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Climate Varies around the Globe: How and Why? Yongqin David Chen (陳永勤) Department of Geography and Resource Management The Chinese University of Hong Kong Shatin, NT, Hong Kong Presentation Outline • Climate controls: latitude dominates, others also work • Classification of irregularly distributed climates • Similar latitudes, but very different climates: Somalia vs Malaysia, Shandong vs Xinjiang Climate controls (1) Latitude – fluctuations in the amount of solar radiation received at earth’s surface, represents the single greatest cause of temperature differences. (2) Land and water – marine and continental climates (3) Ocean currents – poleward-moving warm currents (Gulf Stream and Kuroshio Current in N Hemisphere and Brazilian and East Australian Currents in S Hemisphere, more pronounced impacts in winter) and equatorward-moving cold currents (Canaries and California Currents in N Hemisphere and Peruvian and Benguela Currents in S Hemisphere, chilling effects stabilize air masses) (4) Pressure centers and prevailing winds - air mass movement and gain or loss of heat and moisture, seasonal shifting. More maritime on the west coast of N America and more continental on the east coast (5) Mountain barriers and highlands – windward and leeward sides and rain shadow, Andes and Himalayan in S America and Asia, no mountain barrier in western Europe leading to moderate temperatures and sufficient precipitation in the entire region •Latitudinal heat balance. Average over the entire year, we see that equatorward from 36o , the amount of incoming solar radiation exceeds the loss from outgoing terrestrial radiation. The reverse is true for the middle and high (polar) latitudes, where losses from outgoing terrestrial radiation exceed gains from incoming solar radiation. •Climate is a 3-D phenomenon: latitudinal, longitudinal, and altitudinal •An index of continentality gauges the influence of oceans on air temperature over continents. In this scheme, North America is divided into zones of increasing maritime influence: continental, subcontinental, maritime, and ultramaritime. The greater the maritime influence, the less is the contrast between average summer and average winter temperatures. [Adapted from D.R. Currey, “Continentality of Extratropical Climates.” Annals of Association of American Geographers 64, No.2 91974):274] •Major Ocean Currents: 1. Gulf Stream 2. North Atlantic Drift 3. Labrador Current 4. West Greenland Drift 5. East Greenland Drift 6. Canary Current 7. North Equatorial Current 8. North Equatorial Countercurrent 9. South Equatorial Current 10. South Equatorial Countercurrent 11. Equatorial Countercurrent 12. Kuroshio Current 13. North Pacific Drift 14. Alaska Current 15. Oyashio Current 16. California Current 17. Peru or Humbolt Current 18. Brazil Current 19. Falkland Current 20. Benguela Current 21. Agulhas Current 22. West Wind Drift •Coastal upwellings are important because they provide abundant nutrients and therefore encourage rich fishing grounds. Köppen system - Five principal climate groups A) Humid tropical - Winterless climates; all months have a mean temperature above 18oC. B) Dry - Climates where potential evaporation exceeds precipitation; there is a constant water deficiency. C) Humid middle-latitude, mild winters - the average temperature of the coldest month is below 18oC but above -3oC. D) Humid middle-latitude, severe winter - the average temperature of the coldest month is below -3oC and the warmest monthly mean exceeds 10oC. E) Polar - Summerless climates; the average temperature of the warmest month is below 10oC. •“The zone is a strange climatological phenomenon – a desert (the Somali-Chalbi) on the eastern cost in tropical latitudes. The extreme aridity is caused, in the main, by the fact that the prevailing winds, during most months of the year, have a northeasterly or southwesterly direction, thus making moist air masses over the land an exception rather than the rule (Fig.1).” The Horn of Africa, by J.F. Griffiths Climates of Africa, Vol. 10, World Survey of Climatology, p. 134 • Unusual aridity of the Horn of Africa caused Flohn (1964) to ask: “why are the summer rains missing?” (1) directional divergence produced by overheating at the Ethiopian highlands, (2) speed divergence produced by the northerly increase of the pressure gradient, (3) frictional divergence in coast-parallel winds, (4) deflection of wind-driven ocean surface current and cold upwelling along the coast. Flohn, H., 1964. On the causes of the aridity of northeastern Africa. Würzburger Geograph. Arb., 12: 17 pp. (English translation by East African Meteorological Dept., Nairobi, 1966.) •“why are the summer rains missing?” (1) directional divergence produced by overheating at the Ethiopian highlands; (2) speed divergence produced by the northerly increase of the pressure gradient; (3) frictional divergence in coast-parallel winds; (4) deflection of wind-driven ocean surface current and cold upwelling along the coast. •Amazing fluctuations of annual rainfall amounts at Djibouti for 64 years - Less than 30 mm for five years - 250 mm or more for four years •Seasons defined by rainfall patterns: Jilal – December to March, the northeast monsoon is in dominance and conditions are dry and relatively hot. Gu – April and May, the transition period, relatively wet and hot. Hagar – June to September, the southwest monsoon dominates, bringing relatively cool conditions with showers along the coast but dry inland. Der – October and November, the transition period, similar to the Gu but there is the important rainy season in the north. •Climate of Malaysia Tropical; annual southwest (April to October) and northeast (October to February) monsoons • What is the Malaysia's climate like? Malaysia has two main seasons, the Northeast Monsoon (November to March) and the Southwest Monsoon (May to September), separated by two relatively shorter intermonsoon periods. The mean monthly rainfall amount shows drier weather conditions from May to July and wetter weather conditions from November to January. February is also a relatively dry month. The date of the beginning and end of the monsoon varies from year to year.They are determined by the beginning of rain spell and predominant wind direction.