GEOG100--Lecture 06--Atmospheric and ocean circulation

Chapter 4
Atmospheric and Oceanic
Circulation
Or: What goes around, comes around.

Have you ever noticed
changes in air pressure?

What is air pressure?
Pressure is the force a gas exerts on some specified area
of a container--it is the result of molecular collisions
between the gas and the container

Air pressure changes with altitude, from
place to place—and even in the same place,
changes over time

Pressure, Density, and Temperature

 Pressure (P), density (D), and temperature
(T) are all interrelated
 Pressure is the force of molecular collisions per
unit area (lbs/in2)
 Density is the weight of a material per unit
volume (g/m2)
 Temperature is a measure of molecular motion
 Changes to one of these variables can cause
changes in the others

For example….

Changing Density Pt.I
• There are three ways to change
the density of a gas:
1.Change the size of the container
What happens to pressure?
What happens to temperature?

Changing Density—Pt. II
2. Add or subtract molecules
What happens to the temperature of the balloon
when it’s blown up?
What happens to the pressure inside the balloon
when it’s blown up?
What happens to the pressure when it’s let go?

 What happens when you change the
temperature of a confined gas?

 Let’s take our original container full of
molecules and heat it up!

- What’s happening to the pressure?
- Is density changing, or not?

A little simplification:
 For confined gases:
(if D↑ then P↑)
(if D↑ then T↑)
(if P↑ then T↑)
(if T↑ then P↑--but only if confined)
Note:
(changing T will NOT affect D, if confined)

Changing Density—Pt.III

3. Change its
temperature (if it is
uncontained)
- What will happen
to the density?
- How will pressure
be affected?

In the atmosphere, gases are
uncontained, like this…

A little more simplification:
 For unconfined gases (like in the
atmosphere):
(if T↑ then D↓)
(if D↓ then P↓)
(if D↓ then T↓)

Measuring
Atmospheric Pressure

Measuring
 In 1643 Evangelista Torricelli (a student
of Galileo) invented the first barometer…

Measuring
 In 1643 Evangelista Torricelli (a student
of Galileo) invented the first barometer…

 Today, we use an aneroid barometer

Average Sea Level Air
Pressure
 29.92 in. (inches of mercury)
 14 lbs/in2
 1013.2 mb (millibars of mercury)
 101.32 kPa (kilopascals, where 1
kilopascal is equivalent to 10 millibars)

We will use millibars, as this is the most
commonly used unit of measurement

Isobars
 Lines on a map that connect points of
equal barometric pressure are called
isobars
 Isobars follow the same rules as other
iso- lines (don’t cross, form closed
shapes, etc.)

Wind
 Wind—Air moving horizontally in
response to pressure differences

 The process is called advection

Convection Cell Diagram
 Draw the convection cell diagram and
label it, just like you see it on the board

 Practice drawing a simplified version to
help you remember “out of the high,
into the low” on exam day

 Air always moves from regions of
higher air pressure to regions of
lower air pressure

 In other words:
“Out of the High, Into the Low!”

Local Winds
Convection Cells in Motion
 Land and Sea Breezes
 Mountain and Valley Winds
 Katabatic Winds (a.k.a. Mistral)
 Chinook Winds (a.k.a. Santa Anas,
Diablo Winds, Foehn winds, etc.)

Wind Direction
 Wind direction is determined by where
the wind is coming from

Wind Direction
 For example, an east wind is one that is
coming from the east

Wind Direction
 For example, an east wind is one that is
coming from the east
 A sea breeze is one that is coming from the
sea and moving toward the land

3 Forces Affecting Air in
Motion
 Pressure Gradient Force
 Coriolis Force

 Friction

Force #1:
The Pressure Gradient Force

Force #1:
 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

Force #1:
pressure
 The PGF pulls air out of the high and into the
low at a 90º angle relative to the isobars

Force #1:
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

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 Hemisphere
 to 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 (for the mathematically advanced)

 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 (and air) in
motion over great distances and objects
moving at high speed to be deflected

Coriolis Force: In The Toilet
 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

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

Geostrophic Winds
 These winds occur in the upper
atmosphere, where there is no friction

Geostrophic Winds
 These winds occur in the upper
atmosphere, where there is no friction
 They are known as geostrophic
winds

Putting it together:
3 Forces Affecting Air in Motion

Surface winds:
Make a simple drawing

Surface winds:
Make a simple drawing

 Be able to draw it in your sleep...

Northern Hemisphere and
Southern Hemisphere Winds

A Simplified Global Circulation
Model

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

Between the ITCZ and the SHPs
are the Trade Winds

GEOG100--Lecture 06--Atmospheric and ocean circulation

Rossby Waves:
Undulations in the Jet Stream

World Regions with Monsoon Patterns

Minor Monsoons: Australia and W. Africa

Seasonal Pressure Changes
Cause Seasonal Wind Changes

ITCZ shifts more dramatically over
land than it does over water

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
70

El Niño/Southern Oscillation
(ENSO)

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 72

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 73

Ocean Currents
• Forces driving ocean currents
– Frictional drag of wind
– Coriolis force
– Temperature, density, and salinity differences
– Location of contents and shape of the sea floor
– Tides

74

Warm and Cold Surface Currents
• Direction and temperature

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.
76

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.
77

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.
78

Currents: Thermohaline Circulation

GEOG100--Lecture 06--Atmospheric and ocean circulation

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