关键词: 厄尔尼诺-南方涛动循环, 海表洋流系统, 热带西太平洋, 海表叶绿素a

Abstract:

The sea surface current system in the tropical western Pacific is not only of great significance to local air-sea interactions but also extends its influence to the global level. However, at present, the understanding of its evolution during hydrological, ecological, climatic and environmental changes, as well as its response mechanism under the influence of the El Niño-Southern Oscillation (ENSO) cycle, is still unclear. To explore this issue in depth, this paper focuses on the changes in the ecological-hydrological-climatic environment of the tropical western Pacific from 2003 to 2023 and analyzes its response to ENSO events. The study finds that there are three high-value regions of sea surface chlorophyll a concentration in the tropical western Pacific: the coastal zone (with an average value of 0.152 μg·L^-1), the Halmahera warm eddy zone (with an average value of 0.130 μg·L^-1), and the source region of the North Equatorial Countercurrent (with an average value of 0.109 μg·L^-1). As important carriers of nutrients, the New Guinea Coastal Current and the Mindanao Current transport nutrient-rich coastal seawater and jointly converge into the eastward-flowing North Equatorial Countercurrent, contributing to the formation of these three high-value regions. At the same time, the cyclonic upwelling at the center of the Mindanao cold eddy lifts the nutrient-rich deep seawater to the surface, making the chlorophyll a concentration in this region (with an average value of 0.059 μg·L^-1) slightly higher than that in the general open ocean (with an average value of 0.048 μg·L^-1). The concentration of sea surface chlorophyll a exhibits significant variations in different stages of ENSO events. During El Niño events, the velocity of the New Guinea Coastal Current increases, which indirectly leads to an overall rise in the concentration of sea surface chlorophyll a; during La Niña events, the velocity of the New Guinea Coastal Current decreases, which indirectly causes an overall decline in the concentration of sea surface chlorophyll a. Based on these findings, this paper proposes two brief response mechanisms linking ecological-hydrological-climatic processes in the tropical western Pacific to the El Niño and La Niña events. (1) During El Niño events, the North Equatorial Countercurrent, the New Guinea Coastal Current, and the upwelling in the Mindanao cold dome all enhance significantly. By accelerating the coastal current and strengthening the upwelling, they create a “nutrient pump” effect, which promotes the upwelling of deep nutrients and ultimately leads to a significant increase in chlorophyll a concentrations in most regions. (2) During La Niña events, the North Equatorial Countercurrent, the New Guinea Coastal Current and the upwelling in the Mindanao cold dome all weaken significantly. By slowing down the flow velocity and inhibiting the upwelling, they diminish the “nutrient pump” effect and ultimately reduce the chlorophyll a concentration in most regions. These results not only highlight the broad prospects of utilizing the ENSO cycle in the research of local and global hydrological and climate changes, as well as the carbon cycle, but also provide important theoretical support for a comprehensive understanding of the Earth system and carry profound scientific significance.

Key words: El Ni?o-Southern Oscillation, sea surface current system, tropical western Pacific, sea surface chlorophyll a concentration