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6

Oceanography, An Invitation to Marine Science | 9eTom Garrison

Water And Ocean Structure

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• Water has unusual characteristics• Liquid water helps regulate Earth’s surface

temperatures– Day / night– Summer / winter

• Density affects ocean structure

Familiar, Abundant, and Odd

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• Compound – two or more elements in fixed proportion

• Molecule – atoms held together by chemical bonds

• Chemical bonds – energy relationships that hold elements together

• Water is a polar molecule– One end is positively charged; one end is

negatively charged

The Water Molecule Is Held Together by Chemical Bonds

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Two hydrogen atoms...

share their electrons with one oxygen atom...

to form a water molecule held together by covalent bonds...

which acts as if it has negative and positive ends.

Nucleus (+1 unit of charge)

Electron (–1 unit of charge)

Stepped Art

2 –

105°

The Formation of a Water Molecule

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• Cohesion – water molecules stick to each other, creating surface tension

• Adhesion – water molecules stick to other substances

Hydrogen Bonds Link Water Molecules Together by Electrostatic Forces

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• Heat – energy produced by vibration of atoms/molecules

• Temperature – object’s response to input or removal of heat

• Heat capacity – Measured in calories– Water has high heat capacity

• Can absorb or release large amounts of heat with no change in temperature

Water Has Unusual Thermal Characteristics

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Water’s Temperature Affects Its Density

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Water Becomes Less Dense When It FreezesSpace taken by 24 H2O molecules in solid lattice could be occupied by 27 H2O molecules in liquid state-expands about 9% as the crystal forms.

Graph of temperature vs heat as water freezes, melts, and vaporizes. Line between points C and D -latent heat of fusion, heat is being added or removed but the temperature is not changing. Line between points G and F- latent heat of vaporization, when heat is being added or removed but the temperature is not changing

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• Seawater and pure water have slightly different thermal properties– Dissolved solids in seawater

• Lower freezing point– More dissolved solids, the lower the freezing

point

• Evaporates more slowly

Water Removes Heat from Surfaces As It Evaporates

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• Thermostatic properties – act to moderate temperature changes

• Thermal inertia – tendency to resist change in temperature

• Annual freeze/thaw of ice moderates Earth’s temperature– More than 18,000 cubic kilometers of ice

thaws and refreezes in the Southern Hemisphere each year

Surface Water Moderates Global Temperature

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Movement of Water Vapor from Tropics to Poles

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• Latitude, temperature, and salinity

Global Warming May Influence Ocean Surface Temperature and Salinity

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• Three zones– Surface zone /

mixed layer, warm low-density

– Pycnocline-Density increases rapidly with depth

– Deep zone- of cold, dense water—about 80% of total ocean volume

The Ocean Is Stratified by Density

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• Water mass – body of water with characteristic temperature and salinity– Little vertical movement in the tropics and the

Arctic– South polar ocean only weakly stratified

Density Stratification Usually Prevents Vertical Water Movement

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• Refraction –bending of waves

• Refractive index – Degree that light

is bent traveling from one medium to another

Refraction Can Bend the Paths of Light and Sound Through Water

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• Photic zone – sunlit portion• Aphotic zone – no light• Scattering – light bounced between

molecules• Absorption – structure of water molecules• Water transmits blue light better than red

Light Does Not Travel Far Through the Ocean

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Only a Thin Film of Seawater Is Illuminated by the Sun

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• The relationship between water depth and the speed of sound

Sound Travels Much Farther Than Light Through the Ocean

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• Different density layers cause refraction– SOFAR (sound fixing and ranging)

• Minimum velocity – SOFAR layer

– Shadow zone• Region which little sound energy penetrates

Refraction

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• Sonar systems– Active sonar (sound navigation and ranging)

• Pings – high-frequency sound

– Side-scan sonar– Seismic reflection profilers– Passive sonar – listening only

• Ocean sound used to monitor climate change– Monitor water temperature differences

Sonar Systems Use Sound to DetectUnderwater Objects

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Changes in the Long-Range Speed of Sounds in the North Pacific Ocean

Variations from normal speed range from 20.5 meter per second (dark blue) to 10.5 meter per second (dark red). Such variations relate to the temperature of the seawater.

Data measuring the speed of sound through water to 1,000 meters