Physical Oceanography

Chapter 15

Section 15.1The Oceans

Oceanography is the study of the Earth’s oceansThe depth of the oceans is measured by sonar

The British Challenger Expedition◦ The first research ship to study oceans◦ Investigated ocean currents, water temperature, and chemical

composition of the oceans The German Meteor (1920’s) Expedition

◦ Used sonar to map the seafloor features of the South Atlantic Ocean (including the Mid-Atlantic Ridge)

Topex / Poseidon Satellite Expedition◦ Continually monitors ocean surface temperatures, currents,

and wave conditions Side-Scan Sonar Expedition

◦ Technique that directs sound waves to the seafloor at an angle so the sides of underwater features can be mapped

Modern Oceanography and Advanced Technology

Scientists hypothesize Earth’s water could have come from two places:◦ Comets – when they collide with Earth, the impact

releases water – possibly enough to fill ocean basins over geologic time Meteorites – contain up to 0.5% water. If early Earth

contained same percentage of water, it could have easily filled up the oceans

◦ Volcanism – in early Earth history, violent volcanism occurred over several hundred million years. These volcanoes created/released much water vapor, carbon dioxide, and other gases. Gases gradually condense into oceans (over hundreds of years) as the Earth’s crust cooled

Origins of Oceans

Oceans contain 97% of water found on Earth

Another 3% of water is found in polar ice caps, rivers, lakes, and underground sources

Sea level has risen and fallen by hundreds of meters throughout Earth’s history

Approximately 71% of Earth’s surface is covered by oceans (Blue Planet). Most of the oceans are found in the southern hemisphere

Distribution of Earth’s Water

Major Oceans/Seas on Earth

OCEANS SEAS

There are three major oceans in the world:◦ Pacific Ocean

Contains roughly half of all seawater Larger than all landmasses

combined ◦ Atlantic Ocean

Extends more than 20,000km from Antarctica to the Arctic Circle

North of the Arctic Circle, it is referred to as the Arctic Ocean (but is still the Atlantic)

Surrounding Antarctica, referred to as the Antarctic Ocean. It is severely storm-lashed

◦ Indian Ocean Third largest ocean Located mainly in the southern

hemisphere

Smaller than Oceans in size Land-locked Examples…

◦ Mediterranean Sea◦ Gulf of Mexico◦ The Caribbean Sea◦ The Bering Sea

All seas belong to one global ocean, whose waters are mixed

That said, these waters contain nearly identical proportions of dissolved salts

The Arctic and Antarctic Oceans are covered by sea ice, particularly during the winter. In the summer, the ice “kind of” breaks up.

Ice is less dense than water, so it floats.

Formation:1. Ice crystals form2. Slush develops at the surface of the water3. Thickening of ice solidifies into round pieces called pancake ice4. Pancake ice thickens and freezes into a cover, called pack ice

Salt ions concentrate in the water under the sea ice, as the ions cannot continuously move into the blocks of ice.

Formation of Sea Ice

Section 15.2Seawater

Have you ever swallowed a gulp of seawater? Did you notice its salty taste?

Seawaters differ from freshwater due to various salts

Salt ions are heavy, increasing density of water

Variations in salinity cause the freezing point of seawater to be lower than that of freshwater

Oceans are generally dark, light absorption occurs only in the upper 100m of seawater

Density varies depending on temperature◦ Cold water – more dense◦ Warm water – less dense

So, the most dense water would be cold with high salinity levels

Physical Properties of Seawater

Temperatures range – with average surface temperature of the ocean being 15oC◦ Polar waters average -2oC◦ Equatorial waters average 30oC

Temperatures decrease significantly with depth.

Based on these variations, the ocean is divided into three parts:1. Surface area - relatively warm, sunlit

(100m thick)2. Transitional layer (thermocline) -

rapidly decreasing in temperature with depth. This layer is absent in polar seas

3. Bottom layer – cold and dark, near freezing

Ocean Water Temperatures

Why are ocean floor temperatures near freezing?

Water comes from polar seas. Cold water is more dense and ice does not incorporate salt, this accumulates below the ice causing water to be more salty. The salty water sinks because it is more dense.

Cold, dense masses of salty water from the polar seas migrates toward the equator as a water mass along the ocean floor.

These masses are replaced as surface currents bring salty, subtropical waters into polar regions… which become colder and denser than surrounding waters as winter hits.

There are three main deep water masses within the oceans. The Atlantic Ocean is the only mass of water that contains all three.

Water Masses

1. Antarctic Bottom Water◦ Forms when Antarctic seas freeze during the winter◦ Coldest and densest in all of the oceans

2. North Atlantic Deep Water◦ Forms in similar pattern off of Greenland◦ Warmer and less dense than Antarctic water that overrides

it 3. Antarctic Intermediate Water

◦ Forms when relatively salty water of Antarctic decreases temperature in winter and sinks

◦ Overrides other two masses

Seawater is a solution of about 96.5% water and 3.5% dissolved salts.

The most abundant salt is sodium chloride (NaCl).

