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The Earth's Physical Setting and Oceanography

Explore the dimensions, bathymetric features, and subsea characteristics of Earth's oceans. Learn how ocean depth is measured using echo sounders and satellite altimetry, and understand the role of sound in ocean communication and navigation. Discover important concepts in physical oceanography.

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The Earth's Physical Setting and Oceanography

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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) + 1.675×10-7Z - 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

  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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