Physical Oceanography

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Physical Oceanography
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Outline

• Governing Equations (sorry about this)
• Geostrophy
• Ekman Dynamics
• Vorticity
• Circulation Patterns Water Masses
• Circulation Theories
• Regional Oceanography
• Ocean Factsheet

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Governing Equations

• Navier-Stokes
• The material derivative as modified by Coriolis
  equals the pressure gradient over the density and
  some kind of drag/viscosity.
• Conservation of Mass

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Governing Equations

• Hydrostatic Equilibrium
• The change of pressure with depth equals the
  density times gravity. The negative sign is a
  convention that applies when z increases
  downward.
• Equation of State
• Density is modified by thermal expansion and
  saline contraction.

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Geostrophy

• Applies when the Rossby radius is small. Ro
  U/fL, i.e. when Coriolis is important relative to
  inertia.
• Barotropic flow isobars and isopycnals are
  parallel
• Baroclinic flow isobars are inclined relative to
  isopycnals
• Quasi-geostrophy even when Ro 1, geosptrophic
  equations can be useful if vertical mixing is
  small and you look at timescales gt day in a
  stratified fluid.

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Ekman Dynamics Ekman Spiral
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Ekman Dynamics Coastal
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Vorticity

• Potential vorticity (planetary vorticity
  relative vorticity)/height
• Potential vorticity is ALWAYS conserved
• Planetary vorticity increases poleward
• Relative vorticity increases when
• a) the water column is stretched
• b) shear from a clockwise flow
• Vorticity is positive in the counterclockwise
  direction

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Water Masses

• North Atlantic Deep Water forms sporadically in
  convective cells in N. Atlantic. 17 Sv
• Antarctic Bottom Water forms in polynyas.-1.9 to
  .3 deg C, 34.7 ppt. 17 Sv
• Antarctic Intermediate Water 2-4 deg C, 34.3 ppt

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(No Transcript)
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Ocean Circulation Patterns
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Ocean Circulation Theories
Sverdrup interior the curl of the windstress
generates flow. In the ocean, the curl is
negative, leading to a clockwise circulation.
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Regional Oceanography Currents

• Gulf Stream 30 m/s _at_ Straits of Florida, 70-100
  _at_ Cape Hatteras. Western Boundary Current (WBC)
• Brazil current WBC, 4-20 m/s (WBC)
• Kurishio WBC, 50 m/s (WBC)
• Agulhas 100 m/s, goes around S. Africa.
• Antarctic Circumpolar Current 100 Sv through
  the Drake Passage
• Indonesian Throughflow 10 Sv

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Regional Oceanography Equatorial Currents
Speeds from 20-50 m/s NEC SEC flow to the
west ECC EUC flow to the east
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Regional Oceanography
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Ocean Factsheet

• Pacific Ocean 52 of ocean area, mean depth
  4000m
• Atlantic Ocean 25 of ocean area, mean depth
  3300m
• Indian Ocean 20 of ocean area, mean depth 3900m
• Ocean volume 1.4E9 cubic km of water
• Salinity mainly from Cl (55) and Na (30),
  ranges from 33 to 37 ppt, avg. is 34.9
• Temperature ranges from -2 to 30 deg C, avg is
  3.5 deg C.
• 75 of the ocean has a salinity of 34-35 ppt and
  a temperature of 0-6 deg C.
• Average sea surface temperature 17.5 deg C.

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Ocean Circulation 1 What drives ocean
circulation? Seawater flows along the horizontal
plane and in the vertical. Typical speeds of the
horizontal flow or currents are 0.01-1.0 m/s
vertical speeds within the stratified ocean are
much smaller, closer to 0.001 m/s. Two forces
produce the non-tidal ocean currents the wind
exerting a stress on the sea surface and by
buoyancy (heat and freshwater) fluxes between
the ocean and atmosphere that alter the density
of the surface water. The former induces what we
call the wind driven ocean circulation, the
latter the thermohaline circulation. The wind
driven circulation is by far the more energetic
but for the most part resides in the upper
kilometer. The sluggish thermohaline circulation
reaches in some regions to the sea floor, and is
associated with ocean overturning linked the
formation and spreading of the major water masses
of the global ocean, such as North Atlantic Deep
Water and Antarctic Bottom Water. 2 Wind
induced upwelling The wind stress acting on the
surface layer of the ocean induces movement of
that water. This is called Ekman Layer
transport, which extends to the surface 50 to 200
meters. The Ekman transport is directed at 90
to the direction of the wind, to the right of
the wind in the northern hemisphere, left of the
wind in the southern hemisphere. As the wind
varies from place to place, Ekman transport can
produce divergence (upwelling) or convergence
(sinking) of surface water. 3 Geostrophic
Currents The surface layer is less dense (more
buoyant) than the deeper layers, therefore a
spatially variable Ekman transport field acts to
redistribute the buoyant surface water thinning
the buoyancy surface layer in divergence regions,
thickening the buoyant surface layer in
convergence regions. As the ocean is in
hydrostatic equilibrium, the redistribution of
the buoyant surface layer induces sea level
"valleys" in divergent regions and "hills" in
convergence regions. While these hills and
valleys amount to only a 1.5 meter in
amplitude, they are sufficient to induce
horizontal pressure gradients which initiate the
wind driven circulation following the geostrophic
balance concept. The ocean currents are for the
most part geostrophic, meaning that the Coriolis
Force balances the horizontal pressure gradients.
The wind driven circulation is characterized by
large clock-wise and counter clock-wise flowing
gyres, such as the subtropical and sub polar
gyres. The Antarctic Circumpolar Current is also
a wind driven current in contrast to the
subtropical gyres it reaches the sea floor. 4
Thermohaline Circulation As surface water is
made denser through the removal of heat or
freshwater, the surface layer descends to deeper
depths. If the stratification is weak and the
buoyancy removal sufficient, the descent would
reach the deep sea floor. Such deep reaching
convention occurs in the northern North Atlantic
(North Atlantic Deep Water) and around
Antarctica (Antarctic Bottom Water). The
thermohaline circulation engages the full volume
of the ocean into the climate system, by
allowing all of the ocean water to 'meet' and
interact directly the atmosphere (on a time
scale of 100-1000 years).