Wind-Driven Ocean Circulation
What Drives Circulation of Upper Ocean?
Short answer: Energy from the Sun and
Earth’s rotation
Atmospheric circulation on an
idealized non-rotating Earth
Hadley cell
ƒ Energy from Sun
H
Su
rfa
ce
flo
w
causes differential
heating of ocean
and atmosphere
Equator
L
L
L
H
Hadley cell
Coriolis Effect
Maximum apparent
rotation
60o
30o
830
km/h
1450
km/h
1670
km/h
0o
30o
Arises from motion on a
rotating earth
1450
km/h
Equator
Depends on observer’s
frame of reference
Coriolis Force
From space, we
see that it
arises from the
conservation of
momentum as
the earth
rotates under a
moving object
a= initial north or south wind
velocity of poleward moving air
b= initial eastward velocity minus the
eastward velocity of the earth at a
higher latitude
c= resultant velocity of the wind
a= initial north or south wind velocity
moving toward the equator
b= initial eastward velocity minus the
eastward velocity of the earth at a lower
latitude
c= resultant velocity of the wind
Coriolis Force
Magnitude: F/m = 2Ω sin∅ v
Where: 2 Ω is earth’s
rotational velocity
(constant)
∅ is latitude,
and
v is velocity of the
object.
Direction:
Initial direction
Resultant direction
Right in the Northern Hemisphere
Left in the Southern Hemisphere
Atmospheric circulation on an idealized
rotating earth (with Coriolis Effect)
Polar easterlies
60o
Subpolar low
Westerlies
30o
Subtropical high
0o
NE trade winds
30o
SE trade winds
Subtropical high
Westerlies
Subpolar low
Polar easterlies
60o
Climatic Zones
Vertical Circulation
POLAR
EASTERLIES Earth’s Rotation
Air sinks
POLAR
Polar front
Low
o
SUBPOLAR
PF
TEMPERATE
SUBTROPICAL
“Horse Latitudes”
Air rises:
persistent
cloudiness
60 N
Westerlies
Air sinks: clear
skies, low
rainfall
High
o
30 N
Northeasterly Trades
EQUATORIAL
0
“Doldrums”
Air rises:
persistent
cloudiness and
rain
ITCZ
o
Southeasterly Trades
o
30 S
SUBTROPICAL
“Horse Latitudes”
TEMPERATE
SUBPOLAR
Air sinks: clear
skies, low
rainfall
High
Westerlies
Air rises:
persistent
cloudiness
AF
Antarctic front
POLAR
Antarctica
Air sinks
ITCZ Intertropical
convergence
zone
PF Polar Front
AF Antarctic
Front
Atmospheric
circulation and
source of
planetary winds
The weather patterns of
Temperate Regions
80
80
70
80
70
80
Polar
70
60
60
40
50
Subpolar
50
Temperate
40
40
40
Subtropical
Subtropical
30
80
70
60
Temperate
40
Polar
70
60
50
50
40
80
70
30
Tropical
Tropical
Monsoon
50 60 70 80
90
120
140
30
60
50
Subpolar
60
70
70
80
140
120
100
90
50 40
30 20 10
0
Tropical
Subtropical
80
30
Subtropical
40 40
Temperate
50
160
Tropical
Subtropical
40
180
Equatorial
Tropical
30
160
Equatorial
40
Temperate
50
60
50 50
Subpolar
70
60
70
Polar
80
40
Temperate
80
50
Subpolar
60
Polar
70
80
60
70
80
Atmospheric Pressure Systems
„Land masses heat in summer and cool in winter faster
than ocean, because land has lower heat capacity than
water
„This leads to seasonal effects: formation of cold high
pressure systems over continents in winter and warm
low pressure systems over continents in summer
Atmospheric Pressure Systems:
Northern Hemisphere Winter
180
90
90
90
180
1020
Icelandic low
1026
60
Siberian
1002
High
1032
L
Aleutian low
H
1008
Latitude
30
1014
1008
0
H
1020
H Pacific High
L
H
30
0
L
1014
1014
1020
H
30
H
1014
1008
1002
996
60
90
60
1026
1008
1020
H
H
1014
1008
1014
1020
30
996
1002
1008
1014
1020 H Bermuda High
1020
ITCZ
L
1020
L
180
90
Longitude
180
60
90
Atmospheric Pressure Systems:
Northern Hemisphere Summer
