EVAT 554
OCEAN-ATMOSPHERE
DYNAMICS
LECTURE 9
LARGE-SCALE ATMOSPHERIC
CIRCULATION (CONT)
(Reference: Peixoto & Oort, Chapter 3,7)
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
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
Ascending Air
Descending Air
COLD
THERMALLYDIRECT
CIRCULATION
High
Pressure
Low
Pressure
Ascending Air
Descending Air
WARM
High
Pressure
WARM
THERMALLY
INDIRECT
CIRCULATION
COLD
Low
Pressure
MERIDIONAL CIRCULATION PATTERNS
HADLEY CELLS ARE THERMALLY DIRECT
CIRCULATIONS…
As are the (weak) polar cells
N
Latitude
60
30
0
30
60
Low
High
Low
High
Low
S
Surface Pressure Systems
‘FERREL CELLS’ ARE THERMALLY-INDIRECT CIRCULATIONS
HORIZONTAL ATMOSPHERIC CIRCULATION
Cloud and Precipitation Occur in
Areas of Surface Convergence
and Rising Motion
GLOBAL ATMOSPHERIC CIRCULATION
January
July
There are important seasonal changes in the semipermanent features of the surface atmospheric
circulation
ITCZ migrates with Sun, but lags it...
GLOBAL ATMOSPHERIC CIRCULATION
January
July
There are corresponding seasonal changes in the
upper air (e.g. 500 mb) circulation
GLOBAL ATMOSPHERIC CIRCULATION
Vertical Atmospheric Circulation
N
Latitude
60
30
0
30
60
Low
High
Low
High
Low
Surface Pressure Systems
Zonally-averaged ps
S
GLOBAL ATMOSPHERIC CIRCULATION
Westerly
N
Easterly
60
30
0
30
Surface Wind Systems
Surface Pressure Systems
Zonally-averaged ps
60
S
GLOBAL ATMOSPHERIC CIRCULATION
east-west component of wind at surface consistent with
Geostrophic Balance
uG   1 p / 
fˆa
p /   0  uG  0
(westerlies)
p /   0  uG  0
(easterlies)
Why should there be
any easterlies here??
p /  0
f 0
L’Hopital’s rule
uG   1 2 p /  2
f ' ˆa
f ' 2cos
GLOBAL ATMOSPHERIC CIRCULATION
Geostrophic Balance
1 p / 
ˆacos
fu   1 p / 
ˆa
fv 
VG  1 kˆp
fˆ


1

p
1

p

p 
,

a cos  a  






Hydrostatic Balance
p /z g
Combine these
V
 1 kˆ p
z fˆ
z
G
v  g T
z Tfacos 
u   g T
z Tfa 
THERMAL WIND
  g kˆ
fˆ
u  g 
z fa 
v   g 
z facos 
THERMAL WIND
u   g T
z Tfa 
THERMAL WIND
MERIDIONAL-VERTICAL CIRCULATION
Continuity
  (v)
t
Steady state
(v) 0
(vcos )  w  0

z
v
1  / z
a cos
w  1  / 
a
Streamfunction