Transcript Physical Oceanography - SACS-AAPT

Physical Oceanography
SACS/AAPT Spring Meeting
March 29, 2003
Coastal Carolina University
What is Physical
Oceanography?
Physics applied to the oceans……
Properties and Motion of Seawater, including:
• Density, Pressure, Temperature
• Waves, Tides
• Global Heat Fluxes
• Equations of Motion
(Newton’s 2nd Law Applied to water…)
It’s how you explain this:
Equations of Motion
(Newton’s 2nd Law applied to water)
 F  ma
Oceanographers like to consider the force per volume of water:
a
F
m
V
V
1 F
a  V

Of course, oceanographers never write out F/V, it’s just understood:
Du 1
 F
Dt 
Coordinate System
Used for all calculations from now on….
Forces on a Parcel of Water
•
•
•
•
Gravity
Coriolis
Pressure
Friction
Du 1
 Fg  Fc  Fp  Ff
Dt 

Du 1
  Fx
Dt 
Dv 1
  Fy
Dt 
Dw 1
  Fz
Dt 

Coriolis “Force”
This apparent (or pseudo) force exists
because we all live on an accelerating
reference frame (a rotating planet).
Since our coordinate system is rotating
with the planet, it is indistinguishable
from a ‘real’ force, and we will treat it as
such.
Coriolis Force Simulation
Coriolis “Force”
Coriolis force = m f (speed)
m = mass of water parcel
f = Coriolis parameter = 2 W sin(j)
W = Earth’s angular velocity
j = latitude
Remembering that Oceanographers like their forces per volume:
Fcx   f v
Fcy   f u
Pressure Gradient
A Pressure Gradient is a measure of how the
pressure is changing as you move in the
horizontal direction. It is simply:
p

x
What are its units?
How is this a force?
Pa

m
kg
ms 2 
m
kgm
s2  N
m3
m3
Why this is a force:
F
P
A
Force between 1 and
2:
F12  F1  F2  (P2  P1 )A  PA
F  P y z
But we want force per volume…..
P y z
F
P


V
x y z
x
What happens then?
Because of this force, fluids move from areas of
high pressure towards areas of low pressure:
How do you get a pressure
gradient in the water?
How do you measure P?
P  P2  P1  g(h  h) gh  gh
 g h
P
Fp  

 gi
x
x
(i is the surface
slope)
Equations of Motion
Coming back to the x and y equations of
motion:
Du 1
1
P

  Fx    f v 
 Ff 
Dt 

x


Dv 1
1
P
  Fy    f u 
 Ff 
Dt 

y

Geostrophic Balance
If:
• Only important forces are Pressure Gradient
and Coriolis
• Ocean is in “Steady State” (no acceleration)
1
P 
0  f v 


x 
1
P 
0    f u 


y 
Geostrophic Balance (2)
1 P
f u 
 y
1 P
fv
 x
or
h
fvg
x
h
f u  g
y
Geostrophic Current:
Which way does the current flow if the surface
height is increasing towards the South?
West?
North?
Dynamic Topography
Geostrophy Movie
Gulf Stream
Quiz 1:
Which direction does the water flow
around this pressure feature if it is in the
Northern Hemisphere?
Counter-Clockwise
Cyclonic
Quiz 2:
Which direction does the water flow
around this pressure feature if it is in the
Southern Hemisphere?
Counter-Clockwise
Cyclonic
?
NO….anti-cyclonic