The Indian Ocean Dipole (IOD) is the predominant interannual climate mode and is critical in regulating the biogeochemical cycles of the equatorial Indian Ocean (EIO). However, the dynamics of nutrient supply and the magnitude of biological responses are less understood. Here, by comparing the biophysical in situ observations across the eastern EIO during the decaying period of a positive IOD in 2019 and neutral condition in 2017, we identify that the shoaling thermocline, initiated by reinforced easterly wind, lifts the nutricline into the euphotic layer under positive IOD condition. Coincidently, the strong turbulent mixing induced by the shear instability between the opposite-flowing surface current and subsurface Equatorial Undercurrent increases the upward turbulent nutrient flux into the euphotic zone. Their combined effect triggers a vigorous subsurface biological response over the eastern EIO, with approximately twofold higher integrated chlorophyll-a contents in the water column during a positive IOD than under neutral condition.

中文翻译：

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2019 年极端印度洋偶极子衰变阶段的强大地下生物响应

印度洋偶极子 (IOD) 是主要的年际气候模式，对调节赤道印度洋 (EIO) 的生物地球化学循环至关重要。然而，人们对养分供应的动态和生物反应的幅度知之甚少。在这里，通过比较 2019 年正 IOD 衰减期和 2017 年中性条件期间 EIO 东部的生物物理原位观测，我们确定由加强的东风引发的浅滩温跃层将 nutricline 提升到下面的透光层阳性 IOD 条件。巧合的是，由反向流动的表面电流和地下赤道暗流之间的剪切不稳定性引起的强烈湍流混合增加了进入透光区的向上湍流养分通量。a在正 IOD 期间水柱中的含量比在中性条件下。