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How Detection Frameworks Shape Our View of 2024's Mediterranean Marine Heatwaves

Fernández‑Álvarez, Blanca

Publisher
Zenodo
DOI
10.5281/zenodo.17486877
Repository
zenodo.org
Language
en

Abstract

Abstract Oceans absorb most anthropogenic heat and a large share of carbon emissions, driving sea surface temperature (SST) increases and intensifying marine heatwaves (MHWs). Although rising MHW frequency is well documented, most studies rely on the standard fixed‑baseline approach (Hobday et al., 2016), which uses a static climatology and is sensitive to long‑term warming, thereby amplifying recent extremes and raising concerns about future MHW “saturation” (Rosselló et al., 2023). To address these issues, we compare three detection frameworks—fixed baseline (1982–2011), 20‑year moving baseline (Rosselló et al., 2023), and a detrended method that removes the linear warming trend (Martínez et al., 2023)—across the Mediterranean Sea, a rapidly warming and ecologically vulnerable basin. Using SST data for 1982–2024, all three methods identify 2024 as the peak MHW year of the past two decades, with fixed and moving baselines also registering the longest‑lasting events. Regionally, the eastern Mediterranean, where warming is strongest, recorded the most MHW days in 2024, whereas the western basin experienced exceptionally intense events in 2022. Depending on the detection framework, estimated MHW exposure across the Mediterranean can vary by up to fourfold. Fixed baselines remain useful for assessing thermal stress relative to historical conditions, while moving and detrended methods better track anomalies in a changing climate. Aligning detection frameworks with research and policy goals is therefore crucial for reliable climate risk assessment and regional adaptation planning. The research of this talk has been published on Ocean Science https://os.copernicus.org/articles/21/1987/2025/ Our speaker Blanca Fernández‑Álvarez is a researcher at the IMEDEA CSIC and works with Ananda Pascual. What is a Climate Coffee? #climatecoffees are short (circa 40 min: 20 min talk + 20 min Q&A), relaxed meetings for scientists to share ideas, discuss methods, and communicate new results. They are open to speakers of all seniority; we especially encourage early-career scientists to become speakers. The Coffees are an exciting opportunity for scientists to build a network and disseminate recent results peer-to-peer. We invite researchers from across the climate science community to join us for this series of regular online knowledge exchange events. Video of the talk: https://www.youtube.com/watch?v=3jVrQQ0D9y4&list=PLvVwf0FRZw4oQyHw-6TMm0u2FjFGvnvx6&index=1 Organisation The Climate Coffees are organized by the Horizon Europe projects ObsSea4Clim, OCEAN ICE, the National Center for Climate Research - DMI Danish Meteorological Institute and the European Climate Research Alliance. Sign up for the next climate coffees: https://www.eventbrite.com/cc/climate-coffees-series-talks-on-and-about-climate-2655309 Disclaimer Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or European Research Executive Agency (REA). Neither the European Union nor the granting authority can be held responsible for them.

