Replaces and cancels previous "Reply on RC2"

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%.

Responses of Gulf of Alaska plankton communities to a marine heat wave

A hierarchical approach to defining marine heatwaves

Chapter 5 - Extreme climatic events in the ocean

The adverse impacts of marine heatwaves (MHWs) on marine ecosystems and human activities are well-documented, yet observational studies tend to largely rely on recent records. Long-term records of MHWs can put the recent increase in frequency and intensity of MHWs in the context of past variability. We used long-term monthly sea surface temperature (SST) data and night marine air temperatures to characterise past MHW activity. A persistent increase in the global extent of MHWs is demonstrated, beginning around 1970. The average annual MHW extent post-2010 is estimated to be increased at least four fold compared to that pre-1970. A strong correlation between spatial variance of recorded average monthly SSTs and the average inverse number of monthly observations implies both frequency and amplitude of MHWs is overestimated when the number of monthly observations is low. Nevertheless, many identified early MHWs appear genuine, such as a multi-month event in the North Atlantic in 1851–1852. MHWs are also affected by poorer sampling during the world wars. The most extensive MHW years globally coincide with El Niño years, and MHW extent in the North Atlantic is correlated with the Atlantic Multidecadal Oscillation.

Human-mediated disturbances such as fishing, habitat modification, and pollution have resulted in significant shifts in species composition and abundance in marine ecosystems which translate into degradation of food-web structure. Here, we used a comparative ecological modelling approach and data from two food webs (North-Central Adriatic and South Catalan Sea) and two time periods (mid-late 1970s and 1990s) in the Mediterranean Sea to evaluate how changes in species composition and biomass have affected food-web properties and the extent of ecosystem degradation. We assembled species lists and ecological information for both regions and time periods into stochastic structural and mass-balance food-web models, and compared the outcomes of 22 food-web properties. Our results show strong similarities in structural food-web properties between the North-Central Adriatic and South Catalan Seas indicating similar ecosystem structure and levels of ecological degradation between regions and time periods. In contrast, a comparison with other published marine food webs (Caribbean, Benguela, and US continental shelf) suggested that Mediterranean webs are in an advanced state of ecological degradation. This was reflected by lower trophic height, linkage density, connectance, omnivory, species involved in looping, trophic chain length and fraction of biomass at higher trophic levels, as well as higher generality and fraction of biomass at lower trophic levels. An analysis of robustness to simulated species extinction revealed lower robustness to species removals in Mediterranean webs and corroborated their advanced state of degradation. Importantly, the two modelling approaches used delivered comparable results suggesting that they both capture fundamental information about how food webs are structured.

Marine heatwaves (MHWs) are becoming more frequent as a consequence of climate change. These discrete events are causing widespread stress and mortality in marine ecosystems, including coral reefs. The heat tolerance of different coral species is often complex and depends on a combination of environmental and biological factors, making accurate predictions of the impact of MHWs on individual species challenging. Heating rate has been shown to influence coral bleaching in Acropora species, but it remains unknown how heating rate influences bleaching in other corals with contrasting morphology and bleaching sensitivities. In this study, we explored the sensitivity of Pocillopora damicornis and Plesiastrea versipora, representing branching and encrusting growth forms, respectively, to heating rate. We experimentally simulated MHWs with slow (0.5°C d-1) and fast (1°C d-1) heating rates and measured physiological responses to quantify changes in coral health, including photochemical efficiency, holobiont metabolism, tissue biomass, chl a, and symbiont density. Our results confirm that heating rate is a good predictor of coral bleaching sensitivity for these species, with faster heating rates causing more severe bleaching and declines in coral health. However, bleaching sensitivity differed between P. damicornis and P. versipora, with P. damicornis more affected by the faster heating rate. The use of heating rate, in addition to other metrics such as duration and intensity of heat, will enhance our capacity to predict the local impact of MHW events and their overarching ecological consequences for coral ecosystems.

Marine heatwaves (MHWs) are prolonged extreme oceanic warm water events. They can have devastating impacts on marine ecosystems — for example, causing mass coral bleaching and substantial declines in kelp forests and seagrass meadows — with implications for the provision of ecological goods and services. Effective adaptation and mitigation efforts by marine managers can benefit from improved MHW predictions, which at present are inadequate. In this Perspective, we explore MHW predictability on short-term, interannual to decadal, and centennial timescales, focusing on the physical processes that offer prediction. While there may be potential predictability of MHWs days to years in advance, accuracy will vary dramatically depending on the regions and drivers. Skilful MHW prediction has the potential to provide critical information and guidance for marine conservation, fisheries and aquaculture management. However, to develop effective prediction systems, better understanding is needed of the physical drivers, subsurface MHWs, and predictability limits. Prolonged extreme oceanic warm water events, known as marine heatwaves, can have devastating impacts on marine ecosystems. This Perspective explores the predictability of marine heatwaves, taking into account the physical drivers, monitoring and prediction approaches, and stakeholder considerations.

