Abstract The dynamic of the thermohaline structure of the upper ocean, which depends on ocean–atmosphere interactions, drives most near surface oceanic processes, including the control of gases and heat fluxes, and nutrient availability in the photic layer. The thermohaline structure of the southwestern tropical Atlantic (SWTA), a key region for diagnosing variation of the Atlantic Meridional Overturning Circulation, has prime impact on global climate. Characterising the thermohaline structure is typically based on the application of classical statistical methods on vertical profiles. Such approach has important limitations since classical methods do not explicitly contemplate the vertical nature of the profiles. Functional Data Analysis (FDA) is a new alternative to solve such drawbacks. Here, we apply an FDA approach to characterise the 3D canonical thermohaline structure of the SWTA in austral spring and fall. Our results reveal a clear spatial pattern with the presence of three areas with significantly different thermohaline structure. Area 1, mostly located along the continental slope, reflects the western boundary current system, with low static stability and high frequency of occurrence of barrier layer (BL). Conversely, Area 2, located along the Fernando de Noronha chain, presents strong static stability with a well-marked thermocline. This area, under the influence of the eastern Atlantic, is characterised by a low BL frequency, which is seasonally modulated by the latitudinal oscillation of the Intertropical Convergence Zone, controlling the regime of precipitation. In turn, Area 3 behaves as a transition zone between A1 and A2 with the presence of the water core of maximum salinity in subsurface, and therefore presence of strong-moderate BL. Beyond this study, FDA approach emerges as a powerful way to describe, characterise, classify and compare ocean patterns and processes. It can be applied to in situ data but could also be used to deeply and comprehensively explore ocean model output.

中文翻译：

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从原位剖面的功能数据分析得出的西南热带大西洋温盐结构的 3D 表征

摘要 依赖于海洋-大气相互作用的上层海洋温盐结构的动态驱动着大多数近地表海洋过程，包括气体和热通量的控制，以及光层中养分的有效性。西南热带大西洋 (SWTA) 的温盐结构是诊断大西洋经向翻转环流变化的关键区域，对全球气候有主要影响。表征温盐结构通常基于经典统计方法在垂直剖面上的应用。这种方法具有重要的局限性，因为经典方法没有明确考虑轮廓的垂直特性。功能数据分析 (FDA) 是解决此类缺陷的新选择。这里，我们应用 FDA 方法来表征南方春季和秋季 SWTA 的 3D 典型温盐结构。我们的结果揭示了一个清晰的空间模式，存在三个具有显着不同温盐结构的区域。1区多位于大陆坡沿线，反映西部边界流系，静稳定性低，屏障层（BL）出现频率高。相反，位于 Fernando de Noronha 链沿线的区域 2 具有很强的静态稳定性和明显的温跃层。该区受东大西洋影响，BL频率较低，受热带辐合带纬度振荡的季节性调制，控制降水情况。反过来，区域 3 表现为 A1 和 A2 之间的过渡带，地下存在最大盐度的水核，因此存在强-中度 BL。除了这项研究之外，FDA 方法成为描述、表征、分类和比较海洋模式和过程的有力方法。它可以应用于原位数据，也可以用于深入、全面地探索海洋模型输出。