Abstract
Prolonged marine heatwaves (MHWs) around small island developing states can lead to severe ecological and socio-economic consequences. This study aims at characterizing subsurface MHWs and their potential drivers around the Seychelles and Comoros, rich in biodiversity and prominent to tropical cyclone intensification. Both regions show intensified MHWs in the thermocline, but with a higher intensity and duration in Seychelles. About 84% of subsurface MHWs in Comoros are independent of surface events, suggesting the importance of ocean dynamics whereas in Seychelles, this figure is only 57% due to a shallower thermocline which is more sensitive to atmospheric forcing. Favourable dynamics for subsurface MHWs and sea surface height (SSH) extremes in Comoros tending to occur in May–June include strengthening of the North East Madagascar Current, and prevailing downwelling winds, which deepen the thermocline. Subsurface MHWs and SSH extremes in Seychelles tend to be strongest in February–March but longest-lasting in November–December. In March, intense subsurface MHWs occur due to a relaxation of the winds and downwelling winds. In November–December, long-lasting subsurface MHWs seem linked to the intrusion of the South Equatorial Counter Current, mesoscale activity and downwelling Rossby waves. During the Indian Ocean Dipole events of 1997/98 and 2019/20, long-lived MHW intensities in Seychelles reached 9 °C. Long-lived subsurface MHWs have a higher SSH, and ocean heat content related to a deepening of the thermocline, but lower eddy kinetic energy, characterized by slow-moving and low amplitude anticyclonic eddies. These findings provide insights into the nature of short-lived and long-lived subsurface MHWs, necessary for understanding and mitigating their impacts.