.) •