Tracking Marine Heatwaves in the Balearic Sea: Temperature Trends and the Role of Detection Methods

Abstract. Marine heatwaves (MHWs) are defined as discrete periods of anomalous ocean warming. In the most commonly used MHW determination method, the threshold over which a certain temperature is considered a MHW is calculated using a fixed baseline constructed from a common climatology (1982–2001). By this definition, these phenomena have been increasing in frequency and intensity due to global warming, and it is expected to ultimately lead to a saturation point. Significant efforts have been directed towards developing new ways of defining marine heatwaves motivated by the need to differentiate between long-term temperature trends and extreme events. The Mediterranean Sea serves as an ideal backdrop for comparing different MHW detection methods due to its rapid response to climate change, with higher warming trends than the global ocean. In this work, we evaluate sea surface temperature trends in the Balearic Sea, a subregion of the western Mediterranean, and compare the fixed baseline MHW detection method with two recently developed alternative methodologies. The first alternative employs a moving climatology to adjust the baseline, while the second method involves detrending the temperature data before detecting MHWs with a fixed baseline. Our analysis reveals a statistically significant warming trend of 0.036 ± 0.001 °C per year, which represents an increase of ~10 % compared to previous studies in the same region due to the inclusion of two particularly warm recent years, 2022 and 2023. Regarding MHWs, all three methods identify major events in 2003 and 2022. However, the fixed baseline method indicates an increase in MHW frequency and duration over time, a tendency not detected by the other methodologies, since we are isolating the extreme events from the long-term warming trend. This study underscores the importance of selecting an appropriate MHW detection method that aligns with the intended impact assessments. Studies performed with a moving baseline or detrended data could be more appropriate to analyse species with higher adaptability, while a fixed baseline could be a better option to study species less adaptable and more sensitive to exceeding a critical temperature threshold.