Atmospheric Circulation (Air-Sea Interaction) We live at the bottom of an ocean of air, the atmosphere The atmosphere and the ocean are interdependent;

Atmospheric Circulation (Air-Sea Interaction)

• We live at the bottom of an ocean of air, the atmosphere

• The atmosphere and the ocean are interdependent; what happens in one system causes changes in the other

• Surface currents in the oceans are directly caused by atmospheric winds

Air-Sea Interaction

• Differences in solar energy (heating) across the Earth – combined with the Earth’s spin – create winds

• Winds drive surface currents and create waves• Likewise, certain atmospheric phenomena are

manifested (originate) in the oceans– El Niño – Southern Oscillation– Hurricanes, cyclones

Air-Sea Interaction

• Earth’s atmosphere is composed mainly of Nitrogen, Oxygen, and Water Vapor– Nitrogen: 78%– Oxygen: 21%– Argon, CO2, Neon, Helium, Methane, others: 1%

• Air is never completely dry, however, and water vapor (H2O) can occupy as much as 4% of the air’s volume– Visible as clouds and fog; invisible as water vapor– Enters atmosphere via evaporation, exits via

condensation

Air-Sea Interaction

• Temperature and humidity determine the density of air masses, which in turn determines whether these air masses will rise or sink

• Air containing water vapor is less dense than dry air at the same temperature and density

• Also, when heated, air expands and becomes less dense

• This means that cold air is denser than warm air and cold, dry air is much denser than warm, moist air

Say what?• Warm air is less dense than cold air because

increasing temperature results in greater molecular dispersion

Increasing Temperature

Say what? (Continued)

• Moist air is less dense than cold air because the weight of water vapor (H2O) is less than that of Nitrogen (N2) and Oxygen (O2)

• When water vapor increases, the amount of O2 and N2 decreases per unit volume– Molecular weight of O2 = 16 + 16 = 32

– Molecular weight of N2 = 14 + 14 = 28

– Molecular weight of H2O = 1 + 1 + 16 = 18

Atmospheric Circulation

• Air masses will move from regions of high pressure (dense air) to regions of low pressure (less dense air)

• A low pressure zone results from moist and/or warm air

• A high pressure zone results from dry and/or cold air

• The flow of air from regions of high to low pressure create the winds

Atmospheric Circulation

• Because warm and/or moist air is less dense, it rises (heat rises)

• Likewise, cool and/or dry air is more dense and so it sinks

• As air rises, it expands and cools; water vapor in rising, expanding air will condense into clouds because the cooler air is no longer able to hold as much water vapor

• Precipitation transfers water vapor (AND HEAT!!) from low to high latitudes

Solar Heating Varies with Latitude

• The Earth revolves around the sun in an elliptical path

• The Earth itself is tilted at an angle of 23.5°• The tilt of Earth’s rotational axis results in the

seasons

Solar Heating Varies with Latitude

• Approximately half of the solar energy reaching the Earth is absorbed, but this heat is not evenly absorbed

• The amount of solar energy reaching the Earth’s surface varies with latitude and season

• Because of the Earth’s tilt, solar energy reaching the equator strikes at a low angle, concentrating the radiation in a small area; solar energy reaching the poles, however, does so at a lower angle and so less heat is absorbed in polar zones

Near the poles, light filters through more atmosphere and

approaches at a low angle, favoring reflection rather than

absorption

Got albedo?

• Albedo is the measure of solar radiation that is reflected back into space

• A high albedo indicates that more energy is reflected back into space, while a low albedo indicates that less energy is reflected back to space

• Ice and snow (even clouds) increases albedo, and so much of the light that reaches the polar regions is reflected back into space

Uneven solar heating and atmospheric circulation

• Air is warmed in the tropics and rises• Air is cooled near the poles and falls• It seems logical to suspect then, that air

heated in the tropics expands and becomes less dense as it moves towards the poles, where it will cool (and condense) sinking back towards the poles….– BUT, THIS IS NOT WHAT HAPPENS!

