ERTH 110 Introduction and worksheet Lab 6 Spring 2023

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California State University, Chico *

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110

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Geography

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Oct 30, 2023

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Lab 6: Relationship between Temperature, Salinity, and Density in the Ocean Background Two of the most important characteristics of seawater are temperature and salinity – together they control its density, which is the major factor governing the vertical movement of ocean waters . Although the vertical movement of water has not yet been introduced, we learned about density in our last lab with respect of sea-level rise and when convergent boundaries had been introduced. At convergent boundaries, the denser plate subducts below the less dense plate. The same is true for seawater. This density driven circulation is called the Thermohaline Circulation . As mentioned above, the two main factors that affect density in the ocean are temperature and salinity. Therefore one can expect that seasonality (winter versus summer), storms, evaporation and precipitation play major roles that lead to changes of density during the season. Another factor that affects density is the distance from land, as rivers can bring additional freshwater from land. Learning Goals Enduring Understanding: Whereas temperature typically controls density offshore in the open ocean, salinity plays a larger role in coastal waters. Students who complete this unit will:  Observe temperature, salinity and density data at three locations.  Make a prediction about whether density is controlled by temperature or salinity  Interpret the result of a numerical experiment  Describe how temperature and salinity controls density in coastal and open ocean locations. Tasks 1. Go to the following webpage: https://datalab.marine.rutgers.edu/explorations/2019/density.php?level=application 2. Answer the following questions about the graphs shown: a) Which parameter are shown: b) Write down the time period for which data are plotted!

c) Above the temperature graph you can choose 4 different stations. If you scroll down on the page and click on the most left image under background information, you can see where these 4 stations are located. Please describe in your own words where these 4 stations are located. d) Within the dataset shown, is there a season where temperature fluctuations are occurring? If so, describe the period in terms of seasonality when most of the fluctuations are occurring. e) In which season is the temperature varying less at station Papa? f) If you compare the 3 graphs to each other, can you see a relationship between 2 of the graphs? 3. Based on the information you extracted from the graph, can you predict if temperature or salinity has a stronger effect on the density?

4. To test your prediction, you can click on “Hold temperature constant” or “Hold salinity constant” below the density graph. a. If you click on “Hold temperature constant”, a new model line (red) appears in the density graph. Does the predicted model line follow the original density data? b. What happens if you click on “Hold salinity constant”? Does the model line fit the original data very well? c. Is there a seasonal affect? A seasonal effect would be shown if sometimes the model based on holding the temperature constant and at other times the model based on holding the salinity constant would be better in describing the data. What do you think? The answer is a yes or no. d. Please summarize your answers in table 1. Table 1. REsult Seasonality Stronger effect on density Station yes no temperature salinity Station Papa Flanking Mooring A (30m) Washington Inshore Surface Mooring (7m), part of the Coastal Endurance Array Irminger Sea Flanking Mooring A (30m) Coastal Pioneer Offshore Surface Mooring e. Now choose the other stations and fill out the table (choose from the stations listed above the temperature graph. f. Please consider the locations of the different stations. Which of the four stations is closest to land? If you click on the link you can see where the stations are located: https://oceanobservatories.org/research-arrays/ .

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g. Click on the station (array) which is closest to land. The Washington Inshore Surface Mooring (7m) is part of the Coastal Endurance array. Please read the description and extract the information from the text that points to a factor that leads to strong variation in density ( https://oceanobservatories.org/array/coastal-endurance/ ). In order to help you on what you have to focus while reading the text on the webpage listed above, here are some information on the factors that could affect density in the ocean: Density can change by the following processes: changing the mass of water or changing the temperature. The mass of water is changed if the salinity changes. Increased salinity equals a saltier ocean and, therefore denser ocean water. Lower salinity equals a less salty ocean and, therefore a lower salinity. The following processes affect the salinity and, therefore the density of seawater:  Evaporation -> high evaporation increases salinity if not counter balanced by high precipitation  Precipitation -> precipitation of freshwater to the ocean reduces its salinity if not counterbalanced by evaporation  River -> river add freshwater to the ocean resulting in a lower salinity. The degree on how much the salinities varies is a function of how much water is transported by the river (think Big Chico Creek: sometimes lots of water sometimes completely dry. When is more water in Big Chico Creek?) Again, read the text ( https://oceanobservatories.org/array/coastal-endurance/ ) for the Washington inshore station point to the information you find that is responsible for salinity induced density changes .