Chapter 5 The Properties of Seawater

Physical Oceanography 1:
The physics of seawater
An overview of Ocean Stratification
Chemical and Physical5-5
Structure of the Oceans
• 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.
Chemical 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.
Latitudinal variation in salinity
Salinity Profiles
Chemical and Physical
Structure of the Oceans
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
Density Stratification
Chemical and Physical
Structure of the Oceans
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.
Factors leading to a pycnocline
Density Structure of the Ocean
5-7 System
The Ocean as a Physical
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
The Hydrological cycle
cycling within
the ocean
The Ocean Sciences: Chemical Techniques
• 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
The Ocean Sciences: Other
Physical Properties of Water
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
• In the open ocean, blue light penetrates the deepest.
Light Absorption,
open ocean
The Ocean Sciences: Other
Physical Properties of Water
• 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
• The aphotic zone is the part of the water column below light
penetration and permanently dark.
Light absorption in nearshore, productive water
The Ocean Sciences: Other
Physical Properties of 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
• 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.
Behavior of sound in water
The Ocean Sciences: Other
Physical Properties of 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
The Ocean Sciences: Sea
Surface Microlayer
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
• 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 sea-surface microlayer
Summary of

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