Atmospheric Circulation and Global Weather Systems Dynamics

Mechanisms Influencing Climate: Atmospheric Circulation

Atmospheric Circulation at Height (Jet Streams)

Atmospheric circulation at height involves much more intense wind than at the surface. Strong **westerly winds** circulate in the same direction as the Earth's rotation. These important air flows are called *jet streams* and can be observed in both hemispheres. There are two primary jets or currents:

• The Subtropical Jet Stream
• The Polar Jet Stream

In winter, these currents tend to descend, while in summer they tend to rise. Jet streams have important implications for air navigation; aircraft flying within a stream are more fuel efficient. This is widely utilized by aircraft traveling from America to Europe.

The Role of the Jet Stream

The jet stream moves eastward. This movement causes air swirls downward (subsidence) to the right of the current, and to the left of the current, the air produces an upward movement (ascent), which is responsible for low pressure. The translational motion of the Earth causes the displacement of the jet stream.

Due to these displacements, temperate zones are characterized by having distinct seasons: summer, winter, spring, and fall. The Iberian Peninsula is closer to tropical latitudes. This situation places the peninsula in an area greatly affected by tropical high pressure. In summer, the peninsula remains under the influence of the tropical anticyclone, especially the Azores High.

Surface Air Circulation

The circulation of air in the atmospheric layer that is in contact with the Earth is divided into zones of highs and lows. These pressure fields are represented by **isobars**.

High Pressure Zones (Anticyclones)

Anticyclones are air masses originating from the effects of the jet stream in the presence of cold air masses. The isobars are organized so that pressure increases towards the nucleus. In an anticyclone, the air tends to descend and warm upon contact with the ground. This produces **stable weather**.

Low Pressure Areas (Depressions)

Depressions are air masses originating from the action of the jet stream in hot air masses. If the air pressure is light, the air tends to rise, cool, moisture condenses, and clouds form, which originates precipitation. This results in **unstable weather**.