Chapter 6 Temperature, Salinity, and Density Physical oceanography Instructor: Dr. Cheng-Chien Liu Department of Earth Sciences National Cheng Kung University Last updated: 18October 2003
Introduction • Factors 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 salinity • The 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 • DS 34.60 to 34.80 parts per thousand 200 parts per million • DS 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 DT 10C, and T is easier to measure
Definition of salinity (cont.) • 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 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
Definition of salinity (cont.) • Salinity based on Conductivity electronic • S = -0.08996 + 28.2929729R15 + 12.80832 R215 -10.67869R315 + 5.98624R415 - 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
Definition of salinity (cont.) • Practical Salinity Scale of 1978 • Spsu= 0.0080 - 0.1692 R1/215 + 25.3851 RT + 14.0941 R3/2T-7.0261 R2T + 2.7081 R5/2T + DSRT= C(S, T, 0) / C(KCl, T,0)DS = [(T - 15) / (1 + 0.0162(T - 15))] + 0.005 - 0.0056R1/2T - 0.0066 RT - 0.0375 R3/2T + 0.636 R2T – 0.0144 R5/2T • 2 S 42 • C(S, T, 0): the conductivity of the sea-water sample at temperature T and standard atmospheric pressure • C(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
Definition of salinity (cont.) • Comments • 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 = 0.003 • Small variation in SiO2 • Normal standard water
Definition of Temperature • Absolute temperature T • 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 scale • Difficult, usually made by national standards laboratories • Measurement of T using the interpolating device • In ocean: a platinum-resistance thermometer • A loosely wound, strain-free, pure platinum wire, Resistance = fn(T) • Calibration • Celsius: T[0C] = T[0K] - 273.15 • Accuracy of measuring T = 0.001 0C
Geographical Distribution of Surface Temperature and Salinity • The distribution of T 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 poles • The deviations from zonal are small • Equatorward 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 range • Anomaly < 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 Temperature and Salinity (cont.) • The distribution of S at the sea surface • Zonal fn(longitude) • Fig 6.4: mean SSS • Mid-latitudes: the highest evaporation • Equator: lower raining • High-latitudes: lower ice melting • Fig 6.5: • S = fn(evaporation minus precipitation plus river input) • The Atlantic is saltier than the Pacific • Fig 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 Thermocline • Wind blowing stirs a thin mixed layer • Mixed layer (ML) • S and T are both constants within ML • ML 10 – 200 m • 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 Thermocline (cont.) • Thermocline（躍溫層）and Pycnocline（躍密層） • Fig 6.7: Seasonal variation of ML and thermocline • D is related to T Thermocline Pycnocline • Permanent thermocline • Fig 6.8: • Compare S of ML and thermocline • Mid-latitudes (100 – 400): evaporation > precipitation saltier ML • High-latitudes: rain and melting ice fresher ML • Tropical regions: rain fresher ML
Density, Potential Temperature, and Neutral Density • Trace the movement of water parcel • Need to compare r, but Change P change r • Density and st • Measurement of absolute density of water • Difficult, only measured in lab • The best accuracy is 1: 2.5 × 105 = 4 parts per million • Calculation of density • From in situ measurements of S, T, P • The best accuracy is 2 parts per million • Density anomaly • s(S, T, P) = r(S, T, P) - 1000kg/m3 • st(S, T, P) s(S, T, 0)
Density, Potential Temperature, and Neutral Density (cont.) • Potential temperature q • 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 • q is calculated • Fig 6.9: profiles of T, q, st, sq
Density, Potential Temperature, and Neutral Density (cont.) • Potential density • Definition • The density a parcel of water would have if it were raised adiabatically to the surface without change in salinity • sq = s(s, q, 0) • Same q at the same depth might have different coefficient for thermal and salt expansion sqis not useful for comparing density of water at great depths • Fig 6.10: apparent inversion of density • sq = s(s, q, p, pr) • Not fully solve the problem small discontinuity
Density, Potential Temperature, and Neutral Density (cont.) • Neutral density (Eden & Willobrand, 1999) • Neutral path • A parcel of water moves along a path of constant potential density sr referenced to the local depth r • Neutral surface element • The surface tangent to the neutral paths through a point in the water • No 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 gn • Jackett and McDougall (1997) • Based on the Levitus (1982) atlas • gn = fn(S, t, p, longitude, latitude) • The neutral surface defined above differs only slightly from an ideal neutral surface.
