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: Read Chapter 1 of the notes

ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY

Lecture 3

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 0∘C; seawater, -2∘C;

Why does sea ice melting freshen the sea surface? Freshwater freezes first

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/cm2; 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 (ML) 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 equation 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)

• Fresh water: ~1000kg/m3; • Sea water: 1020-1050 kg/m3.

• At the sea surface: 1020-1029 kg/m3.

Units: kg/m3 ; g/cm3

Density

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:

ρ(T,S,P). (T,S) => water mass. Density is important because: water parcels basically move along isopycnic surfaces.

Equation of state: density as a function of T,S &P

Where is the density of pure water with S=0.

(see Gill, appendix 3 for the equation at pressure p).

At one standard atmosphere (effectively p=0) is:

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~ varies by a few % with (P,T,S). P , C ; (S, T) , C

However, in real ocean: S change is small (~1 psu); T & P change are large…

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 mid-and low latitudes. Near surface in subpolar and polar regions

• SOFAR channel acts as waveguide. Send out beams with moderate angle from the horizontal direction, refraction makes the sound waves channeled.

!Typical T & S profilesin mid-low latitudes;

Sound speed profile calculatedFrom the T&S profiles on the left

SOFAR channel

Physical oceanography application: SOFAR channel

• Acoustic Thermometry of Ocean Climate (ATOC) --measuring large-scale ocean circulation change (gyres, ENSO variability, global warming, etc): 1996-2006http://staff.washington.edu/dushaw/atoc.html

• 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.

• Dushaw, B. D., 2014. Assessing the horizontal refraction of ocean acoustic tomography signals using high-resolution ocean state estimates, J. Acoust. Soc. Am., 136, 1−8. doi: 10.1121/1.4881928

• Dushaw, B. D., P. F. Worcester, W. H. Munk, R. C. Spindel, J. A. Mercer, B. M. Howe, K. Metzger Jr., T. G. Birdsall, R. K. Andrew, M. A. Dzieciuch, B. D. Cornuelle, and D. Menemenlis, 2009. A decade of acoustic thermometry in the North Pacific Ocean, J. Geophys. Res., 114, C07021. doi: 10.1029/2008JC005124

•US Navy Sound surveillance system --array of hydrophones (during cold war).

http://staff.washington.edu/dushaw/atoc.html

Heard Island Feasiblity Test (HIFT): ConsistentWith 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:No obvious short-term Changes; some subtle shiftin distribution of humpbackwhales, etc. away from acousticsources

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• 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)