Subsurface manifestation of Marine Heatwaves in the South West Indian Ocean: Marine heatwaves (MHW) are extreme events of prolonged, anomalously warm ocean temperatures. Globally, marine heatwaves are increasing in frequency and intensity and are responsible for long-term impacts on marine ecosystems, which have devastating socio-economic consequences. A key gap in our understanding of MHWs is how they manifest in the subsurface. This paper uses satellite sea surface temperature (SST) data and in situ subsurface temperature observations from Expendable Bathythermographs (XBTs) to investigate the anomalous water temperature characteristics associated with surface identified MHWs in the South West Indian Ocean (SWIO) and how they progress through the water column. We find that regions of high eddy activity, in the Mozambique Channel and southeast of Madagascarthe eddy corridors through the SWIO, where EKE is high and SST variability is low, are primarily characterised by the occurrence of abrupt and intense MHWs, and that the frequency, duration and intensity of these events are largely associated with mesoscale activity. In these eddy corridors, surface–detected MHW case studies demonstrate a strong, deep–reaching subsurface temperature anomaly signal with maximum intensity below the surface. The majority of these MHWs are associated with anticyclonic eddies, which provide a possible mechanism for the deep extent of the surface MHWs. Improving our understanding of the interaction between mesoscale features and subsurface MHW characteristics will benefit prediction of MHWs and management of the regions’ biodiversity.