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Chapter VII: Ocean Circulation. Essentials of Oceanography, Thurman and Trujillo. Wind-driven surface currents. Ocean Circulation Animation. Figure 7-4. Direct methods Float meters (lagrangian: float with current) Intentional Inadvertent Propeller meters (eularian: stay in one place)

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chapter vii ocean circulation

Chapter VII: Ocean Circulation

Essentials of Oceanography, Thurman and Trujillo

wind driven surface currents
Wind-driven surface currents

Ocean Circulation Animation

Figure 7-4

measuring surface currents
Direct methods

Float meters (lagrangian: float with current)

Intentional

Inadvertent

Propeller meters (eularian: stay in one place)

Indirect methods

Pressure gradients

Satellites

Doppler flow meters

Measuring surface currents

Figure 7B

ocean currents
Surface currents

Affect surface water within and above the pycnocline (10% of ocean water…I think it is more like 25% of ocean water)

Driven by major wind belts of the world

Deep currents

Affect deep water below pycnocline (90% of ocean water…I think it is more like 75%)

Driven by density differences

Larger and slower than surface currents

NO CLEAR CUT DELINEATION

Ocean currents
deep water masses and currents
Deep water masses:

Form in subpolar regions at the surface

Are created when high density surface water sinks

Factors affecting density of surface water:

Temperature (most important factor)

Salinity

Deep currents which transport deep waters are also known as thermohaline circulation

Characteristics of deep waters are determined AT THE SURFACE

Deep water masses and currents
deep ocean characteristics
Conditions of the deep ocean:

Cold

Still

Dark

Essentially no productivity

Sparse life

Extremely high pressure

Deep ocean characteristics
identification of deep water masses
Deep water masses are identified by measuring temperature (T) and salinity (S), from which density can be determined

T-S diagram

Characteristics set at surface

Identification of deep water masses

Figure 7-24

slide10

Understanding the formation of SURFACE currents

4 Primary things that need to be understood

- Ekman transport (and spiral)

- The idea of Convergence

- Conservation of Vorticity

- Geostrophic Balance

What drove Deep Currents?

ekman spiral wind driven
Ekman spiral describes the speed and direction of flow of surface waters at various depths

Factors:

Wind Pushes Water through Wind Stress (τ)

Coriolis effect pushes water to right(left)

Due to shear, water velocity spins to the right(left) with depth.

Ekman spiral: Wind Driven (τ)

Figure 7-6

ekman transport
Ekman transport is the overall water movement due to Ekman spiral

Ideal transport is 90º from the wind

Transport direction depends on the hemisphere

Ekman transport is proportional to the speed of the wind. Higher wind, higher transport!

Ekman transport

Figure 7-6

slide14

Understanding the formation of currents

4 Primary things that need to be understood

- Ekman transport (and spiral)

- The idea of Convergence

- Conservation of Vorticity

- Geostrophic Balance

slide15

Convergence/Divergence

This idea is nothing more then the piling up or moving of water away from a region.

Conservation of VOLUME: (du/dx+dv/dy+dw/dz=0)

Rearranging... du/dx + dv/dy = -dw/dz

If water comes into the box (du/dx + dv/dy)>0 there is a velocity out of the box: dw/dz < 0 DOWNWARD

So lets go back to Ekman…and see where water is piled up and where it is emptied.

slide18

Understanding the formation of currents

4 Primary things that need to be understood

- Ekman transport (and spiral)

- The idea of Convergence

- Conservation of Vorticity

- Geostrophic Balance

slide19

Vorticity (I think the 3rd time we’ve talked about it)

Vorticity is analagous to angular momentum.

Vorticity is a conserved quantity (Conservation of Vorticity)

When we talked about Coriolis we introduced the idea of Planetary Vorticity (f). Every object on earth has a vorticity given to it by the rotation of the earth (except an object on the equator). This vorticity is dependent on latitude.

Each object on earth can have Relative Vorticity as well. An ice skater who is spinning has Relative Vorticity. A skater who becomes more skinny spins faster (greater relative vorticity). But remember that water is incompressible. So if a water column becomes ‘skinny’ it MUST become taller at the same time!

TOTAL VORTICITY is CONSERVED BY FLUIDS.

Planetary (f) + Relative (ξ) = Constant H

H is the (tallness, or depth of water column)

slide20

An example of conservation of vorticity when H stays constant

Right Hand Rule: Curl your fingers on your right hand (northern hemisphere) in the direction of spin. If you thumb points upward the vorticity is positive. If you thumb points downward, vorticity is negative.

North Pole (High planetary Vorticity f)

Off the equator (to the north) Planetary Vorticity (f) > 0. Since (f + ξ )=0, ξ must be < 0. The water begins to spin.

A parcel of water moves off the equator its vorticity on the equator (f+ ξ)=0.

Equator (Zero planetary Vorticity f)

slide21

Right Hand Rule: Curl your fingers on your right hand (northern hemisphere) in the direction of spin. If you thumb points upward the vorticity is positive. If you thumb points downward, vorticity is negative.

