Collaborative Research: Deep Madagascar Basin (DMB) Experiment: A Quest to Find the Abyssal Water Pathways in the Southwest Indian Ocean

深海通过与大气交换热量和化学化合物并在全球范围内移动，在长期调节地球气候方面发挥着至关重要的作用。有时，表层沃茨被带到最深的海洋区域，在那里它们被隔离，在很长一段时间内与大气层进行进一步的交换，但我们对水如何移动的知识是不完整的。例如，20年前，在西南印度洋的马达加斯加盆地，深水沃茨没有受到人类活动的影响。但2018年的最新测量显示，那里有大量的人造化合物。 这些惰性化合物从大气层进入海洋表面，因此当地表水下沉时，它会将这些化合物带到深海。这种化合物在仅仅20年后就出现在马达加斯加盆地深处，这与我们以前对该地区的了解背道而驰。一个可能的解释是，我们所理解的深海洋流可能在过去20年里改变了方向和强度。为了解决这个难题，该项目将首次测量该地区的深海洋流，在为期三周的巡航中使用船上仪器，并使用两种水中机器人技术在几年内跟踪这些洋流。将这些新的测量与计算机模拟相结合，这项研究将确定马达加斯加盆地深层沃茨流动的路径，并研究是什么导致了这种环流模式。它将重点关注从南极洲附近开始的水流，那里的水下沉，通过巨大海底山脉的裂缝进入马达加斯加盆地，然后研究这些深层沃茨如何扩散到盆地。该项目的发现将解释为什么这部分海洋变化如此之快，推进深海环流的知识，并帮助确定热量和化学化合物如何在海洋中移动。拟议的研究将是美国对第二次国际印度洋考察（IIOE-2）的贡献，并将首次直接估计马达加斯加深海盆地的深海环流和温度变化。该项目将支持两名本科生，他们将被选中参加DMB巡航。PI还将主办和指导UCAR在大气研究和科学方面的重要机会，以及每年夏天在教育学生方面的伍兹霍尔伙伴关系。斯克里普斯本科研究奖学金的学生也将每年夏天辅导。此外，整个印度洋社区，特别是来自毛里求斯的海洋学家和学生将可利用这次航行进行附带项目，毛里求斯是这次航行的起点和终点港，拟议的研究将调查基本上不为人知的马达加斯加深海盆深海环流、深海温度在内部的变化情况以及曲折的海底地形对引导深海水流的影响。主要目的是查明通过西南印度洋洋脊深层断裂带进入的较年轻的深海水在海盆中传播的途径，这对于更好地了解印度洋经向翻转环流及其变化至关重要。为了确定海盆内部深海沃茨的路径和变化，将部署一组75个浮标，这些浮标压载在4 000米处漂流两年，并部署3个深海（海面至6 000米）剖面浮标，辅之以海盆中部和断裂带的高分辨率水文剖面图（包括示踪剂分析）。现场观测将与最先进的建模组件配对，该组件将用于调查深层流场的基本动力学和时间演变。在使用新的观测数据集进行验证后，该模型将用于执行粒子跟踪模拟，以回答一些超出现场观测范围的特定问题。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The deep oceans play a crucial role in regulating the Earth's climate on long timescales, by exchanging heat and chemical compounds with the atmosphere and moving them globally. Sometimes surface waters are carried into deepest ocean areas where they are sequestered from further exchanges with the atmosphere over long time periods, but where our knowledge of how water moves is incomplete. For instance, twenty years ago in the Madagascar Basin of the southwest Indian Ocean, deep waters were undisturbed by human influence. But newer measurements in 2018 showed significant amounts of human-made chemical compounds there. These inert compounds enter the ocean at the sea surface from the atmosphere, so when and where the surface water sinks, it carries those compounds to the deep sea. The presence of such compounds in the deep Madagascar Basin after only twenty years counters our previous knowledge about the region. A possible explanation is that deep currents as we understood them may have changed course and strength in the last twenty years. To solve this puzzle, this project will measure the deep currents in the region for the first time, using shipboard instruments during a three-week cruise, and with two types of in-water robotic technologies to follow these currents over several years. Combining these novel measurements with computer simulations, this study will identify the pathways that deep waters travel in the Madagascar Basin, and examine what causes such circulation patterns. It will focus on currents starting near Antarctica, where the water sinks, through fissures in massive seafloor mountain ranges and into the Madagascar Basin, and then on how these deep waters spread to fill the basin. The findings from this project will explain why this part of the ocean is changing so fast, advance the knowledge of deep ocean circulation, and help define how heat and chemical compounds are moved around within the ocean. The proposed research will be a US contribution to the 2nd International Indian Ocean Expedition (IIOE-2) and will provide the first direct estimate of the abyssal circulation and temperature variability in the Deep Madagascar Basin on a basin-scale. The project will support two undergraduate students that will be selected to participate in the DMB cruise. The PIs will also host and mentor UCAR's Significant Opportunities in Atmospheric Research and Science and Woods Hole Partnership in Education students for each summer. Scripps Undergraduate Research Fellowship students will also be mentored each summer. Moreover, the cruise will be available to the Indian Ocean community in general for piggy-back projects, and in particular for oceanographers and students from Mauritius, the start and end port for the cruise.The proposed research will investigate the largely unknown Deep Madagascar Basin (DMB) abyssal circulation, how abyssal temperature varies in the interior, and the effects of the tortuous seafloor topography in steering the abyssal flows. The primary objective is to find out by which pathway(s) the younger abyssal water that enters through deep fracture zones in the Southwest Indian Ridge spreads in the basin, which is crucial for a better understanding of the Indian Ocean Meridional Overturning Circulation and its variability. To determine the pathways and the transformation of the abyssal waters in the basin interior, an array of 75 floats ballasted to drift at 4000-m for two years and 3 deep (sea surface to 6000 m) profiling floats will be deployed, complemented by high-resolution hydrographic sections (including tracer analysis) across the mid-basin and the fracture zones. The in-situ observations will be paired with a state-of-art modeling component, which will be used to investigate the underlying dynamics and time evolution of the deep flow field. After validation using the new observational dataset, the model will be used to perform particle tracking simulations to answer some specific questions that are beyond the scope of the in-situ observations alone.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.