Integrating Oceanographic Data and Benthic Community Structure Temporal Series to Assess the Dynamics of a Marginal Reef

The future evolution of sea surface temperature (SST) extremes is of great concern, not only for the health of marine ecosystems and sustainability of commercial fisheries, but also for precipitation extremes fueled by moisture evaporated from the ocean. This study examines the projected influence of anthropogenic climate change on the intensity and duration of monthly SST extremes, hereafter termed marine heat waves (MHWs) and marine cold waves (MCWs), based on initial-condition large ensembles with seven Earth system models. The large number of simulations (30–100) with each model allows for robust quantification of future changes in both the mean state and variability in each model. In general, models indicate that future changes in variability will cause MHW and MCW events to intensify in the northern extratropics and weaken in the tropics and Southern Ocean, and to shorten in duration in many areas. These changes are generally symmetric between MHWs and MCWs, except for the longitude of duration change in the tropical Pacific and sign of duration change in the Arctic. Projected changes in ENSO account for a large fraction of the variability-induced changes in MHW and MCW characteristics in each model and are responsible for much of the intermodel spread as a result of differences in future ENSO behavior. The variability-related changes in MHW and MCW characteristics noted above are superimposed upon large mean-state changes. Indeed, their contribution to the total change in SST during MHW and MCW events is generally <10% except in polar regions where they contribute upward of 50%.

In the ocean, heat waves are vital climatic extremes that can destroy the ecosystem together with ensuing socioeconomic consequences. Marine heat waves (MHW) recently attracted public interest, as well as scientific researchers, which motivates us to analyze the current heat wave events over the Red Sea and its surrounding sea region (Gulf of Aden). First, a comprehensive evaluation of how the extreme Red Sea surface temperature has been changing is presented using 0.25° daily gridded optimum interpolation sea surface temperature (OISST, V2.1) data from 1982 to 2020. Second, an analysis of the MHW’s general behavior using four different metrics over the study area, together with a study of the role of climate variability in MHW characteristics, is presented. Finally, the main spatiotemporal characteristics of MHWs were analyzed based on three different metrics to describe MHW’s local features. Over the studied 39 years, the current results showed that the threshold of warm extreme sea surface temperature events (90th percentile) is 30.03 °C, providing an additional average thermal restriction to MHW threshold values (this value is changed from one grid to another). The current analysis discovered 28 separate MHW events over the Red+, extending from 1988 to 2020, with the four longest events being chosen as a study case for future investigation. For the effect of climate variability, our results during the chosen study cases prove that ENSO and ISMI do not play a significant role in controlling MHW characteristics (except the MHW intensity, which has a clear relation with ENSO/ISMI) on Red+. Moreover, the chlorophyll concentration decreases more significantly than its climatic values during MHW events, showing the importance of the MHW effect on biological Red Sea features. In general, the MHW intensity and duration exhibit a meridional gradient, which increases from north to south over the Red Sea, unlike the MHW frequency, which decreases meridionally.

Marine heatwaves (MHWs) have been documented around the world, causing widespread mortality of numerous benthic species on shallow reefs (less than 15 m depth). Deeper habitats are hypothesized to be a potential refuge from environmental extremes, though we have little understanding of the response of deeper benthic communities to MHWs. Here, we show how increasing depth moderates the response of seaweed- and coral-dominated benthic communities to an extreme MHW across a subtropical–temperate biogeographical transition zone. Benthic community composition and key habitat-building species were characterized across three depths (15, 25 and 40 m) before and several times after the 2011 Western Australian MHW to assess resistance during and recovery after the heatwave. We found high natural variability in benthic community composition along the biogeographic transition zone and across depths with a clear shift in the composition after the MHW in shallow (15 m) sites but a lot less in deeper communities (40 m). Most importantly, key habitat-building seaweeds such as Ecklonia radiata and Syctothalia dorycarpa which had catastrophic losses on shallow reefs, remained and were less affected in deeper communities. Evidently, deep reefs have the potential to act as a refuge during MHWs for the foundation species of shallow reefs in this region.

