In summer, the Madden-Julian Oscillation (MJO) greatly influences the intraseasonal variability of the South China Sea (SCS). Previous studies have revealed MJO effects on surface chlorophyll concentration, but the impact of the MJO on the ecosystem's structure and functionality remains unexplored. Here, we investigated the marine ecosystem response to the MJO by analyzing phytoplankton pigment data collected in cruises during from 2010 to 2014. The results indicated the strong influence of the MJO on the structure of phytoplankton size classes (PSCs) in the upper 50 m of the SCS basin. To further explore the ecosystem's response to MJO, we utilized a well-calibrated physical-biogeochemical model (ROMS-CoSiNE) of the SCS to conduct numerical experiments with and without MJO forcings. Our model demonstrated that MJO-induced deep mixing and upwelling increased nutrients in the upper layer, increasing the Chl concentration with a higher proportion of nanophytoplankton (15%) and a lower proportion of picophytoplankton (−20%). Moreover, The MJO-forced model experiment exhibited a substantial enhancement in primary production (56%) and export production (23%), resulting in a notable decrease in the e-ratio. This reduction in the e-ratio cannot be attributed to changes in PSCs but can be explained by the time lag between primary and export production. This lag was prolonged by the physical processes of upwelling and mixing, which slows down the particle sinking. Our results emphasize the important role of MJO in regulating the ecosystem at intraseasonal scale, thus improving our comprehension of the nonsteady dynamics of ecosystems in the SCS.
夏季麦登-朱利安振荡（Madden-Julian Oscillation, MJO）对南海具有一定的影响，以往的研究揭示了MJO对海表叶绿素的影响，但MJO对海洋浮游生态系统结构和功能的影响仍需要进一步研究。本文，首先分析了2010年至2014年期间航次浮游植物色素观测数据，研究了海洋浮游生态系统对MJO的响应。结果表明，南海海盆上层水体（水深<50m）浮游植物粒径结构（Phytoplankton size classes, PSCs）对MJO具有很强的响应。为进一步研究海洋浮游生态系统对MJO的响应，我们利用物理生物地球化学模型（ROMS-CoSiNE）进行了有MJO强迫和无MJO强迫的数值实验。模型结果表明，MJO引起的混合加深和上升流增加了上层的营养供给，提升了叶绿素浓度；纳米浮游植物（Nanophytoplankton）比例较高（15%），微微浮游植物 (picophytoplankton) 比例较低（−20%）。此外，MJO强迫下，初级生产力（56%），碳输出（23%）显著提高，但输出效率显著降低。输出效率的降低不能归因于PSCs的变化，而归因于初级生产和碳输出之间的时间滞后来解释。上升流和混合的物理过程减缓了颗粒下沉，从而延长了这种滞后。本文研究结果强调了MJO在季节内尺度上调节海洋浮游生态系统的重要作用，提高了我们对SCS生态系统非稳态动力学的理解。
 

Madden-Julian Oscillation; physical-biogeochemical model; South China Sea; pelagic ecosystem; biological carbon pump,（MJO，物理生态耦合模型，南海，浮游生态系统，生物碳泵）