Lecture 144 February 2005

Atmospheric and Oceanic Circulations(continued)

Chapter 6

Figure Credit: “Earth’s Climate” by W.

Ruddiman

Figure Credit: “Earth’s Climate” by W. Ruddiman

Credit: www.physicalgeography.net

Wind simply put, wind is the horizontal flow of air

in response to differences in air pressurethese pressure differences are usually

due to uneven solar heating at the surface

wind flows because of

pressure gradient

‘heat rises’

Wind

winds are designated as direction fromnot direction to

(oceanographers do it the opposite)

wind compass

so, a westerly wind would be coming from what angular

direction?

Four forces that determine winds

1. Gravity - pulls gas molecules close to Earth density & pressure decrease with height

2. Pressure gradient force - the difference in air pressure between

areas3. Coriolis force - deflects wind from a

straight line to the right or left depending on hemisphere

4. Friction force - the drag on air flow from the Earth’s surface

Pressure vs. Pressure Gradient

•The value of pressure itself is NOT

important

•The CHANGE in pressure over DISTANCE

is

•Change over distance is a GRADIENT

•The GRADIENT in pressure gives winds &

ocean currents their “push”

Pressure Gradient Force (PGF)

isobar - a line of equal pressure (analogous to

isotherm)

gradient is 16 mb(note the closer isobars)

the PGF acts at right (90º) angles to the isobars

Pressure Gradient Force

note the 1008 mb isobar

Wind speed = Const * Pressure Gradient

Here, a 4x increase in PGF corresponds to a 4x increase in wind speed

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Pressure Gradient Force and Isobars

if there were no other forces acting on wind, it would flow in straight lines (perpendicular to isobars) from high to low pressure zones

Coriolis Force (just the facts)

• Rotation of Earth acts to deflect any motion

from a straight line

• Deflection is to right (NH) to the left (SH)

• Coriolis “force” act on a right angle to the

motion

• Coriolis Force is NOT a real “force” but is

caused by viewing motion on a rotating

planet

Coriolis Force

• Show the merry-go-round video

the amount of rotation about a vertical axis

(’spinning’) is maximum at the poles and minimum at

the equator

Figure Credit: “Earth’s Climate” by W.

Ruddiman

Earth’s Rotationevery point on earth rotates around a central

axis at 15 degrees/hour

LatitudeSpeed of

rotation (mph)0˚ 1041

30˚ 902

50˚ 670

60˚ 521

90˚ 0

Coriolis Forcean object with an initial east-west velocity will maintain that velocity,

even as it passes over

surfaces with different velocities

as a result, it appears to be deflected over that surface (right in NH, left in SH)

Coriolis Force and Deflection of Flight Path

Coriolis Force and Deflection of Flight Path

Coriolis Force and Flight Paths II.

Airplane animation

Figure Credit: “Earth’s Climate” by W.

Ruddiman

The Coriolis Force affects air flow in response to pressure gradients in the

atmosphere

geostrophic winds - PGF and Coriolis forces are opposite and balanced

Credit: www.physicalgeography.net

in the northern hemisphere (upper troposphere), the CF deflects the wind to

the right until wind flows parallel to isobars

~7km

Geostrophic Winds

Balance between Pressure Gradient & Coriolis

Forces

Flow along isobars not across

Works for upper atmosphere winds & ocean

currents

500 mb Pressure Map

PGF, CF & isobars in upper troposphereisobars

Friction Force

surface friction reduces wind speed and reduces the Coriolis force (remember CF

increases with wind speed)because of this, it causes winds to

move across isobars at an angle

the friction force operates only in the bottom 0.5-1 km of the atmosphere,

and it acts opposite to the direction of motion

Figure Credit: “Earth’s Climate” by W.

Ruddiman

PGF + Coriolis + Friction Forces

isobars

The inter-tropical convergence zone (ITCZ)solar heating in the tropics expands air and

decreases its density - leading to increased buoyancy

How would this change the average molecular weight of air?

average molecular weight of air is ~29

g/mol

average density of air is 1.3 kg/m^3

what happens to air density if

you add water vapor?

It also gets more humid (adding water vapor)

Convection on your Stove

Convection on Earth

as this air rises, it cools and water condenses out, leading to intense

precipitation

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A satellite (GOES) view of the ITCZ over the eastern Pacific

the position of the ITCZ tracks the sun (it is found in the summer hemisphere) - the

location of the ITCZ determines the rainy season in many tropical countries, especially

those in Africa

the horizontal winds within the ITCZ are calm - the doldrums

The C in ITCZ

the intense uplift of air creates horizontal pressure gradients at the surface

Credit: NASA JPL

as a result, winds converge towards the equator from both hemispheres

what about the complete cycle - where does the uplifted air go?

Equator-to-pole cross section of circulation

Hadley cell circulation

this circulation refers to the complete circulation of rising air in the tropics,

descending air over 30 °N and °S, and trade winds converging at the equator

the descending branch of the Hadley circulation brings hot, dry air to the surface -

leading to high pressure areas and suppressed precipitation

Subtropical high-pressure cellsthese cells occur where the tropical air

descends in either hemisphere

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Figure Credit: physicalgeography.net

Monsoon Circulation

Asian monsoo

nintense, dry winds flow from the Asian interior in response

to the gradient between the

continental high pressure and the equatorial (ITCZ)

low pressure

Asian monsoo

nin summer, the subsolar point

and the ITCZ shift northward,

reversing the pressure gradient

- as the winds flow over the

Indian ocean they gain moisture

Daytime land-sea breeze

results from differential heating of land and sea - not from radiation differences - but

from the different specific heats of land and water

Nighttime land-sea breeze

at night, the land cools more rapidly than the sea and thus overlying air becomes more

dense and has a higher pressure