If freshwater is added to a region, what happens to seawater density?

iii.ii The density of fresh water and seawater

Fresh water and seawater have very different physical properties. Imagine a freshwater lake in winter. As the air temperature falls the temperature of the water at the surface decreases and its density changes (Figure four).

Figure 4 The density of fresh h2o plotted confronting temperature.

• What is the temperature at maximum density?

• The maximum density is at ~4 °C (Figure 4). At both lower and higher temperatures than this the water is less dense.

While information technology is difficult to see on the calibration in Figure 4, the maximum density is at iii.98 °C. As a lake cools to this temperature in winter the surface waters volition sink through convection and warmer water rises to the surface. With continued cooling this warmer surface water also becomes dumbo and sinks. As the surface water is being cooled the lake will become stratified. That is, the density will increase with depth to 1000 kg m⁻³, and the deep h2o will have a temperature of 3.98 °C. Imagine the convection continuing until all the h2o has reached 3.98 °C.

• What volition happen to the h2o in the lake when all the h2o is cooled below to 3.98 °C?

• The water at the surface of the lake will absurd and become less dense. This means that information technology volition not sink abroad from the surface. So the lake volition end up with a cool surface layer with warmer, denser water below.

Eventually the surface layer of water will be cooled to the freezing signal (0 °C) and ice will grade on the surface. At this point the temperature and density of the water will take a structure similar that shown in Figure 5.

Figure 5 (a) Section through a lake showing ice on the surface. (b) Temperature construction throughout the lake: there are three layers: a cold surface layer, an intermediate layer and a warm deeper layer. (c) Density structure mirroring the shape of the temperature profile in (b).

Figures 5b and c show that the temperature and the density increase with depth from the surface to the lesser layer, where the temperature is 3.98 °C and density is at its maximum.

Allow us call up about this a trivial more.

Ordinarily when a liquid is heated the molecules acquire more energy and become more widely spaced, and so in the same volume, the density decreases. In fresh water the opposite may happen, depending on the temperature. When fresh h2o at 3.97 °C (Effigy four) is cooled the density will decrease. You know from the lake (Figure 5) that ice (i.e. solid water) is less dense and floats. But cooling a liquid commonly packs the molecules more closely, which increases the density. This ways that below 3.98 °C cooling results in the water molecules spacing out and both the liquid and the solid h2o beneath this temperature expand. This is an astonishing physical property and is why pipes flare-up and water in cracks shatters rocks in common cold temperatures. The molecular construction of a water molecule is shown in Figure 6.

Figure 6 A molecule of water, H₂O, with the hydrogen and the oxygen atoms sharing electrons.

In H₂O, the oxygen and the hydrogen atoms share electrons, and the angle betwixt the two hydrogen atoms is 105°. This results in a pocket-size net negative charge on the oxygen side of the molecule, and a small net positive charge on the hydrogen side. This is a polar structure in which molecules are weakly attracted to each other and course weak 'hydrogen bonds'. At depression temperatures a more than ordered packing of water molecules develops and the density is reduced. If the temperature is increased but is <iii.98 °C, the hydrogen bonds break, but the molecules notwithstanding pack together closely. Above 3.98 °C, the increase in internal energy means the molecules get more widely spaced and, following Figure 4, the density decreases.

Seawater is saline and the table salt affects the density. Figure seven shows the vertical structure of the temperature, salinity, and density at latitude 20° S in the Atlantic Sea over an abyssal plain almost 5300 thousand deep.

Figure 7 The physical properties of seawater at 20° S in the Atlantic Sea confronting depth. (a) Temperature; (b) salinity; (c) density.

Figure 7a shows 28 °C temperature at the surface to below i °C at the sea floor. The variation in salinity in 7b clearly affects the temperature of maximum density shown in 7c. The seawater density ranges from 1024 to 1028 kg k⁻³, ~24-28 kg 1000⁻³ denser than fresh h2o (Figure 4), and density increases with depth and the h2o is stratified all the fashion to the bounding main flooring (although >3 000 m depth there is simply a small increase). Considering seawater is typically 1024-1028 kg thousand⁻³ there is a density anomaly, σ _t (pronounced 'sigma t') given by:

where ρ is the density of h2o.

• What is the range of σ _t for typical seawater?

• The density anomaly for typical seawater is in the range 24-28 kg m⁻³.

Box ane explains how the physical backdrop shown in Figure vii are measured.

Box 1 Measuring the physical backdrop of the ocean

Much of our knowledge well-nigh how the oceans circulate is based on measuring the various parameters such as temperature and salinity from the surface to the sea floor. The water which makes up this range is called the h2o column. The near important musical instrument used past oceanographers is called a CTD (Figure viii), which measures the conductivity and temperature of the seawater and depth (pressure level).

Figure 8 A CTD: the bottles on the side of the musical instrument collect samples of water and can exist closed past the operator at any chosen depth.

Considering force per unit area in the ocean is proportional to the weight of the h2o higher up, it is given past the hydrostatic equation:

where p is pressure, z is a alter in depth and one thousand is the acceleration due to gravity. The minus sign indicates that the vertical coordinate z (depth) is positive in an upwards direction. And so by measuring pressure, Equation 2 tin be arranged to get depth.

Other parameters can as well be measured, such as sediment particle density, the amount of chlorophyll present (in algae), and and then on. A CTD is lowered from a ship on a winch at a rate of ~60 one thousand min⁻ⁱ. If the sea is 5300 grand deep, as in Figure seven, a round trip is x 600 m and one contour can accept almost three hours.

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Source: https://www.open.edu/openlearn/science-maths-technology/the-oceans/content-section-3.2

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