The oceanographic and climate communities are putting significant effort into reaching a consensus on a common definition for Marine Heatwaves (MHW). The absence of such a unified definition poses a substantial obstacle when it comes to making retrospective comparisons between various MHW studies. This hindrance is critical for achieving a mechanistic understanding of the role of MHWs in marine ecosystems. However, why is it so challenging to characterize and define MHWs? The answer is straightforward: there isn't a single, distinct dynamical mechanism responsible for the persistence of heat anomalies in the ocean, which we refer to as MHWs. Unlike variability associated with phenomena such as large oceanic eddies, oceanic fronts, upwelling systems, tropical cyclones, or climate modes, prolonged heat anomalies do not exhibit characteristic time or spatial scales. As a result, common MHW definitions group together prolonged temperature anomalies lasting from days to years and spanning from a few kilometers to thousands of kilometers in scale. Analyzing sea surface temperature anomalies through power spectra reveals a "red" power spectrum with no discernible time scales. A similar analysis in spatial dimensions similarly shows a lack of any specific scale. Given this absence of emergent scales, we suggest adopting a process-based definition for MHWs. Such an approach would classify all events into a smaller number of categories, each linked to a specific driver or dynamical process operating on certain spatiotemporal scales. This shift could significantly reduce the subjectivity involved in selecting the temporal and spatial scales required for current MHW definitions, ultimately advancing our understanding of these events.

Marine heatwaves (MHWs) are emerging as a severe stressor in marine ecosystems. Extreme warm sea surface temperatures during MHWs often exceed the optimal thermal range for more than one generation of tropical coastal zooplankton. However, it is relatively unknown whether transgenerational plasticity (TGP) to MHWs may shape the offspring's fitness, particularly in an ecologically relevant context with biotic interactions such as predation stress. We addressed these novel research questions by determining the survival, reproductive success, and grazing rate of the copepod Pseudodiaptomus incisus exposed to MHW and fish predator cues (FPC) for two generations (F1 and F2). The experiment was designed in a full orthogonal manner with 4 treatments in F1 and 16 treatments in F2 generation. In both generations, MHW reduced P. incisus survival, reproductive parameters, and grazing by 10%–62% in MHW, but these parameters increased by 2%–15% with exposure to FPC, particularly at control temperature. F2 reproductive success and grazing rate as indicated by cumulative fecal pellets were reduced by 20%–30% in F1‐MHW, but increased by ~2% in F1‐FPC. Strikingly, MHW exposure reduced 17%–18% survival, but transgenerational exposure to MHWs fully ameliorated its lethal effect and this transgenerational effect was independent of FPC. Increased survival came with a cost of reduced reproductive success, constrained by reduced grazing. The rapid transgenerational MHW acclimation and its associated costs are likely widespread and crucial mechanisms underlying the resilience of coastal tropical zooplankton to MHWs in tropical coastal marine ecosystems.

As the Earth's temperature continues to rise, coral bleaching events become more frequent. Some of the most affected reef ecosystems are located in poorly monitored waters, and thus, the extent of the damage is unknown. We propose the use of marine heatwaves (MHWs) as a new approach for detecting coral reef zones susceptible to bleaching, using the Red Sea as a model system. Red Sea corals are exceptionally heat-resistant, yet bleaching events have increased in frequency. By applying a strict definition of MHWs on >30 year satellite-derived sea surface temperature observations (1985-2015), we provide an atlas of MHW hotspots over the Red Sea coral reef zones, which includes all MHWs that caused major coral bleaching. We found that: (a) if tuned to a specific set of conditions, MHWs identify all areas where coral bleaching has previously been reported; (b) those conditions extended farther and occurred more often than bleaching was reported; and (c) an emergent pattern of extreme warming events is evident in the northern Red Sea (since 1998), a region until now thought to be a thermal refuge for corals. We argue that bleaching in the Red Sea may be vastly underrepresented. Additionally, although northern Red Sea corals exhibit remarkably high thermal resistance, the rapidly rising incidence of MHWs of high intensity indicates this region may not remain a thermal refuge much longer. As our regionally tuned MHW algorithm was capable of isolating all extreme warming events that have led to documented coral bleaching in the Red Sea, we propose that this approach could be used to reveal bleaching-prone regions in other data-limited tropical regions. It may thus prove a highly valuable tool for policymakers to optimize the sustainable management of coastal economic zones.

