Chapter VII: Ocean Circulation
Essentials of Oceanography, Thurman and Trujillo
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
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
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 ocean characteristics
Conditions of the deep ocean:
Cold
Still
Dark
Essentially no productivity
Sparse life
Extremely high pressure
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
Atlantic Ocean subsurface water masses
Figure 7-25
Conveyer-belt circulation: Deep Currents
Figure 7-27
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 (τ)
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.
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!
Figure 7-6
More Realistic Climatological (average) Winds
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
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.
Convergence (Divergence) across a mid ocean gyre
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
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)
H
= Constant
H is the (tallness, or depth of water column)
An example of
conservation of vorticity
when H stays constant
North Pole (High planetary Vorticity f)
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.
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)
An example of
conservation of vorticity
when H doesn’t stay
constant
A parcel of water moves east
(constant latitude) in N.Hemis.
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.
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.
Ocean Surface
H
Ocean bottom
What happens when the
parcel leaves the bump?
H
Bump in bottom
An example of
conservation of vorticity
when H doesn’t stay
constant
A parcel of water moves east
(constant latitude) in N.Hemis.
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.
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.
Ocean Surface
H
Ocean bottom
H
H
Bump in bottom
An example of
conservation of vorticity
when H doesn’t stay
constant
A parcel of water moves east
(constant latitude) in N.Hemis.
North
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.
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.
Parcel Moves Equatorward
From ABOVE
H
Bump in bottom
South
H
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!
Coriolis pushes water to
right(left). Piles up water.
current
Sealevel
Pressure force
current
pressure
coriolis
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
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
More Realistic Climatological (average) Winds
Ekman transport creates convergence and
divergence of upper waters.
Divergence
Convergence
Divergence
Convergence
Divergence
Upwelling and Downwelling across a mid ocean
gyre due to Ekman Transport
Convergence causes downwelling! Divergence
causes upwelling!
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).
Ocean Surface
A parcel of water moves into an
area of downwelling. It
becomes shorter (and fatter).
Ekman Convergence
f/H must be
conserved!
Mixed Layer
H
Ocean bottom
H
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.
More Realistic Climatological (average) Winds
Ekman transport creates convergence and
divergence of upper waters.
Divergence
Convergence
Divergence
Convergence
Divergence
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 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
Sea Surface Height and Mean Geostrophic Ocean Circulation
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
Sea Surface Height and Mean Geostrophic Ocean Circulation
L-Subpolar Gyre
H-Subtropical
Gyre
H-Subtropical
Gyre
H-Subtropical
Gyre
L-Subpolar Gyre
H-Subtropical
Gyre
HSubtropical
Gyre
HK
Guam
HA
P37 mean dyht and temperature field
Sea Surface Height
Temperature Field
Salinity Field
SF
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.
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?
North Pole (High planetary Vorticity f)
As the western boundary current
returns north, this should happen,
but it does not. Why?
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)
Back to our example of
conservation of vorticity
when H stays constant
North Pole (High planetary Vorticity f)
Parcel wants to spin
But can’t due to friction
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.
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)
Wind-driven surface currents
Figure 7-4
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
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
Other types of upwelling
Equatorial
upwelling
Offshore wind
Sea floor
obstruction
Sharp bend in
coastal geometry
Figure 7-9
Equatorial upwelling
Other examples of upwelling (Which one looks like San Diego?)
Antarctic surface circulation
Figure 7-13
Ocean surface currents
What Currents do you need to
know?
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
Flows are typically unstable; they meander
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
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 oceanicatmospheric disturbance
Average conditions in the Pacific Ocean
El Nino/La Nina Animation
Figure 7-18a
El Niño conditions (ENSO warm phase)
Figure 7-18b
La Niña conditions (ENSO cool phase;
opposite of El Niño)
Figure 7-18c
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
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
Effects of severe El Niños
Figure 7-21
La Nina
El Nino
End of Chapter VII
Essentials of Oceanography, Thurman and Trujillo
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.
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.
Topex/Poseidon dynamic topography after GEOID has been added
Ocean surface currents
Figure 7-14
North Atlantic Ocean circulation
Sverdrup: measure of flow rate (length3/time) 1 Sv = 106 m3/s
Figure 7-15
Pacific Ocean surface currents
Figure 7-17
Indian Ocean surface currents
Northeast monsoon
Figure 7-23
Southwest monsoon