Chapter 15: Physical Oceanography

Importance of Oceans Oceans are important sources of food, energy, and minerals. Energy sources such as oil and natural gas are found beneath the ocean floor.

Importance of Oceans Approximately one-third of the world’s table salt is extracted from seawater through the process of evaporation. Oceans also allow for the efficient transportation of goods. For example, millions of tons of oil, coal, and grains are shipped over the oceans each year.

Oceanography – study of Earth’s oceans

SONaR – sound navigation and ranging Uses return time of an echo and the known velocity of sound in water to determine water depth Side-scan Sonar – sound waves hit floor at an angle so features are mapped Has mapped large portions of the seafloor

Origins of Water Scientists hypothesize that Earth’s water could have originated from two sources Comets and meteorites colliding with the planet Impacts release water, possibly enough to fill the ocean basins over geologic time Volcanoes – emit significant quantities of gas, mostly water vapor and carbon dioxide Earth cooled and water vapor condensed into oceans

Distribution of Earth’s Water 97% of water in oceans, 3% of water is fresh The percentage of ice on Earth has varied over geologic time from near zero to as much as 10% of the hydrosphere Global sea level has risen and fallen by hundreds of meters in response to melting and freezing ice Tectonic forces that lift or lower portions of the seafloor

The Blue Planet 71% of Earth’s surface is covered by oceans Average depth of an ocean is 3800 meters All the oceans are really one vast, interconnected body of water Divided into specific oceans and seas because of historic and geographic considerations

Major Oceans Pacific Ocean Atlantic Arctic Indian Largest – contains roughly half of Earth’s seawater Atlantic 2nd largest – extends from Antarctica to the arctic circle Arctic Atlantic Ocean north of the arctic circle Indian Located mainly in the southern hemisphere

Minor Oceans Antarctic Storm-lashed region surrounding Antarctica South of 50 degrees south latitude Sometimes referred to as the Southern Ocean

Seas Seas are smaller than oceans and are partly or mostly landlocked 3 major seas are Mediterranean Caribbean Bering

15.2 Seawater

Seawater Seawater is a solution of about 96.5% water and 3.5% dissolved salts. Most abundant salt in seawater is sodium chloride (NaCl) Other salts present are chlorides and sulfates of magnesium, potassium, and calcium Salinity is the measure of the amount of dissolved salts in seawater

Variations in Salinity Average salinity of the oceans is 35 ppt (parts per thousand) Salinities higher in subtropics due to higher rates of evaporation In equatorial regions salinities are lower due to abundant precipitation Polar regions low due to melting sea ice Lowest salinities occur where large rivers empty into the ocean

Sources of Sea Salt Ancient seas salinity not much different from today Volcanic gases contain chlorine and sulfur dioxide Gases dissolve in water and form the chlorine and sulfate ions of seawater Weathering of crustal rocks generates most of the other abundant ions in seawater Sodium, calcium, potassium, iron, magnesium

Removal of Sea Salts Salinity in ocean never really increases Removal involves several processes Some precipitate from seawater near arid, coastal regions Small salty spray droplets from breaking waves are picked up by winds and deposited inland Marine organisms also remove ions from seawater to build their shells, bones, and teeth

Physical Properties of Seawater Physical properties of seawater are quite different than freshwater Salt ions are heavier than water molecules and increase the density The freezing point of seawater -2 degrees C Determining factors for temperature are Depth Latitude Salinity

Temperature Average Sea-Surface Temperature (SST) = 26 C Zones Ocean water temperatures decrease significantly with depth Zones Surface 0-100 meters Little to no temperature change

Thermocline (intermediate) Rapid decrease in temperature Bottom Below 1000 m Temperature levels out and becomes stable because of pressure

Absorption of Light Light intensity decreases with depth Water absorbs light Penetrates only the upper 100m of seawater Photosynthesis will only occur in this first 100 m Some blue light reaches below that, but is not used for photosynthesis

Light Zones Sunlit Zone – Euphotic Zone Twilight Zone – Disphotic Zone 100-200 ft >90% of marine animals live here Twilight Zone – Disphotic Zone About 900 ft Midnight Zone – Aphotic Zone Below 900 ft

Other Zones Littoral Zone Neritic Pelagic Benthic Between tide lines Low tide line to the edge of the continental shelf Pelagic Open ocean Benthic Bottom of open ocean

15.3 Ocean Movements

Oceans are never completely motionless Most obvious movement is the constant motion of the waves Rhythmic movement that carries energy through space or matter Water moves up and down in a circular pattern and returns to its original position

Waves Typical wave characteristics Crest – highest point of a wave Trough – lowest point of a wave Wave height – distance between crest & trough Wave length – distance from crest to crest

Wave Height Depends on 3 things Wind speed Wind duration(how long) Fetch (how much distance wind blows across)

Breaking Waves As ocean waves reach the shallow water near shorelines, they lose energy because of friction with the ocean bottom Waves slow down Incoming waves become higher, steeper, and unstable, and their crests collapse forward Breakers – waves where the crest collapse