Oceanography Lecture Notes Outline
I. Contents - Topics
Covered
The
Atmosphere – Ocean Interface
Wind-Driven
Surface Currents
Geostrophic
Gyres
Countercurrents
and Undercurrents
Other
Important Currents
Upwelling and
Downwelling
Surface
Currents’ Affect on Climate
El Nino and
the Southern Oscillation
Thermohaline
Circulation
II. The ATmosphere – Ocean Interface
A. The Atmosphere and Ocean Are Dynamic Fluid Layers
1. Both are dynamic,
density-stratified, multi-layered, fluid
spheres
·
The Atmosphere
Ø Troposphere (dense, weather layer)
Ø Stratosphere (ozone layer)
Ø Mesosphere (middle layer)
Ø Thermosphere (ionized layer)
·
The Ocean
Ø Surface zone (mixed layer)
Ø Pycnocline (middle
layer with rapid density change)
Ø Deep zone (cold stable layer)
2. Convection in the atmosphere is driven by latitudinal variations in solar input
(uneven heating of
the planet), which in turn powers the
wind-driven ocean
surface currents
·
Convection is the transfer
of energy via mass transfer
·
Equatorial regions have a
heat surplus
·
Atmosphere and ocean act in
concert in an
attempt to redistribute the excess heat
from low to
high latitudes
3. The more fluid atmosphere convects (moves) much more rapidly
than the
underlying ocean
·
Air currents (wind) flow
rates up to 200 kilometers per hour
·
Ocean currents flow rates
up to 10 kilometers per hour
B. The Atmosphere and Ocean are in a Never-Ending Dynamic State
of
1. This exchange is powered by
solar energy
2. Exchange of solar-derived heat between the ocean and atmosphere
is the
heart of the hydrologic cycle
·
Evaporation
·
Condensation
·
Precipitation
C. The Atmosphere–Ocean Interface is a Very Dynamic Interface
1. The great
density difference between bottom of atmosphere and the ocean
surface
2. Large
difference in flow regimes between the two (see A3 above)
3. Friction coupling between moving air
(wind) and water
4. Exchange of heat and gasses
5. Significant changes in surface area as a
function of wind speed
·
Calm conditions – smooth
seas; minimum surface area
·
Stormy conditions - rough seas; much higher surface area
III. Wind-Driven surface currents
A. Surface
Currents Mainly Confined to the Surface Zone
1. Involve about 10% (by volume) of the world ocean
2. Flow horizontally
3. Typically extend down to
about 400 meters (top of the
pycnocline)
4. Driven by wind-driven friction (ocean-air coupling)
Ø Terrigenous materials from land (wind-carried)
Ø Sea salt from ocean surface
B. Wind is the Primary Agent Responsible
for Surface Currents
1. Friction coupling between
wind and ocean surface causes surface water to
get piled up perpendicular to direction of wind
2. Higher pressure on
upwind side of piling up water
3. Piled-up water flows
“downhill” toward low pressure side of pile
·
Net water current flow is in the “downwind” direction
4. A persistent wind can
generate an ocean surface current beneath it.
5. Factors involved in the
initial generation of an ocean surface
current:
·
Wind persistence
·
Wind strength
·
Length of continuous stretch of ocean surface under a persistent wind current (termed a fetch)
6. The prime global-scale
winds responsible for surface current generation are
the powerful
Westerlies and the persistent Trades
(Easterlies)
7. Once generated, the
direction of a surface current will become affected by the
Coriolis effect
·
Surface currents deflected to the right in the Northern Hemisphere
·
Surface currents deflected to the left in the Southern Hemisphere
·
Ocean surface currents not found along the equator tend to follow
curved
paths
8. Continents and ocean basin topography will block surface
current flow and
further
deflect the surface flow into a circular pattern
9. The combination of the Coriolis effect and ocean
basin margins produce
circular surface current flow around the
periphery of ocean basins
·
These circular-flowing surface currents are called gyres.
·
See Figures 9.2 to 9.4 (page 210)
C. The Different Ways Currents Flow
1. Upwelling: ascending water masses
2. Downwelling: sinking water masses
·
Maintain continuity of
flow, vertical movement (0.1 - 1.5m/day)
·
Sinking waters may take
1000 years to reach great depths.
3. Horizontal water movement:
·
Convergence (meeting) and
divergence (spreading out)
D. Influence of Ekman Spiral and Ekman
Transport:
1. Coriolis effect acts on surface current
water
·
Deflects it from the wind
direction
2. Deflected by Earth's rotation
·
Right in N. hemisphere
·
Left in S. hemisphere
3. Transfer through the water column of wind-driven motion with depth to about 100 -150m down
·
Top layer of current
(directly powered by wind) transfers
some of
its kinetic energy to the layer beneath it.
·
This is repeated for
numerous horizontal sheets of water in
the the ocean column down to about 100 meters.