Over the past decades, ocean temperatures have risen substantially, with far-reaching consequences for marine biodiversity and ecosystem functioning. Marine heat waves (MHWs), have increased in both duration and frequency, leading to shifts in species composition and ecosystem processes. We analysed the effects of repeated MHWs on recruitment of benthic communities in a Marine Protected Area (MPA) in Southern Sardinia. We used standardized sampling units, Autonomous Reef Monitoring Structures (ARMS), combined with high-throughput sequencing of mitochondrial cytochrome c oxidase subunit I. This allowed us to measure short-term variations in benthic biodiversity at two sites, Berni and Santa Caterina, which initially had different temperatures. We found 241 eukaryotic families belonging to 22 different phyla recruited on ARMS. Both before and after MHWs, Porifera were the most abundant phylum (21-37%), followed by Bryozoa (27-16%) and Cnidaria (15-18%). Before MHWs, the two sites exhibited different taxa richness and taxonomic composition, with significantly higher alpha diversity in the warmer site. After the MHWs train, alpha diversity did not change significantly. However, the taxonomic composition of the two sites tended to homogenise, resulting in a significant decrease in beta diversity. In both sites, the variation in the benthic assemblage structure after the MHWs train was driven by the decreased abundance of slow-moving and less heat-tolerant taxa (e.g., Bryozoa, Nudibranchia, Rhodophyta). This decrease was counterbalanced by an increase in mobile and more heat-tolerant ones (e.g., Decapoda). The short-term outcome of repeated MHWs is a homogenization of the benthic recruits' community, without any significant change in the number of taxa. Because biodiversity and ecosystem functioning are generally positively linked, the short-term effects of MHWs may have limited impacts on ecosystem functioning. However, they could reshape the benthic community composition, altering the ratio of 'winners' to 'losers' taxa contributing to ecosystem functions.

Marine heatwaves (MHW) are becoming stronger and more frequent across the globe. MHWs affect the thermal physiology of all biological organisms, but wider ecosystem effects are particularly impactful when large habitat-forming foundation species such as kelps are affected. Many studies on impacts from MHWs on kelps have focused on temperature effects in isolation, except for a few studies that have integrated co-occurring stress from grazers, wave exposure and nutrient limitation. It is likely that many stressors act in concert with MHWs and exacerbate their effects. Here we analyzed satellite images over 60 months to assess temporal changes in abundance of surface canopies of the giant kelp Macrocystis pyrifera in the New Zealand coastal zone. The analysis encompassed the most extreme MHW on record (2017/18), across a 6° latitudinal gradient of four regions southward from the northern distributional limit of Macrocystis along mainland New Zealand. We tested the association of surface canopy cover of Macrocystis with sea surface temperature, temperature anomalies, chlorophyll-a (a proxy for nutrient availability) and water clarity (diffuse attenuation coefficient). We found a reduced cover of Macrocystis across all regions during and after the 2017/18 MHW, with least impact at the most southern region where the maximum temperatures did not exceed 18°C. There was also an important and significant interaction between temperature and water clarity, showing that temperature-induced kelp loss was greater when water clarity was poor. These results show that notable negative effects occurred across the coastal range of this foundation species and highlight the importance of studying MHW effects across latitudinal gradients and in concert with other co-occurring stressors.

The spatiotemporal evolution of marine heatwaves (MHWs) is explored using a tracking algorithm called Ocetrac that provides the objective characterization of MHW spatiotemporal evolution. Candidate MHW grid points are defined in detrended gridded sea temperature data using a seasonally varying temperature threshold. Identified MHW points are collected into spatially distinct objects using edge detection with weak sensitivity to edge detection and size percentile threshold criteria at each time step. Ocetrac then uses 3D connectivity to determine if these objects are part of the same event, but Ocetrac only defines the full MHW event after all time steps have been processed, limiting its use in predictability studies. Here, Ocetrac is applied to monthly satellite sea surface temperature data from September 1981 through January 2021. The resulting MHWs are characterized by their intensity, duration, and total area covered. The global analysis shows that MHWs in the Gulf of Maine and Mediterranean Sea are spatially isolated, while major MHWs in the Pacific and Indian Oceans are connected in space and time. The largest and most long-lasting MHW using this method lasts for 60 months from November 2013 to October 2018, encompassing previously identified MHW events including those in the northeast Pacific (2014–15), the Tasman Sea (2015–16, 2017–18), and the Great Barrier Reef (2016). Significance Statement This study introduces Ocetrac, a method to track the spatiotemporal evolution of marine heatwaves (MHWs). It is applied to satellite sea surface temperature data from 1981 to 2021. The method objectively identifies and tracks MHWs in space and time while allowing for splitting and merging. The resulting MHWs are characterized by intensity, duration, and total area covered. Marine heatwaves can have significant ecological consequences, including biodiversity loss and mortality, geographical shifts, and range reductions in marine species and community structure changes when physiological thresholds are exceeded. This results in both ecological and economic impacts. Ocetrac provides a method of tracking the space and time evolution of MHWs that can provide a visualization that demonstrates the global impact of these events.

