1.4 - El Niño & La Niña
Normal conditions in the Pacific Ocean
The Pacific Ocean typically exhibits a consistent pattern of air and water currents that create distinct weather conditions across its eastern and western regions. These normal conditions serve as the baseline from which deviations like El Niño and La Niña occur.
Key features of normal Pacific Ocean conditions
- Pressure distribution - Low pressure dominates the western Pacific, while high pressure prevails in the eastern Pacific.
- Weather patterns under low pressure - In the west, rising air leads to frequent rain and thunderstorms.
- Weather patterns under high pressure - In the east, sinking air results in clear skies and dry weather.
- Wind movements - Trade winds blow from east to west across the surface, while winds high in the atmosphere move from west to east.
- Ocean currents - Warm surface water and strong currents are found in the western Pacific, while cold water rises to the surface in the eastern Pacific.
Characteristics and impacts of El Niño events
El Niño events represent a significant disruption to the normal Pacific Ocean patterns, caused by a weakening or reversal of typical air and ocean currents. This shift leads to dramatic weather changes in surrounding regions.
Changes during El Niño events
- Pressure reversal - Pressure increases in the western Pacific and decreases in the eastern Pacific.
- Wind shifts - Trade winds, which usually blow from east to west, either weaken significantly or reverse to blow from west to east. Winds high in the atmosphere also shift to blow westward.
- Ocean current alterations - Warm surface water currents in the western Pacific weaken or flow in the opposite direction, and the usual rising of cold water in the eastern Pacific stops.
- Weather impacts in the western Pacific - High pressure causes sinking air, leading to clear skies and unusually dry conditions, often resulting in droughts in coastal areas of Southeast Asia.
- Weather impacts in the eastern Pacific - Low pressure causes rising air, bringing heavy rain, thunderstorms, and severe flooding to typically arid coastal regions of South America.
Characteristics and impacts of La Niña events
La Niña events occur when the normal conditions in the Pacific Ocean become more extreme, intensifying the usual patterns of air and water movement. This results in amplified weather effects across the region.
Changes during La Niña events
- Intensified trade winds - Trade winds blow westward with greater strength than under normal conditions.
- Enhanced ocean currents - More cold water rises to the surface in the eastern Pacific compared to typical conditions.
- Weather impacts in the western Pacific - The intensified low pressure leads to increased heavy rainfall and flooding in western regions.
- Weather impacts in the eastern Pacific - High pressure becomes more pronounced, causing reduced rainfall and drought conditions in eastern areas.
Frequency and duration of El Niño and La Niña events
Both El Niño and La Niña events occur irregularly but have recognisable patterns in terms of how often they happen and how long they last.
Occurrence patterns of El Niño and La Niña
- El Niño - These events typically take place every 3 to 7 years and persist for approximately 9 to 12 months.
- La Niña - These events occur at intervals ranging from every 2 to 7 years, often following an El Niño event, though their duration can vary widely.