Physical properties of the oceans

Part of the Ocean engineering course offered by the Wikiversity Department of Marine Engineering, School of Engineering and the Engineering and Technology Portal

Lecture

The oceans of Earth are vast and wild. Our primordial soup, they provide many of the facets that we refer to as existence. The weather of earth is controlled by the heat capacity and circulatory heat transfer mechanisms of the ocean currents. Many of our mineral assets, our food and all of our fresh waters have passed through the oceans at some time.

Temperature

A simplified summary of the path of the Thermohaline Circulation. Darker arrows represent deep-water currents, while lighter arrows represent surface currents

The temperature of the oceans is not a constant, nor is it uniform throughout the oceanic mass. Sunlight imparts its energy to the ocean surfaces all summer long, and in response the ocean heats and holds that energy. Once winter comes, the oceans release that warmth back into the atmosphere in accordance with their heat capacity. This is why it is warmer near the coast in winter, and cooler in summer. Just as the ocean may act as climate control for a local area, the oceans provide convective heat transfer as they circulate, providing climatic adjustment for every latitude via the Thermohaline Circulation. These currents are responsible for the cool weather along the US Atlantic coast, and the balmy high-latitude climate of the Mediterranean. The oceans in the Northern hemisphere are usually warmer in January, long after the summer has passed, and coolest in April, after most of their heat has been given off to the cold winter skies.

Similarly, the temperature of the ocean is not uniform at all depths. Any swimmer may recognize the temperature differential between the water at the surface, where the sunlight is incident, and that deeper water that is much cooler. At medium depths however, there is a greatly increased temperature differential, and the water below a certain point is much cooler than the water above it. This sharp differential is called the thermocline, and is present in most ocean environments, although greater in the tropics and in temperate regions during summer and nearly nonexistent in polar regions. In shallow waters, the cold air of winter can cool the surface so quickly that the water will actually "turn over", reversing the thermocline. The dependence of the thermocline upon salinity and density will be discussed further with those topics.

Density

Density in the oceans is a function of temperature, pressure and salinity, represented by the following equation which is called the Equation of State for Seawater (I'll work at getting you a better eqn. -gustable):

         $\rho (S,T,p)=\rho (S,T,O)/[1-p/K(S,t,p)]\$

where K is the bulk modulus $K=1/\beta$ and $\beta$ is compressibility, and which may be derived from the speed of sound in water C:

         $C={\sqrt {K/\rho }}=1449+4.6T-0.55T^{2}+1.4(S-35)+0.017D$ 

in m/s if D is depth in meters, T is temperature in kelvins and S is salinity in parts per thousand.

Generally speaking, density is roughly proportional to pressure and thus depth, and inversely proportional to temperature. Density is also very dependent upon salinity as well, however, as you will learn in the next segment, this too increases with depth.

Seawater density is nearly uniform at all latitudes, although surface density lessens greatly as one nears the equator, a phenomenon due to tropical climates. This sharp difference in density within these tropical regions is called the pycnocline.

Salinity

The makeup of the oceans is paramount to all these interrelated functions. The ocean is composed of primarily water, however the chemical and biological sediments carried by rivers are present too, as are similar constituents formed from the falling of rain through pollution and the remnants of aquatic life cycles. All of this material amounts to a watery mixture often referred to as salt water, yet more accurately described as saline. Salinity is the measure of how much material is mixed in with water and is defined as the total amount of all material dissolved in seawater or "the total amount of solid materials in grams contained in one kilogram of seawater, when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine and all organic matter completely oxidized". The average salinity of seawater is 35 parts per thousand (35 g/kg).

The two most important factors in salinity determinations are density and concentration. As is indicated above, the density of a substance is contingent upon the local temperature and pressure (and salinity). At a high temperature, seawater expands and is therefore less dense, leading to a lower salinity. At low temperatures, denser water results in greater salinity. Similarly, higher concentrations of salt in the salt-water solution are increasingly saline, while higher concentrations of water are less saline.

