Temperature, Salinity, and Density Physical oceanography Instructor: Dr. Cheng-Chien LiuCheng-Chien...

Temperature, Salinity, and DensityTemperature, Salinity, and Density

Physical oceanographyInstructor: Dr. Cheng-Chien Liu

Department of Earth Sciences

National Cheng Kung University

Last updated: 18 October 2003

Chapter 6Chapter 6

IntroductionIntroduction

Factors that influences S and TFactors that influences S and T• Heat fluxes, evaporation, rain fall, river in

flow, melting and freezing of sea ice

• Change S and T change D convection track water movement

• D = fn(horizontal pressure gradient, currents)

Definition of salinityDefinition of salinity

The simplest levelThe simplest level• The amount of dissolved material [g] in sea water [kg]

not useful the dissolved material is impossible to measure in practice Volatile material like gasses Chlorides are lost in the last stages of drying

• A dimensionless quantity without unit• Fig 6.1: the need for accuracy

S 34.60 to 34.80 parts per thousand 200 parts per million S in the deep North Pacific is even smaller 20 parts per million

If we want to classify water with different salinity, we need definitions and instruments accurate to about one part per million

Notice that T 10C, and T is easier to measure

Definition of salinity (cont.)Definition of salinity (cont.)

A more complete definition (1902)A more complete definition (1902)• Useful but difficult to use routinely

Total amount of solid materials in grams dissolved in one kilogram of sea water when all the carbonate has been converted to oxide, the Br and I replaced by Cl and all organic matter completely oxidized

Salinity based on Chlorinity Salinity based on Chlorinity chemical chemical• S = 0.03 + 1.805Cl

Cl:the mass of silver required to precipitate completely the halogens in 0.328 523 4kg of the sea-water sample

Three reasons The above definition was difficult to implement in practice S Cl Simple and accurate measurement of Cl

• Refine (1966): S = 1.80655 Cl


Salinity based on Conductivity Salinity based on Conductivity electronic electronic• S = -0.08996 + 28.2929729R15 + 12.80832 R2

15 -

10.67869R315 + 5.98624R4

15 - 1.32311R515

• R15= C(S,15,0)/C(35,15,0)C (S, 15 , 0): the conductivity of the sea-water sample at 15°C and

atmospheric pressure, having a salinity S derived from (6.4)C (35 , 15 , 0) is the conductivity of standard "Copenhagen" sea water

• S = fn(R15) is not a new definition of S

• It gives chlorinity as a function of conductivity of seawater relative to standard seawater


Practical Salinity Scale of 1978 Practical Salinity Scale of 1978 • Spsu= 0.0080 - 0.1692 R1/2

15 + 25.3851 RT + 14.0941 R3/2

T-7.0261 R2T + 2.7081 R5/2

T + SRT = C(S, T, 0) / C(KCl, T,0)S = [(T - 15) / (1 + 0.0162(T - 15))] + 0.005 - 0.0056 R1/2

T - 0.0066 RT - 0.0375 R3/2T + 0.636 R2

T – 0.0144 R5/2

T2 S 42C(S, T, 0): the conductivity of the sea-water sample at temperature T and

standard atmospheric pressureC(KCl, T, 0): the conductivity of the standard KCl solution at temperature T

and standard atmospheric pressure The standard KCl solution contains 32.4356 grams of KCl in 1.000 000kg of solution An extension of (6.4) gives salinity at any pressure (Millero 1996)

All water samples with the same RT have the same S


CommentsComments• Table 6.1 Major Constituents of Sea Water

The ratios of the various ions fn(S, location) The various definitions of salinity work well

Except fresh water in estuaries

• Accuracy of measuring S = 0.003Small variation in SiO2

• Normal standard water

Definition of TemperatureDefinition of Temperature

Absolute temperature Absolute temperature TT• Unit: K (Kelvin)• The fundamental processes for defining T

The gas laws relating pressure to temperature of an ideal gas with corrections for the density of the gas

The voltage noise of a resistance R

• Measurement of T using an absolute scaleDifficult, usually made by national standards laboratories

• Measurement of T using the interpolating deviceIn ocean: a platinum-resistance thermometer

A loosely wound, strain-free, pure platinum wire, Resistance = fn(T) Calibration

Celsius: T[0C] = T[0K] - 273.15Accuracy of measuring T = 0.001 0C

Geographical Distribution of Surface Geographical Distribution of Surface Temperature and SalinityTemperature and Salinity

