Atmospheric and Oceanic Circulation

Chapter 4--Part 2

3 Forces Affecting Air in Motion

•Pressure Gradient Force•Coriolis Force•Friction

Force #1:The Pressure Gradient Force

• The pressure gradient force is the force exerted by a gas (in this case, air) at higher pressure trying to move to an area of lower pressure

• The PGF pulls air out of the high and into the low at a 90º angle relative to the isobars

• The greater the “slope”, or gradient, between one pressure region and the next, the faster the air will move

Where the Isobars are Close Together, Winds are Faster & Stronger

HEY… Hold ON.What’s UP with the curving motion?

Force #2:The Coriolis Force

• A force which causes fluids in motion over great distances and objects moving at high speed to be deflected:

to the right in the Northern Hemisphereto the left in the Southern Hemisphere

(Note: Air acts like a fluid in many ways.)

PGF + Coriolis Force = “curving” wind

Coriolis Force—doing the math

• The Coriolis force is a force existing in a rotating coordinate system with constant angular velocity to a reference frame. It acts on a body moving in the rotating frame to deflect its motion sideways.

Formulae (partial - for the mathematically curious)

• In non-vector terms: at a given rate of rotation of the observer, the magnitude of the Coriolis acceleration of the object is proportional to the velocity of the object and also to the sine of the angle between the direction of movement of the object and the axis of rotation.

• The vector formula for the magnitude and direction the Coriolis acceleration is

where (here and below) is the velocity of the particle in the rotating system, and is the angular velocity vector (which has magnitude equal to the rotation rate and is directed along the axis of rotation) of the rotating system. The equation may be multiplied by the mass of the relevant object to produce the Coriolis force:

• The × symbols represent cross products. (The cross product does not commute: changing the order of the vectors changes the sign of the product.)

• The Coriolis effect is the behavior added by the Coriolis acceleration. The formula implies that the Coriolis acceleration is perpendicular both to the direction of the velocity of the moving mass and to the rotation axis.

• A force which causes fluids in motion over great distances and objects moving at high speed to be deflected:

to the right in the Northern Hemisphereto the left in the Southern Hemisphere

Remember:

Coriolis Force: In The Toilet

• Given the definition of coriolis force, is it valid to assume that the water in your toilet, sink, or bathtub will be deflected to the right while draining?

• A force which causes fluids in motion over great distances and objects moving at high speed to be deflected

to the right in the Northern Hemisphereto the left in the Southern Hemisphere

• A force which causes fluids in motion over great distances and objects moving at high speed to be deflected

to the right in the Northern Hemisphereto the left in the Southern Hemisphere

Geostrophic Winds

• When the Coriolis Force and Pressure Gradient Force balance one another, winds spin around a high or low pressure cell, parallel to the isobars

• These winds occur in the upper atmosphere, where there is no friction

• They are known as geostrophic winds

Geostrophic winds

Force #3:Friction

Putting it together:3 Forces Affecting Air in Motion

Surface winds:Make a simple drawing

• Be able to draw it in your sleep...

Northern Hemisphere and Southern Hemisphere Winds

Convergent and Divergent Air

Hadley Cells

A Simplified Global Circulation Model

The ITCZ

Subtropical Highs

Some are so prominent, they even have their own special names

Between the ITCZ and the SHPs are the Trade Winds

The Hadley Cell at Work

The Westerlies

Subpolar Lows

Polar Easterlies

Polar Highs

A Simplified Global Circulation Model

The Jet Stream(s)

Rossby Waves:Undulations in the Jet Stream

World Regions with Monsoon Patterns

Monsoons in India and Asia

Minor Monsoons: Australia and W. Africa

Seasonal Movement of the ITCZ

Seasonal Pressure Changes Cause Seasonal Wind Changes

ITCZ shifts more dramatically over land than it does over water

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Multi-year Atmospheric Oscillations

• ENSO--El Niño-Southern Oscillation–Ocean-Atmosphere connection

• (we will discuss this phenomenon in Chapter 7)

• NAO--North Atlantic Oscillation–Affects Europe, eastern US, Greenland/Canada

region; no defined pattern• AO--Arctic Oscillation

–Associated with NAO• PDO--Pacific Decadal Oscillation

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El Niño/Southern Oscillation El Niño/Southern Oscillation (ENSO)(ENSO)

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NAO--Positive Phase• Stronger Azores

high and deeper Icelandic low

• Stronger winter storms, more of them to the north

• Mild, wet eastern U.S.; warm, wet in N. Europe

• Cold, dry Med., west Greenland, NE Canada

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NAO--Negative Phase• Weaker Azores high,

Icelandic low• Reduced PGF =

weaker storms and less of them

• Cold snaps in eastern U.S. bring more snow; cold, dry in N. Europe

• Wetter Med.; Greenland, NE Canada milder

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Ocean CurrentsOcean Currents•Forces driving ocean currentsForces driving ocean currents

–Frictional drag of windFrictional drag of wind–Coriolis forceCoriolis force–Temperature, density, and salinity Temperature, density, and salinity

differencesdifferences–Location of continents and shape of the sea Location of continents and shape of the sea

floorfloor–TidesTides

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Warm and Cold Surface Warm and Cold Surface CurrentsCurrents•Direction and temperatureDirection and temperature

Ekman Spiral(Northern Hemisphere)

wind Force of wind on surface

Coriolis force

Current

Ekman Transport

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Upwelling Currents

• Where the net movement of water is away from the coast, cold, dense water rises up from the bottom of the ocean to replace the water that has moved away.

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Downwelling Currents

• Where the net movement of water is toward the coast, warmer surface water piles up and pushes down toward the bottom of the ocean, displacing colder water, below.

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Open-ocean Upwelling

• Near the equator, upwelling occurs where surface winds cause ocean water to diverge. As surface waters move apart, cold bottom water rises up to replace what’s been pushed away.

Currents: Thermohaline Currents: Thermohaline CirculationCirculation