| Salinity | • | Temperature | • | Density | • | Light Transmission | • | Sound Transmission |
Salinity is defined as the total amount of dissolved solids in water. These solids are called salts and they constitute approximately 3.5% of the ocean’s mass, the remaining percentage being pure water.
The oceans are very well mixed and the relative abundance of their major constituents is essentially constant. This principle of constant proportion or constant composition, was discovered during the HMS Challenger expedition 1872-1876, one of the most important scientific expeditions of the 19th century. Due to the constancy of composition it is only necessary to measure the concentration of one major constituent. Therefore, the determination of salinity can be made through its most abundant component: chloride, the concentration of which is easily determined.
 This multimetre can be used to measure salinity. The small bottles are for collecting water samples. |
Salinity is measured in Practical Salinity Units (PSU), and for most of the world's oceans has a value between 30 and 40. Where there is mixing of fresh water and sea water, salinity levels are lower. These waters are known as brackish and are often found in estuaries. On the other hand, if water is more saline than normal sea water, it is know as hypersaline. The Dead Sea is an example of a hypersaline water body.
Salinity Distribution
Sea-surface salinity tends to be distributed in a zonal pattern around the world. The saltiest waters are at mid-latitudes where evaporation is high. Less salty waters are found near the equator where rain decreases the salinity concentration, and in polar regions where ice and their melt waters have a low salt concentration.
 Global Sea-surface Salinity. (source: Wikipedia & World Ocean Atlas, 2001). |
In general, salinity increases with water depth. Around Ireland vertical salinity gradients may be seasonally and locally pronounced in near-shore areas and in early summer when warmer stratified water overlies cooler, mixed Atlantic water. However, during the winter months off-shore waters to the west and south of Ireland are generally well mixed from top to bottom with a consequent smaller variation in salinity.
Temperature
The temperature of a body is an indication of how hot or cold it is. In a physical sense, however, it is a measure of the average kinetic energy or motion of the molecules that make up a substance, so the higher the temperature, the faster the molecules of the substance move.
The distribution of temperature at the sea surface primarily depends on the amount of heat from the sun being transferred to the water. Temperatures are highest in equatorial and tropical regions and decrease towards the poles. Moreover the distribution of surface temperature is affected by ocean currents. These move warm water pole-ward along the western side of oceans and cold water towards the equator along their eastern sides, therefore acting as an important determinant of global climate.
Average sea surface temperatures to the west and south of Ireland range from 8-10°C in February-March to 14-17°C in August. To the northeast temperatures are, on average, a couple of degrees colder throughout the year. Water temperatures are about 7-8°C warmer around Ireland than the global average at equivalent latitudes. This is due to the North-Atlantic drift which transports warmer waters from the bay of Mexico.
 Average weekly sea surface temperature around Ireland in 2003 as determined from satellite measurements. The white areas are cloud or land. |
In general, water temperature decreases with ocean depth. However, from December to March the water column is generally well mixed with little variation in temperature from surface to bottom. In April/May, thermal stratification begins to develop in continental shelf and coastal waters and stratification is usually fully developed by mid-July. During summer, bottom temperatures in stratified areas may be 5-6°C cooler than the overlying surface water.
|
Density
Conductivity,
Temperature, Depth (CTD) Meter
 This rosette sampler on board the RV Celtic Voyager contains a CTD profiler and water sampling bottles. A Conductivity, Temperature, Depth (CTD) meter is a device which can measure conductivity (and hence salinity) and temperature in a water column at a range of depths. It is usually lowered at particular locations from a stationary ship in order to retrieve a full depth profile. Recordings are made continually while the sensor is being lowered and raised. The parameterss can be read immediately by an operator or saved digitally for later analysis. The platform on which the CTD is mounted often contains a number of bottles each of which can be filled with a water sample at a particular depth. |
Surface waters tend to be the warmest and hence the least dense. These waters can change temperature and salinity quite easily due to solar heating or evaporation. The gain or loss of heat energy at the ocean surface has a key role in controlling water density, but in the extreme polar areas of the ocean, where temperatures remain relatively constant it is salinity changes that can significantly affect density. The distribution of density depends on latitude, being lowest at the equator and highest at the poles, on account of decreasing temperature in these regions.
