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Chapter 3

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

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Chapter 3

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  1. Chapter 3 The Physical Setting Physical oceanography Instructor: Dr. Cheng-Chien Liu Department of Earth Sciences National Cheng Kung University Last updated: 2October 2003

  2. 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

  3. 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

  4. 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

  5. Bathymetric features • Two types of crust • 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

  6. 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)

  7. 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

  8. 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 • 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

  9. 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

  10. 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) + 1.675×10-7Z7 - 7.139 × 1013 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 psu

  11. 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

  12. Sound in the ocean (cont.) • Absorption of sound • dI = -kIodx • I = Ioexp(-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

  13. 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

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