ENSO Study Notes
Introduction to ENSO
ENSO stands for El Niño Southern Oscillation.
It is a form of interannual variability.
ENSO is renowned in the context of random variability and is widely recognized as one of the most famous phenomena.
Overview of El Niño and Southern Oscillation
The terms 'El Niño' and 'Southern Oscillation' refer to the same phenomenon from different observational perspectives:
El Niño stems from ocean observations.
Southern Oscillation is based on atmospheric observations.
These two observations are now combined and referred to collectively as ENSO.
ENSO and Extreme Weather
ENSO is linked to extreme weather impacts worldwide.
It relates to climatic phenomena, which can be catastrophic, causing havoc in ecosystems, economies, and communities.
Historical Context of El Niño
In the 17th century, Peruvian fishermen observed warmer ocean waters leading to poor fishing conditions, coining the term 'El Niño' in reference to the Christ child.
It wasn't until the 1960s that meteorologists connected El Niño with the Southern Oscillation, defining ENSO.
ENSO's States
ENSO oscillates between three main states:
Neutral ENSO state
El Niño (warm phase)
La Niña (cool phase)
The state is determined by the interaction of sea surface temperatures in the Pacific Ocean and atmospheric pressure shifts.
Mechanisms of ENSO
In the neutral phase:
Trade winds blow from east to west across the Pacific, pushing warm surface water towards Asia, creating areas of cold upwelling.
The thermocline (layer between warm and cold water) is pushed down in the west and rises in the east, leading to rich marine life off the coast of South America.
Transition to El Niño
El Niño occurs when:
Trade winds weaken, ceasing to push warm water west.
Warm water accumulates in the eastern Pacific, inhibiting upwelling of nutrient-rich cold water.
Sea surface temperature must be at least above average to declare an El Niño.
Effects of El Niño
El Niño leads to:
Drier conditions in parts of North America.
Increased rainfall and flooding in parts of South America, particularly Peru.
Drought in regions such as Central America, Australia, and Indonesia.
Major historical event: The 1997-1998 El Niño caused 23,000 deaths and $60-$180 billion in damages.
Relationship with Global Warming
During El Niño, oceans release more heat, elevating global temperatures.
ENSO is a natural phenomenon, and its occurrence is not directly caused by human-induced climate change, though it contributes to the complexity of climate interactions.
The Role of the Walker Circulation
The Walker Circulation is an atmospheric circulation pattern linked to sea surface temperature anomalies.
It consists of east-west convection that influences trade winds and sea level pressure.
Gilbert Walker correlated monsoon rains in India with sea level pressure differences between Darwin, Australia, and Tahiti.
The Southern Oscillation Index (SOI)
The SOI monitors atmospheric pressure differences between Tahiti and Darwin, showing the oscillations between El Niño and normal conditions.
Changes in sea level pressure correspond with fluctuations in rainfall patterns globally.
The Impacts of ENSO on Ecosystems
The relationship between sea surface temperature (SST) and chlorophyll levels (indicating nutrient availability) is critical for marine life.
ENSO events can dramatically shift fishing yields and marine ecosystems dependent on these nutrient flows.
For example, upwelling zones like off Peru's coast are vital for fish populations, which suffer during warm El Niño phases.
Teleconnections of El Niño
ENSO's impacts extend globally (teleconnections), influencing weather patterns in distant regions.
For instance:
The U.S. Southwest experiences warmer, drier conditions during El Niño years.
El Niño-related rainfall can lead to flooding in East Africa and drying in West Africa.
Monitoring and Predicting ENSO Events
NOAA (National Oceanic and Atmospheric Administration) serves as a key monitoring body for ENSO predictions.
While the average occurrence of El Niño is approximately every four to five years, predictions can be complex due to inherent climatic variability.
Other Climate Oscillations
In addition to ENSO, other climate oscillations include:
North Atlantic Oscillation (NAO): Influences weather patterns between Greenland and Denmark.
Variability is measured by sea level pressure anomalies between Azores and Iceland.
Gilbert Walker also discovered the NAO, linking it to atmospheric changes similar to ENSO.
Conclusions and Implications
Understanding ENSO and its variability is crucial for forecasting weather patterns.
Local industries such as agriculture and fisheries, as well as governmental bodies, rely on accurate predictions for managing resources and mitigating risks associated with climate variability.