Brief Review of Lecture 1

1
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 earths crust!

2
What physical properties do we observe?

• Temperature
• Salinity
• Depth
• Sea Surface Height
• Sound
• Light
• Current Velocity
• And many otherswaves, met data, etc.

3
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

4
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

5
Temperature structure with depth

• Warmer at top, cooler with depth
• Thermocline region of rapid change of
  temperature
• permanent
• seasonal
• diurnal
• Higher temperature, lower density

6
Global range of ocean temperature
7
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)

8
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 0.02C

Nansen bottle (1910)
9
Thermistors

• At first-order approximation, resistance is
  linearly proportional to temperature.
• ?R k ?T
• where
• ?R change in resistance
• ?T change in temperature
• k first-order temperature coefficient of
  resistance
• Accuracy 0.1C

Niskin bottle (1966)
10
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 lt 0.06C

11
Expendable Bathythermograph
Accuracy 0.1C (??)
12
IR derived SST Aug 28, 2006
13
Salinity
14
Practical salinity
Rule 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.
15
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

16
Global range of salinity
17
Conductivity-Temperature-Depth sensor

• T accuracy 0.001C
• C accuracy 0.0003 S/m
• 0.0024 on PSS
• Response time Time required for instrument to
  respond to temperature of a new environment.

18
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

19
TAO/TRITON hardware
20
ARGO Program

• Up to 3000 floats in upper 2000 m of ocean
• International collaboration of about 23 countries
• Used with Jason satellite

21
ARGO status
22
ARGO float
23
ARGO simple cycle
24
Current Velocity

• Current meters
• Acoustic Doppler Current Profiler

25
ADCP

• Based on concept of Doppler shift of
  frequency when relative positions of source and
  receiver change

Fd Doppler shifted frequency Fs Frequency
of sound when everything is fixed V Relative
velocity between sound and receiver C Speed
of sound in medium A angle between acoustic
beam and water velocity
The greater the angle of the transducer heads
with the vertical, the more surface data is lost
26
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.

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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?