On the surface where temperatures are high, density is low and salinity is low (S=33-37). Also, in areas where atmospheric temperatures are high, such as the tropics and sub-tropics, there is increased evaporation and therefore low concentrations of water in seawater and increased salinity. The temperature maximum along the equator results in low density seawater and low salinity there as well, as does the influx of fresh water from melted ice in the polar regions.

Salinity variations in the vertical plane are similarly organized. Generally, the highest salinity water is found at the top of the surface, where evaporation occurs. Not far below the surface mixing zone, a sharp decrease in salinity marks the transition to temperature based low density and salinity. Thereafter salinity increases with depth as the temperature drops and the density increases. This profile varies with latitude. At high latitudes (the poles) surface salinity is low and continually increases. In the tropics there is a salinity maximum at the top of the thermocline, which is the result of subtropical salinity sinking a little and flowing towards the equator. In coastal regions, the low salinity runoff creates a sharp contrast with the high salinity seawater which is called the halocline.

An early comparator to salinity was the chlorine concentration. This relation has been labeled as Absolute Salinity $(S_{a})$ and is calculated as:

         $S_{a}\ =1.80655*Chlorinity$

Practical salinity (S), is the measure of salinity with regards to electrical conductivity, which is itself dependent upon temperature and salinity. This definition is as yet not well established, and further research is recommended before any attempt is made to adequately predict salinity, although greater faith has been placed in this method than any other to date.

An interesting addition in the study of salinity is the Conservation of Salt; that is to say that the oceans always contain roughly the same amount of dissolved salts at all times (although this number increases by a very small fraction as time passes).

Chemistry

Hydrothermal vent biogeochemical cycle diagram

Salinity dissolved in seawater comprises the majority of the know elements including ionic species of chlorine (55%), sodium (30%), sulphate (8%), magnesium (4%) and potassium (1%). Interestingly, throughout the oceans, proportions of these constituents are generally consistent, which indicates that the oceans have mixed well over the millenia, even between the oceans, beyond the semi-contained mixing of the thermohaline circulation.

Ocean chemistry is a result of various biological and geological processes. Much of the ocean's mineral makeup is the result of landward erosion, where freshwaters carry minerals down from hilly landlocked regions into the oceans as erosion. The decay and disintegration of calcium shells and bones also plays a role in adding salinity to seawater. Submarine volcanism often contributes to salinity too, by spewing mineral ejecta into solution within the benthic environs. Lastly but not least, anthropogenic waste contributes heavily to the varied and dangerous chemistry of the oceans today. Mercury and PCB contamination is a fixture in our fisheries and our coastal regions today, and the continued use of environmentally destructive industrial processes threatens the future viability of the oceans as a food source.

The oceans can also be extremely caustic requiring structures within them to employ cathodic protection from redox reactions. The vulnerability of materials to seawater may be gauged by referring to its galvanic potential. This is why Naval ships are constantly repainted over and over again; to maintain their integrity against the biting seas.

Assignments

Activities:

1. Select your favorite region of the oceans and research the physical properties of that area
2. Using your newfound knowledge of the subject, why can't freshwater fish live in saltwater environments?
3. If the Thermohaline Circulation were to cease, what would be the resulting impact on the Earth's climate?
4. What current event is potentially slowing the Thermohaline Circulation?
5. What effect does the pycnocline have on ocean currents given that flow velocities are greater near the surface?
6. Use Dimensional Analysis to determine a basic equation for density using only Temperature, Depth and Salinity.

Readings:

1. Peruse the appropriate sections of b:Introduction to Oceanography

Study guide:

1. Wikipedia article:Temperature
2. Wikipedia article:Density
3. Wikipedia article:Salinity
4. Wikipedia article:Chemistry
5. Wikipedia article:Marine geology
6. Wikipedia article:Oceanography
7. Wikipedia article:Chemical oceanography
8. Wikipedia article:Physical oceanography

External Links

• Google Map of the Topography of the Thermohaline Circulation: Follow this course by zooming in close on the volcanoes, hydrothermal vents and ice fields that affect the surface and deep Thermohaline and Brine currents and our climate.