class #7: thursday, july 15 global wind systems

Class #7, Thursday, July 15, 2010 1

Class #7: Thursday, July 15Global wind systems

Chapter 10


Wind: Global Systems

Chapter 10


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



• Single Cell Model– Assume

1. uniform water surface2. Sun always directly overhead the Equator3. Earth does not rotateResult: 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



• 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



• 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.

Class #7, Thursday, July 15, 2010 17Fig. 1, p. 267

Class #7, Thursday, July 15, 2010 18Fig. 2, p. 267


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.


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


Jet Streams

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


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

Class #7, Thursday, July 15, 2010 27Stepped Art

Fig. 10-14, p. 273


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

Class #7, Thursday, July 15, 2010 30Fig. 10-16, p. 275



• 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



• 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


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


Wind: Global Systems

Chapter 10

class #7: thursday, july 15 global wind systems

Documents

general circulation

earths general circulation

global wind systems

comlogin general circulation

average air flow

atmospheregeneral circulation

air rises

surface windflow patterns