·
The Coriolis effect affects
each of the moving horizontal layers
·
The key point is that each
layer responds only to the layer above
it, and since there is a time lag involved, each horizontal layer in the current
will have a unique direction.
·
The overall effect is to
produce a vertically- oriented helix pattern of current directions – Called the
Ekman Spiral
·
See Figure 9.5 on page 211
4. Current speed in the Ekman spiral
decreases with depth
5. Net result:
·
Overall water movement is
at 90° to wind direction
·
Net current motion is
called the Ekman
Transport
·
Dependent on wind
persistence.
6. In nature we find that the overall water
movement is around 45° - not the
theorized 90°
·
Another factor is working
against the Coriolis effect
·
Attributed to a
current-induced pressure gradient (pile-up)
·
See Figures 9.6 and 9.7
(pages 211 and 212)
7. A deflecting surface current converges,
creating a hill of water piling up on
one side of the in the direction of
the deflection
·
Current tends to want to
turn towards the “downhill”
direction from the “hill” –
opposite to Coriolis effect
·
Overall effect is a path
between wind direction and 90° to the
wind
direction
·
See Figures 9.6 and 9.7
(pages 211 and 212)
IV. Geostrophic Gyres
A.
Geostrophic Gyres Defined
1. Circular,
basin-peripheral surface currents that are in balance between the
pressure gradient and the Coriolis effect.
2. Geostrophic gyres
of the Northern Hemisphere are
independent to the ones
in the Southern
Hemisphere
B. Major Geostrophic Gyres of the
1. There are five great Geostrophic gyres in the world ocean
·
South Atlantic
·
North Pacific
·
South Pacific
2. There is another
major surface current that is technically not a gyre:
·
The West Wind Drift or
Antarctic Circumpolar Current
·
Not confined to the
periphery of a single ocean basin
3. The convergence between Northern and Southern Hemispheric gyres does not
coincide with
the geographic equator
·
Coincides with the meteorological
equator
·
Displaced about 5° to 8°
north of geographic equator
4. Pattern of driving winds and positions of continents
shape the gyres
C.
The Major Surface Currents Within Geostrophic Gyres
1. The major currents within a
single Geostrophic gyre have different
characteristics
·
Each current
reflects differences in the factors that shape them
·
Each gyre has a
similar set of unique currents
·
Each current
within a gyre blends into one another
2. Currents are classified by geographic
position within the gyre
·
Western
boundary currents
ü The
ü The
ü The
ü The East Australian Current: South Pacific
ü The Agulhas Current:
·
The Eastern Boundary
Currents
ü The Canary Current:
ü The Benguela Current:
ü The
ü The
ü The West Australian Current:
·
The Transverse Currents
ü North Equatorial Currents:
ü South Equatorial Currents:
3. The Western
Boundary Currents
·
The fastest and deepest of
the three current types
Ø Up to 10 km/hr
Ø Can reach down to 1500 m deep in places
·
Form narrow, deep currents
along the eastern margins of ocean basins
·
Move warm water poleward
·
Each individual current
moves massive amounts of water
Ø Up to 50 million cubic meters per second
·
Maintains its identity for
very long distances
Ø
Sharp boundaries with
coastal circulation system
·
Prone to form warm-and
cold-water eddies
·
Coastal upwelling uncommon
·
Waters derived from trade
wind belts
·
Waters tend to be very
clear and nutrient poor
·
Likely responsible for
unusual abyssal ocean bottom storms
·
See Figures 9.8 to 9.12 for
illustrations (pages 213 to 218)
3. The Eastern
Boundary Currents
·
Have virtually opposite
characteristics compared to the western
boundary currents
·
Slower and more shallow of
the western boundary currents
Ø Up to 2 km/hr
Ø Typically reaches down to less than 500 m deep in places
·
Form broad, shallow
currents along the eastern margins of ocean basins
Ø Up to 1000 kilometers wide
·
Move cold water towards the
equator
·
Each individual current
moves relatively small amounts of water compared to its western counterpart
Ø Up to 15 million cubic meters per second
·
Has diffuse boundaries
separating from coastal currents
·
Coastal upwelling common
·
Waters derived from
mid-latitudes
·
See Figures 9.8, 9.9 and
9.12 for illustrations (pages 213, 214, and 218)
4. The
Transverse Currents
·
Directly derived from the
trade winds and mid-latitude Westerlies
·
Tropical trade winds drive
the east to west transverse currents
Ø
The convergent effect of
the trades cause
the east to west
current to be stronger
than its west to
east counterpart
·
Mid-latitude Westerlies
drive the west to east transverse currents
·
Moderately shallow and
broad
·
Links the western and
eastern boundary currents
5. The West Wind
Drift Current
·
Generated by the unimpeded
Southern Hemisphere mid-latitude Westerlies
Ø No continental interference
·
Carries more water than any
other current in the world ocean
Ø 100 million cubic meters per second
·
Technically a transverse
current
V. Countercurrents and undercurrents
A. Equatorial
Countercurrents