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Chapter 2. Ocean Observations

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Chapter 2. Ocean Observations Chapter 2. Ocean observations 2.1 Observational methods Before we introduce the observational instruments and methods, we will first introduce some definitions related to observations. Accuracy: The difference between a result obtained and the true value. Precision: Ability to measure consistently within a given data set (variance in the measurement itself due to instrument noise). Generally the precision of oceanographic measurements is better than the accuracy. 2.1.1 Measurements of depth. Each oceanographic variable, such as temperature (T), salinity (S), density , and current , is a function of space and time, and therefore a function of depth. In order to determine to which depth an instrument has been lowered, we need to measure ``depth''. Meter wheel. The wire is passed over a meter wheel, which is simply a pulley of known circumference with a counter attached to the pulley to count the number of turns, thus giving the depth the instrument is lowered. This method is accurate when the sea is calm with negligible currents. In reality, ship is moving and currents are strong, the wire is not straight. The real depth is shorter than the distance the wire paid out. Measure pressure. Derive depth from hydrostatic relation: where g=9.8m/s2 is acceleration of gravity and is depth. (i) Protected and unprotected reversing thermometer developed especially for oceanographic use. They are mercury- in-glass thermometers which are attached to a water sampling bottle. The pressure was measured using the pair of reversing thermometers - one protected from seawater pressure by a vacuum and the other open to the seawater pressure. They were sent in a pair down to whatever depth, then flipped over, which cuts off the mercury in an ingenious small glass loop in the thermometer. They were brought back aboard and the difference between the mercury column length in the protected and unprotected thermometers was used to calculate the pressure. Depth accuracy 0.5% or 5m, whichever is the greater. (ii) Electrical strain-gauge pressure transducer which uses the change of electrical resistance of metals with mechanical tension. A resistance wire is firmly connected to a flexible diaphragm, to one side of which the in situ hydrostatic pressure is applied. As the diaphragm flexes with change of pressure, the tension in the wire changes and so does its resistance, which is measured to provide a value for the pressure and therefore depth. Accuracy 0.1%. 1 2.1.2 Measurements of temperature. (a) Bathythermograph. A liquid-in-metal thermometer causes a metal point to move in one direction over a smoked or gold plated glass slide which is itself moved at right angles to this direction by a pressure sensitive bellows. The instrument is lowered to its permitted limit in the water (60, 140 or 270m) and then brought back. Since pressure is directly related to depth, the line scratched on the slide forms a graph of temperature against depth. It is read against a calibration grid to an accuracy of 0.2k and 2m if well calibrated. Advantage: continuous T(z). Less accurate. This is an old method. (b) Expendable Bathythermograph (XBT). Widely used. Uses a thermistor as temperature-sensitive element. The thermistor is in a small streamlined weighted casing which is simply dropped over the ship's side. It is connected by a fine wire to a recorder on the ship which traces the temperature of the water in a graphical plot against depth. The latter is not sensed directly but is estimated from the time elapsed since release, using the known rate of sink of the freely falling thermistor casing. These XBTs are available for depth ranges from 200m to 1800m. Use aircraft: 300m-- 800m. This is an old method. (c) CTD--Conductivity, temperature, and depth (actually pressure). T is measured uses a thermistor mounted close to the conductivity sensor. This will be discussed a bit more in the next subsection. (d) Protected reversing mercury thermometer. These were invented by Negretti and Zamba in 1874. Since it is protected from the sea water pressure, the length of mercury is determined from temperature. As described above, it is attached to a water sampling bottle. When the bottle is closed to collect the sample the thermometer is inverted. Then the mercury is cut off in an ingenious small glass loop in the thermometer. Accuracy is 0.004C and precision is 0.002C. (e) Thermistors chains consisting of a cable with a number of thermistor elements at intervals are sometimes moored along with current meters to record the temperature at number of points in the water column. A ``data logger'' samples each thermistor sequentially at intervals and records temperatures as a function of time. Quality varies significantly. The best thermistors commonly used in oceanographic instruments have an accuracy of 0.002C and precision of 0.0005-0.001C. [Thermistor can also be instrumented on drifting buoys.] (f) Satellite. Direct observations have space and time