The distribution of The distribution of TT at the sea surface at the sea surface• Zonal fn(longitude)• Fig 6.2: mean SST from report and AVHRR

Warmest water is near the equator, coldest water is near the polesThe deviations from zonal are smallEquatorward of 400, cooler waters tend to be on the eastern side of the

basin. North of this latitude, cooler waters tend to be on the western side

• Fig 6.3: SST anomaly and annual rangeAnomaly < 1.50C except in the equatorial Pacific (30C)Annual range:

highest at mid-latitudes, especially on the western side of the ocean cold air blows off the continents in the winter

In the tropics < 20C

Geographical Distribution of Surface Geographical Distribution of Surface Temperature and Salinity (cont.)Temperature and Salinity (cont.)

The distribution of The distribution of SS at the sea surface at the sea surface• Zonal fn(longitude)• Fig 6.4: mean SSS

Mid-latitudes: the highest evaporationEquator: lower rainingHigh-latitudes: lower ice melting

• Fig 6.5: S = fn(evaporation minus precipitation plus river input)

• The Atlantic is saltier than the PacificFig 6.6

More rivers flow into the Atlantic, but 0.32Sv water evaporated from the Atlantic does not fall as rain on land. Instead, it is carried by winds into the Pacific

• Mean: 1.3<T = 3.5<3.8 (0C), 34.6<S = 34.7<34.8 (psu)Half of the waters is in the range

The Oceanic Mixed Layer and The Oceanic Mixed Layer and ThermoclineThermocline

Wind blowing Wind blowing stirs stirs a thin mixed a thin mixed layerlayer• Mixed layer (ML)

S and T are both constants within MLML 10 – 200 m

Variation of mixed layer depth (MLD)Variation of mixed layer depth (MLD)• Response to two processes

Heat fluxes contrast of D work needed for mixing the layer downward

Wind speed intensity of breaking waves turbulence downward mixing

The Oceanic Mixed Layer and The Oceanic Mixed Layer and Thermocline (cont.)Thermocline (cont.)

ThermoclineThermocline （躍溫層）（躍溫層） and Pycnoclineand Pycnocline（躍密層）（躍密層） • Fig 6.7: Seasonal variation of ML and thermocline• D is related to T Thermocline Pycnocline

Permanent thermoclinePermanent thermocline• Fig 6.8:• Compare S of ML and thermocline

Mid-latitudes (100 – 400): evaporation > precipitation saltier MLHigh-latitudes: rain and melting ice fresher MLTropical regions: rain fresher ML

Density, Potential Temperature, and Density, Potential Temperature, and Neutral DensityNeutral Density

Trace the movement of water parcelTrace the movement of water parcel• Need to compare , but Change P change

Density Density and and tt

• Measurement of absolute density of waterDifficult, only measured in labThe best accuracy is 1: 2.5 × 105 = 4 parts per million

• Calculation of densityFrom in situ measurements of S, T, PThe best accuracy is 2 parts per million

• Density anomaly(S, T, P) = (S, T, P) - 1000kg/m3

t(S, T, P) (S, T, 0)

Density, Potential Temperature, and Density, Potential Temperature, and Neutral Density (cont.)Neutral Density (cont.)

Potential temperature Potential temperature • Definition

The temperature of a parcel of water at the sea surface after it has been raised adiabatically from some depth in the ocean

Raising the parcel adiabatically means that it is raised in an insulated container so it does not exchange heat with its surroundings

is calculated

• Fig 6.9: profiles of T, t


Potential densityPotential density• Definition

The density a parcel of water would have if it were raised adiabatically to the surface without change in salinity

• = (s, , 0)Same at the same depth might have different coefficient

for thermal and salt expansion is not useful for comparing density of water at great depths

Fig 6.10: apparent inversion of density

• = (s, , p, pr)Not fully solve the problem small discontinuity


Neutral density (Eden & Willobrand, 1999)Neutral density (Eden & Willobrand, 1999)• Neutral path

A parcel of water moves along a path of constant potential density r referenced to the local depth r

• Neutral surface elementThe surface tangent to the neutral paths through a point in the waterNo work is required to move a parcel on this surface because there is no

buoyancy force acting on the parcel as it moves (if we ignore friction).

• A practical neutral density variable n Jackett and McDougall (1997)Based on the Levitus (1982) atlasn = fn(S, t, p, longitude, latitude)The neutral surface defined above differs only slightly from an ideal

neutral surface.


