Marine Heat Waves, MHW, persistent and anomalously warm events, are known to have significant impacts on biological systems, from shifts in species ranges to biodiversity losses, as well as impacts on ecosystem services and human ocean-based economic activities (e.g., aquaculture and seafood industries). Their detection, attribution and impact are essential if we want to predict changes in ocean health.
The project “deteCtion and threAts of maRinE Heat waves – CAREHeat”, funded by ESA in the framework of the Ocean Health initiative, aims at improving the current MHW detection and characterization methodology, as well as advancing the understanding of the physical processes involved, and the corresponding ecological and biogeochemical changes.
This will be archived - leveraging on the added value of Earth Observation experimental 4D products, using a multi-disciplinary approach combining the novel MHW products, together with satellite-based observations of Ocean Colour (OC), BGC-Argo in situ optical and biogeochemical data and CMEMS biogeochemical model data - developing the following main tasks:

1 Provide a detail assessment of the major gaps in scientific knowledge, existing products and tools in MHW detection and impact assessment; Define technical and scientific requirements, as well as datasets, that will drive CAREHeat methodological development and impact assessment on ecosystems;

2 Develop a novel MHW detection algorithm from space-based SST observations that will optimize existing methodologies and review the today adopted criteria such as minimum duration thresholds of MHW and reference climatology; Investigate the impact of SST trends and prominent climate modes (e.g., ENSO) on MHW detection, and evaluate the role of diurnal warming events in the MHW characteristics;

3 Test and assess a 4D MHW reconstruction algorithm (4D temperature reconstruction and subsurface MHW detection) based on machine learning techniques, on specific test sites; Scaling up the new MHW4D product to generate a global EO experimental dataset covering at least a 10-year recent period;

4 Develop and validate a MHW Global Atlas based on the novel MHW detection methodology and characterize each event in terms of location, duration, intensity and rate of evolution, and as Moderate, Strong, Severe, or Extreme; Complement the Atlas with ancillary meteorological, oceanic geophysical and biological information;

5 Exploit the newly obtained MHW Atlas and MHW4D products to characterize drivers and precursors of MHW; Assess how MHW impact on marine biodiversity (from primary producers to apex predators) and biogeochemistry (dissolved oxygen levels, acidification) from the surface down to the ocean interior, using a multiplatform approach based on Ocean Colour satellite observations, BGC-Argo float measurements and biogeochemical modelling; Identify Compound Events, co-occurring climate and biogeochemical anomaly conditions that are associated with extremely low productivity and low biomass conditions;

6 Assess the impact of new MHW products on the management of key ocean-based human services: fisheries (tunas), aquaculture (sea breams and sugar kelps), and marine protected areas (corals, sea urchins and sea birds); Use of population models and biological observations to assess organism distribution, fertility, mortality, nesting and restoring after MHW events;

These will contribute to the development of a Scientific Agenda and Roadmap and to the establishment of a coordinated European research on Ocean Health and develop synergies with policy makers, NGOs, environmental protection agencies, private companies and the projects belonging to the ESA Ocean Science Cluster. Here, we will present the preliminary results, an overview of the stakeholders engaged, the planned activities and strategies that will be adopted for the scientific development of algorithms, impact assessment of MHW on marine ecosystems, User’s impact assessment, and outreach.

