Geostrophy

1
Geostrophy

• Geostrophy describes balance between horizontal
  pressure Coriolis forces
• Relationship is used to diagnose currents
• Holds for nearly all large scale motions in sea
• Need to know horizontal pressure gradients

2
Horizontal Pressure Gradients
q
Mass same _at_ stations A B (bracket same
isobars) If rA gt rB -gt Height column _at_ B
higher than _at_ A Pressure gradient will push from
B to A Measure by slope of sea surface tanq
rA
rB
A
B
3
Horizontal Pressure Gradients

• Horizontal density differences lead to horizontal
  pressure gradients
• Provides the push for ocean circulations
• Something must balance the pressure gradient push

4
Geostrophy

• What balance HPF?
• Coriolis force!!!!

5
Earth Rotation

• Motions in a rotating frame will appear to
  deflect to the right (NH)
• Deflection will be to the right in the northern
  hemisphere to left in southern hemisphere
• No apparent deflection on the equator

6
Geostrophic Relationship

• Balance Coriolis force fu HPF
  g tanq
• Geostrophic relationship
• u (g/f) tanq
• Know f ( 2W sinf) tanq, calculate u

7
Estimating tanq

• Need to slope of sea surface to get at surface
  currents
• New technology - satellite altimeters - can do
  this with high accuracy
• Traditional methods use sea water properties
  our focus

8
Mapped SSH

• Satellite altimeter
• Direction is along Ds in SSH
• NOT ACROSS!!

9
Mapped SSH

• Satellite altimeter
• Direction is along Ds in SSH
• NOT ACROSS!!

10
Hydrostatic Pressure

• Hydrostatic pressure is simply the weight of
  water acting on a unit area at depth
• Mass seawater in column r A D
• A cross-sectional area of column m2
• D depth of water column m
• Weight column (r A D) g
• Mass acceleration gravity (g 9.8 m s-2)

11
Hydrostatic Pressure

• Hydrostatic pressure is the weight per unit area
• ph r g A D / A
• ph r g D
• Its the D we want

D
ph r g D
12
Dynamic Height

• Hydrostatics give us ph r g D
• Given isobars (ph) average r, D represents the
  dynamic height or
• D(0/1000 db) ph / (g r(0/1000 db))

13
Dynamic Height

• Dynamic height anomaly, DD(0/1500db)

14
Dynamic Height
15
Dynamic Height

• California Cooperative Fisheries Investigations
  (CalCoFI)
• Understand ocean processes in pelagic fisheries
• Started in 1947

16
Dynamic Height

• January 2000 - CalCoFI Cruise 0001

17
Dynamic Height

• DD(0/500db)
• Shows CA Current
• Recirculation in the SoCal Bight

18
Geostrophy

• Barotropic vs. baroclinic flow
• Flow is along lines of constant dynamic height
  (light on the right)
• Baroclinic portion can be diagnosed from CTD
  surveys
• Quantifies the circulation of upper layers of the
  ocean

19
Geostrophy

• Geostrophy describes balance between horizontal
  pressure Coriolis forces
• Relationship is used to diagnose currents
• It is quantitative valid for most large scale
  motions in sea