Brief Review of Lecture 1

Understanding Science, Oceanography, Physical Oceanography

Descriptive or Dynamical Approaches

Eulerian or Lagrangian techniques

History of oceanography

Oceans and ocean basins – oceans are not simply drowned low lying areas on the earth’s crust!

What physical properties do we observe?

• Temperature• Salinity• Depth• Sea Surface Height• Sound• Light• Current Velocity• And many others……waves, met data,

etc.

Considerations for Observation

• Cost

• Ease of measurement

• Time taken – ocean is dynamic!

• Precision – repeat observation without deviation

• Accuracy – in addition, should be consistent with a reference standard

Temperature

• Measure of the heat content of a body (SI unit: Celsius)

• Temperature of ocean can change if heat is lost or gained in situ heat is advected • Source of heat: mainly the sun (surface) geo-thermal (bottom) • Difference between heating of ocean and

atmosphere• Ocean body NOT uniform in temperature

Temperature structure with depth

• Warmer at top, cooler with depth• Thermocline – region of rapid

change of temperature;

permanent

seasonal

diurnal• Higher temperature, lower density

Global range of ocean temperature

Measurement of Temperature

• Expansion of liquid or metal

• Differential expansion (eg bimetallic strip)

• Vapor pressure of liquid

• Thermocouple

• Electric resistance (thermistor)

• IR radiation from sea surface (remote sensing)

Reversing thermometer

• Temperature affected by pressure.

• Reversing thermometers allow flow of mercury in one direction only through special capillary tube– thus when flipped, they retain in-situ temperature.

• Accuracy: ± 0.02°CNansen bottle (1910)

Thermistors

• At first-order approximation, resistance is linearly proportional to temperature.

• ΔR = k ΔT

where

ΔR = change in resistance

ΔT = change in temperature

k = first-order temperature coefficient of resistance

Accuracy = ±0.1°C

Niskin bottle (1966)

Mechanical Bathythermograph

• Liquid in metal thermometer (toluene in copper)

• Many limitations (max depth 300 m, hysteresis and creep, can be deployed at low ship speed only)

• Accuracy < 0.06°C

Expendable Bathythermograph

Accuracy = ±0.1°C (??)

IR derived SST – Aug 28, 2006

Salinity

Practical salinityRule of constant proportions: Ratio between chemical elements more or less constant and range of salinity quite small.

Colligative property – based on number of ions/molecules, not type.

Range of salinity

• 75% of ocean water between 34.5 and 35

• Lowest in coastal waters

• High in enclosed seas and evaporative basins

• Pacific salinity much lower than Atlantic – important repercussions for circulation and climate!

• Higher the salinity, higher the density

Global range of salinity

Conductivity-Temperature-Depth sensor

• T accuracy: ±0.001°C• C accuracy: 0.0003 S/m

~ 0.0024 on PSS

• Response time: Time required for instrument to respond to temperature of a new environment.

TAO/TRITON (formerly TOGA/TAO)

• Real-time data from 70 moored ocean buoys for improved detection, understanding and prediction of El Niño and La Niña.

• Uses ARGOS satellite system• Supported by USA, Japan and France

TAO/TRITON hardware

ARGO Program

• Up to 3000 floats in upper 2000 m of ocean

• International collaboration of about 23 countries

• Used with Jason satellite

ARGO status

ARGO float

ARGO simple cycle

Current Velocity

• Current meters • Acoustic Doppler Current Profiler

ADCP Based on concept of Doppler shift of

frequency when relative positions of source and receiver change

)cos()(2 AFF CV

sd Fd : Doppler shifted frequencyFs : Frequency of sound when everything is fixedV : Relative velocity between sound and receiverC : Speed of sound in mediumA : angle between acoustic beam and water velocity

The greater the angle of the transducer heads with the vertical, the more surface data is lost

Depth/Pressure

• Rope/line over a meter wheel

• Pressure gauge - pressure proportional to depth (hydrostatic balance) – correction for inverse barometric effect (eg. Tide gauge) [1 dB ~ 1 m]

• Echo sounding – time taken for acoustic signal to make trip to sea-floor and back is proportional to distance traveled.

Something to think about…

• Ocean varies on different temporal and spatial scales

• Our ability to understand these variations only as good as our instrumentation

• What processes we resolve depend on our sampling plan (duration, frequency, extent, ….)

• What drives ocean variability?• How does the ocean respond to such forcing?