Marine Heat Waves (MHWs) induce significant impacts on marine ecosystems. There is a growing need for knowledge about extreme climate events to better inform decision-makers on future climate-related risks. The definition, physical drivers and ecological impacts of the extreme temperature anomalies are usually still examined individually, and the prediction of these extreme events is very challenging. In this context, the FEVERSEA project under the ESA Climate Change Initiative (CCI) program aims to provide a global assessment of MHWs in a consistent and comprehensive framework. We first detected MHWs from 1983 to 2016 in the ESA CCI Sea Surface Temperature product (4km of horizontal resolution). The detection analysis highlighted the importance of defining MHWs and their relevant metrics.
We detected MHWs for each grid point using a baseline line climatology that accounts for trend and allows a time-varying seasonality. We found that the magnitude, frequency, and duration of MHW events have a heterogeneous distribution across the global oceans. The MHWs highest intensities are primarily associated with boundary currents like the Gulf Stream or their extensions and with the upwelling regions of the equatorial Pacific cold tongue. The boundary currents experience short and frequent events, while the equatorial pacific experiences long and not frequent MHWs.
Then, taking advantage of advanced statistical methods, we synthesised information and we aggregated punctual (i.e. for each grid point) MHWs in order to identify MHWs Macro-Events in space and in time. We identified MHWs Macro-Events in the ocean basins and we clustered them based on shared metrics and characteristics. The analysis of the identified clusters aims to provide insight on possible common physical and climate drivers across MHWs Macro-Events. The detection of the local and large-scale climate precursors of MHWs are the starting point to build up a prediction framework, which is the final objective of the FEVERSEA project.

Marine heatwaves (MHWs) are discrete warm-water anomalies events occurring in both open and coastal ocean all around the globe. Due to their devastating consequences on ecosystems and marine life (e.g. mass mortality, species migration, failure in reproduction…) and on oceans’ properties (e.g. higher stratification, modification of the oxygen concentrations…), they are considered as extreme climate events. MHWs have drawn researchers’ attention during the last 10 years, as their increasing frequency and duration have been linked to human induced global warming. Only a few portions of the oceans have been subject to MHW long-term studies. Here, we used satellite data to investigate for the first time MHWs in the Southeast Pacific Ocean. We studied over the last 40 years (1982 to 2020) the MHWs tendencies offshore central Chile and Chilean Patagonia (29°S-55°S). We found that the Northern Chilean Patagonia is clearly subject to increasing trends of MHWs frequency, in accordance with increasing SST trends, particularly throughout the last decade during which 45% of the MHWs occurred. Moreover, in North Patagonia, 40% of the MHWs that had an intensity superior to 1°C also occurred during the last decade. To understand which processes trigger MHW conditions in the Southeast Pacific Ocean, we decided to focus on a particular MHW event as a case study. We focused on the longest MHW we recorded over the last 40 years: it appeared in May 2016 offshore Patagonia and lasted for 5 months, until October 2016. During austral fall, warm waters coming from the extratropical South Pacific Ocean were advected close to Patagonian coasts, triggering the MHW in May and maintaining it during the winter. We also analysed the atmospheric parameters and found that, in May, the winds were lower than usual, diminishing the heat fluxes from the ocean to the atmosphere. At that time, the total heat transfer from the ocean to the atmosphere was reduced by 2, and slowly recovered to normal values by the end of the winter. Consequently, during austral fall and winter, the ocean did not cool as much as usual, explaining why a MHW developed. In addition, the year 2016 was particularly affected by strong El Niño Southern Oscillation and Southern Annular Mode positive phases that may have contributed to the persistence of the MHW.

In recent decades, marine ecosystems have been substantially impacted by extreme warm temperatures, termed as Marine Heatwaves (MHWs). In particular, MHWs have driven major coral bleaching events around the world, even in the heat-resistant corals of the Red Sea. With the projected anthropogenic warming in the 21st century, the frequency and intensity of MHWs are expected to increase, which will likely lead to more severe coral bleaching in that region. This study focuses on areas of documented coral bleaching events related to MHWs in the Red Sea that have been identified using a 30-year record of satellite-derived sea surface temperature observations and in situ data. Our objective is to explore the vertical signature of these extreme warming events using a mixed layer heat budget analysis. More specifically, we analyze their dominant physical mechanisms by combining spatial information from Earth Observations and long-term 3D ocean heat content data from a validated high-resolution, regional modeling system model of the Red Sea. We diagnose the relative role of local-scale dynamics (i.e air-sea interactions, ocean currents, entrainment, mixing) on the development and decline of MHWs and across seasons. Preliminary results indicate a key role of wind forcing and atmosphere fluxes for certain events and a predominant horizontal advection presence for others. In addition, we statistically examine changes in the characteristics of ocean- and atmosphere-driven MHWs across seasons and different regions of the Red Sea. Through the synergistic use of Remote Sensing data and climate modeling this study offers an improved understanding of the physical drivers behind MHW capable of causing coral bleaching events. This will aid the sustainable management and mitigation of MHW impacts in the coastal zones of the Red Sea.

