ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY. Lecture 3

Transcription

1 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).

2 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.

3 1 Pressure Pressure is the force per unit area exerted by water (or air for the atmosphere). Units: Pascal or N/M 2, or dyn/cm 2 ; (1 Pascal=1 N/M 2 ; 1N=10 5 dyn) In the atmosphere, pressure is often measured by bars or milibars ; (1bar=10 5 Pascal=10 6 dyn/cm; 1mb=100Pa) Ocean pressure is often measured by decibars (dbar). 1dbar=0.1bar=10 4 Pascal.

4 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).

5 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: o C, Kelvin. 0 o C=273.16K

6 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)

7 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.]

8 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.

9 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.

10 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) The international thermodynamic equaiton of seawater 2010: calculation and use of thermodynamic properties. IOC, UNESCO, pp. 196pp. (salinity TEOS-10_Primer.pdf Units: g/kg

11 Density (ρ) Units: kg/m 3 ; g/cm 3 Fresh water: ~1000kg/m 3 ; Sea water: kg/m 3. At the sea surface: kg/m 3.

12 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:

13 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).

14 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.

15 3. Sound in the sea Detection in the ocean. Frequency: 1Hz ~ thousands of khz. Most instruments: khz, wavelength: 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

16 Sonar echo sounder: Mapping ocean floor Sound in the sea

17 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.

18 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.

19 SOFAR channel SOund Fixing And Ranging (SOFAR). Sound speed minimum: ~1000m ( m) 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.

20 Typical T & S profiles in mid-low latitudes; Sound speed profile calculated! From the T&S profiles on the left

21 SOFAR channel

22 Physical oceanography application: SOFAR channel Acoustic Thermometry of Ocean Climate (ATOC; program ended a few years ago; -- 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.

23 Downloaded from Heard Island Feasiblity Test (HIFT): Consistent With other in situ & satellite observations US Navy Sound surveillance system -- array of hydrophones (during cold war). (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 shift in distribution of humpback whales, etc. away from acoustic sources

24 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.

25 4. Light in the sea (assigned reading) Absorption and penetration. Visible light: ,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.

26 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: m (dysphotic zone; not much light, rapidly dissipates) Midnight zone: >1000m (aphotic zone; no lights) light_travel.html

27 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 ( m), aphotic zone (>1000m)