El Niño–Southern Oscillation (ENSO) is a global climate phenomenon driven by variations in winds and sea surface temperatures over the tropical Pacific Ocean, cycling through El Niño (warming), La Niña (cooling), and neutral phases. The core mechanism is the Bjerknes feedback, named after Jacob Bjerknes in 1969, where weaker trade winds trigger a positive feedback loop that alters ocean temperatures and the atmospheric Walker circulation. These phases disrupt global weather patterns, causing intense storms in some regions and droughts in others, while El Niño events produce short-term global temperature spikes and La Niña events cause cooling. The countries most vulnerable to ENSO are developing Pacific-bordering nations that depend on agriculture and fishing. In the context of climate change, the IPCC's 2021 Sixth Assessment Report states it is very likely that precipitation variance related to ENSO will increase, though future trends remain uncertain. Interestingly, the term "El Niño" (Spanish for "The Boy") was first recorded in 1892 by Captain Camilo Carrillo in Lima, referring to the warm current that appeared around Christmas.