special dates: mpa meeting…6 jul r/v pt sur cruise…14 jul r/v pt sur cruise…25 jul

OC3230-PaduanOC3230-Paduan images Copyright © McGraw Hillimages Copyright © McGraw Hill

Chap 5-6: SeawaterChap 5-6: Seawater

SPECIAL DATES:SPECIAL DATES:

MPA meeting…6 JulMPA meeting…6 JulR/V Pt Sur Cruise…14 JulR/V Pt Sur Cruise…14 JulR/V Pt Sur Cruise…25 JulR/V Pt Sur Cruise…25 JulExam-1 (definite)...2 AugExam-1 (definite)...2 AugExam-2 (Tentative)…1 SepExam-2 (Tentative)…1 SepLabor Day Holiday...5 SepLabor Day Holiday...5 SepFinal Exam...19 SepFinal Exam...19 Sep

(Sp-226, 1300-1450)(Sp-226, 1300-1450)

EX 1EX 1

OC3230 Calendar, Summer 2005version 13 July 2005



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(-)(-)

Water molecules are unique:Water molecules are unique:105˚ angle produces polar molecule105˚ angle produces polar molecule

act as tiny magnets-good dissolveract as tiny magnets-good dissolverHydrogen bonds change the energy require-Hydrogen bonds change the energy require-ments for temperature and phase changesments for temperature and phase changes



Water exists in all three phases at the Earth’s surface temperature Water exists in all three phases at the Earth’s surface temperature range; Energy is required to change phaserange; Energy is required to change phase

Units: 1 cal = 4.18 JouleUnits: 1 cal = 4.18 Joule

680 cal680 cal



Water has a very high heat capacity compared with most otherWater has a very high heat capacity compared with most othersubstancessubstances



Note variationsNote variationsfor water infor water indifferent phasesdifferent phases



Fresh water is unique Fresh water is unique in that its maximum in that its maximum density is not at the density is not at the coldest temperaturecoldest temperature

Also, the phase Also, the phase change from water to change from water to ice leads to an ice leads to an unusual unusual decreasedecrease in in density due to change density due to change in molecular packing in molecular packing (ice has a more (ice has a more regular but less dense regular but less dense tetrahedral structure)tetrahedral structure)

4˚ C water has max density-4˚ C water has max density-implications for freshwater lakes and implications for freshwater lakes and winter-time ecosystemswinter-time ecosystems

Ice floats!-also important for Ice floats!-also important for ecosystemsecosystems

look ahead: freezing problemlook ahead: freezing problem

-30 0.9200 -60 0.9224-100 0.9257



Again, ice floats and 4˚ C water sinks (previous density Again, ice floats and 4˚ C water sinks (previous density versus temperature figure is more useful/important)versus temperature figure is more useful/important)



Dissolving ability of water: molecules can surround and isolateDissolving ability of water: molecules can surround and isolatecharged ions as a function of the polar nature of the moleculecharged ions as a function of the polar nature of the molecule

Seawater has nearly all known elements dissolvedSeawater has nearly all known elements dissolvedwithin itwithin it

What about salt water?What about salt water?



Salinity:Salinity:

““The total amount of solid materials in grams contained in The total amount of solid materials in grams contained in one kilogram of seawater when all the carbonate has been one kilogram of seawater when all the carbonate has been converted to oxide, the bromine and iodine replaced by converted to oxide, the bromine and iodine replaced by chlorine, and all organic matter completely oxidized.”chlorine, and all organic matter completely oxidized.”

This is an operational definition related to the practical This is an operational definition related to the practical methods used to compute salinitymethods used to compute salinity

Units: g/kg; parts per thousand, ppt (‰)Units: g/kg; parts per thousand, ppt (‰)

Open Ocean Range: 34.0–35.5‰Open Ocean Range: 34.0–35.5‰

Open Ocean Average: 35‰Open Ocean Average: 35‰

Measurement Methods: 1) Evaporation, 2) Absolute Measurement Methods: 1) Evaporation, 2) Absolute Salinity, SSalinity, SAA, from Chlorinity, and 3) Practical Salinity from , from Chlorinity, and 3) Practical Salinity from

conductivityconductivity



Why are the oceans salty?Why are the oceans salty?