An example of conservation of vorticity when H doesn’t stay constant

As the parcel hits the bump, H decreases. We know that (f + ξ)/H=Constant. So if H decreases, (f + ξ) must decrease. If f decreases, the parcel moves equatorward. If ξ decreases the parcel spins clockwise.

A parcel of water moves east (constant latitude) in N.Hemis.

Ocean Surface

What happens when the parcel leaves the bump?

H

H

Ocean bottom

Bump in bottom

slide22

Right Hand Rule: Curl your fingers on your right hand (northern hemisphere) in the direction of spin. If you thumb points upward the vorticity is positive. If you thumb points downward, vorticity is negative.

An example of conservation of vorticity when H doesn’t stay constant

As the parcel hits the bump, H decreases. We know that (f + ξ)/H=Constant. So if H decreases, (f + ξ) must decrease. If f decreases, the parcel moves equatorward. If ξ decreases the parcel spins clockwise. Or a combination.

A parcel of water moves east (constant latitude) in N.Hemis.

Ocean Surface

H

H

H

Ocean bottom

Bump in bottom

slide23

Right Hand Rule: Curl your fingers on your right hand (northern hemisphere) in the direction of spin. If you thumb points upward the vorticity is positive. If you thumb points downward, vorticity is negative.

An example of conservation of vorticity when H doesn’t stay constant

As the parcel hits the bump, H decreases. We know that (f + ξ)/H=Constant. So if H decreases, (f + ξ) must decrease. If f decreases, the parcel moves equatorward. If ξ decreases the parcel spins clockwise. Or a combination.

A parcel of water moves east (constant latitude) in N.Hemis.

North

Parcel Moves Equatorward

From ABOVE

H

Bump in bottom

H

South

slide24

Understanding the formation of currents

4 Primary things that need to be understood

- Ekman transport (and spiral)

- The idea of Convergence

- Conservation of Vorticity

- Geostrophic Balance

geostrophic balance
Most large currents are in Geostrophic balance. Which terms from our momentum equation?

All currents are pushed to the right(left).

This piles water up on the right(left).

This creates a pressure force back towards the current.

Eventually a balance is reached. Pressure BALANCES Coriolis!

Geostrophic Balance

Coriolis pushes water to right(left). Piles up water.

current

Sealevel

Pressure force

current

pressure

coriolis

geostrophic balance26
Geostrophic flow causes a hill to form in subtropical gyres

Example in the book of the balance of coriolis and pressure force (gravity).

Current is Perpendicular to slope.

Current is along constant height

Geostrophic Balance

Figure 7-7

slide27

Understanding the formation of currents

We’ve been introduced to the 4 Primary things that need to be understood. Let’s put them all together to understand what drives our ocean currents!

- Ekman transport (and spiral)

- The idea of Convergence

- Conservation of Vorticity

- Geostrophic Balance

slide28

More Realistic Climatological (average) Winds

Ekman transport creates convergence and divergence of upper waters.

Divergence

Convergence

Divergence

Convergence

Divergence

slide29

Upwelling and Downwelling across a mid ocean gyre due to Ekman Transport

Convergence causes downwelling! Divergence causes upwelling!

slide30

With DOWNWELLING, the vertical velocity is downward. This pushes on the column of water, making it shorter (and fatter). What happens when a column of water gets short and fat (Vorticity must be conserved).

A parcel of water moves into an area of downwelling. It becomes shorter (and fatter).

f/H must be conserved!

Ekman Convergence

Ocean Surface

Mixed Layer

We know that (f + ξ)/H= Constant. So if H decreases, (f + ξ) must decrease. I gave examples before that either f or ξ could change. But in this process; it is f that decreases. f can only decrease by the parcel moving equatorward.

H

H

Ocean bottom

slide31

More Realistic Climatological (average) Winds

Ekman transport creates convergence and divergence of upper waters.

Divergence

Convergence

Divergence

Convergence

Divergence

slide32

More Realistic Climatological (average) Winds

Ekman transport creates convergence and divergence of upper waters.

Poleward flow

45o N

15o N

15o S

45o S

Equatorward flow

Complicated flow

Equatorward flow

Poleward flow

geostrophic balance33
Ekman transport has caused a ‘hill’ to form in the sea surface when convergence occurs (subtropical gyre)

Vorticity balance explains equatorward flow (from gyre center to the east)

Geostropic current is along constant height (WARM water to right in N Hemis)

Current must return back to the north (conservation of mass)

Western Boundary Current is that return. Very strong very intense

Geostrophic Balance

Figure 7-7

current gyres
Gyres are large circular-moving loops of water

Subtropical gyres

Five main gyres (one in each ocean basin):

North Pacific, South Pacific, North Atlantic, South Atlantic, Indian

Generally 4 currents in each gyre

Centered at about 30º north or south latitude (I think more like 25o)

Subpolar gyres

Smaller and fewer than subtropical gyres

Generally 2 currents in each gyre

Centered at about 60º north or south latitude

Rotate in the opposite direction of adjoining subtropical gyres

Current gyres
slide36

Sea Surface Height and Mean Geostrophic Ocean Circulation

L-Subpolar Gyre

L-Subpolar Gyre

H-Subtropical Gyre

H-Subtropical Gyre

H-Subtropical Gyre

H-Subtropical Gyre

H-Subtropical Gyre

slide37

HK Guam HA SF

P37 mean dyht and temperature field

Sea Surface Height

Temperature Field

Salinity Field

western intensification of subtropical gyres
The western boundary currents of all subtropical gyres are:

Fast

Narrow

Deep

Western boundary currents are also warm

Western Boundary Currents and Vorticity Conservation…Must conserve.