Water is colder and therefore more dense in the deep ocean. Deep ocean water is layered with the most dense cold waters at the bottom and less dense layers above. Circulation of water within the deep ocean, known as thermohaline circulation, moves water horizontally within these layers of equal density as it cannot easily move vertically across variations in density. Thermohaline circulation, as well as surface currents, are important in the transport of nutrients and warm and cold water masses, around the world’s oceans.
 Thermohaline circulation: the blue arrows indicate movement of cold dense water, which sinks and circulates in the deep ocean. The red arrows indicate the movement of warmer, less dense surface water (source: National Oceanographic Centre, Southampton). |
Light Transmission
Life depends directly and indirectly on energy from sunlight. In seawater, light intensity decreases with depth so it is useful to divide the water column into the photic zone, where plants receive adequate levels of sunlight and can photosynthesize, and the aphotic or dark zone where plants cannot survive.
 Blue and green light penetrates much deeper in clear water than the yellow and red colours. The presence of sediments and other suspended particles in the water can change the light's penetration ability. |
The longer wavelengths of visible light, the reds and yellows, are absorbed by water more readily than are the shorter wavelengths, the greens and blues. This property of water combined with the scattering of the visible light makes the open ocean appear blue, unlike nearshore water, which is typically green and yellow in colour because suspended particles reflect these wavelengths.
Sound Transmission
Sound travels in water about five times faster than in air. Its velocity, however, is modified by temperature, salinity and pressure. Sources of noise in the Earth’s oceans include earthquakes, human activity (e.g. shipping, drilling) and sea creatures. Beneath the warm surface ocean layers there is a zone where sound velocity is a minimum, because of a particular temperature/pressure combination. This horizontal zone, known as the SOFAR channel, traps sound and can conduct it over great distances. It is believed that some cetaceans (whales and dolphins) use this channel to communicate over many kilometres.
| MIDA: | Marine data buoys |
| Sea surface temperatures (2003) |
Marine Institute Web Mapping Services: This web-GIS allows visualisation and download of Conductivity, Temperature and Depth (CTD) information.
SOC: The Southampton Oceanography Centre in the UK, provides links to data sources, images and other resources.
BUBL: The BUBL Information Service, located at Strathclyde University in Glasgow contains links to a range of oceanographic resources.
OceanLink: This web site provides a variety of activities designed to help students with oceanography.
EOS: The department of Earth and Ocean Sciences at NUI Galway carries out oceanographic research around Ireland.
Martin Ryan Institute: Based at NUI, Galway this institute carries out oceanographic research.
Boelens, R.G.V. et al., 1999, Ireland's Marine and Coastal and Adjacent Seas, An Environmental Assessment. Marine Institute. Chapter 2.
Challenger Division for Seafloor Processes, 2003, The Classroom@Sea project, NOCS,
http://www.noc.soton.ac.uk/gg/classroom@sea/general_science/
conveyor.html, [Site visited 04/10/2006].
NOAA, Ocean Explorer, http://oceanexplorer.noaa.gov/explorations/
sound01/background/acoustics/media/sofar.html, [Site visited 04/10/2006].
Office of Naval Research, Science and Technology Focus, http://www.onr.navy.mil/Focus/ocean/water/density1.htm [Site visited 04/10/2006]
Pinet, P. R., 2000, Invitation to Oceanography. 2nd Edition. Jones and Bartlett Publishers. Chapter 5.
Stewart R. H. Introduction to Physical Oceanography. Department of Oceanography. Texas A & M University. 1997-2005. Chapter 6. http://oceanworld.tamu.edu/resources/ocng_textbook/contents.html, [Site visited 04/10/2006].
Tomczak, M. An Introduction to Physical Oceanography 1996-2000. Lecture 3. http://www.es.flinders.edu.au/~mattom/IntroOc/, [Site visited 04/10/2006].
Thurman, H. V., 1996, Essentials of Oceanography, 5th Edition. Prentice Hall. Chapters 5 and 6.
University Corporation for Atmospheric Research, 2001, Windows to the Universe, http://www.windows.ucar.edu/tour/link=/earth/Water/density.html&edu=high [Site visited 04/10/2006]