Extended Data Fig. 1: Coupled oceanographic and atmospheric circulation of the Indian Ocean region.

a, Surface circulation indicated by mean geostrophic velocities and directions. IODP Site U1476 is situated in the Mozambique Channel, which experiences a net southward flow of the Mozambique Channel Throughflow (MCT). The tropical Indian gyre receives and redistributes inflow from the Indonesian Throughflow (ITF). The main components of the tropical Indian gyre: East African Coastal Current (EACC), Southern Equatorial Current (SEC), Southern Equatorial Counter Current (SECC) and Northeast Madagascar Current (NEMC) are highlighted with a dark grey outline marking the mean extension of the tropical gyre^63,64. At ~60°E, the SEC bifurcates into two main branches feeding into the NEMC and the Southeastern Madagascar Current (SEMC) as it crosses bathymetric highs including the Mascarene Plateau, highlighted by the solid grey bathymetric contour at 2,200 m water depth. The SEMC is outside the outline of the tropical gyre as it is part of the anti-cyclonic subtropical gyre. b, Mean wind stress (gray arrows) and wind stress curl of the Indian Ocean indicate the basin-wide negative (positive) wind stress curl forcing the tropical (subtropical) gyre. A black line indicates the zero wind stress curl. c, Mean dynamic sea surface topography indicating a sea-level low between 10-5^°S at the centre of the tropical Indian gyre; the Seychelles–Chagos thermocline ridge (SCTR)⁹. The thicker contour of 1.05 m denotes the northern extent of the subtropical gyre, whereby the blues (reds) highlight lower (higher) sea surfaces, which are associated with the cyclonic (anti-cyclonic) circulation of the tropical (subtropical) gyres. The bathymetric, oceanic (1993-2012) and surface wind data (1979-2019) are derived from: General Bathymetric Chart of the Oceans (GEBCO)⁶⁵, CNES-CLS18 MDT⁶⁶, ERA5 monthly-averaged data on single levels (DOI: 10.24381/cds.6860a573), respectively. These maps are generated with MATLAB and Mapping Toolbox, version 9.8.0.1323502 (R2020a, Natick, Massachusetts: The MathWorks Inc., United States).