The 2023 extreme coastal El Niño event in the eastern equatorial Pacific was driven by northerly alongshore winds and westerly wind anomalies, initially linked to a strong Madden-Julian Oscillation and a cyclonic disturbance off Peru. These anomalies suppressed coastal upwelling and enhanced downwelling Kelvin waves, leading to coastal warming. This warming, in turn, intensified the wind anomalies, creating a positive feedback loop. The event caused severe flooding, record dengue outbreaks, and a significant drop in chlorophyll-a concentrations, impacting marine ecosystems. The coastal El Niño peaked in late April with a SST anomaly of +4°C, followed by a decline in May due to seasonal cooling. The event was predictable at one month lead time, offering valuable insights for disaster preparedness. Observations and model experiments revealed that the coastal warming was driven by atmospheric internal variability in the far eastern Pacific, which also amplified the event. The seasonal cooling of background SSTs decoupled the coastal ocean and atmosphere, leading to the decay of the El Niño. The 2023 event exhibited unique features, including unusual atmospheric perturbations and strong subsurface temperature anomalies, which contributed to the rapid growth of a basin-scale El Niño. The coastal Bjerknes feedback played a crucial role in intensifying and sustaining the El Niño, with the feedback active during February-March-April. This feedback was triggered by the seasonal background warming, suppressed upwelling, and the arrival of the first downwelling Kelvin wave. The feedback was active in March and April, leading to strong wind and rainfall anomalies. After May, the feedback became inactive due to background SST cooling, causing the El Niño to decay. Atmospheric responses to coastal warming included deep convection and enhanced wind anomalies, which further amplified the El Niño. The 2023 event was influenced by a strong phase 8 MJO and a rare cyclonic system, "Cyclone Yaku," which contributed to coastal warming by suppressing upwelling and exciting downwelling Kelvin waves. The event also highlighted the importance of internal atmospheric variability in driving coastal El Niño events. The study underscores the need for continued research to understand the mechanisms behind extreme coastal El Niño events, which are rare but have significant impacts on coastal regions. The 2023 event provides valuable insights into the ocean-atmosphere dynamics involved, highlighting the importance of monitoring and predicting such events for disaster preparedness.The 2023 extreme coastal El Niño event in the eastern equatorial Pacific was driven by northerly alongshore winds and westerly wind anomalies, initially linked to a strong Madden-Julian Oscillation and a cyclonic disturbance off Peru. These anomalies suppressed coastal upwelling and enhanced downwelling Kelvin waves, leading to coastal warming. This warming, in turn, intensified the wind anomalies, creating a positive feedback loop. The event caused severe flooding, record dengue outbreaks, and a significant drop in chlorophyll-a concentrations, impacting marine ecosystems. The coastal El Niño peaked in late April with a SST anomaly of +4°C, followed by a decline in May due to seasonal cooling. The event was predictable at one month lead time, offering valuable insights for disaster preparedness. Observations and model experiments revealed that the coastal warming was driven by atmospheric internal variability in the far eastern Pacific, which also amplified the event. The seasonal cooling of background SSTs decoupled the coastal ocean and atmosphere, leading to the decay of the El Niño. The 2023 event exhibited unique features, including unusual atmospheric perturbations and strong subsurface temperature anomalies, which contributed to the rapid growth of a basin-scale El Niño. The coastal Bjerknes feedback played a crucial role in intensifying and sustaining the El Niño, with the feedback active during February-March-April. This feedback was triggered by the seasonal background warming, suppressed upwelling, and the arrival of the first downwelling Kelvin wave. The feedback was active in March and April, leading to strong wind and rainfall anomalies. After May, the feedback became inactive due to background SST cooling, causing the El Niño to decay. Atmospheric responses to coastal warming included deep convection and enhanced wind anomalies, which further amplified the El Niño. The 2023 event was influenced by a strong phase 8 MJO and a rare cyclonic system, "Cyclone Yaku," which contributed to coastal warming by suppressing upwelling and exciting downwelling Kelvin waves. The event also highlighted the importance of internal atmospheric variability in driving coastal El Niño events. The study underscores the need for continued research to understand the mechanisms behind extreme coastal El Niño events, which are rare but have significant impacts on coastal regions. The 2023 event provides valuable insights into the ocean-atmosphere dynamics involved, highlighting the importance of monitoring and predicting such events for disaster preparedness.