Oceanography Lecture Notes Outline

Atmospheric circulation

I. Contents -  Topics Covered

Structure of the Atmosphere

Composition of Air

Atmospheric Pressure

Winds and the Coriolis Effect

Global Wind Bands

Seasonal Variability in Winds

Effects and Features of Wind

Hurricanes

Clouds, Weather, and Climate

 

II. Structure of the Atmosphere

A. The Atmosphere Consists of a Series of Layers

1. The layers of the atmosphere are defined by temperature shifts

       Troposphere (clouded layer)

    Temperature decreases with elevation

    The layer where weather occurs

 

       Stratosphere (ozone layer)

    Temperature increases with altitude

    Little circulation in stratosphere

    Exceptions are injections of volcanic eruptions

 

       Mesosphere

    Temperature decreases with increasing altitude

 

       Thermosphere

       Temperature increases with increasing altitude

 

2. The atmosphere is warmed at the surface and cooled at the top

 

B. The Atmosphere is Density Stratified

1. The atmosphere is most compressed at the surface

 

2. Pressure decreases with altitude

 

III. Composition and Properties of Air

A. Air is Composed of Transparent, Odorless Gases

1. Nitrogen N2 - (78.1%)

 

2. Oxygen O2 - (20.9%)

 

3. Other gases: Ar (0.9%); CO2 , water vapor, and inert gases (variable ~ avg. 1.4%)

 

4. Variable amounts of dust particles

    Terrigenous materials from land (wind-carried)

    Sea salt from ocean surface

 

B Atmospheric Pressure Varies Both Vertically and Horizontally

1. Measure of air density

 

2. Pressure increases when cooled or when water vapor content decreases

 

3. Air density decreases when air is warmed

 

4. Standard surface air pressure is 760 mm mercury (Hg)

 

5. High pressure zone defined as > 760 mm Hg

 

6. Low pressure zone defined as < 760 mm Hg

 

7. Geographically-continuous regions of equal surface pressure are represented on maps as isobars

 

IV. Winds on a Rotating Earth and THe Coriolis Effect

A. Winds on a Non-rotating Earth Would Be Fairly Simple

1. Hemispherical atmospheric circulation systems

 

2. Large wind cells in each (northern and southern) hemisphere

 

3. Warm air rises at the equator, cools as water vapor condenses as rain

 

4. Dry air rising aloft cools, then sinks at the poles

 

5. Surface winds blow from the equator to the poles

 

B. Winds on a Rotating Earth Are More Complex

1. Equator moves eastward at 1700 kilometer/hour

 

2. Rotational decreases poleward (with increasing latitude)

    850 km/hr at 60o N and S latitude

 

3. Earth rotation causes deflection of moving objects relative to Earths surface

    Moving objects are deflected to the right in the northern hemisphere

    Moving objects are deflected to the left in the southern hemisphere

 

C. The Deflection of Objects on a Rotating Sphere is Called the Coriolis Effect

 

1. The Coriolis Effect is a special term for the effects of global-scale centrifugal forces on Earth

 

2. Moving air and ocean water masses are significantly affected by centrifugal forces

 

3. Moving air masses move along curved paths (deflected) instead of straight paths.

 

4. The Coriolis Effect greatly complicates the Earths wind patterns

    Hemispherical wind cells get divided into several smaller latitudinal systems

 

V. Latitudinal WInd Bands

A. Latitudinal Wind Bands are Deflected by Coriolis Effect

1. Winds veer to the right in northern hemisphere

 

2. Winds veer to the left in the southern hemisphere

 

B. Atmospheric Circulation is Broken Up Into Six Major Wind Bands or Belts

 

1. Northeasterly and Southeasterly Trades

       Termed the Hadley cells (0 - 30 latitude),

       Separated by a belt of low pressure called the equatorial

doldrums

    Coincides with the intertropical convergence zone (ITCZ)

       Bordered on their high latitude side by a subtropical

high pressure belt

       Strong, steady wind system

       Typically associated with warm moist air

 

2. Northern and Southern Westerlies

       Termed the Ferrel cells (30 - 60 latitude)

