Abstract: The sea surface current system in the tropical western Pacific has an important impact on local and even global air-sea interactions. However, its variation process under hydrological and ecological conditions and the response mechanism affected by the El Niño-Southern Oscillation (ENSO) cycle remain unclear. To deeply explore this issue, this paper focuses on the changes in the ecological-hydrological-climate environment of the tropical western Pacific from 2003 to 2023, and analyzes its response relationship to ENSO events. The study found that there are three significant high-value areas of sea surface chlorophyll a concentration in the study area, namely the coastal area (0.152 μg/L), the Halmahera Eddy area (0.130 μg/L), and the source area of the North Equatorial Countercurrent (0.109 μg/L). As important carriers, the New Guinea Coastal Current and the Mindanao Current transport near-shore seawater rich in nutrients and jointly flow into the eastward North Equatorial Countercurrent, contributing to the formation of these three high-value areas. At the same time, the upwelling in the center of the cyclonic Mindanao Eddy area lifts nutrient-rich deep seawater to the sea surface, making the chlorophyll a concentration in this area higher (0.059 μg/L) than that in general ocean areas (0.048 μg/L). During different stages of ENSO events, the sea surface chlorophyll a concentration showed obvious changes. During El Niño, the velocity of the New Guinea Coastal Current increased, indirectly leading to an overall increase in sea surface chlorophyll a concentration; during La Niña, the velocity of the New Guinea Coastal Current decreased, indirectly causing an overall decrease in sea surface chlorophyll a concentration. Based on this, this paper extracts two brief response mechanisms between the ecological-hydrological-climate process in the tropical western Pacific and El Niño and La Niña events. This not only highlights the broad prospects of the ENSO cycle in the study of local and global hydrological and climate changes and carbon cycle, but also provides important theoretical support for a comprehensive understanding of the earth system, with far-reaching scientific significance.

Key words: ENSO, surface currents, tropical western Pacific, surface Chl-a concentration.