1. EVAT 554 OCEAN-ATMOSPHERE DYNAMICS
LECTURE 9
LARGE-SCALE ATMOSPHERIC CIRCULATION (CONT)
(Reference: Peixoto & Oort, Chapter 3,7)

2. HADLEY CELL CIRCULATION
THERMALLY-DRIVEN CIRCULATION
Approximate circulation as steady, linear, two-dimensional (meridional-vertical plane)
HADLEY CELL CIRCULATION
Pole Equator
THERMALLY-DIRECT CIRCULATION FOR AN IDEALIZED
NON-ROTATING EARTH

3. WHAT ABOUT EXTRATROPICS?
THERMALLY-DRIVEN CIRCULATION
Approximate circulation as steady, linear, two-dimensional (meridional-vertical plane)
WHAT ABOUT EXTRATROPICS?
Pole Equator
THERMALLY-DIRECT CIRCULATION FOR AN IDEALIZED
NON-ROTATING EARTH

4. THERMALLY-DIRECT CIRCULATION
Descending Air
High
Pressure
Ascending Air
Low
THERMALLY-DIRECT CIRCULATION
COLD
WARM
Descending Air
High
Pressure
Ascending Air
Low
THERMALLY INDIRECT CIRCULATION
WARM
COLD

5. MERIDIONAL CIRCULATION PATTERNS
HADLEY CELLS ARE THERMALLY DIRECT CIRCULATIONS…
As are the (weak) polar cells 30 60 N S
Latitude
High
Low
Surface Pressure Systems
‘FERREL CELLS’ ARE THERMALLY-INDIRECT CIRCULATIONS

6. HORIZONTAL ATMOSPHERIC CIRCULATION
Cloud and Precipitation Occur in Areas of Surface Convergence and Rising Motion

7. GLOBAL ATMOSPHERIC CIRCULATION
January
July
There are important seasonal changes in the semi-permanent features of the surface atmospheric circulation
ITCZ migrates with Sun, but lags it...

8. GLOBAL ATMOSPHERIC CIRCULATION
January
July
There are corresponding seasonal changes in the upper air (e.g. 500 mb) circulation

9. GLOBAL ATMOSPHERIC CIRCULATION Surface Pressure Systems
Vertical Atmospheric Circulation 30 60 N S
Latitude
Low
High
Low
High
Low
Surface Pressure Systems
Zonally-averaged ps

10. GLOBAL ATMOSPHERIC CIRCULATION Surface Pressure Systems
Westerly 30 60 N S
Easterly
Surface Wind Systems
Surface Pressure Systems
Zonally-averaged ps

11. GLOBAL ATMOSPHERIC CIRCULATION
east-west component of wind at surface consistent with Geostrophic Balance 
(westerlies) 
(easterlies)
L’Hopital’s rule
Why should there be any easterlies here??

12. GLOBAL ATMOSPHERIC CIRCULATION
Geostrophic Balance
Hydrostatic Balance
Combine these
THERMAL WIND

13. THERMAL WIND
THERMAL WIND

14. MERIDIONAL-VERTICAL CIRCULATION
Continuity
Steady state
Streamfunction