-_ ..-.....,... _------------------------------------_... Investigation

SEAWATER TEMPERATURE, SALINITY, 3A: AND DENSITY

Objectives:

Temperature and salinity are two of the most important properties of ocean water; together, they (along with pressure, although to a much lesser extent) control the density (mass per unit volume) of seawater. Salinity is a measure of the amount of salt dissolved in seawater and, on average, ocean water is 96.5% water and 3.5% dissolved salts. This average salinity, 3.5% or 35 parts per thousand, is quoted as a non-dimensional "salinity" of35.0 (or 35 psu). At constant salinity (for values of24.7 or higher, and above, the temperature of initial freezing for seawater of that salinity), the density of ocean water decreases with rising temperature and increases with falling temperature. At constant temperature, the density of ocean water varies directly with salinity, that is, the density increases with increasing salinity and the density decreases with decreasing salinity. Saltwater having a salinity of less than 24.7 is often called brackish water (i.e., seawater diluted with fresh water). Brackish water acts like fresh water in that upon cooling it achieves its maximum density before reaching the temperature at which freeZing begins.

Changes in ocean water density are due largely to processes that take place at the interface between the ocean and atmosphere. For example, solar radiation and ocean/atmosphere heat exchange affect sea surface temperature, which in tum governs the rate of evaporation and

'~j salinity. Precipitation, sea ice fonnation, and freshwater runoff from land also influence salinity and therefore the density of surface waters. Later in this course, we examine the role played by variations in water density (due to temperature and salinity differences) in the (thermohaline) circulation of the ocean.

After completing this investigation, you should be able to:

• Explain how ocean density depends on seawater temperature and salinity. • Identify the various processes that govern the salinity of surface ocean waters. • Describe the general patterns of sea surface temperature and salinity.

Figure I is a Temperature-Salinity (T-S) Diagram with temperature plotted in degrees Celsius on the left vertical axis and the salinity increasing from left to right along the bottom horizontal axis. The solid curves are lines of constant seawater density in grams per cubic centimeter (g/cm3). Lines of constant density are called isopycnals. Variations in seawater density are small (but very significant) so values are given to the fourth decimal place.

1. Examine Figure 1. Place a large dot on the T-S diagram representing seawater with a temperature of 10 DC and salinity of 35.1. According to the diagram, its density (to the fourth decimal place) is g/cm3

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33.5 34.0 34.5 35.0 35.5 36.0 36.5

Salinity

Figure 1. Temperature-Salinity (T-S) Diagram with density contours in g/cm3.

2. Imagine that this seawater is heated to 16°C with no change in salinity. Its density (to the fourth decimal place) is now g/cm3

,

3. This experiment shows that at constant salinity, raising the temperature of seawater causes its density to [(increase) (decrease)].

4. Returning the seawater to its original position (properties) on the T-S diagram shows that with falling temperature and constant salinity, the density of seawater [(increases) (decreases)] .

5. From its original position on the T-S Diagram, now imagine the seawater salinity increasing to 35.7 while" the temperature remains constant. Its new density would be ___ wcm3,

6. This illustrates that at constant temperature, increasing the salinity of seawater causes its density to [(increase) (decrease)].

7. Returning the seawater to its anginal position shows that with decreasing salinity and constant temperature, the density of seawater [(increases) (decreases)].

8. We deteffi1ined in Item 1 (above) that a sample of ocean water having a temperature of .-... 10°C and a salinity of 35. I has, according to the isopycnal passing through that point in the diagram, a density of g/cm3.

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9. Another seawater sample that is positioned on the same Figure 1 isopycnal would have the same density. For example, water with the same density at a temperature of 6 DC-" would have a salinity of . Hence, this demonstrates that seawater samples having different combinations of salinity and temperature can have the same density.

Suppose that two seawater samples having the same density but different temperatures and salinities are mixed in equal portions. Does the resulting seawater mixture have the same density? To find out, plot and labeL points on the T-S Diagram representing Sample A with a temperature of 0 °C and a salinity of 34.2 and Sample B with a temperature of 9 °C and a salinity of 35.45. Both samples have a density of 1.0275 glcm3• Draw a straight line connecting points A and B. If equal portions of the two samples are mixed, the resulting mixture would have temperature/salinity/density properties corresponding to a point halfway along the line connecting points A and B. Mark that new point C.

10. The temperature and salinity of the seawater at point C would be the average ofthe origin'at sample values. However, it can be seen on the T-S diagram that the density of the mixed water Sample C is {(greater than) (the same as) (less than)} the original density of seawater Samples A and B.

11. Caballing is the name given to the mixing ofwater masses to produce a blend that is denser than its original component water masses. Because of the change in density, the newly mixed water [(rises) (sinks)) through the surrounding water column. Caballing is thought to contribute to vertical circulation in some higher latitude Northern Hemisphere

',-" ocean locations.

12. When water evaporates, dissolved and suspended materials are left behind. Temperature largely governs the rate of evaporation because more active water molecules more readily escape the water surface and enter the atmosphere as water vapor. In regions of the ocean where the weather is persistently fair and temperatures are consistently high, evaporation rates are relatively high and consequently the salinity of surface waters is relatively [(hiCh) (low)).

13. At constant temperature, the surface water salinity is likely to be lower where the climate is [(arid) (rainv)], that is, where precipitation is greater than evaporation.

14. Flood waters discharged into coastal waters by a large river are likely to {(increase) (decrease)) the salinity of surface coastal water.

