ATOC 5051 INTRODUCTION TO
PHYSICAL OCEANOGRAPHY
Lecture 3
Learning objective: know and understand the
Properties of sea water:
1 Pressure;
2 Temperature, salinity, density & tracers;
3 Sound in the sea;
4 Light in the sea (reading assignment).
Properties of seawater
•  Pure water:
•  One major difference between pure and sea
water is: salt in seawater (salinity).
•  Pure water physical properties are functions
of Pressure (P) and temperature (T), whereas
those of the seawater are functions of P, T,
and salinity (S).
•  Freezing point: pure water at 0C;
seawater, -2C.
1 Pressure
•  Pressure is the force per unit area exerted by
water (or air for the atmosphere).
•  Units: Pascal or N/M2, or dyn/cm2;
(1 Pascal=1 N/M2 ; 1N=105dyn)
•  In the atmosphere, pressure is often measured
by “bars” or “milibars”; (1bar=105 Pascal=106
dyn/cm; 1mb=100Pa)
•  Ocean pressure is often measured by decibars
(dbar). 1dbar=0.1bar=104 Pascal.
Pressure
•  Why is pressure important for ocean circulation?
Pressure gradient force (PGF)- a major force
govern fluid motion: arises from pressure difference
from one point to another.
•  PGF direction: high to low;
•  Ocean: P(z) - depends on the mass of water above
(hydrostatic equation ~ later classes). (Range: 0 at
z=0 to ~10,000dbar near ocean bottom, if we ignore
atmospheric pressure).
2 Temperature (T)
•  Sea Surface Temperature (SST):
Important variable for air-sea interaction and
driving atmospheric circulation.
T is also important for biological activities.
•  Units: oC, Kelvin. 0oC=273.16K
2 Salinity (S)
•  Original definition: the number of grams of dissolved
matter in one kilogram of sea water.
Old method to measure salinity: evaporate sea water
and weigh the residual.
Significance:
(1) S affects density (how?) => stratification => mixed
layer formation & ML depth => water sinking and
rising=> thermohaline circulation;
(2) Stratification – affect heat content in the mixed layer
– air/sea interaction;
(3) S: an indicator of hydrological cycle (ocean gauge)
Salinity (S)
•  The “law” of constant proportion (Dittmar,
1884): Composition of dissolved matter does
not vary much from place to place. [Reason]
•  Given the constant proportion: measure one
component and then estimate the total
amount of dissolved material, which is S.
[Until 1950s.]
Salinity
•  Main constituent of sea salt: Chlorine ion
(Cl 55% of total); sodium ion (Na 30.6%).
•  In reality, proportion varies slightly with
geographical locations. Correction needs to
be made according to the location.
Salinity: Units
•  Original, g/kg; part per thousand (ppt).
•  Replaced by practical salinity unit (PSU).
•  Later suggestions by SCOR (scientific
committee for oceanic research): Unitless;
•  Thermodynamic Equation of Seawater – 2010
(TEOS-10): g/kg
•  Salt conservation (except for long geological time scales
~≥100,000yrs) in oceans. However, salinity does
change, depends on Precipitation-Evaporation,
river run off, etc.
New in 2010 (most recently)
•  IOC (Intergovernmental Oceanographic Commission),
SCOR (scientific committee for oceanic research), and
IAPSO (international association for the physical sciences
of the ocean) 2010.
•  The international thermodynamic equaiton of seawater –
2010: calculation and use of thermodynamic properties. IOC,
UNESCO, pp. 196pp. (salinity
•  http://www.teos-10.org/pubs/
TEOS-10_Primer.pdf
Units: g/kg
Density (ρ)
Units: kg/m3 ; g/cm3
•  Fresh water: ~1000kg/m3;
•  Sea water: 1020-1050 kg/m3.
•  At the sea surface: 1020-1029 kg/m3.
Density
•
ρ(T,S,P). (T,S) => water mass. Density is
important because: water parcels basically
move along isopycnic surfaces.
Sea water density: (T,S,P). Colder water
is denser. Saltier water is denser. Generally high
pressure increases density. The dependence is
nonlinear. Equation of state, based on laboratory
experiments:
Equation of state: density as a function of T,S &P
At one standard atmosphere (effectively p=0) is:
Where
is the density of pure water
with S=0.