Equation of state of sea water Equation of state of sea water • Relating to T, S, and P

Derived by fitting curves through laboratory measurements The equation has an accuracy of 10 parts per million, which is 0.01

units of ()The equation consists of three polynomials with 41 constants (JPOTS,

1991)

Accuracy of Accuracy of TT, , SS, and , and • For distinguish water masses need an accuracy of a

few parts per million need careful definition, measurement, calibrated instruments and internationally accepted standardProcessing of Oceanographic Station Data (JPOTS, 1991) (UNESCO)

Measurement of TemperatureMeasurement of Temperature

Mercury thermometerMercury thermometer• The most widely used, non-electronic thermometer

In buckets dropped over the side of a ship T of surface watersOn Nansen bottles subsea TIn the laboratory calibrate other thermometers

• Accuracy: 0.0010C (with careful calibration)• Reversing thermometer (Fig 6.11)

Constriction in the mercury capillary break the thread of mercury when the thermometer is turned upside down

Carried inside a glass tube protects the thermometer from the ocean’s pressure

Deployed in pair protected and unprotected (Fig 6.11)Pairs of reversing thermometers carried on Nansen bottles the

primary source of subsea measurements of T = fn(P) (from 1900 to 1970)

Measurement of Temperature (cont.)Measurement of Temperature (cont.)

Platinum Resistance ThermometerPlatinum Resistance Thermometer• The standard of T calibrate other instruments

Thermistor (1970 –)Thermistor (1970 –)• A semiconductor having resistance that varies rapidly

and predictably with temperature• Accuracy: 0.0010C (with careful calibration)

Bucket Bucket TT• Measurement

Mercury thermometer in a bucket lowered into the water sit at a depth for a few minutes equilibrium read

• Accuracy: 0.10C


Ship Injection Ship Injection TT• The temperature of the water drawn into the ship to

cool the engines recorded routinely for decades• Error source: warmed before record• Accuracy: 0.50 – 10C

AVHRRAVHRR• Advanced Very High Resolution Radiometer• NOAA Tiro-N since 1978• Original design measure cloud T, height• Sufficient accuracy and precision measure SST• Sensor description and missions AVHRR


Sources of error using AVHRRSources of error using AVHRR• Unresolved or undetected clouds

Thin clouds such as low stratus and high cirrus impossible to detectClouds smaller in diameter than 1 km, such as trade-wind cumuli

Special techniques have been developed for detecting small clouds (Fig 6.12)

• Water vaporAbsorb part of the energy SSTDifferent influences in channels 10.8 and 12.0 µm reduce the error

• AerosolsAbsorb infrared radiation SST

Stratospheric aerosols generated by volcanic eruptions Dust particles carried over the Atlantic from Saharan dust storms a few 0C

• Skin temperature errorsReduced when used to interpolate between ship measurements of SST

Measurement of ConductivityMeasurement of Conductivity

A conductivity cell (Fig 6.13)A conductivity cell (Fig 6.13)• Platinum electrodes

• Voltage difference current

• Current = fn(conductivity, voltage, volume of seawater)

• Given voltage and the volume of seawater Current = fn(conductivity) = fn(S)Best accuracy of S from conductivity = 0.005 psuBest accuracy of S from titration = 0.02 psu

Measurement of PressureMeasurement of Pressure

UnitUnit• SI unit Pa

• Oceanography dbar1 dbar = 104 Pa1 dbar pressure = 1 meter depth

Strain gageStrain gage• The simplest and cheapest way

• Accuracy = 1%

Measurement of Pressure (cont.)Measurement of Pressure (cont.)

VibrationVibration• Setup

A vibrating tungsten wire stretched in a magnetic field between diaphragms closing the ends of a cylinder

• PrinciplePressure diaphragm wire tension wire frequency

voltage

• Accuracy = 0.1%Better when T is controlledPrecision is 100 – 1000 times better than accuracy

Measurement of Pressure (cont.)Measurement of Pressure (cont.)

Quartz crystal Quartz crystal • The natural frequency of a quartz crystal

cut for minimum temperature dependence

• AccuracyThe best when T is held constant

The accuracy is ±0.015%, and precision is ±0.001% of full-scale values

Quartz Bourdon GageQuartz Bourdon Gage• Has accuracy and stability comparable to

quartz crystals

Measurement of Temperature and Measurement of Temperature and Salinity with DepthSalinity with Depth

Bathythermograph (BT)Bathythermograph (BT)• A mechanical device• Measure T = T(z), MLD before 1970

Expendable Bathythermograph (XBT)Expendable Bathythermograph (XBT)• An electronic device for measure T = T(z)

A thermistor on a free-falling streamlined weightFalling velocity = constantAccuracy

Depth accuracy = ±2% Temperature accuracy = ±0.1◦C Vertical resolution = 65 cm Range = 200 m to 1830 m depth

The most widely used instrument (65,000/year)

Measurement of Temperature and Measurement of Temperature and Salinity with Depth (cont.)Salinity with Depth (cont.)

