Ocean CurrentsOcean Currents

Earth and Space ScienceEarth and Space Science

•Ocean waves carry matter and Ocean waves carry matter and energyenergy

•This thermal energy can be This thermal energy can be transferred to the air, and to the land.transferred to the air, and to the land.

•There are multiple layers in the oceanThere are multiple layers in the ocean

•There are multiple densities of water There are multiple densities of water in the oceanin the ocean

•The ocean’s salinity is not uniformThe ocean’s salinity is not uniform

Seawater Temperature, Seawater Temperature, Salinity and DensitySalinity and Density

CurrentsCurrents

•Water in oceans can sink, rise, and flow horizontally

• A. Ocean currents – predominantly horizontal movement of ocean water

•Carry energy and matter from one part of the ocean to another

•

•Can be local or very broad

•May flow just on the surface Surface current (very fast)

•May flow in deep water slower but much thicker layer

•Movement of deep ocean water is controlled by water density

B. Density is determined by 2 factors

•1. Temperature

•The sun does not warm the ocean water evenly

Ocean Surface TempertureOcean Surface Temperture

•Cold water is more dense and sinks

•Warm water is less dense and stays at the surface

B. Density is determined by 2 factors

•2. Salinity

•Concentration of dissolved salts in the seawater

•Expressed as parts per thousand

•Approximately 35 parts per thousand (35 g salts per 1000 g water)

•Higher salinity = higher density

B. Density is determined by 2 factors

B. Density is determined by 2 factors

•Surface water can change in density by changing salinity due to excess rain or increased evaporation or where rivers enter the ocean

B. Density is determined by 2 factors

II. Water Masses

•The ocean is a single body of water but is not the same everywhere

Temperature and salinity vary from place to place

Water Mass – a large region of water within the ocean that is about the same temperature and salinity

A. Surface layer•Winds mix the water vertically so

temperature and salinity remain relatively constant through the first 100m-200m of depth

B. Deep layers•No vigorous mixing, thus water

masses retain distinctive properties

•C. Water masses are named for the location where they are formed

Examples: Antarctic Bottom Water North Atlantic Deep Water 

•D. Water masses sink at the poles and move along the ocean floor

III. What causes the deep circulation?

A. Giant convection cellsConvection is the rising and falling of heated and cooled fluids due to changing density

•B. How does the cold deep water mix with the warmer surface water?

Still being studied at this time, but it is

that the movements of the tides is a possible cause of mixing

IV. Surface Currents

•Extend down to depths of about 200m A. Surface currents flow for long

distances across the ocean

1. Form gyres, which are circular cells of motion of the surface water

2. Gyres are found in each of the major ocean basins

3. Circulate clockwise in the Northern Hemisphere

4. Circulate counter-clockwise in the Southern Hemisphere

World Ocean CurrentsWorld Ocean Currents

B. Gyres help transport energy and matter from low latitudes to the higher latitudes.

C. Caused by blowing winds near the Equator1. Tradewinds/Easterlies – at low latitudes/tropics

Blow East to West

2. Prevailing Westerlies – in mid-latitudes

Blow West to East

D. Eckman Drift – horizontal movement of water resulting from the balance of the Coriolis effect and wind stress

1. Causes water to move at an angle to the right of the general wind direction (90o)

2. Causes a “mound of water” which balances out the Eckman Drift 

E. Upwelling•Strong upward movement of water

from the deep to the surface

E. Upwelling

•1. As water moves away from the coastline due to the Eckman Drift, more water flows up to replace it from deeper in the ocean.

•2. The upward movement brings nutrient rich deep water to shallow areas for animals and plants to use

E. Upwelling

E. UpwellingE. Upwelling

E. Upwelling

•3. Nutrients come from the “rain” of dead organisms from the warmer surface layer

•4. Deep circulation moves the nutrients around, and then upwelling generally moves the nutrients to the eastern sides of ocean basins.

•5. Most fisheries are supported by this upwelling of nutrients.

F. Downwelling

•Downward motion of surface water to the deep ocean

F. DownwellingF. Downwelling