Page 212 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

176 part I The energy–atmosphere System
location and size of the Asian landmass and its proxim­ ity to the seasonally shifting ITCZ over the Indian Ocean drive the monsoons of southern and eastern Asia.
The difference in the heating characteristics of land and water surfaces creates land and sea breezes. Tempera­ ture differences during the day and evening between val­ leys and mountain summits cause mountain and valley breezes. Katabatic winds, or gravity drainage winds, are of larger regional scale and are usually stronger than valley and mountain breezes, under certain conditions. An elevated plateau or highland is essential, where lay­ ers of air at the surface cool, become denser, and flow downslope.
monsoon (p. 160)
land and sea breezes (p. 162) mountain and valley breezes (p. 162) katabatic wind (p. 162)
18. Describe the seasonal pressure patterns that produce the Asian monsoonal wind and precipitation pat­ terns. Contrast January and July conditions.
19. People living along coastlines generally experience variations in winds from day to night. Explain the factors that produce these changing wind patterns.
20. The arrangement of mountains and nearby valleys produces local wind patterns. Explain the day and night winds that might develop.
21. This chapter presents wind­power technology as well developed and cost effective. Given the infor­ mation presented and your additional critical think­ ing work, what conclusions have you reached?
■ Sketch the basic pattern of Earth’s major surface ocean currents and deep thermohaline circulation.
Ocean currents are primarily caused by the frictional drag of wind and occur worldwide at varying intensi­ ties, temperatures, and speeds, both along the surface and at great depths in the oceanic basins. The circula­ tion around subtropical high­pressure cells in both hemi­ spheres is discernible on the ocean­circulation map— these gyres are usually offset toward the western side of each ocean basin.
The trade winds converge along the ITCZ and push enormous quantities of water that pile up along the east­ ern shore of continents in a process known as the western intensification. Where surface water is swept away from a coast, either by surface divergence (induced by the Coriolis force) or by offshore winds, an upwelling current occurs. This cool water generally is rich with nutrients and rises from great depths to replace the vacating water. In other oceanic regions where water accumulates, the excess water gravitates downward in a downwelling current. These cur­ rents generate vertical mixing of heat energy and salinity.
Differences in temperatures and salinity produce den­ sity differences important to the flow of deep, sometimes vertical, currents; this is Earth’s thermohaline circula- tion. Travelling at slower speeds than wind­driven sur­ face currents, the thermohaline circulation hauls larger volumes of water. Scientists are concerned that increased surface temperatures in the ocean and atmosphere, cou­ pled with climate­related changes in salinity, can alter the rate of thermohaline circulation in the oceans.
western intensification (p. 164) upwelling current (p. 164) downwelling current (p. 165) thermohaline circulation (p. 165)
22. Define the western intensification. How is it related to the Gulf Stream and the Kuroshio Current?
23. Where on Earth are upwelling currents experienced? What is the nature of these currents? Where are the four areas of downwelling that feed these dense bot­ tom currents?
24. What is meant by deep­ocean thermohaline circu­ lation? At what rates do these currents flow? How might this circulation be related to the Gulf Stream in the western Atlantic Ocean?
25. Relative to Question 24, what effects might climate change have on these deep currents?
■ Summarize several multiyear oscillations of air tem- perature, air pressure, and circulation associated with the Arctic, Atlantic, and Pacific oceans.
Several system fluctuations that occur in multiyear or shorter periods are important in the global circulation picture. The most well known of these is the El Niño– Southern Oscillation (ENSO) in the Pacific Ocean, which affects interannual variability in climate on a global scale.
The Pacific Decadal Oscillation (PDO) is a pattern in which sea­surface temperatures and related air pressure vary back and forth between two regions of the Pacific Ocean: (1) the northern and tropical western Pacific and (2) the eastern tropical Pacific, along the U.S. West Coast. The PDO switches between positive and negative phases in 20­ to 30­year cycles.
A north–south fluctuation of atmospheric variabil­ ity marks the North Atlantic Oscillation (NAO), in which pressure differences between the Icelandic Low and the Azores High in the Atlantic alternate between weaker and stronger pressure gradients. The Arctic Oscillation (AO) is the variable fluctuation between middle­ and high­latitude air mass conditions over the Northern Hemisphere. The AO is associated with the NAO, espe­ cially in winter. During the winter of 2009–2010, the AO was at its most strongly negative phase since 1970.
El Niño–Southern Oscillation (ENSO) (p. 168)
26. Describe the changes in sea­surface temperatures and atmospheric pressure that occur during El Niño and La Niña, the warm and cool phases of the ENSO. What are some of the climatic effects that occur worldwide?
27. What is the relationship between the PDO and the strength of El Niño events? Between PDO phases and climate in the western United States?
28. What phases are identified for the NAO and AO? What winter weather conditions generally affect the eastern United States during each phase? What hap­ pened during the 2009–2010 winter season in the Northern Hemisphere?
Answer for Critical Thinking 6.2: The dry season in northern Australia is from about May through October, during the Southern Hemisphere winter. The southeast