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.

High-resolution marine heatwave mapping in Australasian waters using Himawari-8 SST and SSTAARS data

Marine heatwaves (MHWs)—prolonged periods of anomalously warm sea surface temperatures (SST)—pose significant ecological and economic challenges, particularly for aquaculture sectors sensitive to temperature variability around Ireland. This study integrates 43 years of historical daily SST data (1982–2024) from NOAA, ICES, and the Marine Institute to develop a comprehensive deep-learning framework for predicting SST and detecting MHWs in the Irish maritime region.A comparative analysis of two MHW detection methodologies—Hobday et al. (2016) and Darmaraki et al. (2019)—was conducted, highlighting regional trends and spatial patterns of MHW characteristics like frequency, duration, and intensity. The Darmaraki method, with its 99th percentile threshold and flexible event merging criteria, was found to better capture localized and extreme temperature anomalies relevant to aquaculture, while the Hobday method identified a broader range of moderate events. The findings show that MHW frequency has increased significantly over time, particularly in the southeastern and northern waters, with some regions experiencing a doubling of annual MHW events as detected by the Darmaraki method. Long-duration MHWs, exceeding 60 days, are frequently observed along the western and southeastern coasts, demonstrating persistent thermal stress in these areas. The most intense MHWs, with temperature anomalies surpassing 2.5°C above climatological baselines, are concentrated in the southwestern and offshore regions. These areas emerge as critical hotspots, underlining the need for targeted monitoring and adaptive strategies for aquaculture management.Deep learning models were introduced to predict SST and assess MHW risks to address the need for actionable forecasts. Long Short-Term Memory (LSTM) networks are particularly well-suited for analyzing time series data, as they effectively capture temporal dependencies and long-range patterns in sequential datasets. When coupled with the PyTorch framework, these models offer flexibility and scalability, making them ideal for large and complex SST datasets. Furthermore, combining LSTM with Convolutional Neural Networks (LSTM-CNN) enables the integration of both temporal and spatial features, which is crucial for understanding the intricate dynamics of MHWs.The LSTM and LSTM-CNN frameworks demonstrated their effectiveness in forecasting SST across various temporal horizons, with predicted values evaluated against MHW criteria to identify potential events and their impacts. By leveraging these models, this study transitions from reactive to proactive MHW detection, providing early warnings and enabling aquaculture stakeholders to implement timely mitigation measures.This interdisciplinary study bridges marine science and data engineering, combining observational data, machine learning, and robust detection frameworks to enhance the monitoring, forecasting, and management of extreme ocean events. The outcomes provide critical tools for sustainable aquaculture management and contribute to the broader understanding of climate impacts on marine environments.

