physnote_circulation.pdf

John Aubert • American River College Atmospheric and Oceanic Circulation Physical Geography Lecture Notes Page 1 of 8 aubertj@arc.losrios.edu

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Atmospheric and Oceanic Circulation

I. Atmospheric Pressure: Atmospheric pressure is the force exerted by air molecules, sort of like the weight of the air pushing on you. Since the atmosphere is fluid, this force “pushes” in all directions. The force measures approx. 1kg/cm2. If we were to take a perfectly vertical column 1cm2 at the base and rising to space, the collective weight would be 1kg.

A. Density of Molecules: •In high pressure, air molecules are high density (densely packed together). •In low pressure, air molecules are low density (loosely packed together). •Atmospheric pressure is highest at sea level and decreases rapidly toward space. Pressure in space is zero. •This is because gravity is holding the molecules in place.

B. Two Basic Causes of High and Low Pressure at Earth’s Surface

1. Surface Temperature Extremes (called thermal pressure systems)

a) HOT surfaces generate LOW pressure: the air molecules move faster and spread outward. This causes the air to expand and become less dense. Results in LOW pressure.

b) COLD surfaces generate HIGH pressure: the air molecules slow

down and pack together. This causes the air to compress and become more dense. Results in HIGH pressure.

2. Forced Air (called dynamic pressure systems)

a) Air forced INWARD and UPWARD generates LOW pressure: a vacuum is created at Earth’s surface as air is pulled “up and away” from Earth. This results in LOW pressure.

b) Air forced DOWNWARD and OUTWARD generates HIGH

pressure: it “stacks up” at Earth’s surface and exerts a pressurizing force. This results in HIGH pressure.

C. Measuring Atmospheric Pressure

1. Instruments a) Mercury Barometer (see text) b) Aneroid Barometer(see text)

2. Average Sea-Level Atmospheric Pressure a) 29.92 inches Hg b) 760 millimeters Hg c) 1013.2 millibars (mb) (this is the one we will use)

II. Wind: • Wind is the horizontal motion of the air with respect to the Earth's surface. •Wind always moves from HIGH pressure to LOW pressure •Atmosphere always has an uneven pattern of pressure (i.e. there are always highs and lows). •Since fluids always move from high to low, the atmosphere is constantly in motion (wind).

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A. The Pressure Gradient Force: … is the force generated by a pressure difference between two places.

1. Isobars: On a map, these are lines of equal pressure.

2. Closely spaced isobars: …indicate a high pressure gradient (i.e. high pressure difference) and high wind speeds

3. Widely spaced isobars: …indicate a low pressure gradient (i.e. low pressure

difference) and low wind speeds 4. Direction of Force: The pressure gradient force acts perpendicular to the

isobars.

For each example, draw an arrow indicating the direction of wind.

Sample Map A Sample Map B

B. The Coriolis Force: An apparent deflection of wind and ocean currents. (This is

probably the single hardest thing to understand in Physical Geography. If nothing else, memorize this.)

Northern

Hemisphere Southern

Hemisphere Deflection of winds and ocean currents: to the RIGHT to the LEFT

1. Wind Speed: The strength of the Coriolis Force increases as wind speed increases.

2. Latitude: The strength of the Coriolis Force increases as latitude increases. 3. No force at Equator: This apparent force occurs because, as a wind or ocean

current is set in motion in a particular direction, Earth basically rotates out from under it. This makes it seem as though the wind or ocean current takes a left or right turn, depending on the hemisphere. To understand this, you will have to see the in-class demonstration and/or read this section in the text carefully.

