This study investigates the influence of the Southern Annular Mode (SAM) on chlorophyll-a (Chl-a) concentrations and the underlying mechanisms governing their associated environmental variations in the mid-latitude (35–50° S) ocean from 1998 to 2021. The intensification of westerly winds during positive SAM phases influences meridional water transport and mixed layer depth (MLD), which are both critical factors that affect surface nutrient availability. A marked contrast in the relationship between the meridional current anomaly and the SAM was observed, with reduced northward transport of nutrient-rich water in regions north of 50° S during positive SAM phases. This reduction could be attributed to the poleward migration of the westerly winds, which impeded the meridional current from reaching the mid-latitudes. The relationship between SAM and MLD south of 50° S was positive whereas that in the mid-latitude eastern (60–110° E) South Indian Ocean and eastern (90–140° W) South Pacific Ocean was negative or weak. The immediate effect of a more positive SAM on Chl-a in the mid-latitude ocean was reduced productivity caused by enhanced nutrient depletion. However, in the mid-latitude eastern South Pacific Ocean, the northward migration of the zonal mean meridional current anomaly closely aligned with the lagged correlation pattern between SAM variability and Chl-a over time, suggesting that the delayed northward transport of nutrient-rich waters may partially counterbalance the immediate effects of the SAM on ocean productivity. This mechanism was not present in the mid-latitude eastern South Indian Ocean, implying that future climate change may variably affect these regions. Our findings emphasize the importance of considering regional differences and temporal lags when evaluating the influence of SAM variability on ocean productivity and nutrient dynamics in the context of climate change.

中文翻译：

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南环模对南太平洋和印度洋中纬度地区叶绿素a的滞后影响

本研究调查了南环模式 (SAM) 对叶绿素的影响A（叶绿素-A）浓度和控制1998年至2021年中纬度（南35-50°）海洋相关环境变化的基本机制。正SAM阶段西风的加强影响经向水输送和混合层深度（MLD），这都是影响表面养分有效性的关键因素。观察到经向水流异常与 SAM 之间的关系存在显着反差，在 SAM 正相位期间，南纬 50°以北地区富含营养物质的水向北输送减少。这种减少可能是由于西风向极地迁移，阻碍了经向流到达中纬度地区。南纬50°以南地区SAM与MLD呈正相关，而中纬度东部（60-110°E）南印度洋和东部（90-140°W）南太平洋呈负相关或弱相关。更积极的 SAM 对 Chl- 的直接影响A中纬度海洋的生产力因营养物质消耗加剧而下降。然而，在南太平洋东部中纬度地区，纬向平均经向海流异常向北迁移与 SAM 变化和 Chl- 之间的滞后相关模式密切相关。A随着时间的推移，这表明营养丰富的水域向北输送的延迟可能会部分抵消南半球环状模对海洋生产力的直接影响。这种机制在南印度洋东部中纬度地区不存在，这意味着未来的气候变化可能会对这些地区产生不同程度的影响。我们的研究结果强调了在评估气候变化背景下 SAM 变化对海洋生产力和营养动态的影响时考虑区域差异和时间滞后的重要性。