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.
2. Warm surface water is pushed farther west, causing sea levels near Indonesia and the Philippines to rise by up to one metre.
3. The thermocline rises in the eastern Pacific, promoting strong upwelling of cold, nutrient-rich water near South America.
4. The Walker circulation becomes stronger, with more intense convection over the western Pacific and stronger subsidence over the east.

Oceanic and Ecological Effects

1. Strong upwelling increases nutrient supply, boosting marine productivity.
2. Fish stocks and seabird populations often recover after El Niño years.
3. Cooler water temperatures can reduce coral bleaching events.

Atmospheric and Climatic Effects

1. Heavy rainfall and flooding occur in Australia and Southeast Asia.
2. Drought conditions develop along the west coast of South America.
3. Cooler winters occur in Canada and northern USA, while southern USA experiences warmer, drier winters.
4. More hurricanes tend to form in the Atlantic, while the Pacific experiences fewer.

Walker Circulation and Pressure Reversals

1. Under normal conditions, air rises over warm water in the western Pacific and sinks over cold water in the eastern Pacific, completing the Walker Circulation.
2. El Niño weakens or reverses this pattern, shifting rainfall eastward.
3. La Niña strengthens it, intensifying rainfall in the west and drying the east.

Global and Ecological Significance

1. ENSO redistributes heat and moisture globally, acting as a major driver of inter-annual climate variability.
2. It influences agriculture, fisheries, ecosystem dynamics, and public health worldwide.
3. Creates teleconnections, meaning distant regions experience related weather anomalies.
4. Affects the global carbon balance, as warm phases reduce CO₂ absorption by oceans.

Human and Economic Impacts

1. Agriculture

1. El Niño can produce widespread drought, reducing harvests.
2. La Niña can produce floods that damage crops and soils.

2. Fisheries

1. El Niño reduces fish stocks by limiting nutrients.
2. La Niña supports strong fish recovery.

3. Health

1. Wetter conditions during La Niña increase diseases such as malaria and dengue.
2. Drier conditions during El Niño intensify wildfire smoke and respiratory illnesses.

4. Economy

1. Global losses from severe ENSO events can exceed tens of billions of USD.
2. Supply chains, insurance systems, and tourism sectors are disrupted.