1016mb 1020mb 1024mb

LOWER

HIGHER

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C. Cyclones (Low Pressure) and Anti-Cyclones (High Pressure): Spiraling wind systems (NOT storms)

1. Cyclones (Low Pressure): Inward spiraling low pressure wind systems!

Northern Hemisphere (Side View)

Southern Hemisphere (Side View)

Northern Hemisphere (Map view)

Southern Hemisphere (Map view)

2. Anti-cyclones (High Pressure): Outward spiraling high pressure wind

systems! Northern Hemisphere (Side View)

Southern Hemisphere (Side View)

Northern Hemisphere (Map view)

Southern Hemisphere (Map view)

LOW

ascending

converging LOW

ascending

converging

HIGH

descending

diverging HIGH

descending

diverging

L L

H H

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III. General Atmospheric Circulation Model A. Inter-Tropical Convergence Zone (ITCZ) or Equatorial Low Pressure Trough:

Warm and rainy!!! High inputs of insolation along the equator cause warming and convectional lifting (THERMAL LOW PRESSURE). Winds are weak here (called Doldrums); pressure is low.

B. Hadley Cell: Warm air rises from Equator toward the tropopause and then turns poleward in both directions. At approx. 30°N & S Latitude these currents descend from the tropopause toward earth. The circulation is called a Hadley Cell.

C. Subtropical High Pressure: These are zones with generally clear, warm weather. Created where Hadley Cell air descends (is forced back down) to Earth. Wind is weak (called Horse Latitudes). Generates anticyclones. (Since the anti-cyclones are caused by forcing air downward and outward, they are considered DYNAMIC HIGH PRESSURE.)

D. Trade Winds (or Tropical Easterlies): Northeast and Southeast trade winds created as air spirals away from Subtropical High Pressure and into the Equatorial Low Pressure Trough. Found between 5° – 25° N & S Latitude (roughly)

E. Westerlies: Created as air spirals away from Subtropical High Pressure toward the Sub-Polar Low Pressure. Found between 40° – 60° N & S Latitude (roughly)

F. Polar Front Low Pressure: The Polar Front Low Pressure (also called the Sub Polar Low Pressure) is created as warm air from Subtropical High Pressure collides with cold air from Polar High Pressure. This air collides and is forced upward. Polar front cyclones (also called mid-latitude cyclones) form and travel along here. (Since the cyclones are caused by forcing air, they are considered DYNAMIC LOW PRESSURE.)

G. Polar High Pressure: Bitter cold, dense air sinks (descends) at the poles and pushes outward, creating high pressure (THERMAL HIGH PRESSURE).

H. Polar Easterlies: Cold, dry wind at high latitudes (70°+ North and South) which spiral out of the Polar High Pressure.

P O L A R H I G H P R E S S U R E

ITCZ

S U B – T R O P I C A L H I G H P R E S S U R E

P O L A R F R O N T L O W P R E S S U R E

P O L A R H I G H P R E S S U R E

Hadley Cell

Hadley Cell

ITCZ

S U B – T R O P I C A L H I G H P R E S S U R E

polar easterlies

polar easterlies

westerlies

westerlies

northeast trade winds

southeast trade winds

P O L A R F R O N T L O W P R E S S U R E

CRITICAL !!

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IV. Atmospheric Circulation – A Closer Look A. ALL Pressure and Wind Systems Shift: Pressure and wind systems shift north

and south during the year, roughly following the north and south migration (shift) of the declination. In other words, they follow summer!

1. ITCZ Example: Look at a pressure map; note the shift in latitude of the ITCZ

between July and January.

2. Subtropical High Examples a) Pacific Sub-Tropical High: Centered near Hawaii. Farthest north in

July and then shifts southward about 10 degrees by January. b) Azores Sub-Tropical High: Centered near Azores (off the coast of

Portugal). Has roughly the same shift as the Pacific Sub-tropical High.

B. Dynamic vs. Thermal Pressure Systems (review): Thermal pressure systems will develop over hot or cold land masses. They are driven by extreme temperatures. The HIGHS are cold, descending air over land masses in the winter hemisphere, while the LOWS are warm, ascending air over land masses in the summer hemisphere. Pressure systems which develop where air is forced are called dynamic pressure systems.