Marine heatwaves (MHWs) have occurred in all of Earth’s ocean basins over the past few decades, with severe negative impacts on marine organisms and ecosystems. Of particular concern are compound ocean extremes events, i.e., multiple extreme events that occur simultaneously or in close sequence, as their individual effects may interact synergistically. Satellite-derived observations of sea surface temperature and ocean acidity with high temporal and spatial resolution in combination with in-situ observations and advanced Earth system models provide the unique opportunity to understand how these rare and multivariate extreme events unfold in time and space at the global scale and what the physical and biogeochemical drivers are.

Using satellite-derived sea surface temperature data since 1982, we show that MHWs, when defined as the daily sea surface temperature exceeding the local 99th percentile, have doubled in frequency between 1982-2016. The duration, extent and intensity of MHWs has also increased over the last four decades. By combining observations with Earth system model data, we show that the likelihood of seven recent, high impact marine heatwaves increased more than 20-fold due to human-caused climate change. Our analysis reveal that it is almost impossible that the Northeast Pacific 2013-2015 marine heatwave (often called ‘The Blob’) would have occurred without human-cause global warming.

By combining the satellite-derived sea surface temperature data with a novel satellite-based ocean acidity product, we show that globally 1.8 in 100 months (or one out of five present-day marine heatwaves) are compound MHW-OAX events, almost twice as many as expected from 90 percentile extreme event exceedances if MHWs and OAX events were statistically independent. Compound MHW-OAX events are most likely in the subtropics and less likely in the equatorial Pacific and the mid-to-high latitudes. The compound event likelihood results from opposing effects of temperature and dissolved inorganic carbon on [H+]. More compound events occur where the positive effect on [H+] from increased temperatures during MHWs is larger than the negative effect on [H+] from co-occurring decreases in dissolved inorganic carbon.

Our results suggest that some of the observed high impact MHWs were also compound MHW-OAX events, in particular in the low-to-mid latitudes. The reported impacts of the low-to-mid latitude MHWs on marine organisms and ecosystems may therefore be also connected to additional stress from high acidity extremes.

In the complex interacting Earth system, many physical mechanisms can lead to anomalously high ocean temperature: coupled air-sea interactions, atmospheric preconditioning, oceanic preconditioning, climate modes and teleconnections and on top of that anthropogenic warming. In particular, extreme ocean warming events, known as Marine Heatwaves, are becoming more numerous and more severe all along the past 30 years. These events have a disastrous impact on marine ecosystems and on marine industries such as aquaculture and fisheries.

Predicting the occurrence of extreme marine heatwaves events, their spatial extent, depth, intensity and duration is thus essential to carry out preventive actions and thus limit their socio-economic impact.

In order to evaluate an alternative method to numerical models based on a "classical ocean dynamic approach” and used to forecast oceanic events, CLS has developed a model based on advanced Machine Learning approach to predict Marine Heatwaves. This work was conducted in the framework of the ESA project "Digital Twin Ocean precursor". This convolutional neural network takes as input past history of maps data covering the Mediterranean Sea : sea surface temperature (SST), temperature at 40 m depth and sea surface height. The objective of the model is to learn to predict the time series of future SSTs based on this history, hence making use of both spatial and temporal context. These future SSTs are then thresholded to obtain prodictions of Marine Heatwaves. We will present some preliminary results by identifying strenghts and weaknesses of the chosen model.

In parallel to this work, an analysis of some potential drivers of Marine Heatwaves, such as climatic indexes, was performed to better understand the processes that may be involved in the onset of Marine Heatwaves. We also analysed the trends of SSTs observed in the Mediterranean Sea over the last 30 years.