Weekly log: environmental studies: earth science2

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Chapter 5 Global Atmospheric and Oceanic Circulation

Visualizing Physical Geography by Timothy Foresman & Alan Strahler

Chapter Overview

Atmospheric Pressure

Wind Speed and Direction

Global Wind and Pressure Patterns

Local Winds

Oceanic Circulation

Atmospheric Pressure

Atmospheric pressure is pressure exerted by the atmosphere because of the force of gravity acting on the overlying column of air.

Measuring Atmospheric Pressure

Units = inches of mercury (in. Hg) or millibars (mb).

Standard sea level pressure = 1013.2 mb.

Cold, clear night pressure > 1013.2 mb.

Center of a storm with rising warm air will have a pressure < 1013.2 mb.

Barometer is an instrument that measures atmospheric pressure.

Atmospheric Pressure

Measuring Atmospheric Pressure

Radiosonde (balloon) is launched twice a day at key locations in United States

Radiosondes measure:

Pressure

Altitude

GPS location

Temperature

Relative humidity

Wind speed and direction

Would one expect low or higher than standard sea level pressure in a hurricane?

Atmospheric Pressure

Atmospheric Pressure and Altitude

Air density depends on pressure and temperature.

Atmospheric pressure decreases with altitude.

Wind Speed and Direction

Wind:

Horizontal movement of air

Renewable resource

Measured with an anemometer

Wind direction:

Identified by the direction from which the wind comes

West wind blows from west to east

Measured with a wind vane

Wind speed and direction are determined by three factors: pressure gradient, Coriolis effect, and friction.

Courtesy Taylor Instrument Company and

Wards Natural Science Establishment

Wind Speed and Direction

Pressure Gradients

Change of atmospheric pressure measured along a line at right angles to the isobars.

Pressure gradient goes from high to low pressure.

Wind Speed and Direction

Pressure Gradients

Isobar = line on a map drawn through all points having the same atmospheric pressure

Widely spaced isobars  weak gradient and weaker winds.

Closely spaced isobars  strong PG and stronger winds.

Where would you find the greatest pressure gradient on this map?

a. Oklahoma City

b. Southwestern Missouri

c. Memphis

d. Nashville

Wind Speed and Direction

Pressure Gradients

Unequal heating of the Earth’s surface leads to a pressure gradient and causes wind.

Latitude, terrain differences, and land cover can cause uneven heating, pressure gradients and wind.

Wind Speed and Direction

Pressure Gradients

If the island were in the Arctic and covered by glacial ice, would the pressure gradient be the same or different?

Wind Speed and Direction

The Coriolis Effect (CE)

An effect of the Earth’s rotation that acts like a force to deflect a moving object on the Earth’s surface to the:

Right in the northern hemisphere

Left in the southern hemisphere

Wind Speed and Direction

The Coriolis Effect (CE)

Due to Earth’s rotation, a path from the North Pole to New York along 74°W meridian would curve to the right, toward Chicago.

Wind Speed and Direction

Geostrophic wind is wind at high levels (upper levels) above the Earth’s surface moving parallel to the isobars, at a right angle to the pressure gradient.

Wind Speed and Direction

The Frictional Force (FF)

Force exerted by the ground surface that is proportional to the wind speed

Always acts in the opposite direction to the direction of motion

Greatest closest to the surface

Wind Speed and Direction

The Frictional Force

A cyclone is a center of low pressure where surface air converges into a spiral and is uplifted to the upper troposphere.

The PGF, CE, and FF cause the surface wind to spiral, converging inward toward the low-pressure center.

As the inward motion converges, it forces the air to rise (uplift)  cools adiabatically  clouds and precipitation

Wind Speed and Direction

The Frictional Force

An anticyclone is a center of high pressure where upper troposphere winds spins downward (subsidence) and diverges outward at the surface.

Air warms adiabatically as it sinks  inhibiting clouds and precipitation

Global Wind and Pressure Patterns

Global surface winds on an ideal Earth (see Figure 5.11):

Surface winds are shown on the disk of the Earth, and the cross section at the right shows winds aloft.

Global Wind and Pressure Patterns

Tropical Circulation

Warm air over the equator rises and forms low pressure resulting in the equatorial trough (wet weather).

Trade winds converge at the equator.

Air descends near 25 to 30o latitude forming a subtropical high pressure (dry weather) zone.

Global Wind and Pressure Patterns

Tropical Circulation

Hadley cell = A low-latitude atmospheric circulation cell with rising air over the equatorial trough and sinking air over the subtropical high-pressure belts.

Global Wind and Pressure Patterns

Tropical Circulation

Intertropical Convergence Zone (ITCZ):

A zone of convergence of air masses along the equatorial trough

Doldrums

ITCZ shifts with the seasons following the zone of highest insolation:

Over the ocean it shifts a few degrees between January and July.

