Physical Oceanography Unit II

Physical Oceanography Physical oceanography is the study of the properties of seawater. There are 4 main topics: 1.Temperature 2.Salinity 3.Density 4.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 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.