The El Niño-Southern Oscillation (ENSO) is a recurring climate phenomenon that occurs every 2 to 8 years, originating in the tropical Pacific Ocean. It comprises two distinct phases: El Niño, marked by warming of the eastern tropical Pacific’s surface waters, and La Niña, characterized by cooling. These phases result from intricate interactions between the ocean and atmosphere, leaving a significant global impact lasting over a year.
Coastal regions of the Indian Ocean are particularly vulnerable to ENSO’s effects. El Niño often brings droughts and occasional wildfires, while La Niña leads to increased rainfall and flooding. This review delves into the physical processes underlying ENSO and examines its implications for the Indian Ocean’s local climate and socio-economic conditions. Scientists agree that ENSO fluctuations will continue to influence global climate for decades and beyond. Therefore, understanding and predicting ENSO is critical for informed infrastructure planning, policymaking, and trade decisions, benefitting both scientists and the general public.
On local weather
The weather and climate of the Indian Ocean region, particularly in the tropics, are strongly influenced by changes in oceanic and atmospheric circulation, with ENSO events being the most significant driver of year-to-year variability. In tropical areas, these effects are immediately observable, while in extra-tropical regions, they manifest through teleconnections. In India, ENSO, particularly El Niño, has been linked to drought-like conditions occurring every 5 to 8 years. These events often lead to monsoon rainfall falling below 90% of the long-term average, with high temperatures persisting for up to a year.
El Niño typically follows three years of La Niña, which is associated with rainfall deficits and drought conditions in India and neighboring regions. Given that half of India’s population relies on agriculture, the country is particularly vulnerable to these climatic impacts. To mitigate the effects of ENSO-induced droughts, farmers and policymakers must adopt adaptive strategies such as improving irrigation infrastructure, diversifying crop choices, and implementing drought-resistant farming techniques. These proactive measures are essential to safeguard agricultural livelihoods and ensure food security in the face of this recurring climate challenge.
On health
ENSO events have significant health implications in regions near the Indian Ocean, with food shortages, famine, forest fires, and outbreaks of vector-borne diseases like malaria, dengue, and rodent-borne illnesses being prominent concerns. The intensity of mosquito- and rodent-transmitted diseases often increases following extreme weather events caused by ENSO. In Gujarat, India, cities like Ahmedabad and Surat exhibit contrasting health outcomes related to enteric fever (EF) during El Niño and La Niña events. For instance, strong El Niño events increase EF risk by 64% in Ahmedabad but decrease it by 25% in Surat. Conversely, strong La Niña events reduce EF risk in Ahmedabad by 31% but raise it by 21% in Surat during moderate La Niña events. Additionally, extreme precipitation linked to ENSO is associated with a 5% monthly rise in EF cases in Ahmedabad.
The variability in disease risk highlights how regional factors, such as local vector species’ ecological responses to changing rainfall patterns, influence health outcomes. In Indonesia’s West Kalimantan, ENSO-induced droughts have led to rising health problems in small villages, threatening local livelihoods. These findings underscore the need for localized approaches to health management and mitigation strategies, as ENSO impacts vary widely across regions and ecosystems.
On marine life
One significant impact of El Niño is its effect on upwelling in coastal regions, reducing the supply of nutrient-rich cold water to the surface. This disruption diminishes biological productivity and adversely affects fisheries. In the Indian Ocean, El Niño events can notably influence tuna populations and fisheries, critical components of the region’s blue economy. With a coastline of 7,516 kilometers and an Exclusive Economic Zone (EEZ) spanning 2.02 million square kilometers, India relies heavily on its coastal and marine resources. The fisheries sector supports millions of livelihoods, providing approximately 13 million jobs in 2017, with women comprising 32% of the workforce.
Fish production in India reached 11.6 million tonnes in 2017, making the country one of the top producers of fisheries and aquaculture globally. El Niño’s impact on fish species varies: in the eastern Pacific, anchovies and other small pelagic fish are significantly affected, while larger tropical pelagic species like tuna become easier to catch near the coast. Tuna distribution and abundance are closely tied to oceanic conditions, including sea surface temperature variability, sunlight, dissolved oxygen levels, wind velocity, and upwelling.
Climate change further complicates these dynamics, altering marine ecosystems and impacting fish recruitment, migration, abundance, and predator-prey relationships. Observing El Niño events provides valuable insights into how microhabitat changes influence tuna distribution. This knowledge can help refine management strategies for sustainable tuna fishing and ensure the resilience of marine resources in the face of climate change.