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Chapter 2: Ocean Observations

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Chapter 2: Ocean Observations Chapter 2. Ocean observations 2.1 Observational methods With the rapid advancement in technology, the instruments and methods for measuring oceanic circulation and properties have been quickly evolving. Nevertheless, it is useful to understand what types of instruments have been available at different points in oceanographic development and their resolution, precision, and accuracy. The majority of oceanographic measurements so far have been made from research vessels, with auxiliary measurements from merchant ships and coastal stations. Fig. 2.1 Research vessel. 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 deployed, we need to measure ``depth''. Depth measurements are often made with the measurements of other properties, such as temperature, salinity and current. 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, research vessels are moving and currents might be strong, and thus 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 1 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. Pairs of reversing thermometers carried on Nansen bottles were the primary source of subsea measurements of temperature as a function of pressure from around 1900 to 1970. 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%. (iii) Quartz crystal: Very accurate pressure measurements can be made using a quartz crystal, whose frequency of oscillation depends on pressure. This technology is used in modern CTDs. Temperature must be accurately measured for the best pressure accuracy. In CTDs, a thermistor is part of the quartz pressure transducer. The accuracy is _0.01% and precision is _0.0001% of full-scale values. (For more details: See chapter S16 of Talley textbook.) 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 angle 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. 2 (d) Protected reversing mercury thermometer. These were invented by Negretti and Zamba in 1874. Since it is protected from the seawater 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 observations 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), 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 Thermometry of Ocean Climate (ATOC): Acoustic thermometry maps changes in ocean temperature using changes in sound speed along paths between acoustic sources and receivers. It can be used to monitor changes in the average temperature along very long paths at basin or global scale. It has been applied to measure the thermal field in the North Pacific basin from 1996-2006. See http://staff.washington.edu/dushaw/atoc.html: “ATOC is directed at using the travel time data obtained from a few acoustic sources and receivers located throughout the North Pacific basin to study the climatic variability of the thermal field at the largest scale.” They concluded that “…the experiment was a success, with transmissions occurring between 1996 and 2006…The marine mammal/biology problem was formally determined based on extensive scientific studies to be not significant for the acoustic sources employed by ATOC.” 3 Fig. 2.2 Acoustic sources & receivers in the North Pacific. (h) Drifting buoys: 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 (i.e., without losing or gaining 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 solely due to compression or expansion is usually not of much interest to climate scientists, because 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 the effect of adiabatic compression/expansion. Definition:``Potential temperature'' is the temperature that a water parcel has when it moves adiabatically to a reference 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 to the sea surface adiabatically without mixing or diffusion. Since pressure is the 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. Note that when oceanic mixing occurs (without change of external heat flux, e.g. heat flux at air-sea interface), the temperature (or potential temperature) of the mixture of two water parcels with different T &S does not equal to the average temperature of the two original water parcels, while heat content remains the same. This is because heat capacity (specific heat) also varies with varying temperature and salinity values. For this reason, “conservative temperature” is defined in TEOS-10, which more precisely scales with heat content and insensitive to pressure (also see: https://www.nature.com/scitable/knowledge/library/key-physical-variables-in-the- ocean-temperature-102805293/). The difference between potential temperature and conservative temperature is usually well within ±0.05ºC for most ocean waters (although the difference can be large for warm fresh waters).
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