15. At high latitudes in winter, the freezing of seawater to produce ice excludes salts that are trapped in brine cells between ice crystals or incorporated into the underlying seawater. The combination of the low temperatures and increased salinity results in the underlying seawater having a relatively [(hiWz) (low)} density.

Next we continue our inquiry into the basic properties of seawater by considering actual global patterns of sea smface temperature and salinity. We also consider how temperature and salinity vary with depth at a specific location in the ocean.

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..--...Figure 2 is a color-coded global ocean map of mean annual sea surface temperature (SST). Isotherms (lines of constant temperature) are given in degrees Celsius. This map was

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provided by NOAA's National Oceanographic Data Center and is based on satellite, buoy, and ship measurements.

16. The highest whole-degree (solid-line) SST isotherm plotted on the Figure 2 map is ___ °C, located in the western tropical Pacific Ocean.

17. The lowest whole-degree (solid-line) SST isothenn plotted on the Figure 2 map is __ °C at high latitudes.

18. Comparison at the same latitudes shows the mean annual SST is lower along the U.S. [(West) (East)] Coast. This contrast in SST is due to surface ocean currents that flow southward along the West Coast and northward along the East Coast.

19. Mean annual SST is much [(lower) (hie-her)] in the Southern Ocean than in the Indian Ocean.

Figure 3 is a color-coded global ocean map of mean annual sea surface salinity. Isohalines (lines connecting points of equal salinity) are given with a contour interval of 0.2 psu. This map was provided by NOAA's National Oceanographic Data Center.

20. In the open ocean, the highest values of sea surface salinity are located [(aiont: the equator) (in the subtropical ocean basins) (at high latitudes)}.

21. These relatively high values of sea surface salinity are likely due to [(greater precipitation than evaporation) (greater evaporation than precipitation)].

22. The highest open-ocean sea surface salinities appearing on the map are located in the {(central North Pacific) (northern South Atlantic) (southern North Atlantic)).

23. The lowest open-ocean sea surface salinities shown on the map are found in the [(Atlantic) (Pacific) (Indian) (Arctic) (Southern)I Ocean.

24. Generally, the [(lowest) (middle) (hi~hest)] latitudes have the lowest annual mean sea surface salinities.

The ocean maps you have just used, along with numerous others, are available via the course website. Go to the "Earth System" section and click on "NOAA Ocean Atlas 2005." Under "WOA05F Interactive Image Access," you can select from a number of ocean variables such as temperature, salinity, dissolved oxygen, nitrate, silicate, and phosphate. After clicking on a variable, you can specify ocean basin, time period, depth, figure type, and kind of image. Try it, you'll like it! We will return to this valuable resource later in the course.

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Annual temperature roC] at the surface.

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Figure 2. Global ocean map of mean annual sea surface temperatures (SST). Isotherms (Jines of constant temperature) are given in degrees Celsius. (NOAA National Oceanographic Data Center)

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Figure 3. Global ocean map of mean annual sea surface salinity. Isohalines (lines connecting points of equal salinity) are given with a contour interval of 0.2 psu. (NOAA National Oceanographic Data Center)

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Near real-time and archived temperature and salinity data for' the upper one or two kilometers of the world ocean for over 3000 locations are also available online. Figure 4 is an example of such data; it is a plot of temperature and salinity profiles acquired by Argo Profiling Float No. 4900833 on I February 2008 in the Pacific Ocean west of Northem California at 38.4 degrees N, 139.5 degrees W. The left panel is a plot of temperature (in DC along the top) versus pressure (in decibars increasing from the surface downward). The right panel is a plot of salinity (in practical salinity units along the top) versus pressure. Pressure in decibars corresponds numerically to depth in meters so, for example, a pressure of 800 decibars occurs at a depth of 800 m. These data were accessed through the course website in the "Physical and Chemical" section link to "USGOAE profilers." USGOAE is the acronym for "United States Global Ocean Data Assimilation Experiment."

A profileI' is an instrument typically prograrmned for a 10-day cycle during which it sinks to a 1000-m depth where it drifts for approximately nine days before sinking to a maximum depth of2000 m. From its greatest depth it measures temperature and salinity through ocean layers as it ascends to the surface over a period of about six hours. At the surface, data are transmitted to a communications satellite and the float begins another cycle. Figure 5 shows the complete cycle (from http://www.argo.ucsdedu/operationyarkyrofilejpg).

25. The maximum water pressure recorded in the Figure 4 float profile is about 2000 decibars. According to the information mentioned earlier, the pressure value of 2000 decibars occurs at an ocean depth of meters.

26. According to the plotted temperature profile in Figure 4, seawater temperature mostly [(increases) (decreases) (is constant)] from a depth of about 150 m down through the rest of the profile.

27. The temperature at a depth of 1000 m is about [(2.2) (3.,1) (2:,£)] degrees Celsius.

28. This temperature is about [(2...£J (9.0) (J,ld)] Celsius degrees lower than the temperature at the surface.

29. At a depth of 1400 m, seawater temperature is about 2.8 °C and salinity is about [(32.1) (11:.Z) (34.,D].

30. According to the Temperature-Salinity Diagram in Figure 1, this combination of seawater temperature and salinity at a depth of 1400 ill corresponds to a density of approximately [(1.0259) (1.0261) (1.027,D] glcm3•

31. The general decline in temperature and increase in salinity with increasing depth in both ­of the profiles indicate an [(increase) (decrease)] of seawater density with depth.

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Figure 4. Argo Profiling Float No. 4900833 on 1 February 2008 in the Pacific Ocean west of Northern California at 38.4 degrees N, 139.5 degrees W.

--.-­ Figure 5. ARGO profiling float typical operation cycle.

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