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 structureCE 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)
Extratropical stormtrack
Tropical rainfall

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  forms surface 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. The Jet Streams
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

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. 1. Aleutian and Icelandic lows 2. Siberian and Bermuda-Azores highs
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. 2. Hawaiian and Bermuda-Azores highs
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
Extratropical stormtrack
Tropical rainfall

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

20. General circulation of the oceans
Ocean surface currents – horizontal water motions
Transfer energy and influence overlying atmosphere
Surface currents result from frictional drag caused by wind - Ekman Spiral
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