Characteristics of Oceans Help

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By — McGraw-Hill Professional
Updated on Sep 4, 2011


It has been estimated that if all of the oceans’ water were poured off, salt would cover the continents to a depth of 1.5 m. That is a lot of salt!

Salinity is the amount of salt found in 1 kg of water. Salinity, or salt content, is written in parts per thousand (ppt) because there are 1000 g in 1 kg.

The average ocean salinity is 35 ppt. This number varies as rainfall, evaporation, river runoff, and ice formation changes it slightly (32–37 ppt). For example, it is said that the Black Sea is so diluted by river runoff, its average salinity is commonly around 16 ppt.

Freshwater salinity is usually less than 0.5 ppt. Water between 0.5 ppt and 17 ppt is called brackish . In areas where fresh river water joins salty ocean water, like estuaries, the water becomes brackish.

When salt water gets to the polar regions, it cools and/or freezes, getting saltier and denser. Cold, salty water sinks. The level of ocean salinity increases by depth. It is divided into three vertical layers. The surface layer has a mixed salinity depending on rainfall or runoff from the land. The middle layer is called the halocline with a medium range of salinity. The deepest and coldest ocean water has the highest level of salinity.

Fish and animals that live in seawater have worked out ways to survive in a salty environment. Most marine creatures are able to maintain nearly the same concentration of salinity within their bodies as the surrounding environment. When they are moved to an area of much less salinity, they die. You can’t put a saltwater fish in a freshwater aquarium!


The ocean has a broad temperature range from warm (38°C) shallow coastal waters of the equator to the nearly freezing arctic waters.

The freezing point of seawater is about –2°C, instead of the 0°C freezing point of ordinary water. Salt lowers the freezing point of seawater. As seawater increases 5 ppt in salinity, the freezing point decreases by –17.5°C.

The ocean is also divided into three vertical temperature zones. The top layer is the surface layer or mixed layer . This warmest layer is affected by wind, rain, and solar heat. Have you ever been swimming in a deep lake? As you get farther away from the sun-heated surface, it gets colder. Your feet are colder than your upper body.

The second temperature layer is known as the thermocline layer. Here the water temperature drops as the depth increases, since the sun’s penetration drops too.

The third layer is the deep - water layer . Water temperature in this zone sinks slowly as depth increases. The deepest parts of the ocean are around 2°C in temperature, with inhabitants that either like very cold water, or have found specialized environments like a volcanic vent that heats the water dramatically. Figure 13-2 illustrates the thermocline’s location in the ocean’s temperature layers.


Fig. 13-2. The oceans have distinct temperature zones.


Temperature, salinity, and pressure come together to influence water density , which is the weight of water divided by its volume. Cold seawater is denser than warm, coastal water and will sink below the less dense layer.

temperature + salinity = density

The ocean waters are similarly divided into three density zones. Less dense waters form a surface layer. The temperature and salinity of this layer varies according to its contact with the air. For example, when water evaporates, the salinity goes up. If a cold north wind blows in, the temperature dips and that also affects density.

The middle layer is the pycnocline or transition zone . The density here does not change very much. This transition zone is a barrier between the surface zone and the bottom layer, allowing little to no water movement between the two zones.

The bottom layer is the deep zone , where the water stays cold and dense. The polar regions are the only places where deep waters are ever exposed to the atmosphere because the pycnocline is sometimes not present. Figure 13-3 shows the three-transition layers of density (pycnocline), salinity (halocline), and temperature (thermocline) according to depth.


Fig. 13-3. The ocean has transition zones for temperature, salinity, and density.

Where a river meets the ocean, fresh water flows into salty water. The colder river waters are often less dense than the warmer ocean waters. The density differences can create distinct layers in the relatively shallow waters. A submarine or diver operating in this environment and wanting to stay at the same horizontal level would have to alter buoyancy to adjust for density changes. The same thing happens when a river flows into the ocean. It drags along a portion of sediment into the ocean. This changes the shape of the ocean floor by sedimentation and silting and shifts the layering.

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