1 Review: Lectures 7-11 G. Cowles Introduction to Physical Oceanography MAR 555 School for Marine...
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Review: Lectures 7-11
G. Cowles
Introduction to Physical Oceanography
MAR 555
School for Marine Sciences and Technology
Umass-Dartmouth
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Outline• What drives the general circulation• Background: Coriolis, Ekman, Geostrophy • Our subtropic gyre: the North Atlantic• Our WBC: The Gulf Stream• Gulf Stream Instability: Rings!• The subpolar gyre: impact on GoM• Equatorial Current structure and ENSO• Pacific (better in every way) Ocean Circulation• Indian Ocean Variability: the Monsoon• Antarctic Circumpolar Circ • Water Masses - Clues of Origin
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What is driving the ocean?
• Differential Heating: Seasonal and Meridional - Both direct (deep water formation) and indirect (winds)
• Coriolis (technically not driving) but quite influential!
• + Tides, Freshwater, etc.
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Hadley Cells:
Differential Heating (Equator > Poles) + Coriolis + Effects due to
convergence of longitude.
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Global Average Wind Field: Note correlation with Hadley Cells
What is complicating the picture?
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Coriolis: Review
• We want to apply Newton's Law F=ma to see how the ocean will respond (a) to a given force (F)
• We also want our coordinate system to be attached to the spinning earth. That is the reference frame in which we make measurements.
• As we discussed in detail in the last review , Newton’s law doesn’t work in this reference frame because it is non-inertial (accelerating).
• Things that aren’t experiencing any observable force are accelerating, according to our measurements.
• We modify slightly Newton’s Law to give F + Fc = ma where Fc is the Coriolis force. In most engineering problems, Fc is negligible. For geophysical flows, it is extremely important.
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Coriolis: Review. Cont’d
Solution: We modify slightly Newton’s Law to give F + Fc = ma where Fc is the Coriolis force. In most engineering problems, Fc is negligible. For geophysical flows, it is extremely important.
f|V|
|V|
f|V|
|V|
f|V|
|V|
45N ? ?
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Geostrophic Currents: Review
Key Force Balance: Coriolis vs. Pressure Gradient
1.0m
0.5m
0.0m
FPG
Pressure Gradients (not considering influence of density)
FPGFPG
V
Fc
Fc = FPG
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Geostrophic Currents: Quiz
1.0m
0.5m
0.0m
Southern Hemisphere?
FPG
1.0 m
0.5m
0.0m
1010mb
1000mb
990mb
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How a Uniform Wind Drives the Infinite Ocean: Ekman Forcing
At Interface: No Slip Condition
Tau_wind = f( u_wind, wave state, atmospheric
stability, z_obs)
Wind Tugs on the Surface Water
Fwind
Fcoriolis
• Assume the ocean is solid and glides effortless over the Earth
• Coriolis will act as soon as the wind imparts momentum into water column
• Will begin to steer column to the right (Northern Hem).
• Picture at Right has NOT yet reached a Steady State (Forces don’t Balance)
V
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How a Uniform Wind Drives the Infinite Ocean: Ekman Forcing, cont’d
Fwind • Now, Water column is made of many tiny layers each having their own velocity V(t)
• Wind will initiate motion in first layer, Coriolis will steer it.
• First layer imparts a stress on second layer, Coriolis will steer it further to the right
• Picture at Right is NOT yet reached a Steady State (Forces don’t Balance)
Fc for a layer
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How a Uniform Wind Drives the Infinite Ocean: Ekman Forcing, cont’d
Bird’s Eye View
• Our only external force is Fwind (assume no bottom friction)
• When will we reach a steady state?• Steady State: acceleration = 0, thus our
Forces must sum to zero.• Who can possibly oppose the winds
stress? Coriolis?• Once the Coriolis force acting on the
water column as a whole opposes in direction and magnitude the Wind, we have reached a steady state.
• Is our column at the top left at a steady state?
Fc for a layer
FwindV1
V2
V3
Fc2
Fc3
Fc for the column
Fwind
V_avg
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• AVERAGE Current flows at 90 degrees to the Wind Direction: This is the Ekman Transport• Surface Current is 45 degrees to the right (left) of the Wind in the Northern (southern)
hemisphere• Ekman transport confined to the wind-driven layer (depth depends on Turbulence, Coriolis)
Key Results for Ekman forcing
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Complication 1: Non-Uniform Winds
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Complication 2: Non-Uniform Winds
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Complication 1: Non-Uniform Winds
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Complication 2: A Non-Infinite Ocean
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Global Average Wind Field: Note correlation with Hadley Cells
What is complicating the picture?
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Global Ocean Circulation: Principal Features
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Our Subtropic Gyre: The North Atlantic
Pacific Subtropic - Similar Features
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Gulf Stream: The NA WBC
• Width ~100km• Depth ~1000m• Velocities ~100cm/s (up to
200)• Transport near Florida ~ 30 Sv
(what is a Sv?)• Departs coast near Hatteras• Is unstable (small perturbations
in front position will grow)• ‘Gulf’ is from Gulf of Florida• Is our Western Boundary
Current
Some Key Facts
SST: (NOAA)
Pacific has the Kuroshio - Also Unstable
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Instability of the Gulf Stream: Gulf Stream RingsCold Core:
South of Front
See Level Depressed
Isotherms Uplifted Warm Core:
North of Front
Sea Level Uplifted
Isotherms Depressed
Influences Gulf of Maine!
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The Subpolar Gyre:
4.8
0.2
4.8-5.3
0.7
max
4.1
0.60.50.1
0.350.140.26
0.38
GoM is Influenced!!
Intense Cooling of NAC
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Equatorial Currents - Note the Counter Current
Trade Winds cause westward Currents
To “Pile Up” on the West sides of the basins.
This results in a pressure gradient which forces
Eastward currents along the doldrums where
There is limited wind stress to oppose them.
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ENSO
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The Indian Ocean: Monsoons and Variability
NH Winter,, cold in Asia: Dry Winds Blow
Offshore of the Subcontinent
- Similar to Pacific Atlantic
NH Summer,, continent heats up, winds shift to SW,
Dump moisture form Arabian Sea onto Subcontinent:
Key feature - reversal of North Equatorial Current and
Somali Current - Upwells in NH Summer
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Antarctic Circumpolar
No landmass to obstruct
Currents follow the very strong zonal
winds in a loop around Antarctica
Sea Surface is higher or lower as you move North from
Antarctica?
Is this an upwelling or downwelling system?
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Can You Explain The Variability in Primary Productivity?
1
2
34
5
6
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excretion
Deep NO3 pool Vertical profile of NO3
Sea Surface
Mixed layer Pgrazing
NH4
Nutroclines Turbulent diffusion
High nutrients
uptake
NO3
Z
N
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Divergence Opposing Currents Cyclonic:
Upwelling
Coastal Upwelling
Anti-Cyclonic - Downwelling