Pressure System Review Table

Thermal Highs (usually on land)

Thermal Lows (usually on land)

Dynamic Highs also called

Subtropical Highs

Dynamic Lows also called

Polar Front Lows Basic Cause Surface Cooling Surface Heating Forced Air

Cyclone or Anticyclone Anticyclone Cyclone Anticyclone Cyclone

Movement of Air Descending and Diverging

Ascending and Converging

Descending and Diverging

Ascending and Converging

Direction of Spiral (in N. Hem.) Clockwise Counter Clockwise Clockwise Counter Clockwise

Direction of Spiral (in S. Hem.) Counter Clockwise Clockwise Counter Clockwise Clockwise

Associated Temperature Freezing Warm Warm Cool to Cold

Associated Weather Clear and Freezing Warm and

Thundershowers Clear and Warm Cloudy, Rain or Snow

Stability Stable Unstable Stable Unstable

General Location

At N and S Pole and over mid to

high latitude continents in

Winter

Along the Inter Tropical

Convergence Zone (ITCZ) and over

continents in Summer

Over Oceans at approx. 30°N and

S Lat (always present, tend to migrate toward

“summer hemisphere”)

Over Oceans at approx. 60°N or S

Lat (generally travel west to east)

Examples in N. Hem.

Polar Highs Siberian High, Canadian High

South Asian Low, Equatorial Low

Pressure Trough

Pacific Subtropical High, Azores High

Aleutian Low, Icelandic Low

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V. Upper Atmospheric Circulation Features A. Geostrophic Winds: (see text)

B. Polar Front Jet Stream (Geostrophic Wind at the Polar Front): (see text)

C. Rossby Waves: (see text)

VI. Local Winds (Winds on a smaller scale)

A. Land and Sea Breezes: Winds are always designated by where they are coming from; if you are standing at the beach facing the ocean and the wind is in your face, this is a sea breeze. •During a warm day the land surface heats more rapidly and gets hotter than the sea surface, generating a thermal low pressure on land. •As heated air rises by convection, a vacuum is created and cool marine air is pulled into fill the void. •The land breeze occurs at night as the reverse happens (the land cools causing the air above it becomes cooler and denser. This generates a thermal high pressure where cold air sinks and pushes out to sea.)

Sea Breeze

Land Breeze

B. Mountain and Valley Breezes: see text

Valley Breeze

Mountain Breeze (also called Cold Air Drainage or Katabatic Wind)

Cooled air

Heated air

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C. Monsoon Wind Systems – South Asia Example: Like a large scale land / sea breeze system which occurs over an entire year.

1. June – September (monsoon season): Thermal low pressure develops

(the South Asian Low). This thermal low merges with the ITCZ. This thermal low pressure serves as a vacuum for warm, moist, tropical air off the Indian Ocean. Monsoon precipitation is substantial.

2. December – March (dry season): The ITCZ is well south of southeast Asia,

and the area is dominated by the Siberian High (thermal high). Wind blows offshore (out to sea), so there is very little water vapor and very little rain.

VII. Ocean Currents

A. Factors Determining Direction of Current 1. Anti-Cyclones (create gyres): Wind pushes water through the force of

friction between the water and air. The subtropical high pressure centers (anticyclones) form huge spiral ocean currents called gyres.

2. Coriolis Force: Deflects currents to the right in the Northern Hemisphere and the

left in the Southern Hemisphere.

B. Examples 1. Warm Currents: Found on eastern sides of continents at mid latitudes (30-40

degrees N and S); can serve to moderate otherwise potentially very cold climates.

a) Gulf Stream (or Florida Current): Eventually becomes the N. Atlantic Current, warms the British Isles and Scandinavia.

b) Kuroshio Current: Warms Japan and East Asia.

2. Cold Currents: Found on western sides of continents at mid latitudes (30-40

degrees N and S); can serve to moderate otherwise potentially very warm climates. Often brought to the surface by upwelling (see text for definition).

a) Peru-Chile Current: Cold current which travels northward up the west

coast of South America. b) California Current: Cold current which travels from Alaska south down

the coast of California. VIII. El Niño (an ocean temperature anomaly):