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OCTOBER Climate coffee - 30 October 2025 How Detection Frameworks Shape Our View of 2024’s Mediterranean Marine Heatwaves Blanca Fernández-Álvarez Institut Mediterrani d’Estudis Avançats, IMEDEA (CSIC-UIB), Esporles, Spain 1 Introduction They bleach corals, affect kelp forests, and cause mass mortalities of benthic organisms Marine heatwaves, why? Smith et al., 2024 2 Smith et al., 2024 Introduction They bleach corals, affect kelp forests, and cause mass mortalities of benthic organisms Smith et al., 2024 3 Marine heatwaves, why? Smith et al., 2024 Smith et al., 2024 Introduction They bleach corals, affect kelp forests, and cause mass mortalities of benthic organisms 4 Marine heatwaves, why? Smith et al., 2024 Smith et al., 2024 They bleach corals, affect kelp forests, and cause mass mortalities of benthic organisms Introduction So they matter both ecologically and socially Smith et al., 2021 5 Marine heatwaves, why? Introduction High-temperature event where a threshold value is exceeded for a minimum of five consecutive days (Hobday et al., 2016) Threshold: 90th percentile of the distribution at each point over a specific climatology. Climatology: 1982-2023 2022 MHW definition 6 Masters’ Degree Thesis (Started in 2024) Balearic Sea 7 Barceló-Llull et al., 2019 Balearic Basin bounded by the Balearic Islands The western Mediterranean is one of the ecoregions with the greatest risk of species loss due to climate change (Chatzimentor et al., 2023) Processes that happen in the islands can affect commercially important fish stocks (Heslop et al., 2012), like the Atlantic bluefin tuna (Reglero et al., 2018: 2019) Balearic Sea, a subregion of the western Mediterranean Balearic Sea 8 Product: CMEMS Mediterranean Sea - High Resolution L4 Sea Surface Temperature Reprocessed ID: ST_MED_SST_L4_REP_OBSERVATIONS_0 10_021 SST Satellite – Copernicus Marine Service Mediterranean Sea - High Resolution L4 Sea Surface Temperature Reprocessed - Product ID: SST_MED_SST_L4_REP_OBSERVATIONS_010_021 - Spatial resolution: 0.05° (~ 5 km) - Temporal resolution: Daily Balearic Sea 9 Fixed baseline Common climatology (1982-2011) Detrended dataset + fixed baseline Remove trend + common climatology Moving baseline Climatology from previous 20 years Only the fixed baseline presents a significant trend on MHW metrics Results Balearic Sea 16 Expanding the study to the whole Mediterranean…. An annual publication by the CMEMS that provides a comprehensive assessment of the current conditions and changes in the global ocean Mediterranean Sea 17 Backdrop –The Mediterranean Mediterranean Sea Drivers in the Mediterranean: −weakened winds, anomalous heating, and reduced mixing modulated by atmospheric circulation Denaxa et al. (2024) Ecological & socio-economic impacts: −mass mortality events (corals, benthic communities) −shifts in species distribution −fisheries stress Backdrop –The Mediterranean Mediterranean Sea Drivers in the Mediterranean: −weakened winds, anomalous heating, and reduced mixing modulated by atmospheric circulation Denaxa et al. (2024) Ecological & socio-economic impacts: −mass mortality events (corals, benthic communities) −shifts in species distribution −fisheries stress SST Trend (1982-2024) The eastern Mediterranean has a highest warming trend than the western And this trend is ~2 times the trend of the global ocean MED 0.032 ± 0.001 ºC/year Eastern MED 0.034 ± 0.001 ºC/year Western MED 0.027 ± 0.002 ºC/year As previously reported by Pisano et al., (2020), Juza et al., (2022) and Pastor et al., (2020), amongst others Mediterranean Sea 20 Regardless of the method, 2024 has the most MHWs days of the last two decades And in the case of fixed baseline also the longest, and with the moving, the second longest MHWs results MHW days Mean duration Mediterranean Sea 21 WMed: More intense EMed: Many MHW days (especially with the fixed method) MHW days Mean duration Mean intensity MHWs results Mediterranean Sea 22 WMed: More intense EMed: Many MHW days (especially with the fixed method) Consistent with Simon et al. (2023) and Denaxa et al. (2025): MHWs in the EMed are driven by the long-term warming of the Sea MHW days Mean duration Mean intensity MHWs results Mediterranean Sea 23 Saturation!! Fixed baseline Common climatology (1982-2011) Detrended dataset + fixed baseline Remove trend + common climatology Moving baseline Climatology from previous 20 years MHWs results Mediterranean Sea 24 Discussion ★Most MHW days on the last 20 years for the Mediterranean ★Longest mean duration of events ★Particularly long in the Eastern, less so in the Western ★Results consistent across all detection methods, even after accounting for long-term warming Summary of results 2024 presented… 25 This is only one of the many choices the MHW community has identified as variables when quantifying MHWs Change in paradigm Discussion 32 This is only one of the many choices the MHW community has identified as variables when quantifying MHWs Change in paradigm Discussion 33 This is only one of the many choices the MHW community has identified as variables when quantifying MHWs Change in paradigm Discussion 34 This is only one of the many choices the MHW community has identified as variables when quantifying MHWs Change in paradigm Discussion 35 {Input Type} −Raw −Detrended {Climatology} − WMO Climatology −Reanalysis Climatology −30-year window {Threshold Percentile} − 90p −95p −99p {Duration} −5 days −10 days −30 days Marine Heat Waves –Multiple Analysis / Definitions (MHW-MAD): A Multi-Definition Global Marine Heatwave Dataset from Satellite Sea Surface Temperature data Hayward et al., 2025 (in prep.): As part of the an atlas and publication in prep: (Result of the 2025 General Assembly HACKATHON from task 4.5) https://download.dmi.dk/public/MHW/ Product: ESA SST CCI -1982-2025 - Daily - 0.05º global -https://doi.org/10.48670/moi-00169 Discussion 36 Ongoing work Ongoing 37 MonSubHeat Monitoring the Subsurface signature of Marine Heatwaves in the Balearic Sea PI: Bàrbara Barceló-Llull Key questions: - What is the subsurface signature of MHWs? - What is the impact of MHWs in stratification? SOCIB Glider Team Slide adapted from B. Barceló-Llull Subsurface Ongoing 38 MonSubHeat PI: Bàrbara Barceló-Llull - What is the subsurface signature of MHWs? - What is the impact of MHWs in stratification? − 1114 CTD profiles − 42 days (10 June - 21 July) N. Zarokanellos Slide adapted from B. Barceló-Llull Ongoing 39 MAREM In colaboration with Centro Ocenográfico de las Baleares (IEO-CSIC) Monitoring since 2008: invasive algal species and long-term coastal temperature sensors Impacts - Do different definitions really matter when assessing the impacts? Ongoing 40 OCTOBER Climate coffee - 30 October 2025 Questions? 41