Seawater also contains dissolved gases, such as oxygen, nitrogen, and carbon dioxide

Seawater contains nutrients, such as nitrates, phosphates, and silicates

Chemical Properties of Seawater

Salinity is the measure of the amount of dissolved salts in seawater.◦ Levels of salinity have not changed much throughout history◦ Sources of sea salts have remained similar throughout history◦ Salinity is expressed at parts per thousand (ppt)◦ Salinity affects the density of seawater

Average salinity of seawater is 35ppt, but this can vary based on location:

Subtropical regions – may be as high as 37ppt due to rates of evaporation exceeding precipitation

Equatorial regions – slightly lower salinity because rainfall is abundant Polar water – even lower salinity (32-33ppt) because water is diluted by

melting sea ice The lowest salinity levels are found where large rivers

empty into oceans because freshwater is mixing with saltwater

Salinity

Sea salts are removed at the same rate they are added, which is why salinity levels do not rise.

Process of removal:1. Precipitate from seawater near arid, coastal regions

(removes sodium chloride, calcium sulfate & other sea salts)

2. Spray droplets from breaking waves are picked up by winds and deposited inland

3. Marine organisms remove ions from seawater to build their shells, bones, and teeth. As they die, they sink to the seafloor and mix with bottom sediments

Sea Salt Removal

15.3 Ocean Movements

Oceans are never completely motionless. Their most obvious movement is the constant motion of waves.

A wave is the rhythmic movement that carries energy through space or matter.◦ Waves are generated by wind flowing over the water

surface◦ As a wave passes, water moves up and down in a

circular pattern – returning to its original position◦ Only energy moves steadily forward◦ Water moves only in circles but does NOT move

forward

What is a Wave?

A typical ocean wave contains:◦ Crest

Highest point of the wave◦ Trough

Lowest point of the wave

Wave-height is the vertical distance between the crest and trough◦ Depends on three factors:

1. Wind Speed2. Wind Duration3. Fetch

Area of water the wind blows across

Wave Characteristics

Ocean waves lose energy due to friction at the bottom of the ocean

Breaking waves are common along shallow waters and shorelines

As water becomes more shallow, incoming wave crests gradually catch up to wave crests ahead

Wave length then decreases, making incoming waves higher, steeper, and more unstable

Crests then collapse forward and create “breakers”◦ Formation of breakers influence the

motion of wave crests◦ High speed breakers affect

shorelines

Breaking Waves

TidesTides are the periodic rise and fall of the sea level

Tide levels differ based on geographic location:

Highest Level – High TideLowest level – Low Tide

Generally, the daily cycle of high and low tides takes 24 hours and 50 minutes

Causes and Types of Tides

Causes of Tides Types of Tides

Gravitational attraction among the Earth, Moon, and Sun

Lunar tides are more than twice as high as those caused by the Sun – because the Moon is closer to the Earth

Tidal bulges are always aligned to the Moon

Solar Tides can either enhance or diminish lunar tides

Spring Tides◦ Large tidal ranges◦ Occur with a Full or a New Moon◦ High tides are higher than normal◦ Low tides are lower than normal

Neap Tides◦ Small tidal ranges◦ Occur within the First Quarter or

Third Quarter Moons◦ High Tides are lower than normal◦ Low tides are higher than normal

Spring Tides alternate every two weeks with Neap Tides

On average, Spring Tides are 3x higher than Neap Tides

Density currents◦ Currents caused by

differences in the temperature and salinity of ocean water

◦ Move slowly in deep ocean water

◦ Not noticeable due to their depth

Surface Currents◦ Currents that are wind driven

on the surface of the ocean◦ Affect the upper few hundred

meters of the ocean◦ Can move as fast as 100km

per day◦ Move in predictable patters◦ Noticeable to humans

Ocean Currents

Closed circular currents of ocean water Caused by continents deflecting ocean currents to the north and south

Follow circular motions based on the Coriolis Effect◦ Clockwise in Northern Hemisphere◦ Counter-clockwise in Southern Hemisphere◦ Parts closest to equator move towards the west as equatorial currents

Deflect towards poles as they encounter land Poleward flowing waters carry warm, tropical water into higher/colder latitudes Once warm waters reach polar regions, they gradually cool and move back toward the

equator (as they’re deflected by landmasses surrounding the poles) Once they “leave the poles,” they carry cooler water from the polar regions back down

toward the equator

Five types:1. North Pacific2. North Atlantic3. South Pacific4. South Atlantic5. Indian Ocean

Gyres

Upwelling is the upward motion of ocean water Upwelling waters originate from the bottom of

the ocean and are cold Areas of upwelling exist off the western coasts

of continents and trade-wind belts Upwelling waters are rich in nutrients Upwelling waters support abundant

populations of marine life Some of the world’s best fishing grounds are

found in these waters (The Grand Banks – New England / Eastern Canada)

Upwelling

How did the oceans form?◦ Volcanism reached Earth’s atmosphere and created /released water vapor,

carbon dioxide, and other gases. Gases gradually condensed into oceans (over hundreds of years) as the Earth’s crust cooled

What is the Challenger expedition?◦ The first expedition to use scientific measuring devices to study the ocean

What affects the density of seawater?◦ Salinity

What steps are involved in the formation of sea ice?◦ Ice crystals, slush, pancake ice, pack ice

What are the three masses of Atlantic Deep-Water?◦ Antarctic Bottom Water, Antarctic Intermediate Water, Atlantic Bottom

Water What is the basic cause of tides?

◦ Gravitation What is a collapsing wave?

◦ Breaker What is a transitional layer of the ocean characterized by rapidly

decreasing temperatures with depth?◦ Thermocline

Key Review Concepts