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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

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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