Pressure And Winds
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Pressure And Winds

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Pressure And Winds Presentation Transcript

  • 1. Atmospheric Circulation Air Pressure and Winds
  • 2. Overview Air pressure Air pressure and atmospheric circulation Global circulation patterns Local winds
  • 3. Air Pressure The weight of the atmosphere At sea level 1013.2 mb 760 mm of Hg 29.92 in Greatest at surface increases with atmospheric density Decreases with altitude halves every 5.5km
  • 4. Isobars: lines of equal barometric pressure
  • 5. Wind The movement of air under the influence of pressure. Winds influenced by three forces Pressure gradient Winds move from high pressure to low pressure along a straight line Present at both the surface and higher levels of the atmosphere Coriolis Force Angular momentum from the rotation of the earth deflects straight line motion present at both the surface and higher levels (vertically) Weakest at the equator and increases with latitude Friction Force Opposes motion along the path of motion, independent of changes in the path Present at the surface but NOT higher levels
  • 6. Balance of forces produces rotational motion of winds. For Northern Hemisphere Clockwise out from high pressure at surface Clockwise around high, at higher altitudes Anti-cyclone Counterclockwise into low pressure at surface Counterclockwise around low, at higher altitudes Cyclone Geostrophic Winds Geostrophic Winds
  • 7. Refer to the animations for: Coriolis Effect Wind Patterns Cyclones and Anticyclones
  • 8. Global Patterns of Pressure In general: Low pressure occurs where there is lifting of the air. Typically carries up moisture. Lifting mechanisms Convergent Lifting (ITCZ) Convectional Lifting (ITCZ) Frontal Lifting (Subpolar Low) [Orographic Lifting] High pressure occurs where air is falling. Typically very dry air.
  • 9. Equatorial Low Pressure Trough High insolation causes convectional and convergent lifting Provides uplift for Hadley Cells Intertropical Convergence Zone (ITCZ) Follows the subsolar point (with lag time) Trade Winds Subtropical High Pressure Cells Downdraft portion of Hadley Cell System Hot, dry air Westerlies Subtropical Jet Stream Subpolar Low Pressure Cells Frontal Lifting, Cool moist air Polar jetstream
  • 10. Polar High Pressure Polar easterlies Cold Deserts Form over continental masses Canada, Siberia Antarctica
  • 11. Upper Atmosphere Circulation Geostrophic wind Wind flow is parallel to the isobars Wind shear with surface winds contributes to cyclone formation, storms Rossby Waves Jet Streams
  • 12. Rossby Waves Form under the subpolar lows Create tongues of cold air Send cold air to the lower latitudes Stronger during the winter months Subtropical highs are at lower latitudes, exert less force against the cold polar airmasses
  • 13. Jet streams Bands of rapidly moving air 300 km/h Form at the intersection of the tropopause and colliding airmasses Flattened 160 - 480 km wide, 0.9 - 2.15 km thick When passing over cyclones, cause rapid uptake of air (extreme low pressure) and severe weather Polar jet stream 7600 - 10,700 m altitude Forms at the subpolar low pressure zone (where subtropical highs abut the polar highs) 30 – 70o Latitude Subtropical jet stream 9100 – 13,700 m altitude Forms where tropical and midlatitude air masses collide (subtropical high) 20 – 50o Latitude
  • 14. Local Winds Form in response to Form in response to Terrain Differences in temperature patterns, create localized high and low pressure systems Land-Sea breezes Caused by differences between land-sea heating Daytime: Land is hotter than ocean: Low pressure forms over land Winds flow from the oceans inland Nighttime Ocean is warmer than the land: Low pressure forms over ocean Winds flow from the land out to sea Mountain-Valley Breezes Daytime: warm valley air flows upslope Nighttime: rapid cooling of mountain causes it to flow downslope
  • 15. Katabatic Winds Cold air masses that form in upland areas and move down-slope under the influence of gravity (not pressure differentials) Common in Greenland and Antarctica Also called mistral (French Alps and Rhône Valley), bora (Adriatic region), taku (Alaska) Chinook Winds Occurs when a steep pressure gradient develops astride a mountain range, with high pressure on the windward side and low pressure on the leeward side. High winds speeds, descending winds are warm and extremely dry Chinook (Rockies), Santa Anas (California), Foehn (Alps)
