4b_AtmosOceanicCirculation2_Slides

Global Atmospheric Circulation

Key Components

• Intertropical Convergence Zone (ITCZ)

• Trade Winds

• Subtropical Highs

• Westerlies

• Subpolar Lows

• Polar Front

• Polar High

• Polar Easterlies

• Geostrophic Winds

• Jet Streams

Air Movement Towards Poles

• Rising Air in ITCZ:

  • Moves towards poles when aloft.

  • Converges aloft due to Earth’s spherical shape.

• Cooling and Sinking:

  • Air aloft radiates energy into space and cools.

  • Sinks at Subtropical Highs.

• Trade Winds and Westerlies:

  • Some sinking air returns to the equator as trade winds via Hadley Cell circulation.

  • Some also moves northward as Westerlies.

Hadley Cell

• Subtropical Highs:

  • Located at ~30° N and S latitudes.

  • Regions of descending air become warmer and drier.

Subtropical Highs

Nickname

• Also known as "Horse Latitudes":

  • Theory: Ships stalled due to calm winds often led to supplies running low, resulting in horses being eaten or tossed overboard.

Westerlies

• Characteristics:

  • Occur at latitudes higher than trade winds.

  • In Northern Hemisphere, blow from the west towards the northeast.

• Usage:

  • Historically utilized by sailors returning to Europe from North America.

Pressure Cells

• Bermuda High:

  • Strong Bermuda High guides Atlantic hurricanes into the Gulf.

  • A weak Bermuda High directs hurricanes up the East Coast.

• Clockwise Rotation:

  • Allows the formation and movement of trade winds to the south and westerlies to the north.

Polar Front

• Definition:

  • Area of contrast between warm and cold air, where warmer Westerlies rise over colder Polar Easterlies, creating low-pressure cells.

• Effects:

  • Uplift of warm air leads to cloud formation and precipitation.

Subpolar Low-Pressure Cells

• Characteristics:

  • Dominates around 60° N latitude in winter but weakens in summer.

• Key Systems:

  • Aleutian Low:

    • Generates storms and cyclonic systems migrating eastward across North America, most active in late fall to late spring.

  • Icelandic Low:

    • Produces winter winds over the North Atlantic.

Polar High-Pressure Cells

• Characteristics:

  • Form from small polar atmospheric masses, receiving little solar energy.

  • Arctic High: Common in Northern Hemisphere; Antarctic High in Southern Hemisphere.

• Winter Formation:

  • Arctic Highs develop over continental areas (Canadian and Siberian highs) and tend to be less pronounced compared to Antarctic Highs.

• Active Winds:

  • Cold air sinks, diverges near the surface, flowing southward (Polar Easterlies).

Global Wind Patterns and Geostrophic Winds

• Upper Tropospheric Circulation:

  • Generates geostrophic winds which are visualized through isobaric (pressure) surfaces.

  • Undulating Surfaces:

    • Ridges of high pressure form where winds slow and converge; troughs of low pressure form where winds accelerate and diverge.

Constant Isobaric Surface

• Presents pressure variations in the mid to upper troposphere.

Jet Streams

• Definition:

  • High-speed winds located in the upper troposphere.

• Major Jet Streams:

  • Polar and Subtropical Jet Streams that circle Earth at mid-latitudes (30° to 60° N & S).

• Role in Weather:

  • Influence storm development and intensification due to rapid temperature change.

  • Most intense during winter due to greatest temperature contrasts.

North Polar Vortex

• Strong westerly winds enclose a pool of very cold air.

• A stronger polar vortex exists in the Southern Hemisphere.

Polar Jet Stream

• Fast-moving air current that helps contain polar air mass.

• Warming Effects:

  • Can weaken the jet stream, allowing cold Arctic air to move southward.

Rossby Waves

• Develop within geostrophic circulation related to the jet stream.

• Characteristics:

  • Weakened jet stream undulates, moving cold air southward and warm air northward, aiding in cyclonic storm development.

Local Winds

• Land-Sea Breezes:

  • Result from temperature differences between land and sea.

• Mountain-Valley Breezes:

  • Day: Warm air rises from valleys. Night: Cooler mountain air sinks into valleys.

Katabatic Winds

• Description:

  • Stronger winds that occur from cooling air flowing down from highland or plateau areas.

  • Noted Examples:

    • Antarctica and Greenland winds, California's Santa Ana winds.

Monsoonal Winds

• Characteristics:

  • Notable in regions like India and Southeast Asia.

  • Dry season in winter due to stable high pressure.

  • Summer brings torrential rains as warm air draws moisture from oceans, crucial for agriculture.