Speaker

Viviane Menezes, Asst. Scientist, Woods Hole Oceanographic Institution

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

Abstract

This presentation will discuss the Deep Madagascar Basin (DMB) Experiment that began in 2023. The DMB aims to understand the Antarctic Bottom Water (AABW) pathways from the deep fracture zones of the Southwest Indian Ridge (SWIR)– the only gates in the western Indian Ocean for the southern-originated abyssal waters to flow north as part of the lower limb of the Global Meridional Overturning Circulation– to the Deep Western Boundary Current flowing equatorward east of Madagascar. The DMB Experiment comprises a mix of Eulerian (CTD/LADCP/tracers) and Lagrangian (RAFOS and Deep Argo floats) in-situ observations and numerical modeling. Here, I will present the first results based on the Deep Argo floats we deployed near the outputs of the SWIR fracture zones and Eulerian observations within four fracture zones (Gallieni, Atlantis II, Novara, and Melville) — some of them have been measured before.  Distinct from the future global and pilot arrays, our Deep SOLO floats are obtaining profiles at a much higher frequency (3-5 days), with an enhanced vertical resolution of 5 dbar 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 upper layer when the float is descending most rapidly and discrete at slower descent speeds in the deep layers.  Specifically, I will highlight the importance of collecting concurrent high-quality shipboard salinity observations for float salinity calibration and issues related to data collection at high-vertical resolution to obtain salinity observations adequate to investigate changes in the deep ocean. A significant DMB discovery so far is about the Novara and Melville fracture zones. Novara presents an astonishing northward flow below 3500 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.