Marine heatwaves (MHWs), characterized by prolonged periods of anomalously warm ocean temperatures, pose a significant threat to marine ecosystems. These extreme events, while rare by definition, have been increasing in frequency in recent years, leading to devastating and long-lasting impacts across vast scales. The annual number of MHW days has increased globally by over 50% in the last century, primarily due to long-term warming of the upper ocean, although regional variability also plays a role. Future projections indicate a dramatic increase in MHW days by the end of the century, even under optimistic greenhouse gas emission mitigation scenarios. This necessitates a deeper understanding of the mechanisms driving MHW occurrence and their ecological consequences. MHW research is a rapidly evolving field. While there's a growing body of work exploring the definition of MHWs, the local processes involved in their generation and maintenance, large-scale climate drivers, historical trends, future projections, and predictability, a comprehensive analysis of the most extreme events and their common characteristics remains lacking. This study aims to address this gap by identifying and characterizing the most extreme MHWs recorded during the satellite era (1982-2017) using a standardized methodology. The goal is to explore common mechanisms for their occurrence, create a comprehensive database of extreme MHWs, and determine whether previously described events were truly unprecedented or if other high-impact, yet unstudied, MHWs exist. Unlike previous studies focusing on individual events, this study systematically examines commonalities in the local processes and atmospheric conditions initiating and terminating these events, and investigates regionally-dependent responses of surface chlorophyll-a concentration.

The existing literature on marine heatwaves covers various aspects, from defining what constitutes a MHW and the local processes that generate and maintain them to examining large-scale climate drivers and their influence on MHW likelihood. Studies have also explored historical changes, future projections, and the predictability of MHWs. Quantifying MHW characteristics allows for the development of relationships between events and their impacts on marine species and ecosystems. A significant area of research focuses on the dynamics, prediction, and impacts of the El Niño-Southern Oscillation (ENSO). ENSO events often coincide with extreme ocean temperatures that meet the criteria for MHWs. Considerable progress has been made in understanding ENSO dynamics, with operational seasonal forecasts now issued with reasonable skill. Major ENSO events have been linked to negative ecosystem impacts such as mass coral bleaching, distributional shifts in tropical tuna, and other fisheries disruptions. ENSO and deep atmospheric convection can also trigger remote, extratropical MHWs through the propagation of planetary-scale oceanic or atmospheric waves. Proximate causes of MHWs are generally categorized into three groups: changes in ocean heat transport (e.g., boundary current intensification); persistent large-scale atmospheric synoptic systems; and coupled air-sea feedback processes (e.g., ENSO events). This study builds upon this existing knowledge to systematically analyze the most extreme events across the globe.

The study employed a standardized methodology to identify and categorize marine heatwaves (MHWs) based on existing frameworks. A MHW was defined as a period when the local sea surface temperature (SST) exceeded the seasonally varying 90th percentile threshold (PC90) for at least 5 consecutive days. A severity index (S) was used to categorize MHWs based on the SST anomaly relative to local variability, enabling ranking of extreme events. The study utilized daily SST data from NOAA's 1/4° Optimum Interpolation Sea Surface Temperature v2.0, which incorporates satellite retrievals and ship observations. A 30-year climatological period (1983-2012) was used to establish baseline percentile thresholds and mean SST. The analysis focused on identifying the most extreme MHWs, without removing any secular SST trend, to reflect the potential impacts on marine organisms whose thermal thresholds may not immediately adapt to gradual warming. MHW intensity was defined as the SST anomaly relative to the local climatology, and cumulative intensity was calculated by integrating the SST anomaly over the event's duration. The largest contiguous MHWs were identified by finding connected grid cells exceeding MHW conditions and summing their areas. A semi-objective procedure was used to characterize the most extreme events, combining multiple metrics (proportion of the region experiencing MHW, largest contiguous area, integrated intensity) to identify events where most metrics were elevated simultaneously. Start and end dates were manually selected. Local processes (wind speed, sea level pressure, heat fluxes, solar radiation) were examined using data from ERA-interim and ERA5 datasets. Chlorophyll-a concentrations, as a proxy for primary productivity, were analyzed using GlobColor data to explore biological impacts of extreme MHWs. Statistical tests, including Monte Carlo resampling and binomial tests, were applied to assess the significance of observed relationships.