180
90
180
90
90
1008 Icelandic low
L
1008
60
1008
60
Pacific
high
Thermal low
H
L
1026
30
1014
1002
1020
L
L
H
30
Latitude
1020
1014
ITCZ
1014
0
1014
30
1020
1020
H
H
0
1020
H
1026
30
H
1014
1008
60
60
1002
996
990
90
180
90
Longitude
180
90
High and Low Pressure Cells:
Geostrophic Balance
Falling air
A
Coriolis force
effect
Northern
Hemisphere
Rising air
B
Undulations in the Jet Stream
and Polar Front
North Pole
Easterlies
Jet Stream
30o
60o
60o
Upper
Westerlies
Cold
Air
30o
Warm
air
Equator
Warm
winds
L Low
H High
Jet Stream
Atmospheric Pressure Systems:
Northern Hemisphere Summer
180
90
180
90
90
1008 Icelandic low
L
1008
60
1008
60
Pacific
high
Thermal low
H
L
1026
30
1014
1002
1020
L
L
H
30
Latitude
1020
1014
ITCZ
1014
0
1014
30
1020
1020
H
H
0
1020
H
1026
30
H
1014
1008
60
60
1002
996
990
90
180
90
Longitude
180
90
High and Low Pressure Cells:
Geostrophic Balance
Falling air
A
Coriolis force
effect
Northern
Hemisphere
Rising air
B
Undulations in the Jet Stream
and Polar Front
North Pole
Easterlies
Jet Stream
30o
60o
60o
Upper
Westerlies
Cold
Air
30o
Warm
air
Equator
Warm
winds
L Low
H High
Jet Stream
Invasion of temperate regions by warm tropical
air masses from the south and cold polar air
masses from the north
cP
mP
mP
cP
cT
mT
mT
Cyclogenesis
H
L
Cold
L
Warm
N
Warm sector
H
mT
(a)
(b)
L
(c)
L
L
Warm
(d)
(e)
(f)
Cyclogenesis off Cape Hatteras
Iceland
L
Low 3
H
H
Low 1
L
L
H
Bermuda high
Low 2
Blizzard of ‘96
Coupling of the Planetary Wind Field with
Ocean Circulation
The Ekman Spiral
Wind
direction
45o
„Upper layer: Force
from wind shear
balanced by Coriolis
Direction of net
water transport
force and friction
90
(viscosity) from
Northern Hemisphere
underlying layers
Wind
direction 45o
Surface current
o
Spiraling
currents
Direction of net water
transport averaged over
depth of spiral
No water motion
Southern Hemisphere
„Surface transport:
45o to right of wind
„Viscosity transfers momentum between layers;
exponential decline in speed with depth due to frictional
energy losses
„Net Ekman transport 90o to wind
Ekman transport can be used to spin up an
idealized model ocean
Water is piled up until forces are balanced
in an ocean with steady-state circulation
(Geostrophic Balance)
Subtropical Gyre: Circulation in
Geostrophic Balance
Path of ideal
geostrophic flow
s
Corioli
Path of actual
geostrophic flow
Gravity
Geostrophic flow
A
West
rl
ste
e
W
y
Wi
Apex
de
Tra
60
o
30
o
nds
Flow to South
Flow to North
Northern
Hemisphere
Subtropical Gyre
East
nds
i
W
B
0
o
Dynamic
topography of sea
surface as
measured from
space by satellite
altimeters
(TOPEX
Poseidon)
Gulf Stream in the
North Atlantic
Gulf Stream Rings
Continental Shelf
Woods Hole
Cape Hatteras
Warm-Core
Ring
Gu
m
lf Strea
Miami
CU
BA
fS
l
u
G
am
e
tr
Cold-Core
Ring
Bermuda
Bahama
Islands
Sargasso Sea
Slope Water
Gran
d Ba
nks
Convergence and Divergence Zones
Convergence -- Downwelling
Divergence -- Upwelling
CURRENTS
TRANSPORTS
WINDS
Westerlies
o
30 N
20
o
10
o
0
convergence
NE
Trade
Winds
North Equatorial
Current
divergence
Equatorial
Counter-current
convergence
South Equatorial
Current
Equatorial
divergence
Doldrums
o
10
o
20
o
SE
Trade
Winds
Equatorial Upwelling
convergence
o
30 S
Westerlies
Equatorial
North Equatorial Counter-Current
current
Equatorial divergence
Depth (m)
0
50
South Equatorial Current
W
E
W
W
W
W
100
150
W
W
200
o
25 N
Thermocline
20
o
15
o
10
o
5
o
0
o
Latitude
5
o
10
o
15
o
20
o
o
25 S
Coastal Upwelling