Chapter 4: Variations in the Physical Environment

2. Chapter 4: Variations in the Physical Environment • Robert E. Ricklefs • The Economy of Nature, Fifth Edition

3. Earth as a Solar-powered Machine • Earth’s surface and adjacent atmosphere are a giant heat-transforming machine: • solar energy is absorbed differentially over planet • this energy is redistributed by winds and ocean currents, and is ultimately returned to space • there are interrelated consequences: • latitudinal variation in temperature and precipitation • general patterns of circulation of winds and oceans

4. Global Patterns in Temperature and Precipitation • From the equator poleward, we encounter dual global trends of: • decreasing temperature • decreasing precipitation • Why? At higher latitudes: • solar beam is spread over a greater area • solar beam travels a longer path through the atmosphere

5. Warming effect of the sun – greatest at equator • Figure 4.1

6. Temporal Variation in Climate with Latitude • Temporal patterns are predictable (diurnal, lunar, and seasonal cycles). • Earth’s rotational axis is tilted 23.5o relative to its path around the sun, leading to: • seasonal variation in latitude of most intense solar heating of earth’s surface • general increase in seasonal variation from equator poleward, especially in N hemisphere

7. Orientation of earth’s axis relative to the sun causes seasonal climate variation Daylight lasts 24 hours within Antarctic circle Night lasts 24 hours within Antarctic circle

8. Annual range of mean monthly temperatures greatest at high latitude in N Hemisphere; moderating influence of water is less

9. Prevailing Wind Belts of Earth The earth is encircled by several broad prevailing wind belts, which are separated by narrower regions of either subsidence or ascent. The direction and location of these wind belts are determined by solar radiation and the rotation of the earth. The three primary circulation cells are known as the: Hadley cell; Ferrel cell; and Polar cell.

10. Hadley cells.. • On or near the equator, where average solar radiation is greatest, air is warmed at the surface and rises. • This creates a band of low air pressure, centered on the equator known as the intertropical convergence zone (ITCZ).

12. The Intertropical Convergence • Surface currents of air in tropical Hadley cells converge near the equator. • Warm, moist air rising in equatorial regions cools and loses much of its moisture content as precipitation there. • As cool, dry air descends and warms near 30o N and S latitude, its capacity to hold moisture increases, resulting in prevalence of arid climates at these latitudes. • Regions north and south of equator – subtropical high pressure belts • The Intertropical Convergence Zone draws in surface air from the subtropics. When this subtropical air reaches the equator, it rises into the upper atmosphere because of convergence and convection. It attains a maximum vertical altitude of about 14 kilometers and then begins flowing horizontally to the North and South Poles. This rising air comprises one segment of a circulation pattern called the Hadley Cell

13. Intertropical Convergence Zone

14. Hadley Cells • Hadley cells constitute the principal patterns of atmospheric circulation. • IT is the circulation pattern from this rising air comprises one segment of a circulation pattern • warm, moist air rising in the tropics spreads to the north and south • as this air cools, it eventually sinks at about 30o N or S latitude, then returns to tropics at surface • this pattern drives secondary temperate cells (30-60o N and S of equator), which, in turn, drive polar cells (60-90o N and S of equator)

15. Differential warming of earth’s surface creates Hadley cells

17. Distribution of earth’s deserts and wet areas

18. Surface Winds • Surface flow of air in Hadley cells is deflected by earth’s rotation • to the right in N hemisphere • to the left in S hemisphere • Net effect of deflections on surface flows: • air flows toward the west in tropical cells • air flows toward the east in temperate cells • air flows again toward the west in polar cells

19. Global wind patterns

21. Rain Shadows • Moisture content of air masses is recharged when they flow over bodies of water: • rain falls more plentifully in S hemisphere (81% of surface there is water, versus 61% in N hemisphere) • Air masses forced over mountains cool and lose moisture as precipitation. • Air on lee side of mountains is warmer and drier (causing rain shadow effect).

22. Mountain ranges and precipitation patterns

23. Proximity to bodies of water determines regional climate. • Areas downwind of large mountain ranges are typically more arid (rain shadow effect). • Continental interiors are also typically arid: • distant from source of moisture recharge • air masses reaching these areas are likely to have previously lost moisture • Coastal areas have less variable maritime climates than continental interiors.

