Lecture 10:
10: Ocean Circulation
Ekman Transport
Ekman Pumping
Wind-Driven
Wind Driven Circulation
ESS228
Prof. JinJin-Yi Yu
Basic Ocean Structures
Warm up by sunlight!
Upper Ocean (~100 m)
Shallow, warm upper layer where light is
abundant and where most marine life can be found.
Deep Ocean
Cold, dark
Cold
dark, deep ocean where plenty supplies of
nutrients and carbon exist.
No sunlight!
ESS228
Prof. JinJin-Yi Yu
Basic Ocean Current Systems
Upper Ocean
surface
circulation
Deep Ocean
deep ocean circulation
(from “Is The Temperature Rising?”)
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Prof. JinJin-Yi Yu
Vertical Structure of Ocean
Mixed Layer: T and S well mixed by winds
Temperature
Thermocline: large gradient of T and S
Salinity
Deep Ocean: T and S independent of height
cold
salty
high
g nutrient level
(from Climate System Modeling)
ESS228
Prof. JinJin-Yi Yu
Mixed Layer Processes
The depth of the mixed layer
is determined by (1) the rate of
buoyancy generation and (2) the
rate of kinetic energy supply.
The atmosphere can affect the
mixed layer through three
processes: heating,
p
g wind forcing,
g
and freshening (P-E).
The global-average depth of
the mixed layer is about 70 m.
m
(from Global Physical Climatology)
The heat capacity of the mixed
layer is about 30 times the heat
capacity
it off the
th atmosphere.
t
h
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Prof. JinJin-Yi Yu
Seasonal Variation of Mixed Layer
Summer: warm and thin.
Winter: cold and deep
(several hundred meters).
(from Global Physical Climatology)
ESS228
Prof. JinJin-Yi Yu
Two Circulation Systems
density-driven
circulation
i l ti
(Figure from The Earth System)
wind-driven
circulation
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Prof. JinJin-Yi Yu
Global Surface Currents
(from Climate System Modeling)
ESS228
Prof. JinJin-Yi Yu
Six Great Current Circuits in the World Ocean
5 of them are geostrophic gyres:
North Pacific Gyre
South Pacific Gyre
North Atlantic Gyre
South Atlantic Gyre
Indian Ocean Gyre
The 6th and the largest current:
Antarctic Circumpolr Current
(also called West Wind Drift)
(Figure from Oceanography by Tom Garrison)
ESS228
Prof. JinJin-Yi Yu
Characteristics of the Gyres
(Figure from Oceanography by Tom Garrison)
Currents are in geostropic balance
Each gyre includes 4 current components:
two boundary currents: western and eastern
two transverse currents: easteward and westward
Western boundary current (jet stream of ocean)
the fast, deep, and narrow current moves warm
water polarward (transport ~50 Sv or greater)
Eastern boundary current
the slow, shallow, and broad current moves cold
water equatorward (transport ~ 10-15 Sv)
Trade wind-driven current
the moderately shallow and broad westward
current (transport ~ 30 Sv)
Westerly-driven current
the wider and slower (than the trade wind-driven
current) eastward current
Volume transport unit:
1 sv = 1 Sverdrup = 1 million m3/sec
(the Amazon river has a transport of ~0.17 Sv)
ESS228
Prof. JinJin-Yi Yu
Major Current Names
Western Boundary Current
Trade Wind-Driven Current
Gulf Stream (in the North Atlantic)
North Equatorial Current
Kuroshio Current (in the North Pacific)
Brazil Current (in the South Atlantic)
Eastern Australian Current (in the South Pacific)
Agulhas Current (in the Indian Ocean)
South Equatorial Current
Eastern Boundary Current
Canary Current (in the North Atlantic)
California Current (in the North Pacific)
Benguela Current (in the South Atlantic)
Peru Current (in the South Pacific)
Western Australian Current (in the Indian Ocean)
Westerly-Driven Current
North Atlantic Current (in the North Atlantic)
North Pacific Current (in the North Pacific)
ESS228
Prof. JinJin-Yi Yu
Gulf Stream
A river of current
Jet stream in the ocean
Speed = 2 m/sec
Depth = 450 m
Width = 70 Km
Color: clear and blue
(Figure from Oceanography by Tom Garrison)
ESS228
Prof. JinJin-Yi Yu
Surface Current – Geostrophic
p Gyre
y
Mixed Layer
Currents controlled by frictional force + Coriolis force
Æ wind-driven circulation
Æ Ekman transport (horizontal direction)
Æ convergence/divergence
Æ downwelling/upwelling at the bottom of mixed layer
Thermocline
downwelling/upwelling in the mixed layer
Æ pressure gradient force + Coriolis force
Æ geostrophic current
Æ Sverdrup transport (horizontal)
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Prof. JinJin-Yi Yu
Step 1: Surface
S rface Winds
(Figure from Oceanography by Tom Garrison)
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Prof. JinJin-Yi Yu
Winds and Surface Currents
Polar Cell
F
Ferrel
l Cell
C ll
Hadleyy Cell
(Figure from The Earth System)
ESS228
Prof. JinJin-Yi Yu
Step
p 2: Ekman Layer
y
(frictional force + Coriolis Force)
(Figure from Oceanography by Tom Garrison)
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Prof. JinJin-Yi Yu
Ek
Ekman
Spiral
S i l–AR
Result
lt off Coriolis
C i li Force
F
(Figure from The Earth System)
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Prof. JinJin-Yi Yu
Formula for Ekman Transport
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Prof. JinJin-Yi Yu
How Deep is the Ekman Layer?