The study identified the most extreme MHWs from 1982 to 2017, based on intensity, duration, and cumulative intensity. Key findings include: 1. **Spatial and Temporal Distribution:** Every 4-degree grid cell experienced at least one MHW exceeding strong-category conditions. Over 70% of the ocean experienced severe or higher conditions, and 10% experienced extreme conditions. Maximum MHW intensities ranged from 2.5 to 3.7 °C, but some areas exceeded 5 °C, often associated with strong SST gradients. Maximum intensities were greatest at mid-latitudes. 2. **Seasonality:** Most intense and high-category MHWs preferentially occurred in summer due to shallower mixed layers and weaker winds. This seasonal asymmetry persisted even after removing El Niño periods, indicating other factors, such as shallower mixed layers, played a crucial role. 3. **ENSO Influence:** The intensity, duration, and extent of extreme MHWs were strongly modulated by El Niño events, but also sometimes associated with La Niña events in certain regions. The three strongest El Niño events in the record were associated with dramatic increases in the ocean area experiencing high-category MHWs. 4. **Atmospheric Forcing:** Many extratropical MHWs were linked to persistent atmospheric high-pressure systems, leading to reduced wind speeds, suppressed turbulent heat losses, increased insolation, and enhanced poleward warm water advection. Different combinations of these mechanisms were important for different MHWs. Suppressed wind speeds were a common factor during the formation of a large fraction (82%) of the identified MHWs. 5. **Chlorophyll Response:** A majority (72%) of extreme MHWs were associated with anomalously low chlorophyll-a concentrations, particularly at low and mid-latitudes, indicating suppressed primary production. High latitude events often showed increased chlorophyll-a, likely due to reduced sinking of phytoplankton. This response was linked to background nutrient concentrations. 6. **Most Extreme Extremes:** The study identified 30 of the most extreme events, characterized by their large size, duration, and intensity, many of which coincided with El Niño events. Some extreme events occurred during La Niña periods.

This study provides a comprehensive analysis of the most extreme marine heatwaves recorded during the satellite era, identifying common drivers and impacts. The strong association of extreme MHWs with El Niño events highlights the importance of large-scale climate variability in modulating these events, but the influence of other factors like the seasonal variation in mixed layer depth is also substantial. The findings emphasize the role of atmospheric high-pressure systems in triggering subtropical MHWs, primarily through the reduction of wind speeds, leading to decreased turbulent heat losses and changes in ocean heat advection. The significant role of suppressed wind speeds across almost all extreme MHW events underscores the importance of vertical mixing processes in shaping these events. The observed latitude-dependent response of chlorophyll-a to MHWs, with suppression at lower latitudes and enhancement at higher latitudes, provides critical insights into the ecological consequences of these events. The study underscores the need for further research into subsurface processes and for a more thorough understanding of the impact of heat stress accumulated throughout the duration of MHWs. The identification of numerous previously unstudied extreme MHWs highlights the need for further investigation into their formation mechanisms, linkages to climate patterns, and biological impacts.

This study presents a comprehensive analysis of the most extreme marine heatwaves, revealing commonalities in their drivers and impacts. The strong influence of El Niño, seasonal changes in mixed layer depth and atmospheric high-pressure systems in shaping these events, and the latitude-dependent effect on primary productivity, are key findings. The identification of numerous previously unstudied extreme MHWs emphasizes the need for further research into their underlying mechanisms and ecological consequences. Future research should focus on improving subsurface observations to better understand three-dimensional MHW structures and processes and on examining the impacts on a broader range of marine organisms beyond primary productivity.

The study relies on satellite-derived SST data, which may have limitations in certain regions or time periods. The analysis focuses primarily on surface processes, and the lack of comprehensive subsurface data hinders a complete understanding of three-dimensional MHW dynamics. The subjective nature of identifying regions for the "most extreme extremes" could introduce some bias, and the short length of the ARGO data record limits detailed analysis of subsurface processes. The study also primarily uses chlorophyll-a as a proxy for primary productivity, and further investigations using multispectral bands and in-situ data are needed to better understand subtle community-level effects.