1) Rivers dump sediments into the oceans1) Rivers dump sediments into the oceans

2) Mid-ocean ridges provide dissolved material to 2) Mid-ocean ridges provide dissolved material to the oceansthe oceans

Composition of dissolved salts is more like Composition of dissolved salts is more like material in ocean crust than river sedimentsmaterial in ocean crust than river sediments

Material from ridges are more solubleMaterial from ridges are more soluble

Chemical equilibrium keeps salinity constantChemical equilibrium keeps salinity constant

No evidence that salinity has been increasing No evidence that salinity has been increasing over time but, rather, it has been about the over time but, rather, it has been about the same as it is nowsame as it is now

Composition of dissolved salts is not like typical Composition of dissolved salts is not like typical river sedimentsriver sediments



A few major ions account for most of the dissolved materialA few major ions account for most of the dissolved material

The six most common are: ClThe six most common are: Cl--, Na, Na++, SO4, SO42-2-, Mg, Mg2+2+, Ca, Ca2+2+, K, K++

Two diagrams illustrating fraction of major ions dissolved in Two diagrams illustrating fraction of major ions dissolved in seawater (nearly all other elements are also contained in trace seawater (nearly all other elements are also contained in trace amounts):amounts):

??



Major ions in seawater Major ions in seawater are always found in the are always found in the same ratio one to same ratio one to anotheranother

This is known as the This is known as the “Law of Constant “Law of Constant Proportions”Proportions”

SSAA (‰) = 1.80655 x Cl (‰) = 1.80655 x Cl



Plants in the ocean Plants in the ocean require nutrients for require nutrients for cell growth and other cell growth and other chemosynthetic chemosynthetic activitiesactivities

Major nutrients are present in the form of Major nutrients are present in the form of Nitrate (NONitrate (NO33

--), Phosphate (PO), Phosphate (PO443-3-), and ), and

Silicate (SiOSilicate (SiO444-4-) ions) ions

Availability of these nutrients can control Availability of these nutrients can control productivity (e.g., upwelling supplies productivity (e.g., upwelling supplies nutrients and supports growth)nutrients and supports growth)



Plants in the ocean Plants in the ocean require nutrients for require nutrients for cell growth and other cell growth and other chemosynthetic chemosynthetic activitiesactivities

??

Some trace elements, in particular iron (Fe), are now believed to Some trace elements, in particular iron (Fe), are now believed to also be necessary for growthalso be necessary for growth



Back to density of Back to density of water…water…

What happens when What happens when you add salt?you add salt?

Properties of the Properties of the liquid that change liquid that change when the when the concentration of concentration of dissolved materialsdissolved materialschanges are known as “changes are known as “Colligative PropertiesColligative Properties””

Examples:Examples:Freezing point depression, boiling point elevation, and changes Freezing point depression, boiling point elevation, and changes in the temperature of maximum densityin the temperature of maximum density



fresh water fresh water valuesvalues changes with increasing salinitychanges with increasing salinity

At typical ocean At typical ocean salinity values salinity values (>24.70‰), liquid (>24.70‰), liquid water does not have water does not have a temperature of a temperature of maximum densitymaximum density

This has large This has large repercussions for repercussions for freezing of ocean freezing of ocean water versus fresh water versus fresh water (as seen in water (as seen in the relatedthe relatedhomeworkhomeworkproblem)problem)



At typical ocean At typical ocean salinity values salinity values (>24.70‰), liquid (>24.70‰), liquid water does not have water does not have a temperature of a temperature of maximum densitymaximum density

Ocean ranges of Ocean ranges of temperature and temperature and salinity are relatively salinity are relatively narrownarrow

Look ahead: this is an example of a Look ahead: this is an example of a T-S diagramT-S diagram



Homework Problem Homework Problem related to water density, related to water density, freezing points, and heat freezing points, and heat capacitycapacity

Problem set and related Problem set and related MATLAB library of MATLAB library of seawater density routines seawater density routines can be downloaded from can be downloaded from the course web sitethe course web site



How is salinity measured?How is salinity measured?

1)1) Absolute Salinity: Silver Nitrate Absolute Salinity: Silver Nitrate Titration method to determine Titration method to determine Chlorinity:Chlorinity:SSAA (‰) = 1.80655 x Cl (‰) = 1.80655 x Cl

2)2) Practical SalinityPractical SalinityMethod basedMethod basedon conductivityon conductivity(CTD=cond.-(CTD=cond.-temperature-temperature-depth)depth)

Must measure T very Must measure T very accurately along with accurately along with conductivity to get Sconductivity to get S

Must continuously check Must continuously check CTD calibration against CTD calibration against conductivity of “Wormly conductivity of “Wormly Water”Water”



Dissolved GasesDissolved GasesMixing with atmos. +Mixing with atmos. +photosynthesisphotosynthesis

Animal respirationAnimal respiration

Sinking from above +Sinking from above +decreased respirationdecreased respiration



Dissolved GasesDissolved Gases

Deep basins with little physical Deep basins with little physical mixing, due to (or resulting in) mixing, due to (or resulting in) strong vertical density stratification, strong vertical density stratification, can lead to anoxic conditionscan lead to anoxic conditionse.g., Black Sea, some fjords, some e.g., Black Sea, some fjords, some trenchestrenches

