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

The physical setting of the Earth  Prolate ellipsoid Rotation  equatorial bulge  Measurement and unit Latitude  measure distance  1 0 Latitude = 111 km  Meridian Longitude  1 0 Longitude = 111 cos  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 – km Typical depth: 3 – 4 km  Similar scale to a piece of paper Exaggerated vertical scale of plot (Figure 3.4) Dynamical implication  v z  1% v x or v y  2D  vertical vortex lines  little vortex stretching  3D  vortex stretching  turbulence

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

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  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 = T T T Z + ( T ) (S - 35) ×10 -7 Z × T Z 3  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 = -kI o dx I = I o 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