Criscimagna_Ellie_Lab4
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Apr 3, 2024
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MSC 112 Lab 4, Temperature, Salinity, and Density Lab September 21, 2023 Ellie Criscimagna Purpose
: Density is one of the most important properties in relation to motion of the ocean. Even small changes can create strong currents. While density is very important, salinity is another critical specification. Along with temperature, salinity controls density, therefore, controlling the differentiation or stratification of the ocean and its circulation. Salinity is measured as a concentration of 35 grams of salt per kg of water or in other words 35 parts per thousand. The most common units of salinity are PSU or practical salinity units. Salinity is often measured through electrical conductivity, however, conductivity is also affected by temperature, and therefore temperature must be known. With an instrument known as the CTD (conductivity-temperature-depth), the conductivity and temperature are used to measure salinity at different depths. Density is measured as g/cm 3 . Solutes have greater mass than water, so density increases with salinity. Density also varies with changes in temperature, as well as pressure and depth. Density of fresh water is 1 g/cm 3 and the density of seawater is typically just above 1 g/cm 3 , so to simplify the measurement. Sigma-t is calculated as (seawater density - 1.0)*1000. Therefore, a σt of 20.00 is equivalent to 1.020 g/cm 3 . To simplify all of the above, density of the ocean is a crucial property in terms of ocean movement. However, density is affected by salinity, temperature, and pressure/depth, so it is important to measure these three properties to get an accurate representation of the density.
Methods
: Part I. First, a container with a removable divider was used. One half was filled with fresh water and the other half was filled with seawater. A conductivity meter was then used to measure the salinity and temperature of the two solutions. These measurements were then placed on a T-S (temperature-salinity) diagram which calculates density using isopycnals. After connecting the two dots, the average density was predicted. Different dyes were placed in the two solutions to differentiate them before the divider was removed. The behavior of the two different water solutions was observed. Then, the solution was mixed thoroughly and the conductivity meter was used to measure the exact temperature and salinity. A density calculator was used to measure the exact density at a pressure of zero. Part II. A table was provided with pressure (dbar), temperature (˚C), and salinity (‰). Then a density calculator was used to fill in the column labeled ( σt).
Results
: Part I. Figure 1. As seen in Figure 1., the freshwater measured 0.16‰ and 20.2˚C, the saltwater measured 30.8‰ and 8.9˚C, the mixed solution was estimated to be ~16.0‰ and ~15˚C, and the mixed solution was measured to be 17.03‰ and 18.1˚C. In terms of density relative to the isopycnals, the freshwater measured just under 1.000 g/cm 3 , the saltwater measured ~1.025 g/cm 3 , and the mixed solution was ~1.013 g/cm 3 . However with the use of a density calculator, the density of the mixture was measured to be 1.012585 g/cm 3 , or a σt of 12.585.
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Part II. Table 1. Completed density calculated based off pressure, temperature, and salinity profiles. Pressure(dbar) Temperature(˚C) Salinity (‰) Density (σt) 4.6 27.45 35.24 22.756 26.4 27.43 35.24 22.763 50.5 27.41 35.24 22.769 76.8 27.39 35.25 22.783 97.4 27.09 35.38 22.977 124 25.85 35.46 23.429 132 23.25 35.25 24.052 158.4 20.55 35.43 24.944 173.4 18.58 35.32 25.374 200.6 14.42 34.97 26.075 252.4 12.76 34.89 26.356 302.2 12.11 34.85 26.452 351.4 11.37 34.81 26.623 403.5 9.80 34.71 26.76 453.4 8.80 34.66 26.884 504.1 8.30 34.63 26.861 605.7 7.11 34.58 27.073 705.1 6.34 34.56 27.161 805.4 5.62 34.54 27.237 908.6 5.12 34.54 27.297 1008 4.65 34.55 27.358 1211.2 3.92 34.57 27.452 1412.3 3.34 34.59 27.526 1613.6 2.89 34.61 27.584 1818.7 2.58 34.62 27.619 2022 2.29 34.64 27.660 2225.7 2.04 34.65 27.688 2531.7 1.86 34.66 27.710 2836.4 1.69 34.67 27.731 3246.1 1.61 34.68 27.745 3650.1 1.47 34.69 27.764 4062.5 1.42 34.69 27.767 4281.6 1.40 34.69 27.769 4363.2 1.38 34.70 27.778 4400.8 1.38 34.70 27.778
1. Instead of depth you will notice that pressure is used in the above table. One dbar of pressure is ~ 1 meter of depth. Using the above data in the table and Excel, plot: a) temperature versus pressure b) salinity versus pressure Position the two graphs side by side so that you can compare the profiles These are pressure or depth profiles of these parameters. Figure 2. Depth profile of Pressure vs Salinity Figure 3. Depth profile of Pressure vs Temperature 2. Think about the following before doing part 3. What do you think a density vs pressure (or depth) profile would look like? Would density increase or decrease with pressure or depth? Density will increase with depth because the pressure condenses the particles.
3. Calculate the density using the online density calculator. Since the depth or pressure of the water also influences the density (density increases with depth or pressure) you should also enter a pressure value into the online calculator http://gyre.umeoce.maine.edu/physicalocean/Tomczak/Utilities/density.html Remember 1 meter of depth is approximately equal to 1 dbar of pressure. Create another graph plotting the density vs pressure and position it next to the first two graphs. Does density increase or decrease with pressure or depth? Would you expect this to be the case? Why? Figure 4. Depth profile of Pressure vs Density Density increased with depth. This was expected because depth/pressure condenses the particles in water, meaning more particles fit into the same space, increasing the grams per cm 3 .
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Conclusion
: Density is a vital property in terms of movement of the ocean. Density, however, is affected by salinity, temperature, and pressure/depth. These three factors affect the density in different ways and, therefore, they are all related to ocean movement. Salinity adds mass to the water, increasing the density(
Exploring our Fluid Earth
). Temperature is related to density because as water gets warmer, the more space it takes up, and the less mass per area (
Exploring our Fluid Earth
). This means density decreases as temperature increases. Density controls ocean movement such as currents(2013). Currents are important because they redistribute heat, oxygen, water, and nutrients throughout the ocean(2013).
Works Cited Exploring our Fluid Earth
. Density, Temperature, and Salinity | manoa.hawaii.edu/ExploringOurFluidEarth. (n.d.). https://manoa.hawaii.edu/exploringourfluidearth/physical/density-effects/density-tempera ture-and-salinity#:~:text=The%20warmer%20the%20water%2C%20the,will%20therefor e%20be%20less%20dense. NOAA Ocean Explorer: Education - Multimedia Discovery Missions: Lesson 8 - ocean currents: Activities: Currents and marine life
. NOAA Ocean Explorer Podcast RSS 20. (2013). https://oceanexplorer.noaa.gov/edu/learning/8_ocean_currents/activities/currents.html#:~: text=Currents%20are%20important%20in%20marine,and%20carry%20off%20living%2 0organisms.