The South‐East Madagascar Bloom occurs in an oligotrophic region of the southwest Indian Ocean. Phase locked to austral summer, this sporadic feature exhibits substantial temporal and spatial variability. Several studies, with different hypotheses, have focused on the initiation mechanism triggering the bloom, but none has been as yet clearly substantiated. With 19 years of ocean color data set available as well as in situ measurements (Argo profiles), the time is ripe to review this feature. The bloom is characterized in a novel manner, and a new bloom index is suggested, yielding 11 bloom years, including 3 major bloom years (1999, 2006, and 2008). Spatially, the bloom varies from a mean structure (22–32°S; 50–70°E) both zonally and meridionally. A colocation analysis of Argo profiles and chlorophyll‐a data revealed a bloom occurrence in a shallow‐stratified layer, with low‐salinity water in the surface layers. Additionally, a quantitative assessment of the previous hypotheses is performed and bloom occurrence is found to coincide with La Niña events and reduced upwelling intensity south of Madagascar. A stronger South‐East Madagascar Current during La Niña may support a detachment of the current from the coasts, dampening the upwelling south of Madagascar, and feeding low‐salinity waters into the Madagascar Basin, hence increasing stratification. Along with abundance of light, these provide the right conditions for a nitrogen‐fixing cyanobacterial phytoplankton bloom onset.

South Indian Ocean eddies (SIDDIES), originating from a high evaporation region in the eastern Indian Ocean, are investigated by tracking individual eddies from satellite data and co‐located Argo floats. A subsurface‐eddy identification method, based on its steric dynamic height anomaly, is devised to assign Argo profiles to surface eddies (surfSIDDIES) or subsurface eddies (subSIDDIES). These westward‐propagating, long‐lived features (>3 months) prevail over a preferential latitudinal band, forming a permanent structure linking the eastern to the western Indian Ocean, that we call the 'SIDDIES Corridor’. Key features have been revealed in the mean thermohaline vertical structure of these eddies. Anticyclonic SIDDIES are characterized by positive subsurface salinity anomalies, with subSIDDIES not exhibiting negative surface anomalies, as opposed to surfSIDDIES. Cyclonic subSIDDIES also occur, but their related salinity anomalies are weaker. SubSIDDIES exhibit two cores of different temperature polarities in their surface and subsurface levels. Cyclonic subSIDDIES have their cores at around 150‐200 m depth along the 25.4‐25.8 kg m−3 potential density layer with anticyclonic subSIDDIES having their cores at 250‐300 m along the 26‐26.4 kg m−3 density layer. The SIDDIES corridor acts as a zonal pathway for both eddy‐types to advect water masses and biogeochemical properties across the basin. This study provides a new insight on heat/salt fluxes, showing that 58% (32%) of the total heat eddy‐flux is ascribed to cyclonic (anticyclonic) subSIDDIES, respectively, in the eastern South Indian Ocean. Anticyclonic subSIDDIES have also been found to be the sole high‐saline water eddy‐conveyor toward the western Indian Ocean.