The Restless Ocean Chapter 13. Surface Currents Surface Currents – movement of surface waters...

The Restless Ocean

Chapter 13

Surface Currents

• Surface Currents – movement of surface waters closely related to atmospheric circulation and driven by unequal heating of the Earth by the Sun

Ocean Circulation Patterns

• Winds are the driving force of surface currents. When winds change direction, the surface currents also change direction.

Ocean Circulation Patterns

• The Earth’s rotation causes currents to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is called the Coriolis Effect.

Ocean Circulation Patterns

• Surface currents form a “gyre”, or circular vortex, with currents moving west at the equator, deflecting off land towards the poles and circling back down.

Ocean Circulation Patterns

• Large central area in the middle of the gyre is a zone of calmer waters. Around the Antarctic there are no continents to deflect the current so it is a continuous circulation.

Ocean Currents and Upwelling

• Winds may cause vertical movements called upwellings, where colder deeper water is force to the surface.

Ocean Currents and Upwelling

• Usually occurs in areas where the wind blows towards the equator and parallel to the coast (Ex. California, South America, West Africa)

Ocean Currents and Upwelling

• Due to the Coriolis Effect the warmer surface waters are deflected away from the shore and are replaced by cold “upwells”.

The Importance of Ocean Currents

• Navigation – ships could use currents to make quicker ocean crossings and trying to move against the current would cause slower passages.

The Importance of Ocean Currents

• Climate – warm water from the poles moves north and cool water from the poles is circulated towards the equator, moderating climates along the way.

The Importance of Ocean Currents

• Maintains Earth’s Heat Balance

Deep Ocean Circulation

• Controlled by gravity and density differences. • Also called thermo-haline circulation (thermo

– heat, haline – salt)

Deep Ocean Circulation

• Water at the surface can become colder and more salty which will make it denser.

• The dense water sinks toward the ocean bottom, displacing lighter water.

Deep Ocean Circulation

• This mainly occurs in the Arctic and Antarctic, where temperatures are low enough sea ice forms. Salts don’t get frozen in the ice making the surrounding water saltier.

• Deep ocean currents are difficult to measure and so little is known about them.

Tides

• Daily change in the elevation of the ocean surface.

Causes of Tides

• Ocean tides result from the gravitational attraction exerted upon the Earth by the moon and to a lesser extent by the sun.

Causes of Tides

• Tidal bulges remain in place while Earth “rotates” through them. In one day the Earth experiences one high tide and one low tide. The tides migrate as the moon revolves around the Earth, shifting about 50 minutes a day.

• The tide generating power of the Sun is less than half that of the moon

Causes of Tides

• At times of the new and full moons the sun and moon are aligned together and produce higher high tides and lower low tides. These are called the spring tides.

Causes of Tides

• During the first and third quarters the sun and moon act at right angles and offset the influence of each other. These are called neap tides.

Types of Tides

• Semidinural - Two high and two low tides daily, with small differences in the high and low water heights

• Dinural - one high and one low tide• Mixed – usually two high and two low with a

large inequality between high water heights and low water heights

Semidiurnal

Diurnal Tide

Mixed Semidiurnal

Tidal Currents

• Horizontal flow of water accompanying the rise and fall of the tides

• Flood currents – tidal currents that advance into the coastal zone

• Ebb currents – currents generated by seaward moving water

• Slack water – periods of little or no current

Flood Current and Ebb Current

Resources

• http://oceanservice.noaa.gov/education/tutorial_tides/welcome.html