Learning objectives: know and understand the Properties of sea water; specifically:1. Pressure;2. Temperature, salinity, density & tracers;3. Sound in the sea;4. Light in the sea;5. Ice in the sea Reading assignment: Read Chapter 1 of the notesRecommended reading: Chapters 3 & S15 of Talley textbook

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 seawaterare functions of P, T, and salinity (S).

• Freezing point: pure water at 0∘C; seawater, ~ -2∘C with typical ocean salinity;

Critical thinking Question: 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 by 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.

Talley et al. 2011

Pressure• Why is pressure important for ocean circulation?

Pressure gradient force (PGF)- a major force that governs 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 especially regionally, 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

Talley et al. 2011

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 in ocean interior 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 (SOundNavigation And Ranging).

C in water ~ 1500m/s. C~ varies by a few % with (P,T,S) – nonlinear relationship.

P , C ; (S, T) , CHowever, 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

5. Ice in the seaSection 3.9 of Talley et al. 2011

Ice in the sea: ice formed in the sea by freezing of seawater (i.e., sea ice, which is hte majority) and ice broken off from glaciers (ice bergs)

Temperature and freezing point

Sea ice Iceberg

Brine rejection: older ice fresher; new ice: saltier

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)

• Ice in the sea