The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Satellite observations revealed two extremely low surface chlorophyll concentration (SCC) events with a warm sea surface temperature anomaly (SSTA) in the southeastern Arabian Sea (SEAS, 6°–15°N, 72°–77°E) during the summer (July–August–September ) in 2015 and 2019. We find that the physical processes leading to these two similar low SCC events are remarkably different. The low SCC in the SEAS during summer 2019 is mainly related to the weakened upwelling and deepening of the thermocline depth due to the combined effects of the local wind anomaly and the arrival of westward-propagating downwelling coastal Kelvin wave driven by easterly anomalies near the eastern Sri Lanka during an extreme positive Indian Ocean Dipole (IOD) event. In summer 2015, a weaker positive IOD-induced easterly anomaly in the southern Bay of Bengal also drives downwelling coastal Kelvin waves westward, deepening the thermocline in the SEAS. But unlike that in summer 2019, the local wind stress anomalies in the SEAS during summer 2015 favors upwelling, which counteracts the downward motion of the coastal Kelvin waves, leading to weaker downward transport (one-third of that in 2019). Meanwhile, the upper ocean layer in the SEAS experiencs extreme warming during summer owing to the development of 2015/2016 super El Niño. This substantial warming enhances upper oceanic stratification, which results in weaker vertical mixing and reducs the SCC to an extremely low level despite the much weaker IOD strength in 2015.