Measurement of the Physical Properties of Seawater Temperature, Salinity and Pressure (Moored, Towed and Profiled) Feb 04 Introduction - A goal of the ocean scientist/engineer studying the physics of the ocean is to: 1) obtain a quantitative description of the characteristics and circulation patterns of oceanic waters as a function of space and time, and 2) understand the nature of the forces which cause the water movement and ultimately the distribution of the water properties. In the study of the oceans, the most important dynamic property of seawater is its density (ρ), which is a function of the temperature (T), the salt content or salinity (S), and the pressure (P) of the water. In other words, the oceanographer wishes to know T, S, P and as continuous functions of x, y, z, t, where x and y are latitude and longitude on the earth, z is depth, and t is time. Since it is practically impossible to measure all places all the time, we generally try to hold all except one variable constant and sample the quantity as a function of the remaining variable. For example: 1. A moored thermometer at a fixed position x, y, z measures temperature as a function of time, T(t). To get an approximation of the vertical distribution, several temperature sensors may be placed in a vertical array to get T(t,zi). The discrete zi should be chosen so as not to alias the temperature signal variations in the vertical. 2. A vertical profiling instrument (the most used version we refer to as a “CTD” because it samples Conductivity, Temperature and Depth) measures vertical profiles of temperature, T(z), where x, y and t are held nearly constant. Again this is not exactly true because it takes time to make a vertical profile, so t is not exactly constant, and during that time the wind and currents will move the ship so that x and y are not constant. However, in the ocean T varies more strongly with depth and less so with horizontal position and the time it takes to make a profile because the ocean is vertically stratified, so the assumption holds in practical applications. However, in a CTD survey of Massachusetts Bay a few years back, a problem was encountered. A synoptic picture of the density field was desired, but with a single ship the survey took several days to complete. During this time the internal waves at the M2 frequency (the principal lunar tidal frequency of 12.42 hours) significantly distorted the density field with this period so that a synoptic picture was impossible to obtain. A survey of the area would have to be completed in a fraction of the 12.42-hour period, or an average of many profiles taken during the 12.42-hour period would need to be made at each location to average out the tidal variability to get a good survey. 3. An autonomous underwater vehicle such as the Self Propelled Underwater Research Vehicle (SPURV used by APL/UW), ABE (WHOI), ODESY (MIT) or REMUS (WHOI) makes horizontal profiles of temperature, e.g. T(x) or T(y), at constant depth, and nearly constant time. They can also do surveys of T(x,y) in a time relatively fast compared to changes in the water column properties if the area of survey is small enough. 1Since the important water properties are characterized by its temperature, salinity and pressure, let us start our study there. Measurement of seawater Temperature, Salinity and Pressure: Temperature - The temperature of the seawater is the easiest of the three physical quantities to measure, and is a measure of the internal energy or thermal activity of the water. Oceanographers use units of degrees Centigrade - °C. Generally the measure of temperature is in degrees Kelvin, relative to absolute zero where all motion stops. However, in the ocean we see a relatively small temperature change (generally between 0° and 30° C) so we use the Celsius scale, where °C = ° K - 273.15° A. Temperature Measurement - 1. Mercury (or alcohol) glass tube thermometer (nearly outdated in oceanography). These sensors rely on a glass tube of constant cross sectional area, attached to a reservoir of a fluid such as mercury that has a linear coefficient of expansion with temperature. As the temperature rises, the fluid expands and rises in the tube. The tube is marked off in degrees, and the level of the fluid is read relative to the markings to determine the temperature. These sensors have an accuracy of about 0.02 °C when used properly. The oceanographic instrument is the reversing thermometer that is designed to hold the temperature of the thermometer when it is "reversed" or tipped upside down at the depth of the required measurement. The thermometer is designed so that when it reverses, the mercury column breaks at a "y" in the tube and the mercury remaining in the tube is read to get the temperature of the thermometer at the time it reversed. The thermometer does have a thermal expansion problem of the glass tube and separated mercury column as the temperature changes, so the reversing thermometer is also equipped with an auxiliary thermometer, which is used to determine the temperature of the reversing thermometer (glass tube and mercury column) at the time that it is read. When properly calibrated, this then allows the user to correct the temperature to the actual temperature at depth, or "in situ" temperature. The reversing thermometers also come in pressure protected or unprotected versions. In the pressure protected version, the thermometer is completely enclosed in a glass tube so that there are no pressure effects on the reading. In the unprotected version, the seawater is allowed to compress the glass tube and mercury reservoir and so change the reading depending on the pressure. Again with good calibratings, the difference in temperature observed between the pressure protected and unprotected thermometers can be used to estimate the pressure at the depth that the thermometers reversed. In standard hydgrographic casts where several bottles with thermometers are lowered on the wire at one time, the depth of each thermometer is estimated by measuring the amount of wire out and the angle that the wire has at the ship. The reversing thermometers (generally one or two pressure protected, and one unprotected) are mounted in racks on water sample bottles and lowered over the side of 2oceanographic research vessels to obtain temperature and water samples for later analysis in the laboratory or aboard