Abstract. Marine heatwaves (MHWs) are increasingly studied in climate sciences for their ecological impacts, for which accurate real-time bulletins and forecasts are essential. Yet, methodological choices in their detection affect metric estimates, underlining the need to better assess these sensitivities. This study provides a thorough assessment of the impact of sea surface temperature (SST) product choice on MHW statistics, focusing on the tropical Pacific. MHW detection was performed on six daily gridded SST datasets: four widely used blended satellite observational products, one ocean reanalysis, and a multi-dataset ensemble mean computed from the four observational products. Sensitivity to SST products was evaluated for six MHW metrics (MHW days per year, number of events per year, duration, maximum intensity, cumulative intensity and onset rate) and for the degree heating weeks (DHW), a widely used index for coral bleaching risk. Inter-product comparisons revealed a significant dispersion among MHW metric estimates. The reanalysis GLORYS12v1 detected fewer, longer and less intense MHWs while OISST detected more MHWs of shorter duration and higher intensity. likely related to the respectively weak and strong high-frequency SST variability (periods shorter than 2 weeks) of the two products. The sensitivity analysis showed that the onset rate was the most sensitive metric to SST product choice and the maximum intensity the most robust one. Metrics uncertainties were quantified inside seven regions of the basin and were largest in the western Pacific Warm Pool. Co-occurrence analyses of MHWs revealed that, over the basin, 10 % to 80 % of MHW days were detected simultaneously by all products, with the western Pacific Warm Pool showing the lowest agreement (10 %–40 %). Filtering MHWs by size also revealed that the detection of large-scale MHWs (>5°lon×5°lat) was more consistent across products than smaller-scale ones. Finally, over the studied period, inter-product differences tended to decrease with time. The DHW also revealed to be sensitive to SST products, with inter-product differences on DHW annual maximum reaching more than 1 °C weeks and percentages of bleaching alert days (DHW≥4 °C weeks) in common across products reaching 70 % at most across much of the basin. These findings contribute to a better understanding of how SST product choice affects the characterization of MHWs and DHW, and their associated uncertainties.

Marine heat waves (MHWs) are recognized as pervasive drivers of impacts on marine species and ecosystems across the world; however, sub-Arctic areas that are rapidly losing seasonal sea-ice cover remain understudied. In this research, we examine a forty-year time series of MHW characteristics in the seasonally ice-covered James Bay region of the Canadian Inland Seas in central Canada. Through the period 1982 to 2021, we document the trends and investigate past MHW occurrences with respect to their driving processes. After only two MHW events during the early portion of the record (1982-1997), five events occurred in 1998 and signaled both an anomalous year and a step change in the region’s marine climatology. The new marine climate in the region is more variable with longer and more intense MHWs. Four or more MHWs occurred in each of 2001, 2005, 2010, 2012. Events in May and October 2021 lasted over a month in duration, with the former reaching intensities of between 2.5 and 3°C. MHW intensity was correlated with ice breakup date and positive Atlantic Multi-decadal Variability, which are suggested drivers of the increasing trends in sea surface temperatures. While the impacts of MHWs on marine and coastal ecosystems in the region remain unknown because of a lack of monitoring, the 1998 MHW intensification coincides with a massive decline in the region’s seagrass Zostera marina (eelgrass) ecosystem, which has been monitored since 1982. Given projections of more extreme MHWs under global warming and the sensitivity of marine species and ecosystems to warm water events, there is an urgent need to better tracks MHWs and investigate their role in shaping northern ecosystem changes.

Marine heatwaves (MHWs) have recently been at the forefront of climate research due to their devastating impacts on the marine environment. In this study, we have evaluated the spatiotemporal variability and trends of sea surface temperature (SST) and MHWs in the Black Sea. Furthermore, we investigated the relationship between the El Niño–Southern Oscillation (ENSO) and MHW frequency. This is the first attempt to investigate MHWs and their characteristics in the Black Sea using high-resolution remote-sensing daily satellite SST data (0.05° × 0.05°) from 1982 to 2020. The results showed that the spatial average of the SST warming rate over the entire basin was about 0.65 ± 0.07 °C/decade. Empirical orthogonal function (EOF) analysis revealed that SST in the Black Sea exhibited inter-annual spatiotemporal coherent variability. The maximum spatial SST variability was discovered in the central Black Sea, whereas the lowest variability was in the Batumi and Caucasus anti-cyclonic eddies in the eastern Black Sea. The highest SST temporal variability was found in 1994. More than two-thirds of all MHW events were recorded in the last decade (2010–2020). The highest annual MHW durations were reported in 1994 and 2020. The highest MHW frequency was detected in 2018 (7 waves). Over the whole study period (1982–2020), a statistically significant increase in annual MHW frequency and duration was detected, with trends of 1.4 ± 0.3 waves/decade and 2.8 ± 1.3 days/decade, respectively. A high number of MHW events coincided with El Niño (e.g., 1996, 1999, 2007, 2010, 2018, and 2020). A strong correlation (R = 0.90) was observed between the annual mean SST and the annual MHW frequency, indicating that more MHWs can be expected in the Black Sea, with serious consequences for the marine ecosystem.

