The Indian Ocean Dipole (IOD) is a basin-wide ocean–atmosphere phenomenon that has profound impacts on the global climate, land, and ocean. The Dipole Mode Index (DMI), which is defined as the difference of the sea surface temperature (SST) anomaly in the east IOD zone and the west IOD zone has long been used to characterize and quantify the strength of IOD events. In this study, we propose biological dipole mode indices (BDMIs) based on the dipolar observations of chlorophyll-a (Chl-a) anomalies (difference and relative difference) in the east and west IOD zones during the IOD event. The two BDMIs, which are based on Chl-a difference and relative difference, not only represent the dipolar biological activities in the Equatorial Indian Ocean, but also reflect the thermocline dynamics in the east IOD zone and west IOD zone. The in situ measurements in the east and west IOD zones show the clear linkage between the BDMIs and the dynamics of the 20°C isothermal depth. This linkage is attributed to the changes of the nutrient supplies driven by the various ocean physical processes in the IOD event, thus the BDMIs could also act as the surrogate for the thermocline dynamics in the two IOD zones. The BDMIs from satellite ocean color observations show that they can identify and characterize all the major IOD events in the last two and half decades. The SST-based DMI and Chl-a-based BDMIs may depict some different aspects of the IOD events (e.g., surface versus subsurface properties). The performance comparison between the two BDMIs and DMI also shows that the BDMIs and traditional DMI can effectively detect IOD signal for the major IOD events. Indeed, the BDMIs and DMI are complementary for characterizing the IOD events, and the combination of these indices can provide a better understanding of the atmosphere and ocean processes for both surface and subsurface, as well as biological processes in the Equatorial Indian Ocean.

印度洋偶极子（IOD）是一种流域范围内的海洋-大气现象，对全球气候、陆地和海洋具有深远的影响。偶极模式指数（DMI），定义为东 IOD 区和西 IOD 区海面温度（SST）异常的差异，长期以来一直用于表征和量化 IOD 事件的强度。在本研究中，我们根据 IOD 事件期间东西部 IOD 区叶绿素 a (Chl-a) 异常（差异和相对差异）的偶极观测，提出了生物偶极模式指数 (BDMI)。两种基于Chl-a差异和相对差异的BDMI不仅代表了赤道印度洋的偶极生物活动，而且反映了东IOD带和西IOD带的温跃层动态。东部和西部 IOD 区域的原位测量显示 BDMI 与 20°C 等温深度的动态之间存在明显的联系。这种联系归因于IOD事件中各种海洋物理过程驱动的养分供应的变化，因此BDMIs也可以作为两个IOD区域温跃层动力学的替代物。来自卫星海洋颜色观测的 BDMI 表明，它们可以识别和表征过去两年半的所有主要 IOD 事件。基于 SST 的 DMI 和基于 Chl-a 的 BDMI 可以描述 IOD 事件的一些不同方面（例如，地表与地下特性）。两种 BDMI 和 DMI 的性能比较也表明，BDMI 和传统 DMI 可以有效地检测 IOD 信号的主要 IOD 事件。的确，