Marine heatwaves in the Mozambique Channel

Marine heatwaves (MHWs) are becoming more frequent and intense around the world leading to long-term impacts on ecosystems and subsequent socioeconomic consequences. To date, MHWs have not been considered in the Mozambique Channel, a highly biodiverse region with sensitive coral reefs. An assessment of MHW metrics here shows that these events tend to be more intense and last longer in the southeastern part of the channel especially during austral summer. Focus is placed on a particularly sensitive region west of southern Madagascar which shows significant positive trends in MHW metrics over the period 1982–2019 (frequency, intensity and duration). In austral summer 2017, this region experienced the longest and most intense MHW recorded in the past 35 years. This event lasted for 48 days and reached a maximum intensity of 3.44 °C above climatology. The warming largely resulted from anomalous net surface heat fluxes driven by weaker winds and increased insolation but was modulated by horizontal advection and the presence of an anticyclonic eddy. The wind and insolation anomalies were associated with a strong positive Subtropical Indian Ocean Dipole (SIOD), an anomalously weak Mozambique Channel Trough and westward extending Mascarene High. Correlation analysis suggests that long-lasting and intense MHW events occurring between January and March in this region may be associated with positive SIOD events. Changes in winds and surface heat fluxes associated with tropical cyclone Dineo appear to have briefly weakened the event in mid-February whereas those associated with ex-tropical cyclone Enawo appear to have contributed to its end in mid-March 2017.