D ∝ (ν/f)1/2
ν = vertical diffusivity of momentum
f = Coriolis parameter = 2Ωsinφ
(from Climate System Modeling)
ESS228
Prof. JinJin-Yi Yu
Ekman Transport
(Figure from The Earth System)
ESS228
Prof. JinJin-Yi Yu
Step 3: Geostrophic Current
(Pressure Gradient Force + Corioils Foce)
NASA-TOPEX
Observations
Obse
vat o s of
o
Sea-Level Hight
(from Oceanography by Tom Garrison)
ESS228
Prof. JinJin-Yi Yu
Ekman Transport Æ Convergence/Divergence
(Figure from The Earth System)
Surface wind + Coriolis Force
Ekman Transport
Thermocline
Convergence/divergence
(in the center of the gyre)
Pressure Gradient Force
G
Geostrophic
hi Currents
C
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Prof. JinJin-Yi Yu
Geostrophic Current
Forces
Geostrophic Gyre Currents
(Figure from The Earth System)
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Prof. JinJin-Yi Yu
Sverdrup Transport
• Continuity equation for an incompressible flow:
• Assume the horizontal flows are geostrophic:
Ekman layer pumping
Î vertical depth decreases
Î move equatorward to conserve absolute
vorticity.
• Replace the geostrophic flow pressure gradients:
• The continuity equation becomes:
Ekman layer suction
Î vertical depth increases
Î move poleward to conserve absolute vorticity.
ESS228
Prof. JinJin-Yi Yu
Sverdrup Transport
• Continuity equation for an incompressible flow:
• Integrate the equation from the bottom of the upper
ocean (Dw) to the bottom of the Ekman layer (DE):
assume zero
• Assume the horizontal flows are geostrophic:
• Ekman pumping (ѡE) is related to the convergence of
the Ekman transport:
• Replace the geostrophic flow pressure gradients:
• Therefore, we obtain:
• The continuity equation becomes:
geostrophic
transport
• Therefore,
Sverdrup
transport
- (Ekman
Transport)
ESS228
Sverdrup transport = Geostrophic transport
Ekman
transport
Prof.+ JinJin
-Yi Yu
Ekman and Sverdrup Transports
Eq
E
W
E
90°N
Ekman
Layer
y
Ekman Pumping
(w<0)
Ekman
k
S
Suction
i
(w>0)
Equatorward
Sverdrup Transport
Poleward
Sverdrup Transport
Subtropical Gyre
ESS228
Prof. JinJin-Yi Yu
Conservation of Potential Vorticity
Pole
Potential Vorticity
PV = f + ζ
f1
H
(ζ1<0)
f = planetary vorticity = 2Ωsinφ
ζ = relative vorticity = ∂v/∂x- ∂u/∂y
f1 + ζ1 = f2 + ζ2
since f1 > f2 Æ ζ1 < ζ2
f2
H
(ζ2> ζ1)
If ζ < 0,
0 the vortex decreases rotation when
moves toward lower latitudes and increases
rotation when moves toward higher latitudes.
Equator
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Prof. JinJin-Yi Yu
Bo ndar C
Boundary
Currents
rrents
(Figure from The Earth System)
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Prof. JinJin-Yi Yu
Step 4: Boundary Currents
(Figure from Oceanography by Tom Garrison)
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Prof. JinJin-Yi Yu
Boundary Currents
Eastern boundary currents: broad and weak
Western boundary currents: narrow and strong
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Prof. JinJin-Yi Yu
Eastern Boundary Current
Cold water from higher
latitude ocean.
Costal upwelling
associated with subtropical
high
g ppressure system.
y
(from Global Physical Climatology)
Atmospheric subsidence
produce persistent stratiform
clouds which further cool
clouds,
down SSTs by blocking
solar radiation.
ESS228
Prof. JinJin-Yi Yu
Costal Upwelling/Downwelling
A result of Ekman
transport and mass
continuity.
(Figure from Oceanography by Tom Garrison)
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Prof. JinJin-Yi Yu
Global Surface Currents
ESS228
Prof. JinJin-Yi Yu
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