Marine heat waves in the Mediterranean Sea: An assessment from the surface to the subsurface to meet national needs

Abstract

In the Mediterranean Sea, marine ecosystems and the resource-based economy are shared among many countries, making this a regional sea of important geopolitical and economic stakes. Over the last decades, marine heat waves (MHWs) in the Mediterranean Sea have caused mass-mortality events in various marine species and critical losses for seafood industries. MHWs are expected to become more intense, longer and more frequent through anthropogenic warming. This study proposes to better understand how much each Mediterranean country’s Exclusive Economic Zone (EEZ) waters may be affected by MHW changes, to contribute to decision support for management and adaptation at national scale. The variability of surface and subsurface MHWs is assessed over the 1987-2019 period in the Mediterranean EEZs, which are ocean areas where sovereign states have special rights. Combining high-resolution satellite observations and a regional reanalysis, sea surface temperature and ocean heat content are used to define surface and subsurface MHWs. The MHW characteristics selected in this study highlight the important differences between surface and subsurface extreme events. MHW frequency is higher at the surface than in the subsurface and has significantly increased in most EEZs both at the surface and in the subsurface, while MHW duration is longer in the subsurface than at the surface in all EEZs. MHW intensities decrease with depth, while its increase over time is more disparate throughout the basin. MHW maximum intensity displays significant positive trends with higher surface values in the western Mediterranean Sea, while in the subsurface it reaches its extreme values in the EEZs of the Levantine basin. In contrast, MHW cumulative intensity exhibits its extreme trend values in the eastern Mediterranean Sea both at the surface and in the subsurface. The choice of a “Top-Ten” list of EEZs shows that the impact to EEZs is different depending on the MHW characteristics and the depth, emphasizing the need to consider all MHW characteristics and to avoid focusing only on the surface. Overall, the results highlight the necessity of strengthening surface and subsurface observing systems in most national waters to better establish local-scale risk assessments and to respond to diverse stakeholder needs.

Investigating extreme marine summers in the Mediterranean Sea

“…MHW analyses based on satellite SST observations by Ibrahim et al (2021) and Juza et al (2022) for the period 1982-2020 as well as Dayan et al (2023) for the period 1987-2019 are also consistent with the current findings for summer MHWs, despite their shorter reference periods and the season-independent analysis. However, all aforementioned studies report larger MHW durations in the southeastern MS compared to our results.…”

“…MHWs are discrete events lasting at least 5 consecutive days, with temperatures exceeding a percentile-based (90 %) threshold (Hobday et al, 2016). Over the past decades, an increased MHW intensity and frequency have been documented based on observational and modelled SST datasets (Darmaraki et al, 2019a;Juza et al, 2022;Pastor and Khodayar, 2023;Dayan et al, 2023). Further increase of MHW trends is expected in the basin over the 21st century due to anthropogenic forcing and especially under high-emission scenarios (Darmaraki et al, 2019b;Oliver et al, 2019;Plecha and Soares, 2019).…”

Scale‐Dependent Drivers of Marine Heatwaves Globally

“…Atmospheric processes are manifested in the form of anomalous sea surface heat flux into the ocean (Bond et al., 2015; Jacox et al., 2019; Rodrigues et al., 2019; Schlegel et al., 2021), change of ocean advection via the Ekman dynamics (Behrens et al., 2019; Marshall & Hendon, 2014), and suppressed vertical mixing due to the weakened wind stirring and/or enhanced surface buoyancy gain (Alexander et al., 1999; Benthuysen et al., 2014; Kataoka et al., 2017). Representative examples of atmosphere‐driven MHWs include the Blob (Di Lorenzo & Mantua, 2016) and Blob 2.0 (Amaya et al., 2020) in the Northeast Pacific and the 2019 event in the Mediterranean Sea (Dayan et al., 2023), which are concurrent with anomalous wind forcing or solar radiation caused by high‐pressure systems (Sen Gupta et al., 2020). Oceanic processes can drive MHWs directly through anomalous heat advection, especially in the boundary currents as well as their extensions (Bian et al., 2023; Marin et al., 2022).…”

Influence of sea stratification and troposphere stability over the Coastal Squall Lines of Eastern Amazon

“…Thus, the OHC and the BLT include variations in temperature and salinity throughout the water column. These sets of parameters prove to be robust enough to identify regime changes in stratification, beyond considering only SST (Dayan et al, 2023).…”