An Anomalous Decline of the Spring Bloom Chlorophyll Concentration in the Central Pacific is an Early Indicator of El Niño

El Niño Southern Oscillation, a global climate phenomenon, occurs in the tropical Pacific with an anomalous warming phase (El Niño) and a cooling phase (La Niña) spanning for 2–5 years. A substantial decline of Niño3.4 region’s spring bloom chlorophyll-a concentration (Chl-a) is an intrinsic characteristic of the central Pacific during the El Niño onset period, which has been considered as a predictor of the El Niño by earlier studies. This study investigated the concept, a marked decline of spring bloom Chl-a as the predictor of the El Niño, using satellite-measured Chl-a from MODIS Aqua for the period 2003–2023 in relation to the Oceanic Niño Index (ONI) and subsurface properties. It revealed that Chl-a exhibits a significant inverse relationship with ONI during past El Niño events. An intense, continuous decreasing phase of Chl-a during the spring season (February–April) of 2023 may indicate the onset of El Niño. The decline of the spring bloom Chl-a concentration has been explained as the response to the weakening of the upwelling strength in the study domain and associated nutrient depletion in the euphotic zone.