HYPOTHETICAL CIRCULATION ON A NON-SPINNING EARTH

Enter the Coriolis Effect…

• Experience will tell us, however, that winds in the mid-latitudes of the northern hemisphere do NOT flow out of the north, but rather the west

• The hypothetical winds described do not resemble the actual wind patterns of the Earth because we have neglected the effect of the Earth’s rotation

Enter the Coriolis Effect…

• The rotation of the Earth strongly influences the motion of air and water

• This effect is named the Coriolis effect after its discoverer, Gaspard Gustave de Coriolis

• The Coriolis effect changes the intended path of a moving body– Causes moving objects on Earth to follow curved

paths

Coriolis: The Example

• Imagine you and your friend are on a carousel• You are sitting on the inside of the carousel and

your friend is sitting on the outside• You throw a ball to your friend, but are amazed

to find that the ball curves sharply to the right and your friend is unable to catch it (and this is not because you throw like a girl…)

• Other friends watching from a hot air balloon hovering over the carousel confirm that the path of the ball was in fact straight

Coriolis: The Example• If we compare our carousel to the Earth, we

know that the Earth will complete a full rotation every 24 hours

• People living on the equator however must complete a much larger circumference of rotation than would people in middle and high latitudes

• In order for every part of the Earth to complete a rotation in 24 hours, points on the equator MUST travel faster than points near the poles

Back to the carousel

• When you throw the ball to your friend on the carousel, YOU are traveling slower than he/she is riding on the outside of the carousel

• In order for you and your friend to complete a rotation within the same time, the inner riders on the carousel must travel slower than those riding on the outside who must cover more ground in the same amount of time

The Coriolis Effect

• The equator must travel faster than higher latitudes must travel in order for all regions of the Earth to complete 1 full rotation in 24 hours (people in Anchorage, AK and Equator all experience the same 24-hour day)

• Therefore as objects travel from one region of the globe to another, they are subject to changing speeds of travel

rst.gsfc.nasa.gov/Sect14Sect14_1c.html

The Coriolis Effect Will Keep You Up At Night….

The Coriolis EffectYour friend, rotating faster to cover more distance (red line) in same time

You, rotating slower to cover less distance (blue line) in same time

15 mph

8 mph

Carousel rotating counter-clockwise

15 mph

8 mphYou throw ball while moving at 8mph in what you consider to be a straight path

Carousel rotating counter-clockwise

15 mph

8 mph

As ball travels, it carries with it this slower motion with it*

*The carousel beneath the ball is traveling faster than it

Carousel rotating counter-clockwise

15 mph

8 mph

As ball travels, it carries with it this slower motion with it*

*The carousel beneath the ball is traveling faster than it

Carousel rotating counter-clockwise

15 mph

8 mph

As ball travels, it carries with it this slower motion with it*

*The carousel beneath the ball is traveling faster than it

Carousel rotating counter-clockwise

My art is WAY better, but just in case you want the book’s version…

The Coriolis Effect

• Now imagine that you are at the North Pole and your friend is in Rio de Janeiro, Brazil near the equator

• You toss a ball to your friend (yes, use your imagination…) and the same principles apply: you are traveling around the world slower than your friend is. The ball will be deflected to the right due to the rotation of the Earth

• The rotation of the Earth is counter-clockwise

• In the northern hemisphere, objects are deflected to the right relative to their path of motion

• In the southern hemisphere, objects are deflected to the left for the same reason (poles are moving slower than equator)

Coriolis Effect

• As a plane travels from Antarctica towards the equator, it will veer to the left along its path (if it did not alter its course) due to Coriolis effect

• During its northern journey, the plane is flying over land that is rotating eastward at a slower – and ever decreasing – rate compared to that of the jet

• Objects are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere regardless of what direction (N,S,E,W) they are moving in

The Coriolis Effect Influences the Movement of Air in the Atmosphere• Let’s return to our hypothetic model of

atmospheric circulation on the Earth• Air does warm, expand and rise along the

equator• But, instead of traveling continuously from the

equator to the poles, rising air moves poleward and is deflected eastward (to the right) in the Northern Hemisphere, and westward (to the left) in the Southern Hemisphere

The Coriolis Effect Influences the Movement of Air in the Atmosphere• Note that the Coriolis effect does not cause

the winds; it only influences the wind’s direction

• As air rises at the equator, it will lose water vapor by precipitation caused by the expansion (there is decreasing atmospheric pressure w/increasing altitude) and cooling. This drier air travels north or south of the equator and grows denser as it cools

The Coriolis Effect Influences the Movement of Air in the Atmosphere• When the air has traveled ~ one third of the

way from the equator to the pole – to about 30°N or 30°S latitude, the air becomes dense enough to sink back towards the surface, completing the loop

