The sea level variations along the west coast of India (WCI) significantly affect the ecosystems and fisheries, because of their tight coupling with the oxycline depths in this region, which hosts the world's largest natural hypoxic system. Here, we investigate the main causes of the WCI sea level variability. Using idealized experiments with a linear, continuously stratified ocean model, we first demonstrate that there is a direct pathway between the equatorial waveguide and the WCI, in addition to the well-documented pathway that aligns with the coastal waveguide in the Bay of Bengal. This direct connection results from the intersection of equatorial and coastal waveguides near Sri Lanka. The forced and reflected equatorial Rossby waves induce sea level variations at the Sri Lankan coast around 6°N, which propagate directly to the WCI as coastal Kelvin waves, without transiting through the Bay of Bengal coastal waveguide. Using model experiments with realistic coastline and forcing, we then illustrate that this direct pathway is the primary contributor (0.4 regression coefficient) to the WCI intraseasonal (20–150 days) sea level variability, followed by the Bay of Bengal coastal waveguide pathway (0.3) and the wind forcing in a small region near Sri Lanka (0.25). The remote forcing originating from the rest of the Bay of Bengal and the WCI local wind forcing have weaker influence. We conclude by discussing why this direct connection exhibits strong impact on sea level variability at the intraseasonal timescale, while its influence is considerably weaker at the longer seasonal and interannual timescales.

中文翻译：

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斯里兰卡附近赤道和沿海相交波导对印度西海岸季节内海平面变化的作用

印度西海岸 (WCI) 沿线的海平面变化对生态系统和渔业产生了重大影响，因为它们与该地区的眼斜层深度密切相关，而该地区拥有世界上最大的自然缺氧系统。在这里，我们调查了 WCI 海平面变化的主要原因。通过使用线性、连续分层海洋模型的理想化实验，我们首先证明了赤道波导和 WCI 之间存在直接路径，此外还有与孟加拉湾沿海波导对齐的有据可查的路径。这种直接连接是由于斯里兰卡附近的赤道和沿海波导相交而产生的。受迫和反射的赤道罗斯贝波引起斯里兰卡海岸北纬 6° 左右的海平面变化，这些海平面变化作为沿海开尔文波直接传播到 WCI，而不经过孟加拉湾沿海波导。然后，我们使用具有现实海岸线和强迫的模型实验，说明该直接路径是 WCI 季节内（20-150 天）海平面变化的主要贡献者（回归系数为 0.4），其次是孟加拉湾沿海波导路径（0.3 ）和斯里兰卡附近小区域的风力强迫（0.25）。来自孟加拉湾其他地区的远程强迫和WCI局地风力强迫影响较弱。最后，我们讨论了为什么这种直接联系在季节内时间尺度上对海平面变化表现出强烈影响，而在较长的季节和年际时间尺度上其影响却相当弱。