6.7 - Causes of Extreme Weather
The causes and characteristics of El Niño
El Niño is a significant climatic phenomenon that affects the Pacific region every few years. It involves complex interactions between the ocean and atmosphere, leading to unusual weather patterns across vast areas.
Factors contributing to El Niño
- Uncertain origins - The exact causes of El Niño are not fully understood by scientists. Various theories have been suggested, but no definitive explanation exists.
- Sea surface temperature changes - Small increases in sea surface temperatures are closely associated with El Niño events.
- Air-sea interaction - There is a strong connection between atmospheric conditions and ocean currents during El Niño, possibly linked to tropical storms.
- Timing of occurrence - El Niño typically emerges in late December.
Key characteristics of El Niño
- Warmer waters - Unusually warm water appears around northern Peru and Ecuador.
- Wind pattern reversal - Trade winds weaken, stop, or even reverse.
- Regional weather shifts - This reversal leads to drought conditions in Australasia and unseasonal heavy rainfall in South America.
The causes and characteristics of La Niña
La Niña is another climatic event in the Pacific, often following an El Niño episode. It represents an intensification of normal weather patterns, resulting in extreme conditions in certain regions.
Factors contributing to La Niña
- Post-El Niño occurrence - La Niña frequently occurs after an El Niño event.
- Strengthened trade winds - The trade winds become significantly stronger during La Niña.
- Cooling of sea temperatures - Sea surface temperatures around Peru drop to unusually low levels.
Key characteristics of La Niña
- Exaggerated normal conditions - La Niña can be seen as a more exaggerated version of typical Pacific weather patterns.
- Flood risks in Australia - Australia could experience flooding.
- Drought risks in South America - Sea temperatures are unusually cold around Peru.
Comparison of normal weather conditions with El Niño and La Niña events
Understanding the differences between normal weather patterns and those during El Niño and La Niña events is crucial for grasping their impacts.
Normal conditions
Trade winds push warm waters towards the western Pacific near Australia, where warm air rises, cools, and condenses to bring rainfall. In the eastern Pacific, near Peru, air descends creating high pressure. The colder waters are nearer the surface due to warmer water moving westwards, creating favourable conditions for fishing.
El Niño conditions
The weakening or reversal of trade winds causes water that accumulated around Australia to move back towards the eastern Pacific, leading to a sea level rise around Peru. This prevents the cold water rising and reduces fish stocks. An increase in water temperature over Peru leads to rising warm air, low pressure, and more rainfall, increasing the risk of flooding. With descending air over Australia, high pressure dominates, bringing stable, dry conditions, which can lead to droughts.
La Niña conditions
After an El Niño event, La Niña might happen. La Niña can be described as a more exaggerated version of a normal year. While Australia might experience droughts during El Niño, they could experience flooding with La Niña. Sea temperatures are unusually cold around Peru.
Impacts of extreme weather on different regions
El Niño and La Niña events have profound effects on weather patterns, economies, and ecosystems in the Pacific region, with contrasting impacts depending on the event and location.
Effects of El Niño on regional environments
- Australasia - Descending dry air brings warm, dry weather and the possibility of droughts.
- South America - Rising warm, moist air brings rainfall and the possibility of floods. The cessation of cold upwelling reduces fish stocks.
Effects of La Niña on regional environments
- Australasia - Extreme version of normal conditions that could result in flooding.
- South America - Extreme version of the normal conditions that could result in droughts.