Deep Madagascar Basin Experiment: Antarctic Bottom Water Spreading in theSouthwestern Indian Ocean

Speaker: Viviane Menezes, Asst. Scientist, Woods Hole Oceanographic Institution

This presentation will discuss the Deep Madagascar Basin (DMB) Experiment that beganin 2023. The DMB aims to understand the Antarctic Bottom Water (AABW) pathwaysfrom the deep fracture zones of the Southwest Indian Ridge (SWIR)– the onlygates in the western Indian Ocean for the southern-originated abyssal waters toflow north as part of the lower limb of the Global Meridional OverturningCirculation– to the Deep Western Boundary Current flowing equatorward east ofMadagascar. The DMB Experiment comprises a mix of Eulerian (CTD/LADCP/tracers)and Lagrangian (RAFOS and Deep Argo floats) in-situ observations and numericalmodeling. Here, I will present the first results based on the Deep Argo floatswe deployed near the outputs of the SWIR fracture zones and Eulerianobservations within four fracture zones (Gallieni, Atlantis II, Novara, andMelville) — some of them have been measured before.  Distinct from thefuture global and pilot arrays, our Deep SOLO floats are obtaining profiles ata much higher frequency (3-5 days), with an enhanced vertical resolution of 5dbar below 3000 dbar. It is important to highlight that to conserve float batteries,the Deep SOLOs operate in dual vertical sampling mode: continuous in the upperlayer when the float is descending most rapidly and discrete at slower descentspeeds in the deep layers.  Specifically, I will highlight the importanceof collecting concurrent high-quality shipboard salinity observations for floatsalinity calibration and issues related to data collection at high-verticalresolution to obtain salinity observations adequate to investigate changes inthe deep ocean. A significant DMB discovery so far is about the Novara andMelville fracture zones. Novara presents an astonishing northward flow below3500 m (42 cm/s), much larger than in the Atlantis II fracture zone (26 cm/s) —until now considered the main AABW conduit into the Indian Ocean.