Marine heatwaves in the central Mediterranean : frequency and spatial distribution (2008-2025)

Abstract

The Mediterranean Sea is warming faster than the global ocean average, making its semi enclosed, relatively shallow basin particularly exposed to extreme thermal events. Marine Heatwaves (MHWs), prolonged periods of anomalously high ocean temperature, offer an ecologically meaningful way to characterize this warming because they capture disruptive extremes rather than gradual mean trends. This study quantifies the frequency of surface MHWs in the waters around the Maltese Islands, including the 12-nautical-mile territorial sea and the 25-nautical-mile Fisheries Management Zone (FMZ), over a 17-year period from 2008-2025. Daily ultra-high-resolution sea surface temperature analyses from the Copernicus Marine Service were used to construct, for every grid cell, a seasonally varying daily 90thpercentile threshold. Following the minimum-duration criterion of Hobday et al. (2016), periods in which temperature exceeded this threshold for at least five consecutive days were classified as MHW events, and events were counted for every grid cell and mapped across the domain. A representative cell south of Malta was used to validate the detection procedure. Across the domain, the mid-August 90th-percentile threshold ranged from 27.7 °C to 29.6 °C (mean 28.6 °C), and the number of detected events per grid cell over the study period ranged from 26 to 53, with a domain mean of approximately 40. Exposure was spatially uneven. Event frequency was highest in the western and northwestern waters and lowest in the waters immediately south and southeast of the Maltese Islands. At the validation cell, 37 events were detected, and threshold exceedances recurred in most summers, becoming more frequent toward the end of the record, with the warmest peak in 2023. These results show that recurrent surface thermal extremes are pervasive but spatially structured across Maltese waters, with direct relevance to fisheries management, marine spatial planning, and climate-adaptation planning. Methodological limitations, including a simplified climatology, a surface-only focus, and a baseline drawn from a warming period, are discussed, and depth-resolved analysis is identified as a priority for future work.