Class #7: Thursday, July 15 Global wind systems Chapter 10 1Class #7, Thursday, July 15, 2010

Wind: Global Systems Chapter 10 2Class #7, Thursday, July 15, 2010

General Circulation of the Atmosphere General refers to the average air flow, actual winds will vary considerably. Average conditions help identify driving forces. The basic cause of the general circulation is unequal heating of the Earth’s surface – Warm air is transferred from the Tropics to the Poles – Cool air is transferred from the Poles to the Tropics 3Class #7, Thursday, July 15, 2010

General Circulation of the Atmosphere Single Cell Model – Assume 1.uniform water surface 2.Sun always directly overhead the Equator 3.Earth does not rotate Result: huge thermally direct convection cell (Hadley) Three Cell Model – Allow earth to spin = three cells (Hadley, Ferrell, Polar) – Alternating belts of pressure starting with L at Equator – Alternating belts of wind with NE just North of Equator 4Class #7, Thursday, July 15, 2010

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General Circulation of the Atmosphere Average Surface Wind and Pressure: The Real World – Semi-permanent high and lows – Northern vs. Southern Hemisphere – Major features shift seasonally with the high sun North in July South in December 7Class #7, Thursday, July 15, 2010

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General Circulation of the Atmosphere General Circulation and Precipitation Patterns – Rain where air rises (low pressure) – Less rain where air sinks (high pressure) Average Wind Flow and Pressure Patterns Aloft – North-South temperature and pressure gradient at high altitudes creates West-East winds, particularly at mid to high latitudes. 11Class #7, Thursday, July 15, 2010

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Jet Streams kt winds at 10-15km, thousands of km long, several 100 km wide and a few km thick (polar and subtropical) Observations: Dishpan Experiment – Illustrates waves, with trough and ridge, develops in a rotating pan with heat on the exterior and cold at the center. 19Class #7, Thursday, July 15, 2010

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Jet Streams Polar and Subtropical Jet – Established by steep temperature and pressure gradients between circulation cells. – Between tropical-mid-latitude cell (subtropical) and mid- latitude-polar cell (polar) – Gradients greatest at polar jet Topic: Momentum – Low-latitudes: atmosphere gains momentum – High-latitudes: atmosphere losses momentum – Conservation of Momentum 23Class #7, Thursday, July 15, 2010

Jet Streams Other Jet Streams – Tropical easterly jet stream – Low-level jet (nocturnal) – Polar night jet streams 24Class #7, Thursday, July 15, 2010

Atmosphere Ocean Interactions Global Winds and Surface Ocean Currents – Ocean surface dragged by wind, basins react to high pressure circulation forming gyres – Cold current, flowing north to south, on west side of continent – Warm current, flowing south to north, on east side of continent – Oceanic front 25Class #7, Thursday, July 15, 2010

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Stepped Art Fig , p Class #7, Thursday, July 15, 2010

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Atmosphere Ocean Interactions Upwelling – Ekman spiral, Ekman transport – Water moving away from the coast causes upwelling El Nino and the Southern Oscillation – El Nino: irregular warm episode off west coast of South America – Southern Oscillation: rise in pressure over W Pacific, fall in the E Pacific, equatorial countercurrent – ENSO – La Nina – teleconnection 29Class #7, Thursday, July 15, 2010

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Atmosphere Ocean Interactions Pacific Decadal Oscillation – Reversal in Pacific Ocean temperatures – Warm = more Pacific storms – Cool = cool, wet NW North America, wetter over the Great Lakes, salmon fisheries decline 43Class #7, Thursday, July 15, 2010

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Atmosphere Ocean Interactions North Atlantic Oscillation – Reversal of pressure in North Atlantic Ocean affecting weather in Europe and eastern coast of North America – Positive = strong Westerlies, storms in N Europe, wet and mild in eastern US – Negative = wet southern Europe and Mediterranean, cold and dry in eastern US 45Class #7, Thursday, July 15, 2010

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Atmosphere Ocean Interaction Arctic Oscillation – Closely related to NAO – Pressure changes between Arctic and adjacent southern areas causes changes upper-level winds – Positive = mild winter in US and W Europe – Negative = cold US, cold dry Europe, wet Mediterranean 47Class #7, Thursday, July 15, 2010

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Wind: Global Systems Chapter 10 49Class #7, Thursday, July 15, 2010