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Dynamics of the Ocean NOAA Tech Refresh 20 Jan 2012 Kipp Shearman, OSU
Dynamics of the Ocean NOAA Tech Refresh
20 Jan 2012 Kipp Shearman, OSU
January Average Surface Map
The low pressure in the North Pacific (Aleutian Low) and in the North Atlantic (Icelandic Low) these ocean basins. Notice also the highs that are to the south (Pacific High, Bermuda High). Note the position of the ITCZ (center of tropical convection and the base of the Hadley cell).
Northern Hemisphere land masses are dominated by high pressure (on average) during winter. For example, the Siberian High
July Average Surface Map
Notice: Pacific High pressure dominates the North Pacific during the summer. The Bermuda High is also more prominent during summer (it is this feature that steers hurricanes in the Atlantic). These high pressure systems also shift as the ITCZ moves northward
The surface flow in the Southern Hemisphere is much smoother and less wavy due to less prominent land masses.
Outline
• Momentum Equations! • When is Coriolis important? • Geostrophic Balance • Ekman Balance
1
1
1
z
z
z
Du p fv FDt xDv p fu FDt yDw p g FDt z
ρ
ρ
ρ
∂= − + +
∂
∂= − − +
∂
∂= − − +
∂
A B C D E
A → acceleration B → pressure gradient force C → Coriolis force D → gravitational force E → other (friction, tidal, wind forcing, etc.)
2 sinf φ= Ω
Geostrophy Most Important Balance Ever!
Outline
• Momentum Equations! • When is Coriolis important? • Geostrophic Balance • Ekman Balance
When is Coriolis important?
Standard answer: Compare terms in the momentum equations.
( ) ( )
Number"Rossby " Ro~
~Coriolis
Advection
==
⎟⎠
⎞⎜⎝
⎛⎟⎠
⎞⎜⎝
⎛∂∂
=
fLUfULUU
fvxuu
important is Coriolis 1,RoWhen important NOT is Coriolis 1,RoWhen
≤
>>
Coriolis Deflection
UfL
ULfU
atULt
2
21
2
21
221
)(
=
⎟⎠
⎞⎜⎝
⎛=
=
=
δ
Lδ
http://abyss.uoregon.edu/~js/glossary/coriolis_effect.html
U
When is Coriolis important?
Answer: When Coriolis deflection is “big”. One definition of “big”…
Ro==fLUL
δ
UfL2
≈δFirst Recall:
Compare to : Lδ
important is Coriolis 1,RoWhen important NOT is Coriolis 1,RoWhen
≤
>>
Coriolis Effect
Outline
• Momentum Equations! • When is Coriolis important? • Geostrophic Balance • Ekman Balance
1
1
1
z
z
z
Du p fv FDt xDv p fu FDt yDw p g FDt z
ρ
ρ
ρ
∂= − + +
∂
∂= − − +
∂
∂= − − +
∂
A B C D E
A → acceleration B → pressure gradient force C → Coriolis force D → gravitational force E → other (friction, tidal, wind forcing, etc.)
2 sinf φ= Ω
Geostrophy
Geostrophic Balance
• Most common force moving water is PRESSURE (P) difference (gradient), which forces water in the direction from High to Low water pressure.
• But now, with rotation, as soon as particle starts to move down Pressure gradient, a Coriolis force (CF) at right angles starts to build; the stronger the flow, the stronger the force to the right (in the northern hemisphere).
• Eventually, CF and P are balanced, so particle has no force acting (continues at same velocity).
• In northern Hemisphere, particles move with high pressure on the right
• Flow is not down P gradient, but along it.
