How Does Air Move Around the Globe?

1. How Does Air Move Around the Globe?

2. Review of last lecture • Know 3 Forces that affect wind speed /direction • Especially work on Coriolis force, as this is the hardest to understand. Which direction is air deflected to by Coriolis force? • What is the geostrophic balance? At which level is it valid? Difference between upper level and surface winds • Troughs, ridges, cyclones and anticyclones. Do they correspond to high or low surface pressure? Is the air moving clockwise or counter-clockwise around them?

3. The most common atmospheric circulation structure CE H L Cooling or No Heating Heating Friction H L CE Imbalance of heating  Imbalance of temperature  Imbalance of pressure  Wind

4. Introduction • Well-defined heating, temperature and pressure patterns exist across the globe • These define the general circulation of the planet • In describing wind motions: • Zonal winds (east-west): flow parallel to lines of latitude • Flowing eastward: Westerly wind • Flowing westward: Easterly wind • Meridional winds (north-south): flow parallel to lines of longitude • Flowing northward: Southerly wind • Flowing southward: Northerly wind

5. Annual mean precipitation (heating) Extratropicalstormtrack Tropical rainfall Extratropicalstormtrack

6. Primary Highs and Lows Equatorial low Subtropical high Subpolar low Polar high

7. Three-cell model Zonal mean circulation Each hemisphere is divided into 3 distinct cells: Hadley Cell Ferrel Cell Polar Cell

8. Vertical structure and mechanisms • Polar Cell (thermal): • Driven by heating at 50 degree latitude and cooling at the poles • Ferrel Cell (dynamical): Dynamical response to Hadley and polar cells • Hadley Cell (thermal): Heating in tropics  formssurface low and upper level high air converges equatorward at surface, rises, and diverges poleward aloft  descends in the subtropics Polar Hadley

9. Zonal mean structure of temperature Two characteristics: • Horizontally uniform in the tropics • Steep gradient in the extratropics

10. Zonal mean structure of zonal wind Two characteristics: • Westerly winds in the extratropical troposphere • Jet streams: local maximum of winds

11. Westerly winds in the extratropical troposphere • The existence of the upper level pressure gradient  air is being pushed toward poles  Coriolis effect deflects upper air (to the right)  Westerlies dominate upper troposphere • Strongest during winter  thermal gradient is large • Explains why storms move eastward, flight times

12. Caused by steep temperature gradients between cold and warm air masses Polar front - marks area of contact, steep pressure gradient  polar jet stream Low latitudes  subtropical jet stream Stronger in winter, affect daily weather patterns The Jet Streams

13. A Jet Stream seen from satellite The subtropical jet is seen as a band of clouds extending from Mexico on an infrared satellite image

14. Video: The jet streams

15. 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).

16. 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

17. 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

18. Low pressure: clouds and precipitation Extratropicalstormtrack Tropical rainfall Extratropicalstormtrack

19. High pressure: warm surface temperature, drought and desert Global distribution of deserts (all near high pressure cells)

20. 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

21. 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

22. Summary • Three precipitation (heating) belts. Primary high and lows • Three-cell model. Mechanism for each cell • Two characteristics of zonal mean temperature structure • Two characteristics of zonal mean wind structure. Why does westerly winds prevail in the extratropical troposphere? What cause the jet streams? • Semipermanent pressure cells. Low pressure is associated with clouds and precipitation. High pressure is associated with warm surface temperature, drought, and desert. • What drives the ocean surface currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind?

23. Works cited Images • http://pulleysandgears.weebly.com/gears.html • http://visual.merriam-webster.com/earth/meteorology/meteorological-measuring-instruments/measure-wind-direction.php