Chemical and physical properties

Oceanic water is a complex solution containing a greater diversity of dissolved chemicals. The total mass (in grams) of all solid substances dissolved in seawater is expressed as salinity in parts per thousand. The average salinity of waters of the World Ocean is about 35 parts per thousand. Almost all chemical elements known on the earth have been found in ocean water, most of them only as trace elements. The most abundant, and the ones having the greatest influence on the physical properties of seawater, are Cl - , Na + , SO 4 2- , Mg 2+ , HCO 3 - , Ca 2+ , K + , Br - , Sr 2+ , F - and ? 3 ?? 3 . These eleven chemical elements comprise about 99,99 % of the whole weight of dissolved substances. The trace elements have practically no influence on the physical properties of seawater, but play an important role in biochemical processes occurring in the ocean.

Dissolved gases in the oceans are mainly nitrogen, oxygen, carbon dioxide, argon and hydrogen sulphide. The gases derive from the atmosphere, biochemical processes in different layers of the water, at river mouths and estuaries, from degassing of the mantle into the earth's crust and from other geo-chemical processes.

Organic substances are found in dissolved and suspended states in the water column. The greatest concentrations are seen in the surface layer of the ocean, where most biological activities occur.

The average surface temperature of the world's oceans is ca 17°C. The highest temperature is found in the Red Sea (>36°C) and the lowest has been observed in the Weddell Sea, Antarctica (< -2°C). The depth distribution of temperature depends on the amount of solar heating on the surface and intermixing of water masses. Down to depths of 1,000-1,800 m, the temperature gradually decreases, and below 1800 m, cold waters of almost constant temperature exist.

The salinity of seawater differs between different regions of the world's oceans. High salinity (>35 parts per thousand) is encountered in surface waters at tropical latitudes , where evaporation is greater than at other latitudes, and low salinity (ca 29 parts per thousand) is observed in the summer in the Arctic Ocean , when the sun melts the ice. Low salinity is also encountered in coastal regions with significant river run off . The salinity of deep and near bottom waters in the oceans is about 35 parts per thousand . Salinity and temperature together affect the density of water, and many physical characteristics depend on density distribution (e.g. water exchange processes).

Ocean currents

Ocean currents play a mayor role in moving heat over the surface of the earth and thereby influence the climates of the continents. In the ocean basins, cold water tends to move towards the tropics along the western coasts of the continents, and warm water tends to move toward temperate latitudes along the eastern coasts of the continents. The warm Gulf Stream, creating a mild climate far to the north into western Europe, and the cold Peru Current, generating cool, dry environments along the west coast of South America, are both examples of these currents.

The physical conditions of the oceans are as complex as those of the atmosphere. Variation in those conditions is caused by winds , which drive the mayor surface currents of the oceans, and by the bottom topography of the ocean basins. Deeper currents are also established by differences in the density of the water caused by variations in temperature and salinity.

Wherever surface currents diverge, as in the western parts of the Pacific Ocean, vertical, upward movement of ocean water, referred to as upwelling , occurs. As the surface currents move apart, they tend to draw water up from deeper layers. Strong upwelling zones are also established on the western coast of continents where surface currents flow towards the equator. A curious consequence of the rotation of the earth is the deflection of surface currents away from the continents, which is aided by the trade wind. As water move away from the coast, it is replaced by water from greater depths. Because this water tends to be nutrient-rich , upwelling zones are often regions of very high biological productivity and extremely important to marine ecosystems.

Ocean zonation

The oceans can be divided into several zones differentiated by depth or light regime. The two mayor parts of the marine environment are the benthic and the pelagic zone. The benthic zone is the part of the ocean associated with the bottom and the pelagic zone is the water column above the benthos.

The benthic zone can be divided into several parts depending on depth. The littoral zone extends between the highest and lowest tidal levels. Part of this zone is periodically exposed to air, depending on position within the intertidal range. The sublittoral zone is situated beneath the littoral, from the low tide mark to the continental shelf edge, about 200 m deep. Beneath the shelf edge down to about 2000 m, the bathyal zone is located. This area coincides with the continental slope. Deeper down the abyssal (includes the average depth of the deep ocean floor, 2000 to 6000 m), and hadal (includes the trenches, the deepest part of the sea floor, deeper than 6000 m) zones are situated.

The pelagic area is also divided into different zones. The neritic zone is the water column situated above the continental shelf, between the lowest tidal level to the shelf edge. It is often a region of high productivity because the sunlit surface layers of water are not far removed from the regeneration of nutrients in the sediment below. Beyond the neritic zone, were the seafloor drops rapidly to great depths, the oceanic zone is situated. The productivity is usually restricted in this zone due to the low availability of nutrients.

The marine environment can also be divided into zones based on depth of light penetration. The photic zone is the depth where light is sufficient for photosynthesis.   It varies from about 20 m on the shelf to about 100 m in the open ocean, depending upon the clarity of the water. The aphotic zone, below the photic zone, receives no light from the surface because the water above absorbs it all. Down here organisms depend mostly on organic material raining down from above.

El niño-Southern Oscillation event

The cold waters moving north from the Antarctic Ocean along the coast of Chile and Peru, don't only bring cool climate to the west coast of South American, but also create a favourable environment for huge fish populations. Both the Peruvian fishing industry and some of the world's largest seabird colonies thrives on the abundant fish outside the continent.

Each year a warm counter current, known as El niño, moves down the coast of Peru. Some El niños are strong enough to force the cold Peru Current offshore, taking with it the food supply of millions of birds as well as the livelihoods of local fishermen.  

Picture is taken from a site with detailed information about this phenomena. You can also look at an animation of this phenomena

Under "normal" conditions, a steady wind blows across the equatorial central Pacific Ocean from an area of high atmospheric pressure centred over Tahiti to an area of low pressure centred over Darwin, Australia. The difference in atmospheric pressure between the western Pacific and the eastern Pacific normally fluctuates between years in a seesaw pattern. This fluctuation of pressure is called the Southern Oscillation . Occasionally this phenomenon will bring extremely high pressures in the western Pacific resulting in a reversal of the trade winds and, in some instances the normal westward-flowing ocean currents. Such conditions, coupled with a strong El niño, cause warm water to pile up along the western coast of South America and greatly weaken or even halt upwelling there.   When these two simultaneous occurrences (strong El niño and unusually high atmospheric pressure in the western Pacific) take place, an El niño-Southern Oscillation (ENSO) event is the result. The ENSO events do not only affect the western coast of South America, but also have important widespread climate effects.

Even though the term "El niño" has a restricted meaning, many refer it to all the global events that occur in an ENSO year. "El niño" has become synonymous with "ENSO".