DBE Synthesis Supplement: Deep Circulation in the Tropical and Subtropical South Atlantic

0220407 SPEER在世界海洋环流实验的深盆实验过程中，建立了热带和亚热带南大西洋新的水文和示踪气候学，并对水文剖面进行了重新分析。在中大西洋海脊以西(在巴西盆地)的水文剖面上观察到的经向密度梯度表明，在大的空间尺度上持续的纬向流具有显著的规律性，这与新气候学中几个示踪物的等距分布一致。这种交替的纬向流模式在一个最先进的数值模式的输出中也很明显。虽然观测(以及模式)表明平均深部环流的概念是合理的，但它们也表明，不可能通过简单地确定参考速度(正压调整)来确定天气观测的时间平均地转流。巴西盆地天气和时间平均地转纬向速度场之间的差异是斜压的，并且在时间和空间上都是不同的。南大西洋的水文观测和数值纬向流之间的良好一致性表明，可以利用数值结果将观测值分为长期平均和低频时变分量。目前的分析将扩展并应用于南大西洋更大的区域，最重要的目标是定量描述热带和亚热带南大西洋在全球海洋环流期间的深层环流。第一步将包括分析时变模式流的时空结构，以指导将时变模式流与根据水文断面确定的平均地转流分开。当然，该模型似乎真实地模拟了巴西盆地的平均纬向流，这并不意味着可以很好地模拟时变的流。虽然不可能从现有的水文断面直接估计时变水流分量，但对单个剖面的分析以及两个水文断面之间的差异将使PI能够将时变流场的重要方面，如空间尺度和斜压结构与模型输出进行比较。此外，最近对来自燕麦数据的时变流动的分析结果(Hogg等人，，2002)将用于比较。除了揭示深层环流的新方面(对气候、示踪剂扩散、生物地理学等有影响)外，拟议项目的结果还将大大改进对天气学水文断面的分析，对反模式具有重要影响，从而有助于从世界海洋环流数据集中确定平均环流。数值模式产出和海洋数据的直接比较是拟议项目的一个固有部分，它将揭示当前数值模式的优点，更重要的是，揭示其缺点。这些见解将有助于改进模型。

0220407 SpeerIn the course of the Deep Basin Experiment under the World Ocean Circulation Experiment (WOCE) a new hydrographic and tracer climatology of the tropical and subtropical South Atlantic was constructed and a re-analysis of hydrographic sections was carried out. The meridional density gradients observed in hydrographic sections west of the Mid-Atlantic Ridge (in the Brazil Basin) indicate a strikingly regular pattern of persistent zonal flows coherent over large spatial scales, consistent with the isopycnal distributions of several tracers in the new climatology. This pattern of alternating zonal flows is also apparent in the output from a state-of-the-art numerical model. While the observations (as well as the model) indicate that the notion of a mean deep circulation is sensible they also show that it is not possible to determine the time-averaged geostrophic flows from synoptic observations by simply determining a reference velocity (barotropic adjustment). The difference between the synoptic and the time-averaged geostrophic zonal velocity fields in the Brazil Basin is baroclinic and varies both in time and space. The excellent agreement between the hydrographically observed and the numerical zonal flows in the South Atlantic suggests that it possible to use the numerical results to separate the observations into long-term mean and low-frequency time-varying components. The current analysis will be extended and applied it to a larger region of the South Atlantic with the overriding goal to quantitatively describe the deep circulation in the tropical and subtropical South Atlantic during the WOCE period. The first step will consist of analyzing the spatio-temporal structure of the time-varying model flow to guide the separation of the time-varying from the mean geostrophic flows determined from the hydrographic sections. The fact that the model appears to simulate the mean zonal flows in the Brazil Basin realistically does not imply that the time-varying flows are as well represented, of course. While it is not possible to directly estimate the time-varying flow component from available hydrographic sections analysis of individual profiles as well as differences between two hydrographic sections will allow the PI's to compare important aspects of the time-variable flow field, such as spatial scales and baroclinic structure to the model output. Furthermore, the results of a recent analysis of the time-varying flows derived from ?oat data (Hogg et al. , 2002) will be used for comparison. In addition to revealing new aspects of the deep circulation (with implications for climate, tracer dispersal, biogeography, etc.), the results of the proposed project will significantly improve the analysis of synoptic hydrographic sections with important implications for inverse models and, therefore, for determining the mean circulation from the WOCE data sets. Direct comparison of numerical-model output and oceanic data is an intrinsic part of the proposed project and will reveal strengths and, more importantly, weaknesses of current numerical models. These insights will be useful for improving the models.