Increased Frequency but Decreased Intensity of Marine Heatwaves Around Coral Reef Regions in the Southern South China Sea

Marine heatwaves (MHWs) in the South China Sea (SCS) significantly affect both fishery resources and marine ecosystems. The characteristics and causal mechanisms of MHWs occurring in the southern SCS (SSCS) are not yet fully understood. In this study, the properties of MHWs, their long-term trends, and how these compare to those in the coastal shelf region in the northern SCS were explored. It was revealed that the coral reef regions in the SSCS exhibit more frequent but less intense MHWs. Over the past four decades, the average frequency, duration, and total days of MHWs have increased and could be attributed to rising mean sea surface temperature (SST), primarily driven by the horizontal advection, particularly the zonal advection. The associated advections result from the long-term wind stress curl change in the SSCS and strengthened Kuroshio Current, which are due to global warming-induced enhanced vertical density stratification and wind speed acceleration. Furthermore, significant decrease in the MHW intensity around coral reef regions is mainly due to a negative SST-cloud feedback mechanism: during MHW events, enhanced latent heat loss intensifies convection, leading to total cloud formation, which in turn reduces solar radiation and subsequently decreases the MHW intensity. Interestingly, this increase in deep convection and decrease in the MHW intensity appear to coincide with the phase transition of the Interdecadal Pacific Oscillation. Our findings underscore the divergent trends in MHW properties in the SSCS, providing valuable insights into their potential impact on the region's coral reefs. Marine heatwaves (MHWs) are extended periods of abnormally high-temperature ocean events that can have devastating consequences for marine ecosystems, such as coral bleaching. Although recent studies have reported the characteristics and mechanisms of MHWs in the South China Sea (SCS), few studies have focused on the southern SCS (SSCS), which contains numerous coral reefs and is a hot spot for MHWs. In this study, we revealed the long-term trends and driving factors of MHWs in the SSCS over the past four decades. The results suggested that significant increasing trends in the frequency, duration and total days of MHWs occurred over the past 41 years, which could be attributed to the global warming-induced rise in the SST. However, the trends in the mean and maximum intensities decreased, which is a result of the negative feedback process between the SST and clouds. Our results provide a comparative analysis of MHWs in the SSCS and show that a negative SST-cloud feedback-induced decreasing trend in the MHW intensity could reduce the threat of ocean warming to coral reefs. More frequent and prolonged MHWs in the SSCS are primarily attributed to the rising mean SST induced by horizontal advection The significant negative trends in the intensity of MHWs over coral reef zones are mainly driven by SST-cloud negative feedback The combination of global warming and a regime shift in the IPO is the key large-scale forcing linking MHWs in the SSCS