Physical Oceanography Intro

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Physical Oceanography Intro

• Physical properties of water, seawater
• Tides
• Equations of Motion
• Coriolis Force
• Geostrophic Currents
• Ekman Transport
• Thermohaline/Overturning Circulation
• Vorticity, Western Boundary Currents
• Vertical Structure
• Wind-Driven Circulation
• Ocean Factsheet

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Physical Properties of (Sea)Water

• H2O is a weird molecule
• Very high boiling point and freezing point
  relative to molecular weight
• Due to hydrogen bonds and van der Waals force

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Physical Properties of (Sea)Water

• High heat capacity
• High latent heat of fusion and evaporation
• Ice has lower density than liquid water
• High surface tension
• High dissolving power
• Rather transparent

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Salinity
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Salinity

• Freshwater is densest at about 4 C
• Salinity lowers freezing point, also lowers temp
  of max density
• At salinity of seawater, max density is at
  freezing point, ice much less dense, floats

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Ocean Tides

• Semidiurnal tide (12 hr) dominates many
  oceangraphic time series
• Diurnal tide ( 24 hr) also important in many
  places
• Why two high/two low tides per day?

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Ocean Tides
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Ocean Tides
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Ocean Tides with Sun
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Ocean Tides with Sun

• During new and full moon, sun and moon diurnal
  tides add constructivly, strong high and low
  tides
• During half moons, sun and moon add destructivly,
  moon wins, but high and low tides are less extreme

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Ocean Tides with Sun
Spring Tide
Neap Tide
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Governing Equations

• Seven variables u, v, w, pressure, density,
  temperature, and salinity
• Seven equations
• Continuity (mass conservation)?
• 3 momentum equations, one for each dimension
  x,y,z, called Navier-Stokes Equation
• Equation of state relating pressure, temp,
  salinity, and density
• Energy budget
• Salt budget

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Governing Equations - Continuity

• Continuity

• Water is nearly incompressible, so continuity
• can be approximated by

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Governing Equations - Momentum

• Navier-Stokes eq. really just Fma applied to a
  rotating fluid

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Coriolis Force

• Apparent force due to rotation of earth and
  fact that we observe motions on earth from within
  the rotating frame.
• Deflects moving water to right in N.Hem, to left
  in S.Hem.

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Simplifications to Nav-Stokes

• Full equations can be simplified by making
  various assumptions and neglecting certain terms.
• Applicable in real ocean in many places
• Geostrophic Currents Pressure gradient force
  balances Coriolis force
• Ekman Layers Friction balances Coriolis force

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Geostrophic Currents

• Pressure Gradient Force balances Coriolis Force

PGF
PGF
Cor Force
Cor Force
L
H
Resulting Current
Plan View
Side View
Sea Surface Slope
PGF
Cor Force
L
H
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Geostrophic Currents

• North Atlantic Sea Surface Height

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Ekman Layer

• Frictional forces balance Coriolis Force
• Important in boundary layers such as sea surface
  and near seafloor

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Ekman Transport

• Mean transport in surface Ekman layer is to right
  (in N.Hem) of wind direction
• Leads to coastal and equatorial upwelling

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Thermohaline Circ

• Salty Atlantic water is cooled in North Atlantic,
  near Greenland. Becomes cold, dense, sinks to
  bottom of Atlantic, spreads south.

AAIW
MED
NADW
AABW

• Southward deep current must be balanced by
  northward surface current

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Vorticity (spin-yness)?

• Planetary vorticity, f, increases toward poles
• Relative vorticity, ? ? x (u), curl of velocity
• Potential vorticity, q, is always conserved
  (absent external forcing, like wind stress)?

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Western Boundary Current

• Sinking water in N.Atl, must be balanced by
  northward flow.
• Return flow intensified at Western boundary
• Gulf Stream, similar situation in West Pacific
  (Kuroshio)?
• Why??

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Western Boundary Current

• Wind stress adds negative vorticity
• To balance, return flow on western boundary can
  add positive vorticity through friction with
  continent
• Prevents ocean from spinning faster and faster
  and faster and ...

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Vertical Structure
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Wind-Driven Surface Currents
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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 1.4 109 km3 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.
• Gulf Stream 30 cm/s _at_ Straits of Florida, 70-100
  _at_ Cape Hatteras. (Western Boundary Current)?
• Deep currents 1-10 cm/s
• Deep vertical currents 0.1cm/s or less.
  Difficult to measure directly