Compound atmospheric and marine hot extremes coupled with droughts in Madagascar

Madagascar is the fourth largest island, bursting with vast biodiversity as well as unique and plentiful natural resources. However, it is also one of the poorest countries in the world and highly susceptible to natural hazards that affect its subsistence farming system, ranking among the top ten countries impacted by extreme weather events, such as cyclonic storms, droughts. Madagascar has seen an increase in the frequency and duration of meteorological droughts and an intensification of heat extremes, which are expected to worsen under a global warming scenario, with the western and southwestern parts of the island projected to experience the highest temperature rises. This study aims to assess the occurrence of hot atmospheric and marine extreme events - specifically, marine heatwaves (MHWs) and atmospheric heatwaves (AHWs)—in Madagascar, evaluating their trends and the synergistic effects with drought events from 1979 to 2021. We rely on ERA5 reanalysis data to analyze long-term trends in temperature, precipitation, and the frequency of MHWs and AHWs, along with the occurrence of droughts, characterized using the Standardized Precipitation-Evapotranspiration Index (SPEI). A significant warming trend of 0.22°C per decade was observed in atmospheric temperatures (p < 0.05), with a shift toward warmer conditions after 1999, while precipitation showed no significant long-term trend but increasing association with high temperatures in recent decades. Marine and atmospheric heatwaves exhibited statistically significant upward trends in frequency and duration, with the number of events increasing by approximately 2.6 and 4.7 events over the study period, respectively. Heatwave duration was consistently longer and more variable in the ocean, while intensity was higher and more variable in the atmosphere. Drought conditions have become more prevalent and severe since 1999, with 60% of years exhibiting negative SPEI-12 values and statistically significant increases in moderate, severe, and extreme droughts (p < 0.05). The year 2020-2021 was particularly extreme, with nearly complete spatial coverage of severe to extreme drought conditions across Madagascar and adjacent continental regions. These compound extreme events are predicted to significantly affect Madagascar's agricultural systems, amplifying the risks to food security. The study emphasizes the need for integrated strategies in risk assessment and management, highlighting the necessity of considering compound events in future climate adaptation efforts.