Subsurface manifestation of Marine Heatwaves in the South West Indian Ocean

Abstract. 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 the SWIO, which is dominated by the presence of mostly warm anticyclonic eddies, is characterised by moderate MHWs, and that the frequency, duration and intensity of these events are largely associated with mesoscale activity. Surface-detected MHW case studies demonstrated a strong subsurface temperature anomaly signal, with maximum intensity below the mixed layer depth. The spatial distribution of anticyclonic, warm-core eddies closely matched the distribution of the MHWs. This provides a possible mechanism for the deep extent of these 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.