Transcript Lecture2ns - Center for Coastal Physical Oceanography

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?
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Temperature
Salinity
Depth
Sea Surface Height
Sound
Light
Current Velocity
And many others……waves, met data,
etc.
Considerations for Observation
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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
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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°C
Nansen 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 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.
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
Fd  2 Fs ( VC ) cos( A)
Fd
Fs
V
C
A
: Doppler shifted frequency
: Frequency of sound when everything is fixed
: Relative velocity between sound and receiver
: Speed of sound in medium
: 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?