Optimization of the Pacific Regional Equatorial Index for the detection of coastal El Niño in northern Peru

ABSTRACT In 2017, extreme rainfall severely impacted northern Peru, causing floods, infrastructure damage, and human and economic losses. These events, linked to the coastal El Niño (CEN), underscore the need for precise monitoring. This study aimed to enhance early detection of the CEN by optimizing the Pacific Regional Equatorial Index (PREI-V2). Six oceanic indices were assessed, identifying the Trans-Niño Index and Pacific Decadal Oscillation as the most sensitive to precursor conditions. Both were combined using a three-month moving average to reduce temporal variability and highlight early signals. Results show that PREI-V2 outperforms traditional indices, such as Oceanic Niño Index and Niño 1 + 2, by detecting sustained sea surface temperature anomalies earlier and reducing false positives. The optimized PREI-V2 is a robust tool for CEN monitoring, with potential to guide early action and contingency planning. Its integration into national systems is recommended to minimize the socioeconomic impacts on vulnerable coastal regions.