Sea surface temperatures are one of the biggest drivers of global weather patterns. They influence two extreme phases of a naturally occurring climate cycle: El Niño: Southern Oscillation (ENSO) and La Niña. Both phenomena are linked to large-scale variations in sea surface temperatures across the eastern tropical Pacific Ocean, but their effects ripple worldwide, shaping rainfall, droughts, monsoons, and even fisheries.
To fully understand these climate cycles, it’s important to first look at two key concepts: The Intertropical Convergence Zone (ITCZ) and the Southern Oscillation.
What is the Intertropical Convergence Zone (ITCZ)?
The ITCZ is a low-pressure belt near the equator where trade winds from both hemispheres converge, forcing warm, moist air upward. This ascending air leads to cloud formation and heavy rainfall.
- In July, the ITCZ shifts northward around 20°N–25°N (near the Gangetic plain), forming the monsoon trough over India. This trough helps generate thermal lows over northern India, drawing in moisture-laden winds. Due to this shift, southern hemisphere trade winds cross the equator, deflect under the Coriolis force, and turn into the southwest monsoon.
- In winter, the ITCZ moves southward, reversing the wind flow and giving rise to the northeast monsoon.
Thus, the ITCZ plays a critical role in shaping Asia’s seasonal monsoon systems.
What is the Southern Oscillation?
The Southern Oscillation refers to fluctuations in atmospheric pressure over the tropical Indo-Pacific region. It is the atmospheric counterpart of ENSO and can be measured by comparing pressure differences between Tahiti (Pacific Ocean) and Darwin (Australia).
When atmospheric pressure patterns shift, the trade winds and equatorial currents respond, altering sea surface temperatures. This interplay creates the foundation for El Niño and La Niña events.
What is El Niño?
The term El Niño, meaning “the Christ Child”, was coined by Peruvian fishermen who noticed a warm ocean current appearing near Christmas. It occurs every 3–7 years, disrupting normal climate conditions.
Key features of El Niño:
- Warm surface currents replace the cold Humboldt (Peruvian) Current.
- Sea surface temperatures along the Peruvian coast rise by up to 10°C.
- Distortion of atmospheric circulation and irregular evaporation.
- Decline in plankton growth, reducing fish populations.
Globally, El Niño is associated with floods, droughts, disrupted monsoons, and extreme weather.
What is La Niña?
La Niña, or “the little girl”, is the cool counterpart of El Niño. It is marked by cooler-than-average sea surface temperatures in the central and eastern Pacific Ocean.
Impacts of La Niña:
- Strengthened easterly trade winds push warm water westward.
- Cold water upwells near South America’s coast.
- Weather patterns often show the opposite effects of El Niño, such as drought in North America and enhanced monsoons in Asia.
For example, the 1988 La Niña triggered severe drought conditions across North America.
Causes of El Niño and La Niña
Both events are driven by changes in easterly trade winds:
- El Niño: Trade winds weaken or reverse, allowing warm water to spread eastward across the Pacific.
- La Niña: Trade winds intensify, pushing warm water west and enhancing cold water upwelling in the east.
This cycle alters global ocean–atmosphere interactions, leading to weather extremes worldwide.
Global Weather Effects of El Niño and La Niña
- La Niña winters: Drier and warmer conditions in the southern U.S., wetter conditions in the Pacific Northwest, and short cold spells in the Northeast.
- El Niño winters: Wetter conditions from California to the Southeast, stormy winters in the Northeast, and milder conditions in the Northwest.
Beyond North America, ENSO events also impact Asia’s monsoons, rainfall patterns in Australia, and agricultural cycles in Peru.
El Niño and Indian Monsoons
The Indian monsoon is closely tied to ENSO. Normally, high pressure in the eastern Pacific corresponds with low pressure in the Indian Ocean, creating favourable monsoon conditions. However, during El Niño years, this balance reverses:
- The eastern Pacific develops low pressure.
- The Indian Ocean experiences high pressure.
- The result: weakened or delayed Indian monsoons, often leading to drought.
Meteorologists track the Southern Oscillation Index (SOI), the pressure difference between Tahiti and Darwin, to predict monsoon intensity. Negative SOI values often signal poor monsoon performance.
ENSO and Climate Change: Are They Linked?
So far, scientists believe El Niño and La Niña are natural cycles, not direct outcomes of climate change. However, global warming may intensify their effects, making droughts more severe, floods stronger, and weather patterns more unpredictable.
Conclusion
El Niño and La Niña are not just Pacific Ocean events; they are global climate influencers. From disrupting fisheries in Peru to altering Indian monsoons and triggering droughts in North America, ENSO events remind us of the delicate balance between oceans and atmosphere.
Understanding these cycles is crucial for weather forecasting, agriculture, disaster preparedness, and climate research in our rapidly changing world.
For More Readings: BIO-GEOGRAPHICAL CLASSIFICATION OF THE WORLD | ECOREGIONS