Currents and inter-connectivity of the Indian Ocean west of Mascarene Plateau: an IIOE-2 / Monaco Explorations Indian Ocean Expedition collaboration

Insights were obtained on the inter-connectivity of the upper Indian Ocean between Mascarene Plateau and east Africa by tracking 19 sea surface drifter (SSD) and four 15m depth SVP drogues during the Monaco Explorations 'Indian Ocean Expedition' (ME-IOE) of Oct-Nov 2022, an initiative endorsed by the Second International Indian Ocean Expedition. Drogues were deployed along a clockwise track between Mauritius and Seychelles, including clusters immediately west of Aldabra Atoll and skirting around eastern Mascarene Plateau. Interesting circulation features observed included those consistent with dynamics related to the westward flowing South Equatorial Current through the region, the South Equatorial Counter Current, and also meso-scale eddies and dynamical variability in the Mozambique Channel. There was sub-mesoscale eddying evident in some drogue patterns, and lateral divergences of flow passing through deep regions between banks: such as via the ~4000 m deep zone between the relatively shallow (<100m) Saya de Malha and Nazareth banks. There were relatively rapid westward movements and eventual groundings along east Africa (Tanzania) of surface drogues after deployments off Aldabra Atoll: ie grounding in < 3 weeks, at ~ 1-2 m/s. This study complements Crochelet et al's recent inter-connectivity studies of the adjacent Indian Ocean region to the south of the ME-IOE. The results are used to consider the potential for viable ecological inter-connectivity between areas of spawning generation and potential downstream recruitment in the context of biological propagule transport (eg coral larvae). Postulations on transport of undesirable materials such as surface contaminants and debris (eg plastics) are also considered.