Geostrophic Balance
• High Pressure to RIGHT of velocity in northern hemisphere
• High Pressure to LEFT of velocity in southern hemisphere
Barotropic Pressure Gradient
Coriolis Force
Pressure Gradient Force
Top of the ocean (or atmosphere)
Geostrophic Balance Baroclinic Pressure Gradient
Coriolis Force
Pressure Gradient Force
Geostrophic Balance Barotropic + baroclinic pressure gradient
Drawn for northern hemisphere
Coriolis effect on circulation around low and high pressure systems
Low pressure Counterclockwise (N. Hemi.) Cyclonic
High pressure Clockwise (N. Hemi.) Anticyclonic
Huyer (1983)
• Big seasonal changes in the atmosphere
• Winds reverse direction
Aleutian Low
North Pacific High
Wind direction with respect to atmospheric pressure in different season for North Pacific
High pressure is to the right of the direction of the wind.
Within the last 15 years, we can measure the sea surface height using satellite altimetry.
Figure 10.5 Global distribution of time-averaged topography of the ocean from Topex/Poseidon altimeter data from 10/3/92 to 10/6/99 relative to the jgm–3 geoid.
Intro to PO, 2008
Eddies!
• Dense core – cyclonic rotation • Light core – anticyclonic rotation
GFD Trivia: Geostrophic Flow is Non-divergent
ug = −1f ρ
∂P∂y,vg =
1f ρ
∂P∂x
∂ug∂x
+∂vg∂y
+∂w∂z
= 0
⇒w = 0!−1f ρ
∂2P∂y∂x
+1f ρ
∂2P∂x∂y
+∂w∂z
= 0
Thermal Wind
( )
zxfg
zfg
x
zf
zP
x
fzx
Pz
∂
∂−=
∂
∂
∂
∂=−
∂
∂
∂
∂=
∂
∂
∂
∂
∂
∂=⎟⎟
⎠
⎞⎜⎜⎝
⎛
∂
∂
∂
∂
v
v)(1
v1
v1
ρρ
ρρ
ρ
ρ
Outline
• Momentum Equations! • When is Coriolis important? • Geostrophic Balance • Ekman Balance
Ice
Wind
?
The mysterious world of …
The Ekman spiral
Equations of motion 1 1
1 1
1
0
x
y
τu u u u pu v w fvt x y z ρ x ρ z
τv v v v pu v w fut x y z ρ y ρ z
w w w w pu v w gt x y z ρ z
u v wx y z
∂∂ ∂ ∂ ∂ ∂+ + + = − + +
∂ ∂ ∂ ∂ ∂ ∂
∂∂ ∂ ∂ ∂ ∂+ + + = − − +
∂ ∂ ∂ ∂ ∂ ∂
∂ ∂ ∂ ∂ ∂+ + + = − −
∂ ∂ ∂ ∂ ∂
∂ ∂ ∂+ + =
∂ ∂ ∂
We are interested in the balance between Coriolis and wind stress.
1 1
1 1
1
0
x
y
τu u u u pu v w fvt x y z ρ x ρ z
τv v v v pu v w fut x y z ρ y ρ z
w w w w pu v w gt x y z ρ z
u v wx y z
∂∂ ∂ ∂ ∂ ∂+ + + = − + +
∂ ∂ ∂ ∂ ∂ ∂
∂∂ ∂ ∂ ∂ ∂+ + + = − − +
∂ ∂ ∂ ∂ ∂ ∂
∂ ∂ ∂ ∂ ∂+ + + = − −
∂ ∂ ∂ ∂ ∂
∂ ∂ ∂+ + =
∂ ∂ ∂
Balance between Coriolis and Wind stress
10
10
10
0, ,
x
y
x z y z
τfvρ z
τfu
ρ z
p gρ z
u v w u vτ ρA τ ρAx y z z z
∂= + +
∂
∂= − +
∂
∂= − −
∂
∂ ∂ ∂ ∂ ∂+ + = = =
∂ ∂ ∂ ∂ ∂
Wind stress is parameterized
Az is vertical diffusivity.
Oceanic value ~ 10-2 m2s-1
Ekman Transport
= = ∫EU udzEkman Transport on x
Unit: m2s-1
= = ∫EV vdzEkman Transport on y
= yEU f
τ
ρ ( ), Wind stress on x and y
Density of Sea WaterCoriolis parameter (Earth rotation rate)
=
=
=
x y
f
τ τ
ρ−
= xEV f
τρ
Ekman Transport: Due to wind and the Earth’s rotation
Always to the right of the wind in Northern Hemisphere.