• The Coriolis Effect influences the direction of the resulting winds

At the equator, warm, moist air rises, resulting in a low pressure zone

As the rising air becomes colder & drier, its density increases, resulting in a high pressure zone

Deflected to the right

Descending air towards equator is deflected to the right of its path of motion

Descending air towards equator is deflected to the left (Southern H.) of its path of motion

Throwing a monkey wrench into the Coriolis concept…

• The tendency of wind to deflect because of the Coriolis effect increases with its speed and with distance from the equator

• This means that winds in high latitudes deviate much moreso than do tropical winds occurring at low latitudes moving at the same speed

• Likewise, faster winds will be deflected moreso than slower winds in either region

General Wind Patterns

• This means that there is, in fact, no Coriolis effect at the equator, and hence, no deflection of wind

• This is because the change in velocity (speed) of the Earth changes very little near the equator, but changes muchy more at higher latitudes greater Coriolis effect

General Wind Patterns

• A column of warm, low density air rises away from the surface and creates a band of low pressure at the equator

• The weather in areas of low pressure is characterized by cloudy conditions with lots of precipitation because rising air cools and cannot retain (hold onto) its water vapor

• This region is clothed in tropical rain forests

General Wind Patterns

• A column of cool, dense air moves towards the surface and creates high pressure zones. Descending air is quite dry and so these regions are characterized by dry, clear, fair conditions

• Sinking air is very arid (dry) and the great deserts of the world are centered along this band of high pressure (30°N and 30°S)

General Wind Patterns

• Sailors have a special term for the calm, equatorial regions where low pressure persists and little winds exist; the doldrums

• Sailors also have a special term for the regions within the high pressure band, where winds are light and variable; the horse latitudes

• Places between the high and low pressure bands, on the other hand, experience rapidly moving air, and are characterized by strong, dependable winds

(Horse latitudes)

Winds are named for the direction in which they originate

Storms and fronts

• Different air masses meet at fronts• When warm air meets cold air, the warm air

rises gently, resulting in mild precipitation

Storms and fronts

• When cold air moves into warm, the warm air rises quickly, resulting in LOTS of precipitation

Tropical Cyclones (Hurricanes)

• Tropical cyclones are huge rotating masses of low pressure characterized by strong winds and torrential rain

• In North and South America, tropical cyclones are commonly called hurricanes

• In the western North Pacific, they are called typhoons

• In the Indian Ocean, they are called cyclones

Tropical Cyclones (Hurricanes)

• Tropical cyclones carry tremendous amounts of heat from one region of the world to another

• The energy contained in a single hurricane is greater than that generated by all energy sources in the United States in one year!

• Hurricanes are powered by the release of water’s latent heat of condensation (when water evaporates, it stores tremendous amounts of heat; when water condenses into a liquid, it releases this stored heat into the surrounding atmosphere)

Tropical Cyclones (Hurricanes)• The conditions required to form a hurricane

are as follows:– Ocean temperature greater than 25°C (77°F),

which provides an abundance of water vapor to the atmosphere via evaporation (summer and fall)

– Warm, moist air, which supplies vast amounts of heat as the water vapor condenses and fuels the storm

– The Coriolis Effect, which causes the hurricane to rotate counterclockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere

More hurricanes in Northern Hemisphere; warmer weather in tropics there b/c greater amount of land

Tropical Cyclones (Hurricanes)

• In fact, there are no hurricanes that can occur directly over the equator because the Coriolis Effect is zero there

• Hurricanes can not form below 8° N or S latitude; Coriolis effect is not strong enough to cause deflection

• Hurricane season: June 1 – Nov 30: 97% of cyclones (hurricanes) occur during this time

Tropical Cyclones (Hurricanes)

• As air rises (in the northern hemisphere), it is deflected to the right

• This results in a counter-clockwise rotation

http://en.wikipedia.org/wiki/File:Hurricane_isabel_and_coriolis_force.jpg

Pressure gradient (moving towards the low pressure center) is represented by blue arrows; the Coriolis deflection is represented by red arrows

L

EYE

Sinking of air occurs at the eye as dry air moves in from the atmosphere

The 2005 Hurricane Season

• The 2005 hurricane season was the most extensive on record, and actually persisted into January 2006!

• A record 27 tropical storms formed, a record 15 of which became hurricanes– 5 became Category 4– 4 became Category 5 (the highest category)

• $100 billion in damages and >2000 deaths• Coincidentally, 2005 was the hottest year on

record!