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Chapt 6 water & ocean structure


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Chapt 6 water & ocean structure

  1. 1. Chap 6 Water & Ocean Structure Physical Oceanography
  2. 2. Water and Heat Sun is exclusive source of energy driving ocean and atmospheric currents. The Sun radiates throughout the electromagnetic spectrum, but principle radiation is in visible part of the spectrum. Visible light is strongly absorbed by seawater
  3. 3.  Heat – energy produced by random vibration of atoms or molecules. Temperature – object’s response to an input or removal of heat. Specific heat – heat required to raise the temperature of 1 g of substance by 1 °C. (calories / g)
  4. 4. Infrared Energy in clear water, only 10% reaches 25 m, only 0.5% reaches 100 m, .0025% reaches 200 m essentially all energy gain in the oceans takes place in upper 10 -100 m of water
  5. 5. Concept of Steady State Averaged over the globe and over a year, the Earth loses as much energy as it gains. Green house effect may be changing the steady state
  6. 6.  There is a net gain of energy at low latitudes and a net loss of energy at high latitudes. This latitudinal difference is energy gain and loss drives both ocean and atmospheric circulation. All energy exchange by the oceans occurs at the surface This exchange of energy controls the temperature of ocean water masses
  7. 7. Density Mass / volume (g / cm3) Density depends on temperature and salinity Ocean density ranges from 1.02 to 1.03 g/cc. Density differences, together with winds, are the principal factors determining ocean currents.
  8. 8. Freezing Water Density curve (6.6) shows the relationship between the temperature or salinity of a substance and its density.
  9. 9.  Water density decreases as the water freezes Angle between water molecules expand from 105 ° to 109 ° Forms a crystalline lattice – less dense, hence ice float.
  10. 10.  Sensible heat loss – detectable decrease in heat, measured with a thermometer, before ice freezes Latent [hidden] heat of fusion – amount of heat removed to form ice per g of water (80 calories) This process of freezing and thawing moderates global temperature swings. Why?
  11. 11. Review the Concepts Heat is transmitted in the ocean in which wave length? Define density The density of a parcel of seawater will be affected by which factors
  12. 12. Evaporating Water Latent heat of evaporation – amount of energy required to break hydrogen bonds 585 cal / g at 20 °C Why such a big difference between latent heat of evaporation and the latent heat of fusion?
  13. 13. Sea Water vs. Pure Water Solids dissolved  No solids, water lowers specific heat requires 1 cal to by 4% (heats faster) heat up vs. 0.96 cal Ions also interfere sea water. with the freezing  No ions to interfere point, the saltier the with the freezing lower the freezing point point
  14. 14. Solar Energy Inputs the sun makes a direct hit at equator, while the same sunlight is spread over a larger area at the poles. This is just another way of showing that the equator is heated up more than the north or south poles of the Earth. This uneven heating of our round globe causes the air at the equator to rise, cool, and then wring out its moisture as rain.
  15. 15.  The equator, then, is a zone of low pressure systems and lots of rainfall. This zone extends from roughly 5°N to 5°S of the equator. The air doesnt keep rising forever. It eventually reaches an altitude where it is the same temperature (and density) as the surrounding air.
  16. 16.  Itthen spreads out laterally, both in a north direction, and in a south direction. As it moves poleward (either north or south from the equator), the air continues to cool, and finally, sinks. Where it sinks, the pressure is high. Heat budget is balance (p.163, f. 7.10)
  17. 17. Density Structure of the Ocean Winds are the primary driving force of the surface circulation, which is also called wind- driven circulation, density differences drive the deep, or vertical, circulation of the oceans. The density of seawater is controlled by temperature and salinity, so the deep circulation is also called the thermohaline circulation.
  18. 18. Review the Concepts Contrastsea water and fresh water What causes the seasonal changes? Why the poles are cold?
  19. 19.  Temperature differences as small as a few hundredths of a degree and salinity differences of a few parts in a hundred thousand can be important. Both temperature and salinity are conservative properties of seawater, that is, there are determined by processes occurring at the surface.
  20. 20. Salinity Salinity refers to the weight fraction of dissolved solids in water. Average salinity of seawater is about 35‰ (‰ and ppt mean “parts per thousand). Principal processes that change salinity are: 1. dilution (by rainwater and river water)
  21. 21.  2. Evaporation freezing (& thawing) of sea ice Salinity changes occur only at the surface of the ocean
  22. 22.  Because temperature and salinity change only at the surface density changes occur only at surface Water masses can be identified by their temperature-salinity characteristics. Density, together with winds, govern ocean currents
  23. 23. Ocean Structure Upper 100-500 m to have uniform temperature and salinity because of mixing by waves.(6.13) Below this, to a depth of ~1000 m, Temp., Salinity, and density change ( Thermocline, Halocline, Pycnocline) (6.12)
  24. 24.  Indeep water, temperature, salinity and density are relatively uniform This structure varies latitudinally. At mid-latitudes, it also varies seasonally: upper mixed layer will deepen in summer; thermocline might largely disappear in winter
  25. 25. Sound Is a form of energy transmitted by rapid pressure changes in an elastic medium. Intensity decreases as it travels through seawater until eventually is absorbed and converted into heat Speed is 1,500 m / s, almost five time the speed in air
  26. 26. Echolocation Marine mammals use sound rather than light to “see” in the ocean Echolocation –use of reflected sound to detect environmental objects MM use echolocation to detect prey and avoid obstacles
  27. 27.  Speed of sound increases as temperature and pressure increases (6.21) Travels faster at the surface than in deeper, cooler water. Minimum speed at 600 – 1,200 m Below this depth the pressure offsets the temperature and speed increases again
  28. 28. SOFAR Layer Sound Fixing and Ranging Transmission of sound in this minimum- velocity layer is very efficient because refraction tends to cause sound energy to remain within the layer (6.20) Loud sounds made at this depth can be heard for thousands of kilometers Sound generated in the India Ocean was hear as far a way as the Oregon Coast (Box 6.1)
  29. 29. SONAR Sound Navigation and Ranging Active SONAR – projection of short pulses of high frequency sound to search for objects in the ocean. Operator can tell direction, size, heading and even the composition by analyzing the composition of the returned ping
  30. 30.  Side-Scan Sonar – towed behind a vessel (6.22) Used for geological and archeological studies, and the location of downed ships and airplanes
  31. 31. Review the Concepts What kind of temperature does most of the world ocean has? What is characteristic about the oceans deep sound channel (sofar layer) ? What is called a zone in which the oceans salinity increases rapidly with increasing depth? Which zone does the most pronounced or marked all year around thermoclines exist?