Mixing with atmos. +Mixing with atmos. +photosynthesisphotosynthesis

Animal respirationAnimal respiration


Chap 5-6: SeawaterChap 5-6: Seawater The Equation of StateThe Equation of State

The relationship between Temperature, Salinity, Pressure The relationship between Temperature, Salinity, Pressure and the resulting density, i.e., and the resulting density, i.e., = = ff(T,S,P)(T,S,P)

Recall in the atmosphere (in an ideal gas): P = Recall in the atmosphere (in an ideal gas): P = RTRTP P also, P Talso, P T

Related: Related: Potential TemperaturePotential Temperature = that temperature a water = that temperature a water parcel would have if it were raised from its actual depth to parcel would have if it were raised from its actual depth to some reference depth adiabatically (without exchanging some reference depth adiabatically (without exchanging heat)heat)

Computation requires knowledge of “adiabatic lapse rate”Computation requires knowledge of “adiabatic lapse rate”Several online calculators are available to giveSeveral online calculators are available to give = = ff(T,S,P,P(T,S,P,Prefref); usually ); usually = = ff(T,S,P,0)(T,S,P,0)



Potential Temperature, Potential Temperature, , is less than or equal , is less than or equal to the in situ (i.e., to the in situ (i.e., observed) temperature, observed) temperature, TT

Difference is not Difference is not significant in shallow significant in shallow waters (above ~500m)waters (above ~500m)



What about the equation of state for seawater?What about the equation of state for seawater?

It is very complicated and cannot be expressed byIt is very complicated and cannot be expressed bya simple formulaa simple formula

An empirical formula has been developed andAn empirical formula has been developed andagreed upon by the international communityagreed upon by the international community



Empirical Empirical Equation of Equation of State for State for sea water sea water (EOS90);(EOS90);

Expressed Expressed in terms of in terms of the surface the surface density and density and KK



Next: Sound in the OceanNext: Sound in the Ocean

The secant bulk modulus, The secant bulk modulus, KK, is related to the compressibility , is related to the compressibility of sea water and, thereby, to the speed of soundof sea water and, thereby, to the speed of sound

Conclusion: Equation of State for sea water is complex but Conclusion: Equation of State for sea water is complex but empirical formulae give give us consistent results given T,S,Pempirical formulae give give us consistent results given T,S,P

T T S S P P


Chap 5-6: SeawaterChap 5-6: Seawater Sound in the OceanSound in the Ocean

The speed of sound, The speed of sound, cc, is related to the density and the , is related to the density and the compressibility, compressibility, , of the medium according to:, of the medium according to:

where, where, KK = 1/ = 1/ is the bulk modulus is the bulk modulus

€

c = 1βρ( )

= κρ( )





Sound travels as a “Longitudinal” or compression wave:Sound travels as a “Longitudinal” or compression wave:

To be compared with “Transverse” waves (i.e., surface waves):To be compared with “Transverse” waves (i.e., surface waves):

€

c = 1βρ( )

= κρ( )

Animations courtesy of Dr. Dan Russell, Animations courtesy of Dr. Dan Russell, Kettering UniversityKettering University

xx

yy

xx

zz





Note that Note that cc is inversely proportional to compressibility; is inversely proportional to compressibility;

While water is much less compressible than air, sound travels While water is much less compressible than air, sound travels through water much more quickly than it does through airthrough water much more quickly than it does through air

waterwater << << airair ccwaterwater >> >> ccairair

€

c = 1βρ( )

= κρ( )

ccwaterwater ~ 1500 m/sec ~ 1500 m/sec

ccairair ~ 330 m/sec ~ 330 m/sec





Note that Note that cc is inversely proportional to compressibility; is inversely proportional to compressibility;

While water is much less compressible than air, sound travels While water is much less compressible than air, sound travels through water much more quickly than it does through airthrough water much more quickly than it does through air

waterwater << << airair ccwaterwater >> >> ccairair

€

c = 1βρ( )

= κρ( )

ccwaterwater ~ 1500 m/sec ~ 1500 m/sec

ccairair ~ 330 m/sec ~ 330 m/sec

Given Given cc, the wavelength, , the wavelength, , and frequency, , and frequency, ff, of sound waves , of sound waves are related by the wave equation:are related by the wave equation:cc = = ff

(seawater)(seawater) ff Low FrequencyLow Frequency 30Hz30Hz 50m50mHigh FrequencyHigh Frequency 1.5MHZ1.5MHZ 1mm1mmInstruments:Instruments: 10-100kHz10-100kHz 14-1.4cm14-1.4cm



