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

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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.