•This is not a violent storm or large waves; it is an ocean temperature anomaly (difference from the average). •In the strictest sense, an El Nino event occurs when the trade winds, which normally push warm Pacific water along the Equator toward Australia and Indonesia, weaken. •This allows the warm water to flow (or "slosh") back toward South America (Coastal Peru). The net result is that a large body of unusually warm water forms off the coast of South America. •This raises the atmospheric water vapor content in this area; sometimes this vapor gets channeled toward California causing heavy rains. •Conversely, Australia and Indonesia experience cooler ocean waters and dryer weather than normal. • A La Nina is essentially the opposite…

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Summary of Global Cause and Effect

Sample Questions: Questions similar to these will be on your exam. As you study you should anticipate how I might use these questions to create new questions on the same concepts. 1 Which value is closest to the Earth’s average sea level pressure? A. 1mb B. 10mb C. 100mb D. 1000mb E. 10,000mb

2 What is an isobar?

3 Which condition is most likely to produce high wind speeds? A. High pressure B. Low pressure C. High pressure gradient D. Low pressure gradient

4 A cyclone is best described as: A. a violent tornado or waterspout B. a hurricane-like storm C. a converging and ascending system D. an outward spiraling high pressure system

5 Which of the following will shift south during the Southern Hemisphere summer? A. ITCZ B. sub-tropical high pressure systems C. Polar Front Jet Stream D. the path of the westerly winds E. all of the above

6 What type of weather do we associate with the ITCZ?

7 Which pressure system forms where the Hadley Cell descends back down toward Earth? A. Equatorial low pressure trough B. Sub-tropical high C. Polar front low D. Polar high E. all of these

8 On what type of surface will a large thermal high pressure system most likely develop? A. high latitude ocean B. high latitude land C. low latitude land D. low latitude ocean

9 If the Azores Sub-Tropical High Pressure system is found at about 40°North Latitude in July, at what latitude would it likely be found in January? A. 40°South Latitude B. 30°North Latitude C. 5°North Latitude D. 60°North Latitude E. 25°South Latitude

10 What is one reason, discussed in class, why geostrophic winds travel faster than surface winds?

11 Which of the following will drive (or cause) a sea breeze in a coastal area? A. Thermal low pressure B. Dynamic low pressure C. Thermal high pressure D. Dynamic high pressure

12 What are the main differences between a mountain and valley breeze? 13 What is the basic reason why mountain breezes and katabatic winds occur?

14 During which month will Darwin, Australia most likely experience monsoon conditions? (Hint: Darwin is in the Southern Hemisphere) A. July B. September C. March D. May E. January

15 Which is the ocean current that transfers warm water toward the North Pole? A. West African Current B. Benguela Current C. Kuroshio Current D. California Current

16

What is the feature pictured at right? A. Northern hemisphere cyclone B. Northern hemisphere anti-cyclone C. Southern hemisphere cyclone D. Southern hemisphere anti-cyclone

17 On the map provided (at the time of the test) identify the locations of major midlatitude (NOT polar and NOT equatorial) warm and cold ocean currents.

18 Possible Essay/Diagram Question: Be prepared to diagram and/or explain BOTH land AND sea breezes.

19 Possible Essay/Diagram Question: Be prepared to diagram and/or explain BOTH mountain AND valley breezes.

20 Critical Diagram Question: Be prepared to fully and completely diagram the global circulation model (global wind and pressure systems).

GOOD NEWS!!! The practice questions above – plus many, many more – can be found in the online practice quizzes discussed in your syllabus and in class. You can take each quiz multiple times, and each time you will get some new questions. Once submitted, the quizzes are graded automatically, with the correct answers provided immediately. This is a great way to prepare for the exams!!!

Earth / Sun Relationships

Sun angles are high at low latitudes and low at high

latitudes.

Net Radiation Absorbtion

Low latitudes experience radiation surpluses, while high latitudes experience radiation

deficits.

Temperature

Low latitudes become warm, while high latitudes become

cold.

Atmospheric Pressure

Low latitudes develop low pressure, while high latitudes

develop high pressure.

Atmospheric Circulation (Wind) Air moves from high to low pressure, generating large

scale, semi-permanent circulation patterns.

Oceanic Circulation

Wind pushes (drags) water, causing large scale, semi-permanent ocean currents.