Chap 6 Water & Ocean Structure Physical Oceanography
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
– 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)
Heat
Infrared Energy
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
in
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
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
Density
/ 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.
Mass
Freezing Water
Density
curve (6.6) shows the relationship between the temperature or salinity of a substance and its density.
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.
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?
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
Evaporating Water
Latent
heat of evaporation – amount of energy required to break hydrogen bonds
cal / g at 20 °C Why such a big difference between latent heat of evaporation and the latent heat of fusion?
585
Sea Water vs. Pure Water
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
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.
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.
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)
Density Structure of the Ocean
Winds are the primary driving force of the surface circulation, which is also called winddriven 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.
Review the Concepts
Contrast
sea water and fresh water What causes the seasonal changes? Why the poles are cold?
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.
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)
2.
Evaporation freezing (& thawing) of sea ice Salinity changes occur only at the surface of the ocean
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
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)
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
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
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
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
SOFAR Layer
Sound Fixing and Ranging Transmission of sound in this minimumvelocity 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)
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
Sonar – towed behind a vessel (6.22) Used for geological and archeological studies, and the location of downed ships and airplanes
Side-Scan
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?