How El Niño and La Niña cause extreme weather

Current and Recent Extreme Weather Context

• Western New York experienced record-breaking snowfall measured in feet rather than inches.
• Recent global events included record-breaking rainfall and the UK's highest recorded temperature.
• Scientists forecast the potential development of a "Godzilla El Niño," potentially the strongest on record.

Nature and History of ENSO

• ENSO (El Niño Southern Oscillation) comprises three states: neutral, El Niño (warm), and La Niña (cool).
• Peruvian fishermen first identified El Niño ("the boy") in the 17th century due to unusually warm Pacific waters appearing around Christmas.
• The term ENSO was coined in the 1960s after linking surface ocean temperatures with the Southern Oscillation atmospheric pressure changes between Darwin, Australia, and Tahiti.
• ENSO is driven by interactions between Pacific Ocean sea surface temperatures and atmospheric pressure, specifically involving trade winds and the thermocline depth.

El Niño Mechanics and Impacts

• El Niño occurs when trade winds weaken or reverse, allowing warm water to remain in the eastern and central Pacific.
• The thermocline flattens or deepens in the east, suppressing upwelling and reducing nutrient-rich cold water flow.
• Definition criteria: Sea surface temperatures in the Niño Region 3.4 must be at least 0.5°C above average.
• Historical Benchmark (1997-1998):
  • Triggered devastating droughts in Indonesia, Malaysia, and the Philippines.
  • Caused heavy rains and floods in Peru.
  • Resulted in approximately 23,000 global deaths.
  • Generated economic damages estimated between $60 billion and $180 billion.
  • Contributed to record global temperatures as the oceans released stored heat.
• Typical Weather Patterns During El Niño:
  • Northern US and Canada experience drier, warmer conditions.
  • Central America and parts of Australia/Asia face drought and increased fire risk.
  • Western coast of the Americas sees increased intensity of tropical storms.
  • East Africa typically receives heavy rainfall and potential flooding; West Africa tends toward drier conditions.

La Niña Mechanics and Impacts

• La Niña occurs when trade winds strengthen, pushing warm water further west toward Asia.
• The thermocline becomes shallower in the eastern Pacific, intensifying upwelling and increasing fish populations (e.g., anchovies off Peru).
• Definition criteria: Sea surface temperatures in Niño Region 3.4 must be at least 0.5°C below average.
• Recent Trends:
  • By early 2023, La Niña persisted for three consecutive northern hemisphere winters, a duration unusual for the typical 9–12 month cycle.
  • Despite La Niña's global cooling effect, Earth continued to hit near-record temperatures due to broader global warming.
• Notable Disasters Linked to La Niña:
  • East Africa: Severe drought threatening food security for tens of millions.
  • Pakistan (2022): Severe flooding affecting 33 million people, causing nearly 2,000 deaths, and economic losses exceeding $30 billion (9% of GDP).
  • Hurricanes: Increased frequency on the East Coast of the Americas.
• The 2022 Pakistan floods were exacerbated by the "double whammy" of La Niña and glacier melting from climate change.

Interaction with Climate Change

• Since 1950, ENSO phases have become stronger, leading to more significant sea surface temperature changes.
• Historical environmental records show ENSO variability spanning tens of thousands of years, making it difficult to definitively attribute recent 50-year changes solely to human-caused climate change.
• Climate Model Consensus:
  • Models disagree on whether ENSO-related sea surface temperatures will increase or decrease over the next century.
  • Models agree that global heating will increase rainfall variability in the East-Central Tropical Pacific.
  • Projected outcome: Even heavier rains during El Niño and drier conditions during La Niña in this region.

Forward-Looking Statements and Recommendations

• ENSO is expected to remain one of the most powerful forces influencing Earth's climate.
• The world faces a risk of even higher global temperatures if a major warming phase follows the current cooling La Niña.
• A critical recommendation is to advance resources to vulnerable tropical regions (which house over one-third of the global population) ahead of predicted weather patterns.
• While ENSO is somewhat predictable, its combined effects with global warming create a complex and difficult-to-forecast disaster landscape.