2.4E El Nino (HL) Notes

The El Niño Southern Oscillation (ENSO) Cycle

1. The El Niño–Southern Oscillation (ENSO) is a natural climate phenomenon involving periodic fluctuations in sea-surface temperatures (SSTs) and air-pressure patterns across the tropical Pacific Ocean.
2. ENSO consists of two extreme phases: El Niño (warm phase) and La Niña (cool phase), with transitional or neutral conditions between them.
3. ENSO strongly influences global weather, precipitation, and temperature patterns, impacting agriculture, fisheries, and ecosystems worldwide.
4. The frequency and intensity of ENSO events are irregular, typically occurring every 2 to 7 years and lasting 9 to 18 months.

The ENSO Cycle: Three Phases

1. Normal (Neutral) Pacific Conditions

1. Trade Winds (Easterlies) push warm surface water westward toward Indonesia and Australia.
2. Warm water accumulation in the western Pacific leads to:
   1. High sea-surface temperatures (SSTs) (~28 °C).
   2. Rising air and low pressure, causing heavy rainfall and convection.
3. The eastern Pacific (Peru, Ecuador) experiences:
   1. Cooler SSTs due to upwelling of cold, nutrient-rich water.
   2. High pressure and dry conditions.

2. El Niño (Warm Phase)

Development

1. The trade winds weaken or sometimes reverse, disrupting the normal flow of warm surface water.
2. Warm water spreads eastward across the Pacific, raising sea-surface temperatures in the central and eastern Pacific Ocean.
3. The thermocline becomes deeper in the east, which reduces the upwelling of cold, nutrient-rich water.
4. Low pressure develops over the eastern Pacific, while high pressure forms over the western Pacific.
5. The Walker circulation weakens or reverses, shifting rainfall toward the eastern Pacific.

Oceanic and Ecological Effects

1. Reduction in upwelling leads to lower nutrient availability.
2. Phytoplankton populations decline, which reduces fish stocks such as anchoveta near Peru.
3. Seabirds and marine mammals suffer food shortages.
4. Warmer sea water triggers coral bleaching, for example, in the Great Barrier Reef.

Atmospheric and Climatic Effects

1. Heavy rainfall and flooding occur along the western coasts of the Americas.
2. Droughts and wildfires occur in Australia and Indonesia due to lack of rain.
3. Reduced monsoon rains occur in India, affecting agriculture.
4. Warmer winters occur in Canada and the northern USA.
5. Flooding can occur in East Africa as convection zones shift eastward.

3. La Niña (Cold Phase)

Development

1. Trade winds strengthen beyond normal levels.