Nansen Bottles (Fig 6.16)Nansen Bottles (Fig 6.16)• Hydrographic stations

Measure water properties from the surface to some depth, or to the bottom, using instruments lowered from a ship

• MeasurementUsually 20 bottles were attached at intervals of a few tens to hundreds

of meters to a wire lowered over the side of the shipT a protected reversing thermometer along with an unprotected

reversing thermometerS determined by laboratory analysis of water sample A lead weight was dropped down the wire tripped a mechanism on

each bottle the bottle flipped over reversing the thermometers shutting the valves trapping water in the tube releasing another weight

The deployment and retrieval typically took several hours

Measurement of Temperature and Measurement of Temperature and Salinity with Depth (cont.)Salinity with Depth (cont.)

CTDCTD• Replacement of Nansen bottles from 1960s

• An electronic instrument

• Measure C, T, DC inductionT thermistorD P quartz crystal

• Accuracy: Table 6.2

Light in the ocean and absorption of Light in the ocean and absorption of lightlight

Significance of LightSignificance of Light Transmission of Light in the seawaterTransmission of Light in the seawater

• Index of refraction n = 1.33• Reflectivity = (n – 1)2 / (n + 1)2

Attenuation of lightAttenuation of light• dI / dx = -c I I2 = I1exp(-cx)

• Fig 6.17: c()

RadianceRadiance• The power per unit area per solid angle (W m-2 Sr-1)

Light in the ocean and absorption of Light in the ocean and absorption of light (cont.)light (cont.)

Water colorWater color• Jerlov’s classification (Fig 6.18)

Type I water the clearest water 10% light to 90m e.g. Kuroshio (black water)

Type II, III water chlorophyll dominate blue-green More turbid tropical and mid-latitude waters Can be seen from space remote sensing of ocean color Fig 6.19

Type 1 – 9 waters coastal water Turbid, optically complex water

Light in the ocean and absorption of Light in the ocean and absorption of light (cont.)light (cont.)

AbsorptionAbsorption• Water

• Chlorophyll

• CDOM

• Others

CZCS algorithmCZCS algorithm SeaWiFS missionSeaWiFS mission MODISMODIS

Important ConceptsImportant Concepts

• Density in the ocean is determined by temperature, salinity, and pressure.

• Density changes in the ocean are very small, and studies of water masses and currents require density with an accuracy of 10 parts per million.

• Density is not measured, it is calculated from measurements of temperature, salinity, and pressure using the equation of state of sea water.

• Accurate calculations of density require accurate definitions of temperature and salinity and an accurate equation of state.

Important Concepts (cont.)Important Concepts (cont.)

• Salinity is difficult to define and to measure. To avoid the difficulty, oceanographers use conductivity instead of salinity. They measure conductivity and calculate density from temperature, conductivity, and pressure.

• A mixed layer of constant temperature and salinity is usually found in the top 1–100m of the ocean. The depth is determined by wind speed and the flux of heat through the sea surface.

• To compare temperature and density of water masses at different depths in the ocean, oceanographers use potential temperature and potential density which remove most of the influence of pressure on density.


• Water parcels below the mixed layer move along neutral surfaces.

• Surface temperature of the ocean was usually measured at sea using bucket or injection temperatures. Global maps of temperature combine these observations with observations of infrared radiance from the sea surface measured by an AVHRR in space.


• Temperature and conductivity are usually measured digitally as a function of pressure using a CTD. Before 1960–1970 the salinity and temperature were measured at roughly 20 depths using Nansen bottles lowered on a line from a ship. The bottles carried reversing thermometers which recorded temperature and depth and they returned a water sample from that depth which was used to determine salinity on board the ship.

• Light is rapidly absorbed in the ocean. 95% of sunlight is absorbed in the upper 100m of the clearest sea water. Sunlight rarely penetrates deeper than a few meters in turbid coastal waters


• Phytoplankton change the color of sea water, and the change in color can be observed from space. Water color is used to measure phytoplankton concentration from space.

Temperature, Salinity, and Density Physical oceanography Instructor: Dr. Cheng-Chien LiuCheng-Chien...

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