Sound speeds in the ocean are ~1500 m/sec but they vary by Sound speeds in the ocean are ~1500 m/sec but they vary by about 5% as a function of T,S, and P:about 5% as a function of T,S, and P:

T T cc S S cc P P cc

Variations in Variations in cc lead to bending (turning) lead to bending (turning)of sound waves according to Snell’s Lawof sound waves according to Snell’s Law

Animation courtesy of Dr. Dan Russell, Animation courtesy of Dr. Dan Russell, Kettering UniversityKettering University

Snell’s Law applies equally to Snell’s Law applies equally to surface waves and light waves;surface waves and light waves;

If If cc changes in the medium, changes in the medium, wave crests will wave crests will bend toward bend toward the region of slower speedthe region of slower speed

€

sinθ1sinθ2

=c1c2



Sound waves spreading out Sound waves spreading out uniformly for uniformly for cc = constant = constant

Animations courtesy of Dr. Dan Russell, Animations courtesy of Dr. Dan Russell, Kettering UniversityKettering University

Bending upward toward lower Bending upward toward lower cc

Bending downward toward lower Bending downward toward lower cc



The distri-The distri-bution of c bution of c can lead to can lead to shadow zones shadow zones where sound where sound energy does energy does not penetratenot penetrate



A more complicated distribution of A more complicated distribution of cc(z) and resulting shadow (z) and resulting shadow zoneszones

The process of bending or turning of waves due to variations in The process of bending or turning of waves due to variations in their speed is called “their speed is called “refractionrefraction””

SOFAR: Sound Fixing and Ranging channel; energy is trapped SOFAR: Sound Fixing and Ranging channel; energy is trapped by the effect of minimum by the effect of minimum cc around 1000m around 1000m



SOFAR: Sound Fixing and Ranging channel; energy is trapped SOFAR: Sound Fixing and Ranging channel; energy is trapped by the effect of minimum by the effect of minimum cc around 1000m around 1000m



There is no mid-There is no mid-depth minimum or depth minimum or sound channel at sound channel at high latitudeshigh latitudes

Overview:Overview: Temperature and Pressure are most critical to Temperature and Pressure are most critical to ccSalinity is important at high latitudesSalinity is important at high latitudes

Look Ahead:Look Ahead: Global distributions of T(z), S(z)Global distributions of T(z), S(z)



Match the Match the sound speed sound speed profiles on the profiles on the left with the left with the refraction and refraction and shadow zone shadow zone results on the results on the rightright



Attenuation:Attenuation:

1)1) Spreading (Spherical vs Spreading (Spherical vs Cylindrical)Cylindrical)

2)2) Absorption, I = IAbsorption, I = Iooee-ax-ax (energy (energy

transfer to medium; transfer to medium; a=absorption coefficient)a=absorption coefficient)

3)3) Scattering (redirection by Scattering (redirection by particles)particles)

Absorption CoefficientAbsorption Coefficientversus frequencyversus frequency

““Relaxation”Relaxation”

ff aa IILower frequencies travel furtherLower frequencies travel further


Chap 5-6: SeawaterChap 5-6: Seawater Sound in the OceanSound in the OceanTarget Strength (active acoustic applications)Target Strength (active acoustic applications)

Related to effectiveness of the reflection of soundRelated to effectiveness of the reflection of soundMeasured by the Measured by the Acoustic ImpedanceAcoustic Impedance, Z = , Z = cc

The Reflectivity, R, determines how much acoustic energy is The Reflectivity, R, determines how much acoustic energy is reflected from a discontinuity and how much is transmitted reflected from a discontinuity and how much is transmitted through it; R = (Z1 - Z2)/(Z1 + Z2) x 100%through it; R = (Z1 - Z2)/(Z1 + Z2) x 100%

closely closely matched matched impedance impedance valuesvalues


Chap 5-6: SeawaterChap 5-6: Seawater Sound in the OceanSound in the OceanAmbient Noise (passive acoustic applications)Ambient Noise (passive acoustic applications)

Related to backgroundRelated to backgroundsoundsound

Environmental processes makeEnvironmental processes makedistinctive noisesdistinctive noises

Can cause a detectionCan cause a detectionproblem for someproblem for someapplicationsapplications

Can also be inverted toCan also be inverted toremotely monitor theremotely monitor theenvironment (e.g., rainenvironment (e.g., rainrate, wind speed)rate, wind speed)



Acoustic Thermometry of Ocean Climate (ATOC)Acoustic Thermometry of Ocean Climate (ATOC)

Exploits SOFAR channel to send sound great distancesExploits SOFAR channel to send sound great distancesTravel time is tied to the average T along the pathTravel time is tied to the average T along the path

special dates: mpa meeting…6 jul r/v pt sur cruise…14 jul r/v pt sur cruise…25 jul

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