Evolution of marine heatwaves in warming seas: the Mediterranean Sea case study

Abstract

Anomalous warming of the upper ocean is increasingly being observed in the Mediterranean Sea. Extreme events, known as marine heatwaves (MHWs), can have a profound impact on marine ecosystems, and their correct detection and characterization are crucial to define future impact scenarios. Here, we analyze MHWs observed over the last 41 years (1982–2022) in the Mediterranean sea surface temperatures (SSTs). We show that the intensification in frequency, intensity, and duration of Mediterranean MHWs in recent years is mainly due to a shift in SST mean that occurred in the last two decades and largely reduced when analyzing detrended SST data. Detrending thus allows the use of a fixed climatology without overestimating MHW properties over time and distinguishes long-term warming (i.e., trend) from transient and abrupt SST changes. Analogous results are also found over a shorter temporal period, by analyzing 13 years (2007–2020) of in situ data collected at different depths (5 to 40 m) at Columbretes Islands. Additionally, the in situ analysis reveals that atmospheric summer heatwaves could affect a layer of 10 m in depth. Lastly, a catalogue of the major Mediterranean MHWs that have occurred since 1982 is presented. This catalogue evidences an exceptionally long-lasting and intense MHW, starting in May 2022 and persisting, at least, until the end of the year, resulting in the event with the highest cumulative intensity just after the well-known 2003 MHW event.

Characterization of marine heat waves in the eastern tropical Atlantic Ocean

“…Likewise, a category III or Severe event corresponds to an anomaly 3 times greater than the difference and a category IV or Extreme event corresponds to an anomaly 4 times greater than the difference. Authors such as Martìnez et al (2023) suppressed the SST trend before applying the Hobday method for detecting MHWs. They consider that this suppression made it possible to use a fixed climatology without overestimating the properties of MHWs over time.…”

Attributing daily ocean temperatures to anthropogenic climate change

“…The 2022 Mediterranean MHW began in May and persisted through at least the end of the year (Martínez et al 2023). SST anomalies (figure 10, red) increased markedly in late spring and remained elevated through the remainder of 2022.…”

The Heatwave of Summer 2022 in the North-Western Mediterranean Sea: Some Species Were Winners

“…There is general agreement [60][61][62] that the intense warming of the surface layer started early, during May 2022, since by June SSTs (sea surface temperatures, based on satellite data) were already above normal in the western Mediterranean. Martinez et al [10] even compute that the first 2022 severe MHW started by 15 June 2023. Then, in the NW part of the Mediterranean as well as throughout the Mediterranean, the SST has constantly been higher than the climatological baseline until spring 2023 (at least).…”

“…At the Mediterranean scale, the SST anomalies ranged between 1.5 and 2 • C during the meteorological summer, and until April 2023 all anomalies ranged between 0.5 and 1.5 • C. At the local scale, some SST anomalies higher than 2 • C persisted in the south-western part. In their Mediterranean main MHW catalogue, Martinez et al [10] found two MHW events classified as 'severe' (as in 2003): from 15 June to 21 August 2022 and from 24 October to 22 November 2022. Whatever the domain considered and the criteria applied, this extreme event had a record-breaking duration.…”

“…The T-MedNet website ( [64]; https://t-mednet.org/visualize-data/temperature?view= tfigure, accessed on 7 December 2023) clearly showed that, by the end of 2022, the temperature anomalies reached deeper than 40 m. This resulted in the event with the highest cumulative intensity just after the well-known 2003 event [10].…”