L4 - Atmospheric General Circulation

Wind Conventions

• Westerly winds: Blow from west to east and primarily influence the weather patterns across the mid-latitudes by transporting warm air poleward. These winds are essential for the functioning of the Ferrel Cell, which plays a crucial role in atmospheric circulation.

• Easterly winds: Blow from east to west, particularly prominent in the tropics, and are associated with the trade winds. They are important drivers of oceanic currents and play a key role in the El Niño-Southern Oscillation (ENSO) phenomena.

• Oceanic current conventions: Differ from atmospheric winds.

  • Westward current: Flows towards the west, transporting warmer water and affecting climate in the regions they flow through.

  • Eastward current: Flows towards the east, often bringing cooler temperatures and influencing weather patterns along coastlines.

Meridional Circulation (Hadley, Trenberth & Caron 2001)

• The atmosphere and ocean transport approximately 6 ext{ PW} (Petawatts) of energy poleward due to factors such as:

  • Excess incoming radiation in the tropics, resulting from the Earth's tilt and solar energy concentration.

  • Deficit of outgoing radiation at high latitudes, where less solar energy is absorbed, leading to cooler conditions.

• Meridional heat transport framework: Based on a non-rotating planet model, helps us understand poleward energy transfer and highlights the importance of sea surface temperatures in driving weather patterns and climate.

Angular Momentum Conservation

• Angular momentum must be conserved during poleward air movement.

  • Equation: mvr = mWr^2 where:

    • m = mass

    • v = velocity

    • r = radius from the rotation axis

• As air moves poleward, rings of air closer to the rotation axis must spin faster due to decreased radius, resulting in westerly winds that are strong in the mid-latitudes.

• As air moves equatorward at the surface, rings of air expand, generating easterly winds, crucial in the trade wind belt.

Coriolis Force Impact

• As ascending air in the tropics flows poleward, the Coriolis force causes deflection: right in the Northern Hemisphere and left in the Southern Hemisphere.

  • This deflection is vital for creating prevailing wind patterns and helps balance pressure gradient forces related to temperature differences from pole to equator.

Atmospheric Circulation Dynamics

• Zonal mean zonal winds: Vary with seasons (DJF - December, January, February & JJA - June, July, August) and are influenced by varying thermal structures throughout the year.

• Ferrel Cell: Part of the mid-latitude circulation, opposite in direction to the Hadley Cell, operates between 30° to 60° N/S, and is responsible for significant poleward energy transport and mid-latitude storm tracks.

• Observations: Actual atmospheric behavior is 3D and variable over time; it is essential to utilize satellite imagery and computer modeling to better understand this complexity.

Instability and Weather Systems

• Critical speed in zonal flow: The strong zonal flow in mid-latitudes becomes unstable when critical meridional temperature gradients are met, leading to significant weather changes.

• Resulting phenomena include:

  • Development of waves that grow into meanders, forming closed eddies (termed baroclinic instability) which enhance storm intensity and frequency.

Heat Transport Mechanisms

• Eddies in the jet stream are crucial for heat transport.

  • Waves and eddies: Move warm air poleward, directly influencing seasonal weather patterns, and cold air equatorward.

  • This lateral heat transport is key to maintaining balance in temperature gradients, as well as contributing to total atmospheric circulation.

  • Eddies also carry zonal momentum, redistributing it poleward and impacting marine ecosystems.

Climate Zones and Features

• Near Equatorial Regions: Characterized by convergence of trade winds, leading to high rainfall;

  • Formation of tropical rainforests, rich in biodiversity and vital for global oxygen and carbon cycles.

• Subtropics (20° - 30°):

  • Features the descending branch of Hadley circulation, characterized by dry and hot conditions that lead to desert belts, where unique ecosystems have adapted to limited water availability.

• Mid-latitudes (>30°): Dominated by westerly winds that create a dynamic atmosphere;

  • Variable weather influenced by eddies with calm conditions in anticyclones and stormy conditions in cyclones, impacting agriculture and human settlements.

Seasonal & Zonal Variations

• Seasonal variations significantly alter weather patterns, leading to phenomena such as monsoons, hurricanes, and polar vortex events.

• Zonal asymmetry (land-sea contrast): Affects wind patterns, sea-level pressure, and temperature, leading to significant local climate variations.

Storm Tracks

• Identified as belts of the strongest eddy activity in mid-latitudes, which guide the movement of low and high-pressure systems across continents.

• Includes storm track dynamics observed over decades, revealing patterns that contribute to forecasting and understanding climate change effects.

Additional Notes

• Important to consider the atmospheric properties on a zonally-uniform rotating planet subject to zonally-uniform differential heating for a comprehensive understanding of circulation patterns and climate zones, and to anticipate future shifts due to climate change.