The Global Atmospheric Circulation System

The Global Atmospheric Circulation System

Have you ever wondered why the world's dense rainforests cluster around the equator, while vast deserts line up at 30° North and South? This pattern is driven by Global Atmospheric Circulation, the worldwide large-scale movement of air that transports heat from the equator to the poles.

The atmosphere circulates in three distinct loops, or cells, in each hemisphere:

• Hadley cells (0°–30°): Driven by intense insolation at the equator, warm air rises and flows towards the poles, sinking at 30°.
• Ferrel cells (30°–60°): These are friction-driven gears that sit between the Hadley and Polar cells, forcing air to move from 30° towards 60°.
• Polar cells (60°–90°): Cold, dense air sinks at the poles and flows towards 60°, where it meets warmer air and rises.

As this air moves, the Coriolis Effect (caused by the Earth's rotation) deflects the winds to the right in the Northern Hemisphere and left in the Southern Hemisphere. This deflection causes weather systems to spin and creates the Trade Winds, which blow from the subtropics toward the equator.

Climatic Zones and Pressure Belts

Where air rises or sinks, it creates distinct pressure belts that determine global climate zones. Normal atmospheric pressure ranges from intense low pressure at 980 mb to strong high pressure at 1050 mb.

• Equatorial Low (0°): Warm air rises rapidly, creating a Low Pressure System (Depression). As air rises, it undergoes adiabatic cooling, causing water vapour to condense into heavy clouds. This brings high temperatures (25–30°C) and heavy rainfall (>2000 mm/year), creating tropical rainforests.
• Subtropical High (30° N/S): Air sinks, creating a High Pressure System (Anticyclone). Sinking air undergoes adiabatic warming, which prevents cloud formation. This results in arid desert zones (like the Sahara) with rainfall under 250 mm/year and extreme daily temperature ranges due to the lack of insulating cloud cover.
• Temperate Low (60° N/S): Warm air from the Ferrel cell rises over cold air from the Polar cell. This creates low pressure, bringing moderate temperatures and frequent frontal rainfall (e.g., the UK).
• Polar High (90° N/S): Cold, dense air sinks, creating highly stable, dry high-pressure conditions. These form polar deserts with temperatures often dropping below -40°C.

Mechanisms of Extreme Weather

Global circulation can sometimes generate Extreme Weather events, such as tropical storms or droughts.

Tropical storms (like hurricanes or typhoons) require specific conditions to form:

1. They need sea surface temperatures of at least 26.5°C to 27°C and an ocean depth of at least 50–60m to sustain massive evaporation.
2. Crucially, they do NOT form directly on the Equator; they form between 5° and 30° latitude where the Coriolis effect is strong enough to induce rotation.
3. As warm, moist air rises via convection, it cools and condenses, releasing latent heat.
4. This acts as "fuel", forcing air to rise even faster, lowering the surface pressure (often below 950 mb) and intensifying the storm.

Droughts are often caused by sinking air in high-pressure anticyclones, which blocks cloud formation. During an El Niño event (every 2–7 years), reversed trade winds cause cold water to surface near Australia. The air above this cool water sinks, creating intense, long-lasting high-pressure systems that cause severe droughts.

Contrasting Weather Extremes: UK vs Australia

What counts as an extreme heatwave? A 30°C day is a dangerous extreme in the UK, but it is a perfectly normal summer day in Australia. An event is defined as extreme based on its Weather Anomaly — how far it departs from the long-term average.

In both the UK and Australia, a heatwave is generally considered extreme when temperatures spike 7–10°C above their usual summer maximum.

Comparing Extremes:

• Temperature: The UK's average summer maximum is ~20–23°C, with extremes over 30°C. Australia's average summer maximum is ~30–33°C, with extremes routinely exceeding 40°C.
• Precipitation: The UK averages ~1150 mm/year. An "extremely wet" year exceeds 1210 mm, and an "extremely dry" year is below 950 mm. In contrast, Australia is much drier, averaging ~465 mm/year. A severe drought year drops below 360 mm.
• Wind: UK gales rarely exceed 62 km/h, usually driven by Atlantic depressions. Australia regularly experiences cyclones with winds over 118 km/h, holding a record extreme of 407 km/h.

Worked Example: Calculating a Weather Anomaly

In the exam, you may need to calculate a temperature anomaly using data provided.

Formula:

$$\text{Anomaly} = \text{Current Temperature} - \text{Long-term Average}$$

London's long-term average July maximum is 23°C. During a recent heatwave, temperatures reached 31°C. Calculate the temperature anomaly.

• Step 1: Identify the current temperature and the long-term average. Current = 31°C, Average = 23°C

• Step 2: Substitute the values into the formula.

$$\text{Anomaly} = 31 - 23$$

• Step 3: Calculate the final answer with units and the correct sign (+/-). Answer: +8°C

Extreme Weather Case Studies

OCR Geography requires you to know two contrasting extreme weather case studies (one UK, one non-UK).

• UK Case Study (Heatwaves): Persistent, blocking high-pressure anticyclones caused severe UK heatwaves in 2003, 2018, 2019, and 2022. The 2022 event shattered records, reaching an extreme of 40.3°C.
• Non-UK Case Study (Australia's "Big Dry"): From 2002–2009, an El Niño event caused severe drought. Rainfall dropped 40–60% below average, and temperatures remained above 40°C for weeks in some regions.
• Alternative Non-UK Case Study (Typhoon Haiyan): In 2013, the Philippines was hit by a devastating tropical storm. Extremely low atmospheric pressure (895 mb) generated sustained winds of 315 km/h.