Wind: Global Systems

1. Chapter 10 Wind: Global Systems

2. 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

3. General Circulation of the Atmosphere • Single Cell Model • Assume • uniform water surface • Sun always directly overhead the Equator • 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

6. 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

10. 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.

16. Jet Streams • 100-200 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.

20. 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

21. Jet Streams • Other Jet Streams • Tropical easterly jet stream • Low-level jet (nocturnal) • Polar night jet streams

22. 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

24. Stepped Art Fig. 10-14, p. 273

26. 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

37. 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

39. 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

41. 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