Warm temperature anomalies are increasing in frequency in the global ocean with potential consequences on the goods and services provided by marine ecosystems. Recent studies have analyzed the distribution and dynamics of marine heat waves (MHWs) and evaluated their impacts on marine habitats. Different drivers can generate those anomalies and the emerging attributes can vary significantly both in space and time, with potentially different effects on marine biology. In this paper we classify MHWs based ontheir attributes and using different baselines, to account for different adaptive responses in phytoplankton dynamics. Specifically, we evaluate the impacts of the most extreme, long-lasting and high-intensity MHWs on phytoplankton communities using remote sensing data. We demonstrate marginal impacts on total chlorophyll concentrations which can be different across different ocean regions. These contrasting effects on phytoplankton dynamics are most likely the results of the different mechanisms generating the MHWs in the first place, including changes in front dynamics, shallower mixed layers, and eddy dynamics. We conclude that those drivers producing extreme MHWs can also induce different phytoplankton responses across the global ocean.

Marine heatwaves have been linked to negative ecological effects in recent decades1,2. If marine heatwaves regularly induce community reorganization and biomass collapses in fishes, the consequences could be catastrophic for ecosystems, fisheries and human communities3,4. However, the extent to which marine heatwaves have negative impacts on fish biomass or community composition, or even whether their effects can be distinguished from natural and sampling variability, remains unclear. We investigated the effects of 248 sea-bottom heatwaves from 1993 to 2019 on marine fishes by analysing 82,322 hauls (samples) from long-term scientific surveys of continental shelf ecosystems in North America and Europe spanning the subtropics to the Arctic. Here we show that the effects of marine heatwaves on fish biomass were often minimal and could not be distinguished from natural and sampling variability. Furthermore, marine heatwaves were not consistently associated with tropicalization (gain of warm-affiliated species) or deborealization (loss of cold-affiliated species) in these ecosystems. Although steep declines in biomass occasionally occurred after marine heatwaves, these were the exception, not the rule. Against the highly variable backdrop of ocean ecosystems, marine heatwaves have not driven biomass change or community turnover in fish communities that support many of the world's largest and most productive fisheries.

Marine heatwaves (MHWs) are extreme warming events in the ocean that can significantly impact marine ecosystems and economies. While such extreme events have garnered significant research attention in recent years, most research to date took an Eulerian perspective, that is, they considered MHW as stationary objects. This disregards the fact that MHW are three dimensional objects that tend to propagate in space and time. In this study, we overcome this limitation and employ marEX, a novel method inspired by Ocetrac, to analyze and track MHWs globally. We limit outselves here to surface MHW, but the method is designed to extend to depth. Starting from a grid-level 95th percentile threshold-based detection following (A.J. Hobday et al., 2016), this method first creates coherent two-dimensional MHW objects by detecting contiguous regions of extreme sea-temperature anomalies at each time step. These regions are then linked over time using robust criteria that account for both merging (when two or more MHW regions combine into one) and splitting (when a single MHW divides into separate regions). This is achieved by tracking each piece of an event individually while maintaining links between them. This ensures that the full lifecycle of each MHW, including interactions between different events, is accurately recorded.We test this method using ~40 years of simulated sea-surface data from the European Eddy RIch Earth System Models (EERIE) project. We find that most MHWs tend to form and dissipate in the same regions, indicating persistent hotspots for their development and termination. But there are also a substantial number of MHWs that persist for several months and propagate substantially, akin to the North Pacific “Blob event”. While the tracked MHW have, on average, a duration of ~14 days, the longest one propagated for as long as 520 days. These findings underscore the value of tracking MHWs as dynamic events, demonstrating that their propagation pathways and lifecycles hold crucial information.

Marine heatwaves are expected to become more frequent, intense, and longer-lasting in a warming world. However, it remains unclear whether feedback processes could amplify or dampen extreme ocean temperatures. Here we impose the observed atmospheric flow in coupled climate simulations to determine how the record-breaking 2019 Northeast Pacific marine heatwave would have unfolded in preindustrial times, and how it could unravel in a +4°C warmer world compared to present-day conditions. We find that air-sea interactions, involving reductions in clouds and ocean mixed-layer depth and air advection from fast-warming subpolar regions, modulate warming rates within the marine heatwave. In a +4°C warmer climate, global oceans are +1.9°C warmer than present levels, and regional mean warming in the Northeast Pacific can reach +2.3–2.7 ± 0.25°C. Our identified feedback processes are projected to further amplify the intensity and spatial extent of analogous Northeast Pacific summer marine heatwaves beyond those thresholds, with a warming reaching +2.9 ± 0.15°C above present levels. Such an event-specific amplification would place even greater stress on marine ecosystems and fisheries.