(see Gill, appendix 3 for the equation at
pressure p).
Tracers
•  Dissolved oxygen, nutrients (nitrate,
phosphate, silicate,etc) are often used as
tracers for water masses.
•  Caution: non-conservative (consumed).
•  Salinity is often a good tracer.
3. Sound in the sea
•  Detection in the ocean.
•  Frequency: 1Hz ~ thousands of kHz. Most
instruments: 10-100 kHz, wavelength: 14 –
1.4 cm.
•  a) Echo sounding. Detect ocean depth.
D=(C t)/2 (C: speed)
•  b) Sonar-echo sounder. SONAR (SOund
Navigation And Ranging).
C in water ~ 1500m/s. C~(P,T).
P , C
;
T
,C
Sound in the sea
Sonar echo sounder:
Mapping ocean floor
•  High frequency (500khz-1MHz)
SONAR=>better resolution (small
objects and fine features) but
propagates for a short distance.
•  Lower frequency
(50-100KHz)=>lower resolution, but
propagates for a longer distance.
Sound in the Sea: SONAR
•  Detect submarine or school of fish.
•  Eco-sounder, emit sound beams and reflect
back. Can turn 360 degrees, reach hundreds
of meters in distance.
SOFAR channel
•  SOund Fixing And Ranging (SOFAR). Sound
speed minimum: ~1000m (600-1200m) in midand low latitudes. Near surface in subpolar and
polar regions (board demo)
•  SOFAR channel acts as waveguide.
Send out beams with moderate angle from the
horizontal direction, refraction makes the
sound waves channeled.
Sound
speed profile calculated
!
Typical T & S profiles From the T&S profiles on the left
in mid-low latitudes;
SOFAR channel
Physical oceanography application: SOFAR
channel
•  Acoustic Thermometry of Ocean Climate
(ATOC; http://atoc.ucsd.edu/: program ended a few years
ago; http://aog.ucsd.edu/thermometry/index.htm) -measuring large-scale ocean circulation
change (gyres, ENSO variability, global
warming, etc).
•  Place sound sources & receivers in SOFAR
channel. Based on the fact that C depends on
T. Increased T will result in faster C and thus
it takes a shorter time for the beam to arrive
at the receiver.
Heard Island Feasiblity
Test (HIFT): Consistent
With other in situ &
satellite observations
(small scale
structures, such as
Eddies, internal waves, etc. did not
have much impacts in scattering
the sound signals)
Marine Mammal Research
Program (MMRP) Results:
Downloaded from http://atoc.ucsd.edu/
No obvious short-term
Changes; some subtle shift
in distribution of humpback
whales, etc. away from acoustic
sources
• US Navy Sound surveillance system -array of hydrophones (during cold war).
Other applications
•  Tracking of vessels in distress (i.e., During World
War II, dropping into the ocean a small metal sphere (SOFAR bomb)
specifically designed to implode at the SOFAR channel – secret distress signal
by drowned pilots)
•  Humpback whales use the SOFAR channel
to communicate.
4. Light in the sea (assigned reading)
•  Absorption and penetration.
•  Visible light: 0.39-0.76
,from violet to
red, most absorbed within the upper a few
meters.
•  Light attenuation law:
- vertical attenuation coefficient.
- Clear water, k-0.02/m; turbid water: 2/m.
Light penetration: some through mixed layer.
Attracts modeler’s attention.
•  Euphotic zone: 0~200m (sunlight zone) –
contains the vast majority of commercial fisheries and is home
to many protected marine mammals and sea turtles
•  Twilight zone: 200-1000m (dysphotic zone;
not much light, rapidly dissipates)
•  Midnight zone: >1000m (aphotic zone; no
lights)
http://oceanservice.noaa.gov/facts/
light_travel.html
Summary
•  Salt – distinguish seawater from pure water; Pure water
physical properties – (T,P); seawater – (T,S,P);
•  Pressure: PGF – important for ocean circulation;
•  Temperature; salinity; density; salinity – good tracer;
•  Sounds in the sea – detecting objections
•  Light in the sea – strong absorption near the surface…
Euphotic zone (<200m), dysphotic zone (200-1000m),
aphotic zone (>1000m)