  • 16. Monsoons A seasonal variation in wind patterns and rainfall Summer months characterized by onshore flow of moisture-laden maritime winds Extremely high rainfall ITCZ has passed over the land, drawing in winds Winter months characterized by offshore flow of dry continental winds Very little rainfall ITCZ has passed back out to sea, Subtropical Highs form over continents, driving winds out to sea The seasonal pattern is characterized by marked wet and dry seasons Half of the world’s population lives in areas dominated by monsoons Developing countries, dependent on agriculture and rainfall
  • 17. Ocean Currents Surface Currents Driven primarily by surface winds Equatorial currents driven by trade winds Gyres are rotational currents driven by the subtropical highs Western Intensification: As equatorial currents reach the Western edge of oceans (i.e., the Eastern edge of continents) water piles up against the shore (up to 15 cm), drives water northward or southward Gulf Stream and Kuroshio current Upwelling Currents: When currents flow away from continents, pulls cooler, deeper water up from below nutrient rich, prime fisheries Downwelling Currents: where water accumulates, some is driven downwards
  • 18. Thermohaline currents Driven by differences in salinity, density and temperature Slow moving (takes 1000 years to complete a circuit) Moves enormous quantities of water Downwelling in the Labrador Sea, due to cooling Upwelling in the Indian Ocean and North Pacific May help to regulate climate, by absorbing and redistributing heat energy Global warming my disrupt downwelling If so, then increased surface heating would compound global warming (positive feedback)
  • 19. Oscillations Alternating weakening and strengthening of high and low pressure systems that disrupt the general circulation pattern Produce droughts and floods Four identified major oscillations ENSO NAO AO PDO
  • 20. ENSO (El Niño - Southern Oscillation) Weakening and reversal of South Pacific equatorial and subtropical high and low pressure Affects surface currents, upwelling El Niño produces warmer ocean temperatures, La Niña produces colder conditions Effects (El Niño unless specified otherwise) Disrupts fisheries off South American coast Intense hurricanes in the South Pacific Droughts: Southern Africa, Southern India, Australia, Phillipines Floods: USA, Bolivia, Cuba, Ecuador, Peru El Niño produces more rain in SW US La Niña produces more rain in the Pacific NW.
  • 21. NAO (North Atlantic Oscillation) Variations in strength between Icelandic low (subpolar low) and Azores high (subtropical high) Positive phase: weakening of low, strengthening of high Increased pressure gradient, strong winds (westerlies) Moderates winters in the US European winters characterized by storms and high rainfall Mediterranean areas are drier than usual. Negative Phase: weakening of pressure gradient low winds cold air masses further South US has cold, snowy winters Europe has dry, cold winters Mediterranean areas are wetter than usual Patterns appear to fluctuate with the AO
  • 22. AO (Arctic Oscillation) Warm Phase: Lower pressure than normal over the North Pole, relatively higher pressures to the south Milder winters in the Northern Hemisphere Stronger westerlies Influx of warm Atlantic water into the Arctic ocean Cold Phase: Higher pressure over North Pole and lower pressure over the Atlantic Colder winters in Northern Hemisphere Thickening of sea ice
  • 23. PDO (Pacific Decadal Oscillation) Longer period of variation than ENSO PDO: 20-30 years ENSO: 2-12 years Variations occur between two regions Region 1: Northern and Tropical West Pacific Region 2: Eastern Tropical Pacific, along East Coast of Central and South America Positive Phase Lower than normal temperatures in Region 1 and higher than normal in Region 2 1977-1990 Negative Phase Higher than normal temperatures in Region 1, lower than normal in region 2 1947 – 1977, 1990 – present Causes dry conditions in the southwestern US Possibly linked to AO