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.