Analysis of magnitude, drivers, and mechanisms of salinity variability in the tropical Atlantic

海洋在全球水循环中发挥着至关重要的作用。在DFG Forschergruppe的第一阶段，焦点在赤道大西洋，在第二阶段，焦点在热带北大西洋，这是一个受强降水影响的地区。该区域的海面盐度在空间尺度上变化很大，范围从与对流雨区有关的小尺度到与例如横跨南美洲至非洲的热带辐合带有关的盆地尺度。时间变率再次与时间尺度为几小时的零星降雨事件、近地表层结和混合的日循环、时间尺度为数周至数月的海洋中尺度动力学、主要与热带气圈季节迁移有关的季节变率(图1)以及与热带大西洋变率的主要气候模式有关的年际至年代际变率有关。拟议的项目依赖于卫星数据和在包括SFB754在内的不同项目框架内在热带北大西洋获得的数据。在这里，我们使用了自2006年以来拍摄的大量CTD剖面，主要是沿着北纬15°到S 5°之间的23°W剖面(图2)，以及在热带北大西洋更广阔的区域拍摄的船上和滑翔机数据。大量的盐度剖面可以描述近地表盐度的变异性，并将这一信息与卫星遥感的SSS结合起来。滑翔机数据特别是包括大量近地表海洋微结构剖面的数据非常适合于研究近地表昼夜淡水通量。拟议项目的主要目标是增进我们对北大西洋热带盐度最低区域的水文循环的了解，重点放在海洋淡水通量和近地表淡水收支方面，这对理解气候变化的长期影响具有重要意义。更具体地说，我们将讨论以下专题：1)与大尺度气候模式有关的ITCZ以下区域SSS的年际变化；2)季节性混合层淡水收支，重点是昼夜混合和在ITCZ附近由高度可变的风强迫的近惯性波的作用；以及3)与降雨事件和日至多日暖层有关的近地表层结变率及其对卫星遥感观测的SSS的影响。所获得的知识和可用的数据集将用于评估在进行数据同化和不进行数据同化的情况下模式模拟的质量，并将有助于增进对SSS变异性的驱动因素和机制的了解。这项拟议的研究与国际项目Ssps-2密切相关，该项目将重点关注2016-2017年热带东太平洋的盐度最低区域。

The ocean plays an essential role in the global water cycle. While during the first phase of the DFG Forschergruppe, the focus was on the equatorial Atlantic, during the second phase it is on the tropical North Atlantic, a region affect by intense precipitation. Sea surface salinity (SSS) in this region is highly variable on space scales ranging from small scales, associated with convective rain cells, to the basin scale, associated for example with the Intertropical Convergence Zone (ITCZ) spanning from South America to Africa. Temporal variability again is associated with sporadic rainfall events with time scales of few hours, daily cycles in near surface stratification and mixing, ocean mesoscale dynamics with time scales of weeks to few months, seasonal variability mainly associated with the seasonal migration of the ITCZ (Fig. 1) and interannual to decadal variability associated with the dominant climate modes of Tropical Atlantic Variability. The proposed project relies on satellite data and on data acquired in the tropical North Atlantic in the frame of different projects, including the SFB 754. Here we use the large amount of CTD profiles taken since 2006 mainly along the 23°W section between 15°N and 5°S (Fig. 2), together with shipboard and glider data taken in the wider region of the tropical North Atlantic. The large amount of salinity profiles allows to characterize the near-surface salinity variability and to combine this information with SSS from satellite remote sensing. The glider data that particularly include a large amount of microstructure profiles of the near-surface ocean are ideally suited to study near surface diapycnal freshwater fluxes. Main goal of the proposed project is to improve our understanding of the hydrological cycle in the salinity minimum region of the tropical North Atlantic with focus on oceanic freshwater fluxes and near surface freshwater budgets with implications for the understanding of long-term climatedriven changes. More specifically we will address the following topics: 1) interannual variability of SSS in the region below the ITCZ associated with large-scale climate modes, 2) the seasonal mixed layer freshwater budget with focus on diapycnal mixing and the role of near-inertial waves forced by highly variable winds close to the ITCZ, and 3) the near surface stratification variability associated with rainfall events and diurnal to multi-diurnal warm layers and its effect on SSS as observed by satellite remote sensing. The gained knowledge and available datasets will be used to assess the quality of model simulations with and without data assimilation and will contribute to increase the understanding of drivers and mechanisms of SSS variability. The proposed study is strongly related to the international program SPURS-2 that will focus on the salinity minimum region of the eastern tropical Pacific in 2016-2017.