Marine heat waves: Characterizing a major climate impact in the Mediterranean

In recent years, global sea surface temperature (SST) has risen steadily, with 2023 and 2024 breaking successive historical observation records, thus rendering marine heatwaves (MHWs) an unignorable new marine disaster. To scientifically mitigate and assess the impacts of MHW disasters on China’s coastal waters, this study developed a monitoring and weekly forecast product for MHWs based on the OSTIA (Operational SST and Ice Analysis) SST observational fusion data and SST numerical forecast data. Evaluation shows the following: the quarterly average of the RMSE for the weekly MHWs intensity forecasts is 0.52 °C; and the quarterly average score for the weekly MHW’s category forecasts is 94.4. Characteristic analysis of 2024 MHWs reveals 93.7% of China’s coastal waters and adjacent areas experienced MHWs throughout the year, and the average monthly impact rate of MHWs is 43.8%. High-value areas of total days and cumulative intensity are concentrated in the central-eastern part of the Yellow Sea, which makes it the most severely affected area by MHW disasters in 2024. The weekly MHW’s forecast product developed in this study provides deterministic weekly forecasts of MHWs intensity and categories for China’s coastal waters. This product can serve as a guidance basis for MHW disaster prevention and mitigation, and help reduce losses caused by MHWs to the marine environment and marine economy.

The Mediterranean Sea (MS) has been experiencing significant surface warming over the past decades, greater than the global ocean and particularly higher during summers. The present study proposes the concept of Extreme Marine Summers (EMS) and investigates their characteristics in the MS in a climatological framework, based on ECMWF ERA5 daily sea surface temperature (SST) data for the period 1950-2020. Main objectives are to explore the SST substructures within EMSs, the contribution of Marine Heatwaves (MHW) during EMSs and the driving role of air-sea heat fluxes in the EMS formation.EMSs are defined as the summers (July-August-September) exhibiting a mean SST greater than the 95th percentile of the mean summer SST values within the study period. A marine summer may evolve as extreme under different SST substructures within the season, e.g., due to uniformly increased SST values throughout the summer or due to higher than usual SSTs of a specific part of the SST distribution during the season. Results suggest that EMSs identified in the greatest part of the basin are formed due to the warmest part of the ranked daily SST distribution being warmer than normal. SSTs within EMSs are organised under high dependency on the climatological SST variability: locations where the warmest (coldest) part of the ranked daily SST distribution is more variable climatologically, experience EMSs primarily due to the contribution of the warmest (coldest) part of the SST distribution.MHWs in EMSs present greater intensity, duration and occurrence frequency with respect to mean MHW conditions, in the northern flanks of the Mediterranean basin and particularly in the Aegean and Adriatic Seas. Although the north-western part of the basin experiences the most intense EMSs and summer MHWs, the role of MHWs in the formation of EMSs appears more pronounced in the central and eastern MS. In the rest of the basin, and particularly in southern MS regions, MHWs in EMSs are less intense but longer lasting and more frequent than usual.To quantify the driving role of the net surface heat flux (Qnet) in the EMS formation, a metric is proposed based on the surface heat budget equation. The proposed metric represents the mean contribution of Qnet during summer sub-periods within which SST is kept above climatology via a) faster warming or b) slower cooling compared to the corresponding climatological period. Results show that EMSs are largely driven by Qnet in the northern MS regions: a latitudinal gradient is generally observed in the basin with increasing contribution percentages while moving northerly. In areas where the observed SST anomalies are not entirely explained by surface heat fluxes, negative wind speed and mixed layer depth seasonal anomalies relative to climatology are commonly observed, suggesting that wind-induced mixed layer shoaling is a complementary EMS contributing mechanism. Moreover, results reveal a strong link between MHW properties and surface heat fluxes during EMSs, suggesting that Qnet modulates particularly the intensity of MHWs.