Over land, the zone shifts 20o to as much as 40o in Asia.

Global Wind and Pressure Patterns

Tropical Circulation

Monsoon = seasonal reversal of the wind

January = north wind (dry)

July = warm, moist air (wet)

Global Wind and Pressure Patterns

Considering the direction of the winds compared to the isobars, which statement is most correct?

Because this area is near the equator, the Coriolis effect has no influence on these winds.

Because the pressure gradients are great, friction has no influence on these winds.

Because some of the winds are over the ocean, neither the Coriolis effect nor friction has an influence on these winds.

Because the alignment of the wind direction is at 45°to the isobars, both the Coriolis effect and friction are important.

Global Wind and Pressure Patterns

North American monsoon

Global Wind and Pressure Patterns

Area of high atmospheric pressure centered at about 30° N and 30° S

Stable and dry weather

Trade winds and westerlies

Hawaiian and Azores high

Shift with the seasons

East and west coast differences

Subtropical high-pressure cells

Global Wind and Pressure Patterns

In the days of sailing ships, which pattern of navigation made the most sense, considering prevailing wind directions?

a. United States to Africa to England back to the United States

b. United States to England to Africa back to the United States

c. United States to England back to the United States

d. United States to Africa back to the United States

Global Wind and Pressure Patterns

Midlatitude Circulation

Westerlies

Between about 30° and 60° latitude

Polar front = boundary between cold polar air masses and warm subtropical air masses

Global Wind and Pressure Patterns

Midlatitude Circulation

Jet stream = high-speed airflow in a narrow band within the upper-air westerlies and along certain other global latitude zones at high altitudes:

Polar-front jet stream

Shifts equatorward in the winter

Subtropical jet stream

Global Wind and Pressure Patterns

What a Geographer Sees

Jet Streams and Air Travel

If an airplane flying in the center of this subtropical jet stream travels east at 1000 km/hr (621 mi/hr), how fast will the same airplane go, with the same fuel expenditure, when it travels west in the jet stream on its return flight?

Courtesy NASA

Global Wind and Pressure Patterns

Jet stream disturbances

Rossby waves

Baroclinic instability

Zonal flow (west to east)

Global Wind and Pressure Patterns

Jet stream disturbances

Growth of disturbances in the jet stream

Global Wind and Pressure Patterns

High-Latitude Circulation

January

July

Courtesy John E. Oliver

Global Wind and Pressure Patterns

Global Circulation at Higher Altitudes

Global Wind and Pressure Patterns

Global surface winds on an ideal Earth (Review)

Global Wind and Pressure Patterns

Global air cells: Ferrel, Hadley, or Polar? (Review)

Local Winds

Daily Cycles of Winds

Daily reversal of the winds as a result of uneven heating

Sea breeze

Land breeze

Local Winds

Daily Cycles of Winds

Mountain breeze

Valley breeze

Local Winds

Other Topographic Winds

Chinook: a dry wind

Santa Ana winds

For north–south mountain ranges in midlatitude regions (30° to 45° latitude), dry regions will be found on the ____ side in the northern hemisphere and on the ____ in the southern hemisphere.

a. east; east

b. west; west

c. east; west

d. west; east

Local Winds

Other Topographic Winds

Santa Ana winds can create fire hazards. In this photo, wildfires have already begun in some areas, as is apparent from the smoke drifting off the Southern California coast.

Courtesy NOAA

Oceanic Circulation

Ocean Currents

A persistent, dominantly horizontal flow of water controlled by wind patterns

Gyres: large circular ocean movements

What relationship do you notice with the northern hemisphere ocean current and the pressure type typically located at 30o N?

Oceanic Circulation

Ocean Currents

Ocean circulation and energy transport:

Warm surface waters in the tropics move poleward.

Thermohaline circulation: Cold and dense waters in the N. Atlantic sink, flow equatorward, and eventually upwell to the surface at far distant locations to cool surrounding regions and complete the circuit.

Oceanic Circulation

Circulation and Energy Transfer

Energy surplus

Energy deficit

In order to maintain the Earth’s energy balance, absorbed solar energy is moved from regions of excess to regions of deficit, carried by ocean currents and atmospheric circulation

Oceanic Circulation

Cycles in Atmospheric and Oceanic Circulation

El Niño–Southern Oscillation (ENSO)

La Niña

Oceanic Circulation

Cycles in Atmospheric and Oceanic Circulation

Climate effects of El Niño events

Oceanic Circulation

Cycles in Atmospheric and Oceanic Circulation

North Atlantic Oscillation (NAO)

Pacific Decadal Oscillation (PDO)