Density, Potential Temperature, and Neutral Density (cont.) • Equation of state of sea water • Relating s 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 s(q) • The equation consists of three polynomials with 41 constants (JPOTS, 1991) • Accuracy of T, S, and r • For distinguish water masses need an accuracy of a few parts per million need careful definition, measurement, calibrated instruments and internationally accepted standard • Processing of Oceanographic Station Data (JPOTS, 1991) (UNESCO)
Measurement of Temperature • Mercury thermometer • The most widely used, non-electronic thermometer • In buckets dropped over the side of a ship T of surface waters • On Nansen bottles subsea T • In 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.) • Platinum Resistance Thermometer • The standard of T calibrate other instruments • Thermistor (1970 –) • A semiconductor having resistance that varies rapidly and predictably with temperature • Accuracy: 0.0010C (with careful calibration) • Bucket T • Measurement • Mercury thermometer in a bucket lowered into the water sit at a depth for a few minutes equilibrium read • Accuracy: 0.10C
Measurement of Temperature (cont.) • Ship Injection T • 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 • AVHRR • 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
Measurement of Temperature (cont.) • Sources of error using AVHRR • Unresolved or undetected clouds • Thin clouds such as low stratus and high cirrus impossible to detect • Clouds smaller in diameter than 1 km, such as trade-wind cumuli • Special techniques have been developed for detecting small clouds (Fig 6.12) • Water vapor • Absorb part of the energy SST • Different influences in channels 10.8 and 12.0 µm reduce the error • Aerosols • Absorb infrared radiation SST • Stratospheric aerosols generated by volcanic eruptions • Dust particles carried over the Atlantic from Saharan dust storms a few 0C • Skin temperature errors • Reduced when used to interpolate between ship measurements of SST
Measurement of Conductivity • 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 psu • Best accuracy of S from titration = 0.02 psu
Measurement of Pressure • Unit • SI unit Pa • Oceanography dbar • 1 dbar = 104 Pa • 1 dbar pressure = 1 meter depth • Strain gage • The simplest and cheapest way • Accuracy = 1%
Measurement of Pressure (cont.) • Vibration • Setup • A vibrating tungsten wire stretched in a magnetic field between diaphragms closing the ends of a cylinder • Principle • Pressure diaphragm wire tension wire frequency voltage • Accuracy = 0.1% • Better when T is controlled • Precision is 100 – 1000 times better than accuracy
Measurement of Pressure (cont.) • Quartz crystal • The natural frequency of a quartz crystal • cut for minimum temperature dependence • Accuracy • The best when T is held constant • The accuracy is ±0.015%, and precision is ±0.001% of full-scale values • Quartz Bourdon Gage • Has accuracy and stability comparable to quartz crystals
Measurement of Temperature and Salinity with Depth • Bathythermograph (BT) • A mechanical device • Measure T = T(z), MLD before 1970 • Expendable Bathythermograph (XBT) • An electronic device for measure T = T(z) • A thermistor on a free-falling streamlined weight • Falling velocity = constant • Accuracy • 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 Salinity with Depth (cont.) • 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 • Measurement • Usually 20 bottles were attached at intervals of a few tens to hundreds of meters to a wire lowered over the side of the ship • T a protected reversing thermometer along with an unprotected reversing thermometer • S 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 Salinity with Depth (cont.) • CTD • Replacement of Nansen bottles from 1960s • An electronic instrument • Measure C, T, D • C induction • T thermistor • D P quartz crystal • Accuracy: Table 6.2
Light in the ocean and absorption of light • Significance of Light • Transmission of Light in the seawater • Index of refraction n = 1.33 • Reflectivity = (n – 1)2 / (n + 1)2 • Attenuation of light • dI / dx = -c I I2 = I1exp(-cx) • Fig 6.17: c(l) • Radiance • The power per unit area per solid angle (W m-2 Sr-1)
Light in the ocean and absorption of light (cont.) • Water 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 (cont.) • Absorption • Water • Chlorophyll • CDOM • Others • CZCS algorithm • SeaWiFS mission • MODIS
Important 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.) • 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.
Important Concepts (cont.) • 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.
Important Concepts (cont.) • 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
Important Concepts (cont.) • 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.