Trends and variability of marine heatwaves in Portuguese coastal waters

Marine heatwaves (MHWs) represent a significant threat to marine ecosystems, particularly in vital fisheries regions such as the Portuguese coast. Understanding MHW dynamics at a finer spatial scale is essential for comprehending their impacts on the ecosystems. This study addresses this gap by analyzing historical MHW events from 1982 to 2023 and projecting future scenarios based on different greenhouse gas emission pathways. The primary objective is to characterize the spatiotemporal patterns of MHWs along the Portuguese coast and understand their temporal evolution. Using historical sea surface temperature (SST) data, MHW metrics were analyzed across various coastal regions. Future projections, utilizing Shared Socioeconomic Pathways (SSP) 245 and SSP 585, assessed changes in MHW frequency, duration, and intensity in the near future (2024-2060) and far future (2061-2100). Results indicate no significant spatial differences in average annual MHW metrics among study areas, but significant temporal variations were observed. The progression of MHW metrics is often more than twice as fast in the slopes and southern areas compared to the west shelf. Record-breaking MHW events in 2023 were consistent with global trends, highlighting a positive linear correlation between historical warming and increased MHW metrics. Projections under SSP 585 indicate a 2.5 to 4.5-fold increase in the annual number of MHW events, lasting 1 to 3 months, with nearly year-round MHW conditions by 2100, reaching peak intensities of 10.7 °C. These findings highlight the need for climate action to mitigate the impacts of intensified MHWs on coastal ecosystems. This research provides foundational insights into MHW dynamics along the Portuguese coast, laying the groundwork for future studies and highlighting the implications of climate change on marine environments.