24. Ocean currents…

25. Western coasts have cold currents. • Oceanic water circulation: • cold polar water forced equatorward from the poles along west coasts of major continents • this water acts as a barrier to warm, moist air • net result is coastal deserts, especially on west coasts of South America and Africa • Equatorward flows are deflected to W in both hemispheres, causing upwelling of cold, nutrient-laden water in these regions.

26. Wind…

27. + rotation of the Earth

29. Major ocean currents: wind + earth’s rotation

31. Welcome back

32. A few administration points • Reminder: • Exam 1 – April 8 • Make-up sessions • I will be absent on March 25 and March 27 • Make-up scheduled for Thursdays • March 19. 12.30-2.00 Khoury 132 • April 2. 12.30 – 2.00 Khoury132 • So: since we need the make-up sessions before the exam, and since you have conflicts, then make-up session will be: March 14 (Saturday), Khoury 132, 9 am to Noon (with break)

33. Three more points • Quiz 1 (chapters 1 to 4) on Friday March 13. • Evolution (no need to nag anymore) before MCAT • Syllabus to be revised. Stay tuned for new syllabus on Friday.

34. Last point • No more mobile phone usage in class • No writing text messages • No reading text messages • No taking calls • No making class. • Put your phone away.

37. Review: Ocean currents redistribute heat and moisture. • Ocean surface currents propelled by winds. • Deeper currents established by gradients of temperature and salinity. • Ocean currents constrained by basin configuration, resulting in: • clockwise circulation in N hemisphere • counterclockwise circulation in S hemisphere • Warm tropical waters carry heat poleward.

38. Seasonal Variation in Climate • Seasonal progression of sun’s zenith causes familiar patterns of temperature. • Intertropical convergence also migrates seasonally: • region of high precipitation shifts N or S with intertropical convergence • regions of arid conditions (30o N and S of intertropical convergence) shift accordingly

39. Seasonality of Rainfall in Tropics

40. Similar Patterns Outside Tropics • At 30oN in Chihuahuan Desert: • at northward limit of intertropical convergence, summer rainfall, winter drought

41. Similar Patterns Outside Tropics • At 32oN in Sonoran Desert • Winter and summer peak of rainfall

42. Similar Patterns Outside Tropics • At 35oN in San Diego • West of summer rainfall belt • Winter-rainfall, summer-drought climate • Mediterranean climate • (similar climates also found in western South Africa, Chile, and Western Australia)

43. Seasonal Cycles in Temperate Lakes • The four seasons of a small temperate lake - each season has its own characteristic temperature profile: • winter: coldest water (0oC) at surface, just beneath ice layer, increasing to 4oC near bottom • spring: ice melts; as surface warms, denser water sinks, resulting in uniform 4oC profile, with little resistance to wind-driven spring overturn

44. Seasonal Cycles in Temperate Lakes • summer: continued warming of surface results in thermal stratification, a stable situation and resistant to overturn; strata established: • epilimnion - warm, less dense surface water • thermocline - zone of rapid temperature change • hypolimnion - cool, denser bottom water (may become oxygen-depleted) • fall: water cooling at surface sinks, destroying stratification, once again permitting wind-driven fall overturn

46. Climate Sustains Irregular Fluctuations • El Niño is an annual event which can assume extreme proportions, with implications for worldwide climate. • Background: • annual El Niño events involve a warm oceanic countercurrent flowing southward toward Peru • reversal of high/low pressure areas in central Pacific Ocean (Southern Oscillation) accentuate this effect leading to El Niño “event” (ENSO)

47. El Niño brings severe weather. • Severe El Niño events occur irregularly, about once every 10-12 years. • Consequences of severe El Niños: • drought in tropical South America, Africa, and Australia • increased precipitation outside of tropics • disruption of fisheries and seabird populations

48. Changes that occur during ENSO events Low pressure over Tahiti and high pressure over Darwin, Australia  increased cloud formation  weak trade winds  warmer waters move east  subtropical jet stream carries moisture east  decreased development of Atlantic hurricanes

49. ENSO events correlated with sea surface temperatures in South American coastal waters Irregular intervals of 2 to 10 years; El Nino followed by La Nina (strong winds  increase normal oceanic and upwelling currents)

50. Heavy rainfall in many tropical regions, drought in north-temperate regions, increase in hurricane activity in North Atlantic Ocean High pressure over Tahiti and low pressure over Darwin, Australia  increased cloud formation  strong trade winds  warmer waters move west  subtropical jet stream separated  increased development of Atlantic hurricanes