       Bordered on their high latitude side by the Polar Front

       Bordered on their low latitude side by a subtropical

high pressure belt

       Winter storm systems typically ride this belt from west to east

 

3. The Northern and Southern Polar Easterlies

       Termed Polar cells (60 - 90 latitude)

       Bordered on their low latitude side by the polar jet stream

       Consists of very cold dry air

       Winter storm systems typically develop at the low latitude edge

of this belt

 

C. Air Either Rises or Falls Where Latitudinal Wind Bands Meet

1. Equatorial warm air rises, sinks at ~30N

2.       At 30N some moves back towards equator as trade winds

       NE in northern hemisphere

SE in S. hemisphere (directions from which they blow)

3. Trade winds converge at equator

       Intertropical convergence zone (ITCZ). 

 

4. At 30N the remaining air flows towards poles as the Westerlies

 

5. Westerlies meet colder, dense air flowing from poles towards equator

 

    These air masses converge at Polar (Antarctic) Front

 

D. Jet Streams are Narrow Bands of Strong Winds at the Polar Front

1. Jet streams vary seasonally, as the cells migrate

 

2.    The jet streams are typically found along the Polar Front

 

E. Winds Influence Climate

1. Climate is the long-term averaged weather

 

2. Winds influence at mid-latitudes: 

       Low rainfall coupled with high evaporation

       Typically light and variable winds (horse latitudes)

       Dominated by high atmospheric pressure

 

3. Winds influence in equatorial regions: 

       High rainfall coupled with cloudiness

       Typically light and variable winds (doldrums)

       Dominated by low atmospheric pressure

 

VI. Seasonal variabilty in winds

A. Causes of Seasonal Changes:

1. Caused by differential solar heating of ocean and land

2. Product of high heat capacity of water

 

B. Weather Characteristics of Summer

1. Low pressure areas over land caused by warm rising air

2. High pressure over ocean

 

C. Weather Characteristics of Winter

1. Winter produces the opposite effect

       High pressure areas over land caused by cold sinking air

       Low pressure over ocean

 

D. Characteristics of Monsoons

1. Regional seasonal changes in winds

 

2. A result of continent configurations

3. Summertime pattern: 

       Warming land with rising air draws cooler, moist air from ocean

       Result yields monsoon rains

 

4. Wintertime pattern: 

       Winds reverse, cool continental air is drawn towards ocean

       Result is dry weather. 

 

E. Characteristics of Local Wind Patterns Near Coastlines

1. Sea breeze pattern: 

       Warm land air rises, replaced by cool sea air

       Called onshore winds

       Typically a daytime phenomena

 

2.    Land breeze pattern

       sea air rises, replaced by cool land air

       Called offshore winds

       Typically a nighttime phenomena

 

3. Fluctuation between sea and land breezes a daily occurrence

 

VII. effects and features of wind

A. Mountain topography Has an Effect on Surface Winds

1. Winds rise and cool, leading to condensation of water vapor 

2. Precipitation occurs on windward side of topographic high

 

3. Dry air found on leeward side (this area termed the rain shadow) 

 

B. Characteristics and Significance of Jet Streams:

1. High speed winds of upper troposphere

2. Polar jet streams found at 60N and 60S

 

3.    Steers storm systems within the Westerlies

 

4. Sub-tropical jet streams at 30N and 30S

 

5. Greatest oscillation in winter

 

VIII. Hurricanes

A. Hurricanes Form From Trade Winds in Equatorial Regions 

1. Initiated over warm waters (>27C) 

 

2. Begins as pressure disturbance (termed easterly wave)

 

3. Involves the convergence of rotating winds

 

4. Starts as low pressure with moist winds at 10 - 20N and S

       Starts as tropical depression

       Builds to become tropical storms

 

5. Becomes hurricanes when wind speed >75 knots

 

6. Moves westwards

 

7. Dissipates over land or cold water

IX. Clouds, Weather, and CLimate

A. Clouds Heat and Cool the Earth

1. Require condensation nucleii 

2. Types of clouds: 

       cold air with precipitation (cumulonimbus)

       warm air rises (nimbostratus, altostratus, cirrostratus and cirrus).

 

X. Vocabulary Terms