The Seychelles-Chagos Thermocline Ridge (SCTR) is a biologically important region of open ocean upwelling within the south west Indian Ocean (5–10°S and 45–90°E), driven by the tropical gyre. The SCTR refers to an elongated feature that joins two local minima in thermocline depth; the Seychelles Dome (SD) and Chagos Dome (CD). Entering the ocean basin from the east, the Indonesian Throughflow (ITF) has been shown to interact with the upwelling region, although the relationship between the phytoplankton bloom associated with the SCTR and the ITF are so far unexplored. Using in situ observations and remotely sensed data, the buoyancy fluxes from the ITF are shown to strongly condition surface chlorophyll-a (chl-a) concentrations over the Chagos Dome, the eastern extreme of the SCTR, at seasonal and interannual scales. Accordingly, we find a significant inverse correlation (r = −0.43) between the altimeter-derived volume transport of the ITF and the surface chl-a concentrations. This inverse correlation increases (r = −0.61) when only the 10th and 90th percentile of the ITF volume transport anomalies are considered, indicating the influence of the ITF may be overcoming other physical drivers, especially under extreme ITF events. We hypothesise that the buoyancy flux of a strong ITF input ‘caps’ the Chagos Dome with warm, less saline waters, suppressing surface phytoplankton and reducing the surface chl-a concentrations. This hypothesis is supported by a strong, significant correlation (r = 0.66) between remotely sensed surface salinity and surface chl-a over the region. This relationship is not found over the Seychelles Dome, where the ITF has a weaker direct impact over the bloom. These results suggest that the westward travel of ITF waters may condition the eastward expansion of the SCTR and, therefore, the zonal extent of the associated chl-a bloom. This happens at seasonal and interannual time-scales concomitantly with the propagation of downwelling Rossby waves, deepening the thermocline and facilitating the westward advance of ITF waters. This is visible through a combination of remotely sensed and in situ observations at depth from the RAMA mooring array at the eastern domain of the SCTR, where intrusions of warm, less saline waters, typical of ITF waters, coincide with downwelling Rossby waves deepening the thermocline. Thus, both the westward travel of ITF waters and the propagation of downwelling Rossby waves shape the eastward expansion of the SCTR and, therefore, the zonal extent of the associated surface chl-a bloom on a year-to-year basis.

中文翻译：

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印度尼西亚流量与塞舌尔-查戈斯温跃层海脊内叶绿素水华之间的关系


塞舌尔-查戈斯温跃层海脊 (SCTR) 是西南印度洋（南纬 5-10°和东经 45-90°）内受热带环流驱动的开放海洋上升流的重要生物学区域。 SCTR 是指连接温跃层深度中两个局部最小值的细长特征；塞舌尔圆顶 (SD) 和查戈斯圆顶 (CD)。印度尼西亚贯穿流（ITF）从东部进入洋盆，已被证明与上升流区域相互作用，尽管与 SCTR 相关的浮游植物大量繁殖和 ITF 之间的关系迄今尚未探索。利用现场观测和遥感数据，ITF 的浮力通量在季节和年际尺度上强烈影响 SCTR 东端查戈斯圆顶上空的表面叶绿素-a (chl-a) 浓度。因此，我们发现高度计导出的 ITF 体积传输与表面 chl-a 浓度之间存在显着的负相关性 (r = -0.43)。当仅考虑 ITF 体积运输异常的第 10 个和第 90 个百分位时，这种负相关性会增加 (r = -0.61)，这表明 ITF 的影响可能会克服其他物理驱动因素，特别是在极端 ITF 事件下。我们假设强 ITF 输入的浮力通量用温暖、盐度较低的海水“覆盖”了查戈斯圆顶，抑制了表面浮游植物并降低了表面叶绿素 a 浓度。这一假设得到了该地区遥感地表盐度和地表叶绿素 a 之间强烈、显着的相关性 (r = 0.66) 的支持。在塞舌尔穹顶上没有发现这种关系，ITF 对水华的直接影响较弱。 这些结果表明，ITF 水域向西移动可能会影响 SCTR 向东扩展，从而影响相关叶绿素 a 水华的地带范围。这种情况发生在季节和年际时间尺度上，同时伴随着下降的罗斯比波的传播，加深了温跃层并促进了 ITF 水域的西进。通过结合 SCTR 东部区域 RAMA 系泊阵列的遥感和现场观测可以看出这一点，那里典型的 ITF 水域的温暖、盐度较低的水域的侵入，与加深温跃层的下降罗斯贝波相一致。因此，ITF 水域的向西移动和下降的罗斯比波的传播都影响了 SCTR 的向东扩张，从而影响了每年相关表面叶绿素水华的纬向范围。