New Recording 4

Introduction to Atmospheric Circulation

Atmospheric circulation refers to the large-scale movement of air across the globe, which plays a crucial role in weather patterns and climate. This note breaks down the fundamental concepts and details of atmospheric circulation for better understanding, especially for someone new to the subject.

Overview of Topics:

1. Driving forces in the atmosphere

2. Importance of the Intertropical Convergence Zone (ITCZ)

3. Major winds and circulation patterns

Global Atmospheric Circulation

Global circulation is caused primarily by differential heating by the Sun, meaning that different parts of the Earth receive varying amounts of sunlight. Let’s dive into the key aspects:

The Sun as the Main Heat Source

The Sun is essential as it provides energy that warms the Earth. This warmth is not evenly distributed:

• Radiation Absorption: Clouds, gases, and the surface of the Earth absorb sunlight, but some of this energy is lost as heat.

• Curvature of the Earth: Due to its round shape, sunlight strikes the equator more directly, meaning it's concentrated in a smaller area. In contrast, at higher latitudes like the poles, sunlight is spread out over a larger area, leading to less warmth.

• Pole vs. Equator: Since the atmosphere is deeper near the poles, more of the sunlight is scattered, leading to reduced heating in those regions.

• Earth’s Tilt: The tilt of Earth’s axis affects how much daylight each area receives, especially in polar regions. This results in long winters with significantly reduced sunlight and increased heat loss.

Impact of Snow and Ice

Snow and ice reflect a lot of the Sun's energy, which is referred to as high albedo (reflectivity). This means that polar regions stay colder as they are less effective at absorbing heat from sunlight.

Thermal Gradient

A thermal gradient between the equator and poles generates a significant difference in temperature. The poles lose more heat than they receive from the Sun, which drives the global atmospheric circulation, acting as a system to redistribute heat to help maintain conditions suitable for life on Earth.

Formation of Atmospheric Cells

There are three major atmospheric circulation cells in each hemisphere related to this heating:

1. Hadley Cells: These are formed due to warm air rising near the equator and cold air sinking around 30 degrees North and South latitude.

2. Ferrel Cells: Located between Hadley and Polar cells, they have varying characteristics as they deal with both warm and cold air.

3. Polar Cells: These are found at the poles and feature cold, descending air.

Driving Forces in Atmospheric Circulation

Atmospheric winds are driven by several key forces:

1. Centripetal Force: This is based on Newton’s First Law, where an object in motion will stay in motion unless acted upon by an outside force. Winds typically move in straight lines unless other forces affect them.

2. Pressure Gradient Force: Wind blows from areas of high atmospheric pressure to those of low pressure. If you imagine a balloon, when you let some air out, air rushes in from surrounding areas of higher pressure.

3. Gravitational Force: This pulls air down toward the Earth and helps stabilize vertical airflow; as a result, air mostly moves horizontally across the surface.

4. Coriolis Force: This results from Earth's rotation. In the Northern Hemisphere, winds are deflected to the right, while in the Southern Hemisphere, they deflect to the left. This effect results in circular wind motions.

5. Friction Force: This force acts at the Earth’s surface, slowing wind down and influencing wind behavior close to the ground.

Intertropical Convergence Zone (ITCZ)

The ITCZ is a significant area situated around the equator defined by low pressure that shifts seasonally. Here’s how it works:

• Seasonal Movement: The ITCZ shifts north and south depending on the season and geographical features.

• Warm Air Rises: With high solar radiation at the equator, warm air rises, creating a low-pressure area leading to the formation of clouds and precipitation.

• Sinking Cold Air: Cold air sinks around 30 degrees North and South, leading to the formation of high-pressure systems that suppress cloud formation.

• Hadley Cells Formation: The process of rising and sinking air defines the structure of Hadley Cells, which are vital to global circulation.

• Historically, the ITCZ has been known as the "doldrums" due to the calm conditions present during certain seasons, making it a critical area for hurricane formation under specific conditions.

Major Wind Patterns

Different winds develop from the atmospheric circulation, categorized as follows:

1. Trade Winds: These winds blow from east to west towards the equator. They are warm and dry.

2. Westerlies: These winds blow from west to east and move towards the polar regions.

3. Polar Easterlies: Here, winds blow from the poles and can create convergence zones around 60 degrees latitude.

4. Horse Latitudes: Found around 30 degrees latitude, characterized by calm winds and significant high pressure, leading historically to maritime travel delays.

Jet Streams

Jet streams are narrow bands of strong winds at high altitudes:

• Polar Jet Stream: Stronger due to significant temperature differences between polar regions and the warmer air below.

• Subtropical Jet Stream: Generally weaker than polar jet streams but still play a role in weather.

• Rossby Waves: These are undulating patterns in the jet stream significant for weather systems.

• Effects of Global Warming: Increased temperatures may create instability in the jet streams, leading to unpredictable weather patterns.

Summary

Understanding atmospheric circulation means grasping the connections among temperature variations, pressure systems, and wind forces. This fundamental knowledge is essential for predicting weather and understanding how Earth's climate dynamics operate.