Ekman (1905)
• Wind-forcing can generate currents and waves, as wind transfers some of its momentum into the ocean"
• Wind acts via friction at the surface: wind stress τ"
Stresses have units of N/m2, (force/area), like pressure.Stresses are forces parallel to a surface, pressure is force perpendicular to the surface."
• Force/Area depends on the square of the wind speed u, and it points in the same direction as the wind: "
"
"
Wind Forcing at the Ocean Surface
2u∝ττ = ρaCD
u u 3
3
/ 1.3 air ofdensity 104.1tcoefficien drag mkg
C
a
D
≈=
×≈= −
ρ
Example: 20kt wind ≈ 10 m/s → 0.18 N/m2 = 1.8 dyne/cm2"
Vertical structure of u and v (Ekman spiral)
cos sin
cos sin
2
zsyδ
zsyδ
z
τ z zu eρδ f δ δ
τ z zv eρδ f δ δ
Aδf
−
−
⎡ ⎤± ⎛ ⎞ ⎛ ⎞= − − −⎜ ⎟ ⎜ ⎟⎢ ⎥
⎝ ⎠ ⎝ ⎠⎣ ⎦
⎡ ⎤⎛ ⎞ ⎛ ⎞= − + −⎜ ⎟ ⎜ ⎟⎢ ⎥
⎝ ⎠ ⎝ ⎠⎣ ⎦
=Don’t memorize u and v.
δ is Ekman depth: Decay depth of Ekman spiral. Depth of frictional influence. You want to understand the meaning of this depth.
1. Ocean at surface is dragged by wind, but then acted on by Coriolis Force. Current at surface are 45° to right of wind in Northern Hemisphere.
2. Friction transmits stress (drag) downward within water column: upper layer rubs on layer below and moves it. But response will be weaker (frictional losses) and further to the right. 3. Process continues down through water column.
4. Creates a spiral, decaying with depth. This is the Ekman spiral.
5. Typical decay depths are 10-30 m.
Ekman Spiral
2
4
2 2 10 1510
−
−
×= ≈ ≈zA m
fδ
Lentz, 1992
Q, surface heat flux
near-surface shear layer – not well understood, strongly affected by waves, some indication it is like the bottom “log layer” that we’ll discuss later
Ekman layer < 5 m
up to about 50 m
What does the real oceanic surface boundary layer look like?
Example: Calculate Ekman Transport on y Wind data by QuickSCAT from OrCOOS (http://agate.coas.oregonstate.edu/data_index.html)
τx = 0.2 N/m2
τy = 0.1 N/m2
ρ = 1025 kg/m3
f = 2Ωsinφ = 1.03×10-4s-1
UE = τy/ρf = +1 m2/s VE = −τx/ρf = −2 m2/s
Summary • Ekman spiral is due to wind stress and
the Earth’s rotation which is decaying with depth. Decay depth is Ekman depth.
• Current at surface are 45° to right of wind in Northern Hemisphere.
• Vertical integration of Ekman spiral is Ekman transport (UE and VE).
• Ekman transport is always to the right of the wind in Northern Hemisphere.
Upwelling/Downwelling driven by the presence of a coastal boundary:
Coastal Upwelling: Wind to South. Ekman transport in surface layer is to right of wind (West). Flow is divergent at the coast. Deeper water is upwelled into near-surface.
Primarily seen during spring/summer off Oregon coast.
Coastal Downwelling: Wind to North. Ekman transport in surface layer is to right of wind (East). Flow is convergent at the coast. Deeper vertical velocity is downward.
Upwelling/Downwelling with Stratification
T=20 T=18
T=16
Upwelling • Cold deep water brought to surface near coast • Nutrients (max near bottom) brought up to surface • Creates fronts in T,S
Downwelling • Surface water transported to coast • Warm surface water forced downward
Coastal Upwelling: Sea Surface Temperatures
Coldest temperatures near coast.
Surface water at the coast came from deeper in the water column.
