11.4 El Niño and La Niña phenomena

El Niño and La Niña are key players in the tropical climate dance. These ocean-atmosphere partners take turns heating up or cooling down the Pacific, shaking up weather patterns worldwide.

These phenomena are crucial to understanding tropical weather systems and hurricanes. They influence storm formation, intensity, and paths, making them essential knowledge for predicting and preparing for extreme weather events.

El Niño and La Niña in ENSO

Defining El Niño and La Niña

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• El Niño manifests as abnormally warm sea surface temperatures in the central and eastern equatorial Pacific Ocean, occurring every 2-7 years
• La Niña exhibits abnormally cool sea surface temperatures in the central and eastern equatorial Pacific Ocean, often following El Niño events
• El Niño-Southern Oscillation (ENSO) cycle encompasses periodic fluctuations between El Niño and La Niña conditions, including the neutral phase between extremes
• ENSO functions as a coupled ocean-atmosphere phenomenon, involving interactions between sea surface temperatures, atmospheric pressure, and wind patterns across the tropical Pacific
• Southern Oscillation component of ENSO represents the seesaw pattern of atmospheric pressure between the eastern and western tropical Pacific
• ENSO events typically peak during the Northern Hemisphere winter and persist for 9-12 months or longer

ENSO Cycle Characteristics

• ENSO cycle involves complex feedback mechanisms (Bjerknes feedback) amplifying initial perturbations in the system
• Ocean-atmosphere coupling during ENSO events creates a self-reinforcing cycle of temperature and pressure changes
• ENSO events can be monitored and quantified using indices (Oceanic Niño Index (ONI), Southern Oscillation Index (SOI))
• ONI measures sea surface temperature anomalies in the Niño 3.4 region of the Pacific
• SOI calculates the difference in atmospheric pressure between Tahiti and Darwin, Australia

ENSO's Atmospheric and Oceanic Conditions

El Niño Conditions

• Trade winds weaken or reverse during El Niño, allowing warm water to spread eastward across the equatorial Pacific
• Thermocline deepens in the eastern Pacific during El Niño events
• Sea surface slope flattens across the Pacific basin in El Niño conditions
• Higher atmospheric pressure develops in the western Pacific and lower pressure in the eastern Pacific
• Walker Circulation shifts eastward, altering global wind patterns and precipitation distribution
• Convection and rainfall increase over the central and eastern Pacific

La Niña Conditions

• Trade winds strengthen during La Niña, pushing warm surface waters westward and enhancing upwelling in the eastern Pacific
• Steeper sea surface slope forms across the Pacific basin in La Niña conditions
• Thermocline becomes shallower in the eastern Pacific during La Niña events
• Lower atmospheric pressure occurs in the western Pacific and higher pressure in the eastern Pacific
• Walker Circulation intensifies, leading to stronger easterly winds along the equator
• Convection and rainfall increase over the western Pacific and Indonesia

Global Impacts of El Niño vs La Niña

Temperature and Precipitation Effects

• El Niño typically leads to warmer than average global temperatures (0.1°C to 0.2°C increase)
• La Niña tends to have a cooling effect on global mean temperatures (0.1°C to 0.2°C decrease)
• ENSO events significantly alter global precipitation patterns
• El Niño often causes drought in Southeast Asia and Australia (20-50% reduction in rainfall)
• El Niño increases rainfall in parts of South America (Peru, Ecuador) (200-400% increase)
• La Niña generally produces opposite precipitation anomalies in these regions
• ENSO influences monsoon systems (Asian and Australian monsoons)
• Teleconnections associated with ENSO lead to climate anomalies in remote regions (altered jet stream patterns affecting North American weather)

Tropical Cyclone Activity

• El Niño suppresses Atlantic hurricane activity due to increased vertical wind shear (20-30% reduction in named storms)
• El Niño enhances cyclone formation in the eastern and central Pacific (50-100% increase in named storms)
• La Niña tends to increase Atlantic hurricane activity (20-40% increase in named storms)
• La Niña suppresses eastern Pacific cyclone formation (20-30% reduction in named storms)
• ENSO affects the spatial distribution of tropical cyclone formation and intensification

Monitoring and Predicting ENSO Events

ENSO Monitoring Systems

• Network of buoys (TAO/TRITON array) measures oceanic and atmospheric parameters across the tropical Pacific
• Satellites (NOAA-20, Suomi NPP) provide sea surface temperature and ocean color data
• Argo floats collect subsurface temperature and salinity data in the global oceans
• Tide gauges measure sea level variations along coastal areas
• Ship-based observations contribute to data collection efforts

ENSO Forecasting and Applications

• Advanced coupled ocean-atmosphere models forecast ENSO conditions up to 6-9 months in advance
• ENSO forecasts crucial for long-range weather prediction, indicating likely temperature and precipitation patterns globally
• Seasonal hurricane forecasts in various ocean basins rely on understanding ENSO's influence on tropical cyclone activity
• ENSO predictions inform decision-making in sectors (agriculture, water resource management, disaster preparedness)
• Economic impacts of ENSO events can be substantial (billions of dollars in agricultural losses or gains)
• Ongoing research aims to improve ENSO prediction skill and understand climate change effects on future ENSO events
• Climate change may alter the frequency, intensity, and impacts of ENSO events in the coming decades