Deserts and Desert Environments

Transcription

1 Deserts and Desert Environments

2 Introduction Aeolian geomorphology is the study of the effect of wind on Earth surface processes and landforms Wind: the movement of air from one place to another, specifically from an area of high pressure to an area of low pressure in an attempt to balance out the pressure differences Though this occurs everywhere, our focus will be on desert landscapes

3 What is a Desert? Desert: Regions of very low annual rainfall (less than 300mm), meagre vegetation, extensive areas of bare and rocky mountains and plateaux, and alluvial plains (Huggett, 2007) Covers approx. 1/3 of Earth's land surface Aridity forms the basis for the classifications of deserts (Huggett, 2007) Driest place in the world- Dry Valleys (Antarctica) Temperature not only indicator Some deserts are hot and some are cold Highest recorded temperature 58 C (Libyan desert) Lowest recorded temperature C (Antarctica)

4 What is a Desert? Aridity index (Hugget, 2007): Variables: AI = Aridity Index PE = Potential evapotranspiration P = Average annual precipitation Four types of desert Hyperarid: AI < 0.05 Arid: 0.05 < AI < 0.2 Semi-arid: 0.2 < AI < 0.5 Dry Subhumid: 0.5 < AI < 0.65 P AI = PE

5 What the Aridity Index Tells Us Evapotranspiration Transpiration (from vegetation) + Evaporation Precipitation (from clouds) (from surface) Surface The aridity index is therefore an expression of the relationship between the amount of water reaching the surface (from precipitation) vs. the amount that gets lost back to the atmosphere (evapotranspiration). If they are equal, the aridity index would be 1

6 What is a Desert? Lush vegetation Abundant water High rainfall Desert? Sparse vegetation Scarce water Low rainfall Desert? Absence of vegetation Water in solid state, therefore unavailable Very low rainfall Desert?

7 (Some Global Deserts Indicated) Sahara Syrian Arabian Iranian Karakum Gobi Taklimakan 8. Namib 9. Kalahari 10. Atacama 11. Great Basin 12. Chichuahnan 13. Simpson 14. Great Victoria 15. Antarctica

8 Distribution of Deserts Low-Latitude Deserts In the vicinity of the tropics of Cancer and Capricorn Result due to the global distribution of air pressure and winds Heated air at equator (equatorial low) rises and spreads out sinking between 20 and 30 lat (north and south) known as the sub-tropical highs Air that sinks is compressed and warmed Examples Namib desert, Sahara desert, Iranian desert

9 Distribution of Deserts Mid-Latitude Deserts Not controlled by the subsiding air masses associated with high pressure Exist because the are sheltered in the interiors of landmasses Far removed from ocean (source of moisture) Presence of mountains (rain shadow effect) Examples Gobi desert, Great Basin, Patagonian desert

10 Southern African Deserts (Adapted from Laity, 2008)

11 Evolution of Deserts (Laity, 2008) Owe their origin to climatic, topographic, and oceanic factors that prevent the incursion of moisture bearing weather systems Most deserts are arid due to a combination of these factors Global considerations Sub-tropical high pressure belts Continental interiors Polar deserts Regional considerations Cold-current influences Rain shadow effect Edaphic considerations

12 Evolution of Deserts (Laity, 2008) Global considerations Sub-tropical high pressure belts World's arid and semi-arid regions mainly sub-tropical in distribution Influenced by descending air associated with Hadley cells (sub-tropical high pressure belt) Aridity not present in all longitudes due to presence of anticyclone cells (influenced by the coriolis force) associated with descending air masses Example: Sahara (Northern Africa)

13 Sub-tropical High Pressure Belts ❶ Warm moist air rises from the equatorial region, cooling as it rises. ❷ It then spreads out at the upper troposphere where it moves towards the polar regions. 30 N 0 30 S ❸ ❶ ❸ ❷ ❸ As the air moves to higher latitudes it becomes cooler and more dense causing it to descend. Theses are masses descend in the sub-tropics, causing high pressure systems and ultimately contributing to arid conditions. The descending sub-tropical high pressure belt is broken up into anticyclonic cells (local high pressure cells) which results in the subsidence being discontinuous and consequently means that the aridity will not be present in all longitudes

14 Evolution of Deserts (Laity, 2008) Global Considerations (cont) Continental Interiors Arid due to distance from sea and sources of moisture Greater range in annual temperature than coastal deserts Example: Karakum Desert (Turkmenistan) Polar deserts Low levels of solar radiation at poles result in very cold temperatures Therefore the atmosphere contains little water for precipitation Example: Dry Valleys (Antarctica)

15 Karakum Desert

16 Dry Valleys

17 Evolution of Deserts (Laity, 2008) Regional Considerations Cold current influences Cool coastal deserts form adjacent to cold currents on the western margin of continents Often long and narrow and may be bounded in the east by north-south mountain ranges Climate moderated by the cold waters (rainlessness, fog and dew, cold temperatures) Examples: Atacama (Chile), Namib (Namibia)

18 Namib Cold Benguela Current Air masses derived from cold water currents are generally lower in moisture. There is therefore little moisture that can lead to the formation of precipitation.

19 (Some Global Cold Currents Indicated) ❹ ❸ ❶ ❷ ❻ ❻ ❶ Benguela Current ❷ Peru (Humbolt) Current ❸ California Current ❺ ❻ ❹ Canaries Current ❺ West Australia Current ❻ West Wind Drift

20 Evolution of Deserts (Laity, 2008) Regional Considerations (cont) Rainshadow When air crosses mountain barriers it rises on the windward side and descends on the leeward side Subsidence prevents convection and produces adiabatic heating which has a drying effect In addition as the air rises on the windward side it cools and looses a lot of moisture in the form of precipitation Examples: Patagonian Desert (Patagonia), Great Basin (USA)

21 The Rainshadow Effect ❷ As this moist air encounters an obstacle (e.g. mountain) it is forced upward. As the air rises it cools. ❸ Cool air cannot hold as much moisture as warm air which leads to cloud formation along with precipitation. ❶Moist air from the ocean (water body) moves towards the interior ❸ ❹ At this point the air has lost the overall majority of its moisture. The dry air then descends on the leeward side of the obstacle, heating as it does so, leading to dry conditions prevailing. ❹ ❶ ❷

22 Evolution of Deserts (Laity, 2008) Edaphic factors Result in large measure due to the influence of soil Sandy soils and high evapotranspiration makes surface waters scarce in areas with relatively high rainfall Example: Kalahari Desert