EMC Experiment: Examining the Fate of the East Madagascar Current

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2). The system of ocean currents that forms the global Meridional Overturning Circulation (GMOC) plays a central role in regulating Earth's climate. One of the least understood yet critical aspects of this system is the upper-layer circulation within the East Madagascar Current (EMC) in the South Indian Ocean. This project will use surface drifters, subsurface RAFOS floats and particle tracking in a numerical model of the region to examine pathways after the EMC separates from the continental shelf near the southern tip of Madagascar: whether it turns east into the Madagascar Basin or west into the Mozambique Basin, or both. The project will take advantage of several RAFOS sound sources associated with the already-funded Deep Madagascar Basin project (NSF/OCE/PO #1924431), adding one additional sound source to facilitate triangulation of the mid-depth RAFOS floats of the new project. The project will provide observations in an under sampled ocean region, help define the fate of Indonesian Throughflow (ITF) waters, and may inform understanding of the as yet unexplained Southeast Madagascar Bloom -- a global 'hotspot' for primary production in an otherwise oligotrophic subtropical region. The project will support the PI, an early career investigator, and fund the participation of six early-career Malagasy scientists (including a collaborator) in the research cruises. The project will also organize a 2-week summer school in Madagascar. Undergraduate WHOI summer interns will be supported each summer, including funding for their participation in scientific meetings. The proposed research will be a US contribution to the 2nd International Indian Ocean Expedition. To observe fluid parcel pathways of the detached EMC, the project will simultaneously release surface drifters and subsurface acoustically-tracked isopycnal RAFOS floats within the EMC four times over a year. This Lagrangian experiment, the first of its kind in the EMC, will be paired with particle tracking simulations to investigate surface and subsurface pathways associated with the detached EMC, its connectivity with the Agulhas Current and the South Indian Countercurrent, and respective transit times. The research will assess the impacts of EMC strength, downstream propagating eddies, and seafloor topography on the detached EMC pathways. The specific objectives are to 1) determine the advective pathways and transit times of the EMC extension at surface and pycnocline levels and investigate the physical mechanisms that control those pathways, 2) characterize the evolution of any EMC-originated eddies that transit westward, 3) evaluate potential EMC retroflection modes and investigate controls on them, and 4) determine the fates and advective transit times of EMC originated parcels following measured and modeled potential paths. This knowledge will inform understanding the ITF water journey within the South Indian Ocean, the Southeast Madagascar Bloom development, southern Madagascar coastal upwelling cells, and impacts on the Agulhas Current and the GMOC. 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.