limitations. Satellite obs can provide large spatial and temporal scale data. Advanced Very High Resolution Radiometer (AVHRR) on board of NOAA satellite, can measure SST with accuracy of 0.1-0.3k. [Multi-channel: 0.58-0.68 (visible), 0.725-1.10 (near-infra-red), 2 thermal infra-red (3.65-3.93 , 10.3-11.3 ,11.5-12.5 ). Problem: Cloud vapor absorption. Inaccurate when there are clouds. Tropical Rainfall Measuring Mission (TRMM)--Microwave Imager (TMI), measure SST, 0.2C difference compare with buoy data. Spatial and temporal resolutions: 25x25 km and daily since 1997. TMI can penetrate clouds and thus are not contaminated by clouds; but the data quality can be affected by strong rainfall. [Polar orbiting: 500- 800km height. Geostationary: 36,000km.] (g) Acoustic tomography. Acoustic tomography maps changes in ocean temperature using changes in sound speed along paths between acoustic sources and receivers. It is currently used in two somewhat different modes - in concentrated regional experiments where an attempt is made to reconstruct the full three-dimensional temperature field, and over global paths to monitor changes in the average temperature along very long paths. The first has been and is being used to good effect in winter convection regions, where in situ ship observations have been very difficult to obtain due to the small size of convection features and the poor weather in the interesting part of the year. The second is being used for global climate monitoring. Acoustic Thermometry of Ocean Climate (ATOC) website. (http://atocdb.ucsd.edu/index.html) (h) Buoy: Some Drifters are also instrumented to measure T. Pressure of the ocean increases greatly downward. A parcel of water moving from one pressure to another will be compressed or expanded. When a parcel of water is compressed adiabatically, that is, without exchange of heat, its temperature increases. (This is true of any fluid or gas.) When a parcel is expanded adiabatically, its temperature decreases. The change in temperature which occurs solely due to compression or expansion is not of interest to us - it does not represent a change in heat content of the fluid. Therefore if we wish to compare the temperature of water at one pressure with water at another pressure, we should remove this effect of adiabatic compression/expansion. Definition:``Potential temperature'' is the temperature which a water parcel has when moved adiabatically to another pressure. In the ocean, we commonly use the sea surface as our "reference" pressure for potential temperature - we compare the temperatures of parcels as if they have been moved, without mixing or diffusion, to the sea surface. Since pressure is lowest at the sea surface, potential temperature (computed at surface pressure) is ALWAYS lower than the actual temperature unless the water is lying at the sea surface. 2.1.3. Measurements of salinity. (a) Laboratory. Evaporate and weigh residual (oldest method). 3 (b) Laboratory. Classical (Knudsen) method. Determine amount of chlorine, bromine and iodine to give "chlorinity", through titration with silver nitrate. Then relate salinity to chlorinity: S = 1.80655 Cl. Accuracy is 0.025. This method was used until the International Geophysical Year in 1957. Water sample. Not convenient on board ship. (c) Measure conductivity. Conductivity of sea water depends strongly on temperature, somewhat less strongly on salinity, and very weakly on pressure. If the temperature is measured, then conductivity can be used to determine the salinity. Salinity as computed through conductivity appears to be more closely related to the actual dissolved constituents than is chlorinity, and more independent of salt composition. Therefore temperature must be measured at the same time as conductivity, to remove the temperature effect and obtain salinity. Accuracy of salinity determined from conductivity: 0.001 to 0.004. Precision: 0.001. The accuracy depends on the accuracy of the seawater standard used to calibrate the conductivity based measurement. How is conductivity for calculating salinity measured? (c.1) For a seawater sample in the laboratory, an ``autosalinometer'' is used, which gives the ratio of conductivity of the seawater sample to a standard solution. The standard seawater solutions are either seawater from a particular place, or a standard (Potassium Chlorine) KCl solution made in the laboratory. The latter provides greater accuracy and has recently become the standard. Because of the strong dependence of conductivity on temperature, the measurements must be carried out in carefully temperature-controlled conditions. (c.2) CTD. From an electronic instrument in the water, either inductive or capacitance cells are used, depending on the instrument manufacturer. Temperature must also be measured, from a thermistor mounted close to the conductivity sensor. In a CTD, a unit consisting of conductivity, temperature, and pressure sensors is lowered through the water on the end of an electrical conductor cable which transmits the information to indicating and recording units on board ship.
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