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Processes of India’s offshore summer intraseasonal sea surface temperature variability

机译：印度近海夏季季节内海表温度变化的过程

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摘要

Active and break phases of the Indian summer monsoon are associated with sea surface temperature (SST) fluctuations at 30–90 days timescale in the Arabian Sea and Bay of Bengal. Mechanisms responsible for basin-scale intraseasonal SST variations have previously been discussed, but the maxima of SST variability are actually located in three specific offshore regions: the South-Eastern Arabian Sea (SEAS), the Southern Tip of India (STI) and the North-Western Bay of Bengal (NWBoB). In the present study, we use an eddy-permitting 0.25° regional ocean model to investigate mechanisms of this offshore intraseasonal SST variability. Modelled climatological mixed layer and upper thermocline depth are in very good agreement with estimates from three repeated expendable bathythermograph transects perpendicular to the Indian Coast. The model intraseasonal forcing and SST variability agree well with observed estimates, although modelled intraseasonal offshore SST amplitude is undere-stimated by 20–30 %. Our analysis reveals that surface heat flux variations drive a large part of the intraseasonal SST variations along the Indian coastline while oceanic processes have contrasted contributions depending of the region considered. In the SEAS, this contribution is very small because intraseasonal wind variations are essentially cross-shore, and thus not associated with significant upwelling intraseasonal fluctuations. In the STI, vertical advection associated with Ekman pumping contributes to ∼30 % of the SST fluctuations. In the NWBoB, vertical mixing diminishes the SST variations driven by the atmospheric heat flux perturbations by 40 %. Simple slab ocean model integrations show that the amplitude of these intraseasonal SST signals is not very sensitive to the heat flux dataset used, but more sensitive to mixed layer depth.
机译：印度夏季风的活跃期和中断期与阿拉伯海和孟加拉湾在30-90天时标上的海表温度（SST）波动有关。先前已经讨论了造成海盆尺度季节内海表温度变化的机制，但是海表温度变化的最大值实际上位于三个特定的近海区域：东南阿拉伯海（SEAS），印度南端（STI）和北部-孟加拉西部海湾（NWBoB）。在本研究中，我们使用允许涡流的0.25°区域海洋模型来调查这种近海季节内海表温度变化的机制。模拟的气候混合层和较高的跃层深度与垂直于印度洋沿岸的三个重复的消耗性热线断面的估计值非常吻合。尽管季节内离岸SST振幅被低估了20％至30％，但季节内强迫和SST变异性模型与观测值吻合得很好。我们的分析表明，地表热通量变化沿印度海岸线驱动了大部分季节内海表温度变化，而海洋过程根据所考虑的区域却产生了相反的影响。在SEAS中，这种贡献非常小，因为季节内风的变化基本上是跨海岸的，因此与明显的上升季节内波动无关。在STI中，与Ekman抽水有关的垂直对流贡献了约30％的SST波动。在NWBoB中，垂直混合减少了由大气热通量扰动驱动的SST变化40％。简单的平板海洋模型积分表明，这些季节内SST信号的幅度对使用的热通量数据集不是很敏感，但对混合层深度更敏感。

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