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，地表水被带到海洋最深处，在那里它们被隔绝了很长一段时间，无法与大气进一步交换，但我们对水是如何运动的了解还不完整。例如，20年前，在西南印度洋的马达加斯加盆地，深水未受人类影响。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进入海洋表面，所以当地表水下沉时，它会将这些化合物带到深海。这些化合物在马达加斯加盆地深处仅20年后就出现了，这与我们之前对该地区的认识相悖。一种可能的解释是，在过去的二十年里，我们所理解的深海洋流可能改变了方向和强度。为了解决这个难题，该项目将首次测量该地区的深层洋流，在为期三周的巡航中使用船上仪器，并使用两种水下机器人技术在数年内跟踪这些洋流。将这些新的测量方法与计算机模拟相结合，这项研究将确定马达加斯加盆地深水的流动路径，并研究导致这种环流模式的原因。它将关注从南极洲附近开始的洋流，在那里，海水下沉，穿过巨大的海底山脉的裂缝，进入马达加斯加盆地，然后研究这些深水如何扩散到盆地。这个项目的发现将解释为什么这部分海洋变化如此之快，推进对深海环流的了解，并帮助定义热量和化合物是如何在海洋中移动的。这项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，并将首次在盆地尺度上对马达加斯加盆地深处的深海环流和温度变化进行直接估计。该项目将支持两名大学生，他们将被选中参加DMB巡游。每年夏天，pi还将接待和指导UCAR的大气研究和科学重大机会以及伍兹霍尔教育伙伴关系的学生。斯克里普斯大学本科生研究奖学金学生也将在每年夏天接受指导。此外，这艘游轮还将向印度洋社区开放，供他们进行背船项目，特别是来自毛里求斯的海洋学家和学生，毛里求斯是游轮的起点和终点。拟议的研究将调查大部分未知的深马达加斯加盆地（DMB）深海环流，深海温度如何在内部变化，以及曲折的海底地形对深海流动的影响。研究的主要目标是找出通过西南印度洋脊深部断裂带进入的较年轻的深海水在盆地中传播的途径，这对于更好地理解印度洋经向翻转环流及其变异性至关重要。为了确定盆地内部深海水域的路径和转变，将部署75个浮子阵列，在4000米的深度漂流两年，并部署3个深度（海面至6000米）剖面浮子，并在盆地中部和裂缝带进行高分辨率水文剖面（包括示踪分析）。现场观测将与最先进的建模组件相结合，该组件将用于研究深层流场的潜在动力学和时间演化。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，以回答一些超出原位观测范围的特定问题。该奖项反映了美国国家科学基金会的法定使命，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.