Chapter VII: Ocean Circulation

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Chapter VII Ocean Circulation
Essentials of Oceanography, Thurman and Trujillo
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Wind-driven surface currents
Ocean Circulation Animation
Figure 7-4
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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

Figure 7B
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Ocean currents

• Surface currents
• Affect surface water within and above the
  pycnocline (10 of ocean waterI 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
  waterI think it is more like 75)
• Driven by density differences
• Larger and slower than surface currents
• NO CLEAR CUT DELINEATION

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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

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Deep ocean characteristics

• Conditions of the deep ocean
• Cold
• Still
• Dark
• Essentially no productivity
• Sparse life
• Extremely high pressure

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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

Figure 7-24
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Atlantic Ocean subsurface water masses
Figure 7-25
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Conveyer-belt circulation Deep Currents
Figure 7-27
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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?
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Ekman spiral Wind Driven (t)

• Ekman spiral describes the speed and direction of
  flow of surface waters at various depths
• Factors
• Wind Pushes Water through Wind Stress (t)
• Coriolis effect pushes water to right(left)
• Due to shear, water velocity spins to the
  right(left) with depth.

Figure 7-6
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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!

Figure 7-6
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More Realistic Climatological (average) Winds
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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
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Convergence/Divergence
This idea is nothing more then the piling up or
moving of water away from a region. Conservation
of VOLUME (du/dxdv/dydw/dz0) Rearranging...
du/dx dv/dy -dw/dz If water comes into the
box (du/dx dv/dy)gt0 there is a velocity out of
the box dw/dz lt 0 DOWNWARD So lets go back
to Ekmanand see where water is piled up and
where it is emptied.
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Convergence (Divergence) across a mid ocean gyre
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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
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Vorticity (I think the 3rd time weve 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 (?) Constant

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

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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) gt 0. Since (f ? )0, ? must be
lt 0. The water begins to spin.
A parcel of water moves off the equator its
vorticity on the equator (f ? )0.
Equator (Zero planetary Vorticity f)
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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
doesnt stay constant
As the parcel hits the bump, H decreases. We know
that (f ?)/HConstant. So if H decreases, (f
?) must decrease. If f decreases, the parcel
moves equatorward. If ? 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
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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
doesnt stay constant
As the parcel hits the bump, H decreases. We know
that (f ? )/HConstant. So if H decreases, (f
? ) must decrease. If f decreases, the parcel
moves equatorward. If ? 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
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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
doesnt stay constant
As the parcel hits the bump, H decreases. We know
that (f ? )/HConstant. So if H decreases, (f
? ) must decrease. If f decreases, the parcel
moves equatorward. If ? 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
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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
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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!

Coriolis pushes water to right(left). Piles up
water.
current
Sealevel
Pressure force
current
pressure
coriolis
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Geostrophic Balance

• 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

Figure 7-7
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Understanding the formation of currents
Weve been introduced to the 4 Primary things
that need to be understood. Lets put them all
together to understand what drives our ocean
currents! - Ekman transport (and spiral) - The
idea of Convergence - Conservation of Vorticity -
Geostrophic Balance
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More Realistic Climatological (average) Winds
Ekman transport creates convergence and
divergence of upper waters.
Divergence
Convergence
Divergence
Convergence
Divergence
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Upwelling and Downwelling across a mid ocean gyre
due to Ekman Transport
Convergence causes downwelling! Divergence causes
upwelling!
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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 ?)/H Constant. So if H
decreases, (f ? ) must decrease. I gave
examples before that either f or ? could change.
But in this process it is f that decreases. f
can only decrease by the parcel moving
equatorward.
H
H
Ocean bottom
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More Realistic Climatological (average) Winds
Ekman transport creates convergence and
divergence of upper waters.
Divergence
Convergence
Divergence
Convergence
Divergence
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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
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Geostrophic Balance

• 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

Figure 7-7
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Sea Surface Height and Mean Geostrophic Ocean
Circulation
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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

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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
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HK Guam
HA SF
P37 mean dyht and temperature field
Sea Surface Height
Temperature Field
Salinity Field
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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
  ConservationMust conserve.

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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.
Back to our example of conservation of vorticity
when H stays constant
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) gt 0. Since (f ? )0, ? must
be lt 0. The water begins to spin.
A parcel of water moves off the equator its
vorticity on the equator (f ? )0.
Equator (Zero planetary Vorticity f)
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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) gt 0. Since (f ? )0, ? must
be lt 0. The water begins to spin.
But cant due to friction
A parcel of water moves off the equator its
vorticity on the equator (f ? )0.
Equator (Zero planetary Vorticity f)
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Wind-driven surface currents
Figure 7-4
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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

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Coastal upwelling and downwelling

• Ekman transport moves surface water away from
  shore, producing upwelling
• Ekman transport moves surface water towards
  shore, producing downwelling

Figure 7-11
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Other types of upwelling

• Equatorial upwelling
• Offshore wind
• Sea floor obstruction
• Sharp bend in coastal geometry

Figure 7-9
Equatorial upwelling
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Other examples of upwelling (Which one looks
like San Diego?)
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Antarctic surface circulation
Figure 7-13
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Ocean surface currentsWhat Currents do you need
to know?
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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.

Figure 7-16
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Flows are typically unstable they meander
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Currents and climate

• Warm current ? warms air ? high water vapor ?
  humid coastal climate
• Cool current ? cools air ? low water vapor ? dry
  coastal climate

Figure 7-8a
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El Niño-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

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Average conditions in the Pacific Ocean
El Nino/La Nina Animation
Figure 7-18a
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El Niño conditions (ENSO warm phase)
Figure 7-18b
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La Niña conditions (ENSO cool phase opposite of
El Niño)
Figure 7-18c
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The 1997-98 El Niño

• 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

Figure 7-19a
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El Niño recurrence interval

• Typical recurrence interval for El Niños 3-12
  years
• Pacific has alternated between El Niño and La
  Niña events since 1950

Figure 7-20
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Effects of severe El Niños
Figure 7-21
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El Nino
La Nina
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End of Chapter VII
Essentials of Oceanography, Thurman and Trujillo
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Measuring currents through satellite
Red High sea levelHigh sea level is warmer
water (water expands when warm)In N Hemisphere
warm water is on the right. ONLY measures
anomaly, Must add GEOID.
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Equatorial Currents are complicatedbut 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 isnt the driving force. The
process is just ekman convergence/divergence and
pressure forces.
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Topex/Poseidon dynamic topography after GEOID has
been added
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Ocean surface currents
Figure 7-14
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North Atlantic Ocean circulation
Sverdrup measure of flow rate (length3/time) 1
Sv 106 m3/s
Figure 7-15
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Pacific Ocean surface currents
Figure 7-17
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Indian Ocean surface currents
Northeast monsoon
Southwest monsoon
Figure 7-23