Physical Oceanography 1:The physics of seawater

An overview of Ocean Stratification

• Tropical and subtropical oceans are permanently layered with warm, less dense surface water separated from the cold, dense deep water by a thermocline, a layer in which water temperature and density change rapidly.

• Temperate regions have a seasonal thermocline and polar regions have none.

5-5Chemical and Physical Structure of the Oceans

Salinity displays a latitudinal relationship related to precipitation and evaporation.

• Highest ocean salinity is between 20-30o north and south or the equator.

• Low salinity at the equator and poleward of 30o results because evaporation decreases and precipitation increases.

• In some places surface water and deep water are separated by a halocline, a zone of rapid change in salinity.

• Water stratification (layering) within the ocean is more pronounced between 40oN and 40oS.

5-5 Chemical and Physical Structure of the Oceans

Latitudinal variation in salinity

Salinity Profiles

Density of sea water is a function of temperature, salinity and pressure.

• Density increases as temperature decreases and salinity increases as pressure increases.

• Pressure increases regularly with depth, but temperature and salinity are more variable.

• Higher salinity water can rest above lower salinity water if the higher salinity water is sufficiently warm and the lower salinity water sufficiently cold.

• Pycnocline is a layer within the water column where water density changes rapidly with depth.

5-5Chemical and Physical Structure of the Oceans

Density Stratification

The water column in the ocean can be divided into the surface layer, pycnocline and deep layer.

• The surface layer is about 100m thick, comprises about 2% of the ocean volume and is the most variable part of the ocean because it is in contact with the atmosphere.

• The surface layer is less dense because of lower salinity or higher temperature.

• The pycnocline is transitional between the surface and deep layers and comprises 18% of the ocean basin.

• In the low latitudes, the pycnocline coincides with the thermocline, but in the mid-latitudes it is the halocline.

• The deep layer represents 80% of the ocean volume.

• Water in the deep layer originates at the surface in high latitudes where it cools, becomes dense, sinks (convects) to the sea floor and flows outward (advects) across the ocean basin.

5-5Chemical and Physical Structure of the Oceans

Factors leading to a pycnocline

Density Structure of the Ocean

Water is recycled from the ocean to the land and returned to the sea.

• The reservoirs of water include:

– Oceans - cover 60% of the northern hemisphere and 80% of the southern hemisphere and contains 97% of Earth’s water.

– Rivers, lakes and glaciers.– Groundwater - contains a larger volume of water than all of the water in lakes and rivers.

• The hydrologic cycle describes the exchange of water between ocean, land and atmosphere.

– On land precipitation exceeds evaporation.– In the ocean evaporation exceeds precipitation.

• The ocean is part of a biogeochemical system in which land undergoes weathering and weathered products are transported to the sea where they may be deposited directly or used by organisms and later deposited as organic remains or organic wastes. Deposits are buried, lithified and recycled by plate tectonics into new land which is weathered and the cycle repeats.

5-7The Ocean as a Physical System

5-7The Hydrological cycle

Resource cycling within the ocean

• Reversing thermometers automatically record the temperature of the water from which the sample is taken. As the sample bottle and thermometer turn over, a gap forms at the base of the mercury column which prevents the temperature reading from changing. Temperature can also be determined electronically.

The Ocean Sciences: Chemical Techniques

Sea ice is ice that forms by the freezing of sea water; icebergs are detached parts of glaciers.

• As sea water freezes, needles of ice form and grow into platelets which gradually produce a slush at the sea surface.

• As ice forms, the salt remains in solution, increasing salinity and further lowering the freezing point of the water.

• Depending upon how quickly the ice freezes, some salt may be trapped within the ice mass, but it gradually is released.

• Pancake ice are rounded sheets of sea ice that become abraded along the edges as ice masses collide.

The Ocean Sciences: Other Physical Properties of Water

Amount of light entering the ocean depends upon the height of the sun above the horizon and the smoothness of sea surface.

• 65% of light entering the ocean is absorbed within the first meter and converted into heat. Only 1% of light entering the ocean reaches 100m.

• Water displays the selective absorption of light with long wavelengths absorbed first and short wavelengths absorbed last.

• In the open ocean, blue light penetrates the deepest.

The Ocean Sciences: Other Physical Properties of Water

Light Absorption, open ocean

• In turbid coastal waters light rarely penetrates deeper than 20m. and the water appears yellow to green because particles reflect these wavelengths.

• The photic zone is the part of the water column penetrated by sunlight.

• The aphotic zone is the part of the water column below light penetration and permanently dark.

The Ocean Sciences: Other Physical Properties of Water

Light absorption in nearshore, productive water

The speed of sound in water increases as salinity, temperature and pressure increase, but in the ocean, the speed of sound is mainly a function of temperature and pressure.• Above the pycnocline increasing pressure with depth increases

the speed of sound despite the gradual decrease in temperature.

• Within the pycnocline, the speed of sound decreases rapidly because of the rapid decrease in temperature and only slight increase in pressure.

• Below the pycnocline the speed of sound gradually increases because pressure continues to increase, but temperature only declines slightly.

The Ocean Sciences: Other Physical Properties of Water

Behavior of sound in water

• SOFAR Channel is located where sound speed is at a minimum. Refraction of sound waves within the channel prevents dispersion of the sound energy and sound waves travel for 1000s of kilometers within the channel.

The Ocean Sciences: Other Physical Properties of Water

The sea surface microlayer is the water surface to a depth of a few hundred micrometers. It is critical for the exchange between the atmosphere and the ocean.

• Neuston layer is the habitat of the sea surface microlayer and is inhabited by the neuston, all organisms of the microlayer.

• Processes that transport matter to the surface layer from below are:

– Diffusion - random movement of molecules.– Convection - vertical circulation resulting in the transfer of heat and matter.– Bubbles - the most important process because bubbles absorb material and inject it

into the air as they bursts.

• Processes within the microlayer can be divided into the:

– Biological - bacteria and plankton are much more abundant in the layer than below.– Photochemical effect - the interaction of ultraviolet light and organic compounds.

The Ocean Sciences: Sea Surface Microlayer

The sea-surface microlayer

Summary of resource cycling