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The major wind systems

The major wind systems. Review of last lecture. Primary high and lows Three-cell model. Mechanism for each cell Two characteristics of temperature structure Two characteristics of wind structure. Why does westerly winds prevail in the extratropical troposphere? What cause the jet streams?

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The major wind systems

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  1. The major wind systems

  2. Review of last lecture • Primary high and lows • Three-cell model. Mechanism for each cell • Two characteristics of temperature structure • Two characteristics of wind structure. Why does westerly winds prevail in the extratropical troposphere? What cause the jet streams? • What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind? Two types of ocean upwelling

  3. Semipermanent Pressure Cells Instead of cohesive pressure belts circling the Earth, semipermanent cells of high and low pressure exist; fluctuating in strength and position on a seasonal basis. These cells are either dynamically or thermally created. Sinking motions associated with the subtropical highs promote desert conditions across specific latitudes. Seasonal fluxes in the pressure belts relate to the migrating Sun (solar declination).

  4. South Pacific high South Atlantic high South Indian high For NH winter: 1. Aleutian and Icelandic lows 2. Siberian and Bermuda-Azores highs 3. South Pacific, Atlantic, Indian highs

  5. South Pacific high South Atlantic high South Indian high For NH summer: 1. Tibetan low 2. Hawaiian and Bermuda-Azores highs 3. South Pacific, Atlantic, Indian highs

  6. High pressure: Desert and Drought Global distribution of deserts (all near high pressure cells)

  7. The 2011 East African drought

  8. Video: Horn of Africa drought 2011 http://www.youtube.com/watch?v=HXYFvx0SAMI

  9. Ocean surface currents – horizontal water motions Transfer energy and influence overlying atmosphere Surface currents result from frictional drag caused by wind - Ekman Spiral General circulation of the oceans • Water moves at a 45o angle (right) • in N.H. to prevailing wind direction • Due to influence of Coriolis effect • Greater angle at depth

  10. Global surface currents • Surface currents mainly driven by surface winds • North/ South Equatorial Currents pile water westward, create the Equatorial • Countercurrent • western ocean basins –warm poleward moving currents (example: Gulf Stream) • eastern basins –cold currents, directed equatorward

  11. is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. Ocean upwelling • Coastal upwelling: Due to Coriolis effect • Equatorial upwelling: Due to Coriolis effect

  12. Foehn Winds over the Alps • A Foehn wind is a type of dry down-slope wind that occurs in the lee (downwind side) of a mountain range. • They are initiated when mid-latitude cyclones pass to the southwest of the Alps. • Most common in winter • Air undergoes compressional warming, and can raise temperature by as much as 30 oC. • Central Europe enjoys a warmer climate due to Foehn winds. Foehn Winds

  13. Central Europe enjoys a warmer climate due to Foehn winds.

  14. Chinook Winds • Chinooks are similar to Foehn winds but happen on the eastern side of the Rocky Mountains and form when low pressure systems occur east of the mountains. • Most common in winter • Can exceed 90 mph. • Warm up the temperature for western Great Plains Chinook winds

  15. Effects on 1988 Winter Olympics • Chinook winds melted the snow • For the first time, Winter Olympics extended to 16 days • Sent a ski jumper flying into a camera tower

  16. Santa Ana Winds • Occur in California during the transitional seasons, especially autumn, when high pressure is located to the east. • Often contribute to the spread of wildfires. Santa Ana winds

  17. Santa Ana winds and spread of wildfire

  18. Katabatic Winds • Katabatic winds warm by compression but originate when air is locally chilled over high elevations. The air becomes dense (with low temperature) and flows downslope. • Can reach 60-120 mph. • Common along Antarctica and Greenland ice sheets. • Also referred to as Boras winds of the Balkan Mountains and Mistral winds in France.

  19. Sea and Land Breezes • Caused by temperature differences between land and sea. Strongest in spring/summer. • During the day (night) land is hotter (colder) than water. • A thermal low develops over the warmer region. • Air converges into the low, ascends, and produces clouds and possibly precipitation.

  20. Sea breeze over Hawaii

  21. Valley and Mountain Breezes • Diurnal variation similar to a land/sea breeze occur in mountainous areas and are called valley and mountain breezes. • Mountains facing the sun heat more intensely than shaded valley areas. This develops a thermal low during the day which produces a valley breeze. • At night the situation reverses producing a mountain breeze.

  22. Summary • Semipermanent pressure cells. High pressure causes desert and drought. • What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind? Two types of ocean upwelling • Foehn winds over the Alps and Chinook winds over the Rockies. Warm up the eastern slope • Santa Ana winds. Caused by high pressure over the mountain. Spread wildfires • Katabatic winds • Sea and land breezes. Caused by temperature difference between land and sea. • Valley and mountain breezes. Caused by temperature difference between mountain slope and valley.

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