Has the frequency of Mediterranean Marine Heatwaves really increased in the last decades?

Marine Heat Waves (MHWs) are events of prolonged anomalously warm water, in portions of the oceans, which have severe impacts on the local marine ecosystems. In a climate change scenario, with increasing temperatures and more frequent atmospheric extreme events, the frequency and intensity of the MHWs are expected to increase. In such a scenario, the choice of the long-term baseline used to compute SST anomalies in such a non-stationary system, becomes a critical issue, since it can significantly affect the frequency and intensity of the events. Here, we critically address the problem of how to characterize and define MHWs in the present warming climate scenario by evaluating the impact of different SST climatic baselines, and the effects of removing climate trends from the original SST time series. We focus on the Mediterranean Sea, a hot spot region for climate change, where a strong mean SST increasing trend (about $0.045 {}^\circC/year)$ has been observed in the last 40 years. This case study focuses on examining the conditions during 2022/2023 and the sensitivity of the detection under different preprocessing methods that account for trends and climate variability.