Ocean Currents

• Ocean Density

Energy in = energy out

Half of solar radiationreaches Earth

The atmosphere is transparent to shortwave but

absorbs longwave radiation

(greenhouse effect)

The atmosphere is heated from the

bottom by longwave radiation and convection

I. Uneven heating of Earth’s surface causes predictable latitudinal variation in climate. Why? - Angle of incidence… equator vs. poles

Climatic Variation & Seasons on Earth

Thus, radiation is more intense near the equator compared to the poles. For this reason, it’s warmer near the equator than at the poles.

North Pole

Earth

Equator

South Pole

Uneven heating of Earth’s surface causes atmospheric circulation

Greater heating at equator than poles 1. sun’s rays hit more directly 2. less atmosphere to penetrate

Therefore 1. Net gain of energy at equator 2. Net loss of energy at poles

II. What about seasons? Why do we have them?

Tilt!Because of the tilt of Earth’s axis, the amount of radiation received

by Northern and Southern Hemispheres varies seasonally

A. Northern Hemisphere has summer when it tilts toward the sun, winter when it tilts away

B. Southern Hemisphere has summer when it tilts toward the sun, winter when it tilts away

Earth’s distance from the sun varies throughout the year – doesn’t that cause the seasons?

I. Tilt of the Earth’s axis towards or away from the sun creates the seasons

Earth’s Seasons

North Pole

Earth

When the north pole tilts toward thesun, it gets more radiation – more warmth

during the summer

SUMMER (Northern Hemisphere)

South Pole

WINTER (Southern Hemisphere)

When the north pole tilts toward thesun, the south pole tilts away

So when it’s summer in the north, it’s winter in the south

Equator

I. Tilt of the Earth’s axis towards or away from the sun creates the seasons

Earth’s Seasons

When the north pole tilts away from the sun, it gets less radiation –

So it’s colder during the winter

North Pole

Earth

WINTER (Northern Hemisphere)

South Pole

SUMMER (Southern Hemisphere)

When the north pole tilts away from thesun, the south pole tilts toward it…

When it’s winter in the north, it’s summer in the south

Equator

Air rises and falls in Hadley, Ferrel, and

Polar cells(vertical circulation)

Circulation cells explain global distribution of

rainfall

Earth’s rotation determines

wind direction (horizontal circulation,

Coriolis force)

High heat capacity of water and ocean currents buffer ocean temperatures

Land temperatures fluctuate more, especially in higher latitudes

These differences in surface energy balance influence air movements, and create prevailing winds

• Salinity – The difference in the salinity of hot and cold water drives the currents. The higher the salinity of the water, the more dense the water is.

• Density - the state or quality of compactness; closely set or crowded condition.

• Water Mass – Area of different salinity density in the ocean.

At 30º N & S, air descends more strongly over cold ocean than over land

In January…

These pressure gradients create geographic variation in prevailing windsAt 60 º N & S, air descends more strongly over cold land than over ocean

In summer at 60 º N & S, air descends over cold ocean (high pressure) and rises over warm land (low pressure)

Cool equator-ward flow of air on W coast of continents Warm poleward flow of air on E coasts of continents

Ocean currents are similar to wind patterns: 1. Driven by Coriolis forces 2. Driven by winds

Ocean currents move 40% of “excess heat” from equator to poles

Driven by circulation of deep ocean waters Deepwater formation occurs near Greenland and in Antarctic

60% of heat transport is carried by atmosphere through storms that move along pressure gradients

The Pacific Ocean strongly influences the climate system becauseIt is the largest ocean basinNormal ocean current and wind direction in central Pacific is easterly

Winds and surface water

• Wind blowing over the ocean can move it due to frictional drag.

• Waves create necessary roughness for wind to couple with water.

• One “rule of thumb” holds that wind blowing for 12 hrs at 100 cm per sec will produce a 2 cm per sec current (about 2% of the wind speed)

Top-down drag

• Wind acts only on the surface water layer.• This layer will also drag the underlying water,

but with less force.• Consequently, there is a diminution of speed

downward.• Direction of movement is also influenced by

the Coriolis Effect and Ekman Spiral

Geostrophic Flow Surface currents generally mirror average planetary

atmospheric circulation patterns