Western intensification of subtropical gyres
slide39

Back to our example of conservation of vorticity when H stays constant

Right Hand Rule: Curl your fingers on your right hand (northern hemisphere) in the direction of spin. If you thumb points upward the vorticity is positive. If you thumb points downward, vorticity is negative.

Remember this example?

As the western boundary current returns north, this should happen, but it does not. Why?

North Pole (High planetary Vorticity f)

Off the equator (to the north) Planetary Vorticity (f) > 0. Since (f + ξ)=0, ξ must be < 0. The water begins to spin.

A parcel of water moves off the equator its vorticity on the equator (f+ ξ)=0.

Equator (Zero planetary Vorticity f)

slide40

Back to our example of conservation of vorticity when H stays constant

As the water moves up the coast in the VERY Narrow WBC, it rubs against the coast. It removes vorticity through friction.

The WBC MUST be narrow, it must get close to the coast.

Conservation of Vorticity is valid as an idea. But once an outside force like friction is applied, conservation is not going to happen.

North Pole (High planetary Vorticity f)

Parcel wants to spin

Off the equator (to the north) Planetary Vorticity (f) > 0. Since (f + ξ)=0, ξ must be < 0. The water begins to spin.

But can’t due to friction

A parcel of water moves off the equator its vorticity on the equator (f+ ξ)=0.

Equator (Zero planetary Vorticity f)

upwelling and downwelling
Vertical movement of water ()

Upwelling = movement of deep water to surface

Hoists cold, nutrient-rich water to surface

Produces high productivities and abundant marine life

Downwelling = movement of surface water down

Moves warm, nutrient-depleted surface water down

Not associated with high productivities or abundant marine life

Upwelling and downwelling
coastal upwelling and downwelling
Ekman transport moves surface water away from shore, producing upwelling

Ekman transport moves surface water towards shore, producing downwelling

Coastal upwelling and downwelling

Figure 7-11

other types of upwelling
Equatorial upwelling

Offshore wind

Sea floor obstruction

Sharp bend in coastal geometry

Other types of upwelling

Figure 7-9

Equatorial upwelling

the gulf stream and sea surface temperatures
The Gulf Stream is a warm, western intensified current

Meanders as it moves into the North Atlantic

Creates warm and cold core rings

Rings move west. Argue as given in book for westward intensification.

The Gulf Stream and sea surface temperatures

Figure 7-16

currents and climate
Warm current  warms air  high water vapor  humid coastal climate

Cool current  cools air  low water vapor  dry coastal climate

Currents and climate

Figure 7-8a

el ni o southern oscillation enso
El Niño = warm surface current in equatorial eastern Pacific that occurs periodically around Christmastime

Southern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El Niño

ENSO describes a combined oceanic-atmospheric disturbance

El Niño-Southern Oscillation (ENSO)
average conditions in the pacific ocean
Average conditions in the Pacific Ocean

El Nino/La Nina Animation

Figure 7-18a

the 1997 98 el ni o
Sea surface temperature anomaly map shows warming during severe 1997-98 El Niño

Internet site for El Niño visualizations

Current state of the tropical Pacific

The 1997-98 El Niño

Figure 7-19a

el ni o r ecurrence interval
Typical recurrence interval for El Niños = 3-12 years

Pacific has alternated between El Niño and La Niña events since 1950

El Niño recurrence interval

Figure 7-20

slide59

El Nino

La Nina

end of chapter vii

End of Chapter VII

Essentials of Oceanography, Thurman and Trujillo

slide61

Measuring currents through satellite

Red: High sea level…High sea level is warmer water (water expands when warm)…In N Hemisphere warm water is on the right. ONLY measures anomaly, Must add GEOID.

slide62

Equatorial Currents are complicated…but they are still driven EXACTLY THE SAME WAY as the gyres. The currents are complicated because the winds are complicated and the equator is present (Why would the equator be important?) f is nearly zero near the equator so swashing and stretching of water columns isn’t the driving force. The process is just ekman convergence/divergence and pressure forces.

north atlantic ocean circulation
North Atlantic Ocean circulation

Sverdrup: measure of flow rate (length3/time) 1 Sv = 106 m3/s

Figure 7-15

indian ocean surface currents
Indian Ocean surface currents

Northeast monsoon

Southwest monsoon

Figure 7-23