Negative surface chlorophyll concentration anomalies in the Southeast Arabian Sea during 2015 and 2019 summers

Satellite observation revealed two extremely low surface chlorophyll concentration events in the southeast Arabian Sea (SEAS, 6^oN-15^oN, 72^oE-77^oE) during the summer monsoons (June to October) of 2015 and 2019. The results indicate that the physical processes leading to negative SEAS chlorophyll anomalies during the 2015 and 2019 summer monsoons were inconsistent. In the 2019 summer monsoon, the warm SSTA and low chlorophyll in the SEAS are mainly related to the weakened upwelling and deepened thermocline depth due to the combined effects of local wind anomaly and the arrival of westward-propagating downwelling coastal Kelvin wave driven by easterly anomalies near eastern Sri Lanka during extreme positive Indian ocean dipole (IOD) event. Deeper thermocline depth and stronger downward movement during the 2019 summer made it difficult to transport nutrients upward, which in turn led to reduced surface chlorophyll concentrations in the SEAS. Positive IOD-induced easterly anomaly in the southern Bay of Bengal during the 2015 summer drove downwelling coastal Kelvin wave to propagate westward deepening the thermocline in the SEAS. Due to local wind stress anomalies favored upwelling and counteracted the downward motion of the downwelling coastal Kelvin wave. The downward transport (thermocline depth anomaly) in the SEAS during the 2015 summer was only one-third of (half of) that in 2019. Meanwhile, the upper ocean layer in the SEAS experienced extreme warming (the SSTA exceeded +0.8^oC) due to the development of super El Niño in 2015. This significant warming enhanced marine stratification and prevented the subsurface nutrients from reaching the surface, which is unfavorable for the chlorophyll bloom. Deeper thermocline and weaker mixing allowed chlorophyll concentrations to reach extreme negative anomalies despite weaker IOD strength in 2015 than in 2019.