Chap 6 Water & Ocean Structure

1. Chap 6 Water & Ocean Structure Physical Oceanography

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. 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. 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. 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. 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. 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. Freezing Water • Density curve (6.6) shows the relationship between the temperature or salinity of a substance and its density.

10. 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.

11. 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?

12. 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

13. 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?

14. Solids dissolved lowers specific heat by 4% (heats faster) Ions also interfere with the freezing point, the saltier the lower the freezing point No solids, water requires 1 cal to heat up vs. 0.96 cal sea water. No ions to interfere with the freezing point Sea Water vs. Pure Water

16. 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.

18. 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 doesn't keep rising forever. • It eventually reaches an altitude where it is the same temperature (and density) as the surrounding air.

19. It then 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)

21. 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.

22. Review the Concepts • Contrast sea water and fresh water • What causes the seasonal changes? • Why the poles are cold?

23. 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.

24. 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)

25. 2. Evaporation freezing (& thawing) of sea ice • Salinity changes occur only at the surface of the ocean

26. 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

27. 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)

29. In deep 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

31. 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

32. 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

33. 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

35. 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)

37. 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

39. Side-Scan Sonar – towed behind a vessel (6.22) • Used for geological and archeological studies, and the location of downed ships and airplanes

40. Review the Concepts • What kind of temperature does most of the world ocean has? • What is characteristic about the ocean's deep sound channel (sofar layer) ? • What is called a zone in which the ocean's salinity increases rapidly with increasing depth? • Which zone does the most pronounced or marked all year around thermoclines exist?