Wind: Global Systems Chapter 10

Transcription

1 Wind: Global Systems Chapter 10

2 General Circulation of the Atmosphere General circulation of the atmosphere describes average wind patterns and is useful for understanding climate Over the earth, incoming solar radiation balances outgoing radiation In tropics, incoming solar radiation > outgoing radiation; poles, incoming radiation < outgoing Need net transfer of radiation from tropics to poles - move warm air northward and cold air southward 2

3 Single cell model- hot air rises at equator, moves toward poles high in atmosphere, sinks at poles, moves equator ward at low levels of atmosphere Assumptions: 1. uniform water surface 2. Sun always directly overhead the Equator 3. Earth does not rotate 3

4 3-Cell Model: rotation of earth prevents single cell model- actual pattern more like 3-cell model 4

5 5

6 Observations on 3-Cell Model Near equator over water, horizontal pressure gradients light, winds weak- doldrums In tropics, rising air condenses and gives off latent heat- moves poleward at high altitude- convergence produces high surface pressure at 30º north- subtropical highs Subsidence produces, warm dry conditions at surface- deserts At surface some of subsiding air moves toward poles, some back to equator Coriolis deflects equatorward moving surface air to right in Northern Hemisphere, left in Southern Hemisphere- trade winds Convergence of trade winds- Intertropical convergence zone (ITCZ)- convergence produces rising air, completes 6 cycle

7 Observations on 3-Cell Model Poleward moving air deflected to east, producing prevailing westerly winds in midlatitudes Convergence of cold air moving away from poles and from subtropical highs- subpolar low- 60º latitude (polar front) Polar easterlies due to Coriolis force deflection of surface air moving away from poles In mid-latitudes, most energy transfer due to synoptic-scale high and low pressure systems (cyclones and anticyclones) Patterns shift and change seasonally Actual patterns affected significantly by distribution of oceans and continents, ocean currents, land elevation 7

8 3 Cell Model: Bottom Line Does a fair job describing most of the realworld patterns, but not all Hadley cell describes real patterns Ferrel and polar cells are not quite well represented in real world especially in upper levelsl The Ferrel cell implies easterly winds aloft but we see westerlies We don t see large overturning cells outside of Hadley zones 8

9 Seasonally averaged winds Now we take a look at some observations Average sea-level pressure patterns reveal seasonally varying areas with high and low pressure January - Aleutian and Icelandic low- strong storms often Siberian i and weaker Canadian High from very cold air- relatively weak Bermuda High and pacific High July - Strong Pacific High, thermal low over Desert southwest, weak Icelandic low, no Aleutian low, thermal low northern India-summer Monsoon Northward shift of ITCZ from January to July 9

10 Winter 10

11 Summer 11

12 General Circulation and Precipitation Patterns Low latitudes more rainfall when sun overhead (ITCZ) (summer), drier when subtropical high overhead (winter) Summer, Pacific subtropical high moves northward, causing subsidence along Pacific Coast- little rain Winter, Pacific High moves south, Storms along polar front coming off Aleutian low bring rain to west coast Clockwise rotation around Bermuda High brings warm humid air into eastern US in summer 12

13 Movement of Summer Highs H approximate winter location 13

14 Average Annual Precipitation 14

15 Averaged 500 mb heights, January 15

16 Averaged 500 mb heights, July 16

17 Jet Streams Jet streams long, narrow currents of very fast flowing air Typically 100 to 200 knots and found near tropopause at 10 to 15 km altitude North-south temperature t gradient causes pressure gradient to increase with height-and less density so stronger winds V g =(1/ρf) Δp/Δs Often have a subtropical jet and a polar front jet Pressure gradient strongest t at fronts where temperature gradient is greatest

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19 Example location of jet stream

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21 Formation of jet stream

22 Low-level jets Low level jets commonly form at night at top of temperature inversion Inversion limits mixing with air below that is travelling slower due to friction at the earth s surface Slope from Rocky Mountains to Missisippi Valley causes colder air to west at same elevation as areas to the eastlower pressure to the west and pressure gradient force causes flow from the south During summer, the low-level jet help transport warm, moist air from Gulf of Mexico northward, enhancing thunderstorms, especially at night (nocturnal jets)

23 Atmosphere- Ocean Interactions Wind blowing over ocean causes movement of the water near the surface Pressure differences from surface water movements cause water down to a few hundred meters to move also Circulations called gyres are set up in the oceans Coriolis force deflects water to right in northern hemisphere, left in southern hemisphere 40% of latitudinal (south-north) heat transfer in northern Hemisphere by ocean surface water

24 Ocean Circulations

25 El Nino (boy child) Trade winds blow from southeast along west coast of South America causes upwelling of cool nutrient rich water During El Nino pressure decreases over eastern Pacific and increase to the west- trade winds weaken or reverse direction less upwelling and warm water in eastern tropical Pacific La Nina (girl child)- colder than normal waters over eastern tropical Pacific Surface atmospheric pressure patterns changes every few years- called Southern Oscillationrelated to ocean water temperatures (El Nino)- combined effect referred to as ENSO- El Nino/Southern Oscillation

26 Non El Nino Conditions

27 El Nino Conditions

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30 Ocean Nino Index

31 Typical climatology of El Nino/La Nina El Nino- wet winter in southern US and southern California and southern Nevada with late winter rains in CA,NV El Nino- weak Polar Jet over Canada, mild over northern US La Nina- wet in Pacific Northwest, warm, dry southeast US

32 El Nino Conditions

33 La Nina Conditions