Coastal El Niño events off Peru associated with cold ENSO background conditions

The Peruvian upwelling system is one of the most productive coastal marine ecosystems, powered by persistent winds that bring cold, nutrient-rich water to the surface. However, it can be severely disrupted by rapid coastal warming episodes known as Coastal El Niño events, which alter marine ecosystems and regional climate. Although often linked to ENSO, evidence from events like 1925 and 2017 shows that some warmings arise without basin-wide equatorial warming or strong tropical Pacific coupling, prompting the need to assess how common they are and whether they represent a recurring climate mode.

Using an objective, pattern-based method rather than traditional Niño indices, this study identifies coastal warming events defined by an eastern-Pacific warming and central-Pacific cooling dipole while the canonical ENSO mode remains weak. This approach reveals that ENSO-independent coastal warmings are more frequent and diverse than previously thought, typically driven by subtropical atmospheric variability that weakens both far-eastern equatorial trade winds and alongshore coastal winds, reducing upwelling, deepening the nearshore thermocline, and amplifying surface warming; some events may later transition into full El Niño as equatorial feedbacks develop. Overall, coastal warming emerges as a distinct mode of tropical Pacific variability triggered by remote atmospheric forcing and strengthened by local ocean–atmosphere processes.