physical oceanography unit ii. physical oceanography physical oceanography is the study of the...
TRANSCRIPT
Physical Oceanography
Unit II
Physical Oceanography
• Physical oceanography is the study of the properties of seawater.
• There are 4 main topics:1. Temperature2. Salinity3. Density4. Pressure
Temperature
• Water has the highest specific heat (heat capacity) of any liquid: a high amount of energy is needed to raise or lower the temperature of water.
• As a result, sea water takes a long time to heat up or cool down.
• Earth’s climate is kept moderate because of this property of water.
• Compare the differences in extremes of temperature on land and water:– Land: Libya (64ºC) to Antarctica (-89ºC)– Ocean: Persian Gulf (36ºC) to Southern Ocean
(-2ºC)
Temperature and Depth
• Temperature decreases with depth, but not at a constant rate.
• On our coast, the surface temperature fluctuates with the season, and the temperature changes only a few degrees from the surface to about 200 m deep.
• Below 200 m, it is too deep for the temperature to be influenced by the sun and temperature drops very rapidly until about 1000 m.
• This region is called the permanent thermocline.
Permanent Thermocline
• The average ocean temperature is about 4ºC and the average surface temperature is about 18ºC.
• In the tropics, day and night are about equal in length and the surface temperature rarely falls below 25ºC.
• In the Arctic, winter nights are long, so water loses a lot of its heat resulting in ice formation.
Methods of Temperature Measurement
1. Expendable Bathythermograph (XBT)
It is launched from a ship. It records temperature as it falls and depth (by measuring density).
• The XBT is used for depths down to 2000 m (usually 410-760 m).
• Two very small wires transmit the temperature data to the surface.
• The probe is designed to fall at a constant rate, so depth can be inferred from time since it was launched.
2. Niskin Bottles
• Used for collecting water samples from various depths.
• Usually arranged in a rosette of up to 36 bottles. Lowered to maximum depth, then pulled up slowly, taking samples in a series as it is raised.
• Can have reversing thermometers along the side to record temperature.
• A CTD (conductivity, temperature, depth) probe is often attached to the rosette to measure salinity, temperature and pressure all at once as a water sample is taken.
3. Infra-red Photography• From airplane or satellite, for
measuring surface temperature over a wide area (ex: North Atlantic on July 15), then colorized.
4. Stationary and Drifting Buoys• Data often includes surface and
subsurface water temperatures, air pressure, air temperature, wind speed and direction.