Chapter 3
The Physical Setting
Physical oceanography
Instructor: Dr. Cheng-Chien Liu
Department of Earth Sciences
National Cheng Kung University
Last updated: 2 October 2003
The physical setting of the Earth
 Prolate ellipsoid
• Rotation  equatorial bulge
 Measurement and unit
• Latitude  measure distance
10 Latitude = 111 km
Meridian
• Longitude
10 Longitude = 111 cosf km
• Meter
• Mile
• Nautical mile
Ocean and seas
 Oceans
• The Atlantic Ocean (Fig 3.1)
• The Pacific Ocean (Fig 3.2)
• The Indian Ocean (Fig 3.3)
 Seas
• Mediterranean Seas
General definition
The Arctic Sea
The Caribbean Sea
• Marginal Seas
The Arabian Sea
South China Sea
Dimensions of the oceans
 Dimension
• Area
70.8%
Order by size (Table 3.1)
• Width: 1500 – 13000 km
• Typical depth: 3 – 4 km
 Similar scale to a piece of paper
• Exaggerated vertical scale of plot (Figure 3.4)
• Dynamical implication
vz  1% vx or vy
2D  vertical vortex lines  little vortex stretching
3D  vortex stretching  turbulence
Bathymetric features
 Two types of crust
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•
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Oceanic crust: denser, 10km
Continental crust: lighter, 40km
Histogram of elevations: Fig 3.5
Plate tectonics  relative motion of crust  subsea
features (Fig 3.6)
 Influences of subsea features
• Ocean circulation)
Separate oceans (deeper waters)
Interrupt ocean currents
Produce turbulence
Lead to vertical mixing
Subsea features
 Basins (Fig 3.6)
 Canyon
 Continental shelf (Fig 3.7)
 Continental slopes
 Plains
 Ridges
 Seamounts (Fig 3.8)
 Sills
 Trenches (Fig 3.9)
Measuring the depth of the ocean
 Echo sounder
• Principle: Fig 3.10
• Measurements
1922 US Navy Destroyer Steward
1925 – 1927 German Meteor research and survey ship
Uneven distributed maps (Fig 3.11)
• Error sources
Sound speed 4% (table of mean sound speed  1%)
Hill regions  shallower depths off to the side
Ship position
Schools of marine zooplankton or fish  remapping
Gaps
Measuring the depth of the ocean
(cont.)
 Satellite altimetry
• Principle
Sea level (geoid)  ellipsoid
Variation of gravity  geoid undulations ( 60m) (Fig 3.12)
 Depend on the strength of the seafloor and the age of the seafloor feature
 Vary from region to region
Restless ocean  topography ( 1m)
 Sea level  gravity (Fig 3.13)
Echo sounder
 Measure the regional relationship between gravity and bathymetry
Satellite-altimeter
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Measure the height of the sea surface relative to the center of mass of the Earth
Accuracy: GEOSAT (few meters), Topex/Poseidon ( 5cm)
Interpolate between echo sounder measurements  bathymetry
First measured and classified by US Navy, released in 1996
Maps of the geoid with 3km spatial resolution
Bathymetric charts and data sets
 Maps of the sea floor with 3km spatial
resolution
• BODC
GEBCO digital atlas  1:10 million
• US NGDC
ETOPO-5 CDROM
 5-minute (5 nautical mile) grid
• Smith and Sandwell 1997
2-minute grid
Vertical accuracy: 100m
Fig 3.14
Sound in the ocean
 Sound
• Communication
• Measurement
• Navigation
 Sound speed
• Equation
C = 1448.96 + 4.591 T - 0.05304 T2 + 0.0002374 T3 + 0.0160 Z + ( 1.340 0.01025 T ) (S - 35) + 1.675×10-7Z - 7.139 × 10-13 T Z3
Range: 1450 – 1550 m/s (Fig 3.15)
Sensitivity
 4 m/s per degree
 16 m/s per km
 1.5 m/s per 1 increase in salinity
Sound in the ocean (cont.)
 Sound channel
• At a depth around 1000m
• Fig 3.16
• Practical importance
Propagate to great distances
• Range: 10 – 1200 m
Sound in the ocean (cont.)
 Absorption of sound
• dI = -kIodx
• I = Io exp(-kx)
• Range of k
0.08 dB/km at 1000 Hz
50 dB/km at 100,000 Hz
• Experiment in 1960
15 Hz  halfway around the world
 Use of sound
• 1950s  microphone on the sea floor  submarine
• Whales, subsea volcanic eruptions
Important concepts
 Scale of ocean is similar to a piece of paper
 Only three official oceans
 Exceed volume of ocean water 
continental shelves
 Echo sounder  maps of bathymetry with
poor spatial resolution
 Satellite altimeter  maps of bathymetry
• 3km spatial resolution and 100m depth accuracy
 Sound speed, channel, usage