Abstract. We present a new ultra-high resolution 1/30° (∼3 km) regional ocean model of the eastern Australian region and evaluate the performance of this model against two 1/10° (∼10 km) models as well as a suite of satellite and in situ observations. We evaluate model biases in the context of (i) submesoscale-permitting (∼3 km) vs. mesoscale-permitting (∼10 km) horizontal resolution and (ii) differences between version 5 and version 6 of the Modular Ocean Model (MOM5 vs. MOM6) to assess the added value in each case and determine the suitability of our higher resolution model for scientific research. There are some consistent biases shared by the two regional MOM6 configurations, and also in the higher resolution configuration that are not seen in the lower resolution models. These biases are further investigated with two sets of sensitivity experiments to understand the effect of submesoscale eddy parameterization and imposed dynamic viscosity at a submesoscale-permitting resolution. The high-resolution simulation has much higher variance compared to the lower resolution simulations across all evaluation metrics, indicating that the greater spectrum of length scales also manifests in more variability in the temporal domain. The two MOM6 regional configurations of differing resolution appear to be more aligned than the regional (MOM6) and global (MOM5) 1/10° configurations in most results, reflecting the substantial changes made to the MOM between version 5 and version 6. Importantly, we also show that higher resolution is not a panacea: in regions where key dynamics are quasi-linear and well-captured captured by coarser grids (e.g., the EAC jet), further refinement may offer limited benefit – and actually degrade the performance if parameterizations are not appropriately tuned.

In 2023–2024, widespread marine heatwaves associated with record ocean temperatures impacted ocean processes, marine species, ecosystems and coastal communities, with economic consequences. Despite warnings, interventions were limited. Proactive strategies are needed for inevitable future events.

Intensifying marine heatwaves (MHWs) are pervasive and destructive manifestations of anthropogenic climate change. Over the past two decades, MHWs have driven biological, ecological and socioeconomic change in almost all oceans and seas. In this Review, we assess the impacts of MHWs on marine organisms and the benefits they provide to people, highlight knowledge gaps and consider opportunities to mitigate MHW impacts. Globally, MHWs have become increasingly intense and frequent, and result in mortality or movement of species when acute temperature thresholds are exceeded. Vulnerability and resilience to MHWs vary among species, but these mortality events have been prominent for habitat-forming foundation species such as corals, kelp and seagrass, causing many cascading indirect impacts on ecosystem functioning and biodiversity. Poleward species shifts produce novel and complex species interactions and altered ecosystem functions, which have considerable consequences for people and their livelihoods. Reducing greenhouse gas emissions remains essential and urgent to address impacts long term, but increases in MHW intensities and duration will be unavoidable and prominent for the foreseeable future. As such, closing the current knowledge gaps around MHWs and their impacts on biodiversity, as well as proactive management strategies, are urgently needed to mitigate further damage to ecosystems and people, and to build resilience into the future. Marine heatwaves (MHWs) have become more intense and widespread globally, affecting species, ecosystems and people. After summarizing how and why MHWs are changing, this Review explores these impacts and their underlying mechanisms, highlights knowledge gaps and considers opportunities to mitigate the effects of MHWs.

Marine heatwaves (MHWs) off Western Australia (WA), including the extreme example in 2011, can substantially impact local marine ecosystems. Understanding WA MHW predictability is key to improving forecast systems and enabling early warnings. Easterly wind stress anomalies in the western and central equatorial Pacific termed "easterly wind extremes (EWEs)" can trigger both WA MHWs and La Ni & ntilde;a via ocean waveguides. However, the role of La Ni & ntilde;a as either merely a preconditioning symptom in EWE-generated WA MHWs or a direct trigger remains unclear. Here, we identify the WA MHWs and La Ni & ntilde;a using the vertically averaged temperature (VAT) from sea surface to about 300-m depth. We investigated the relationships between WA VAT MHWs, EWEs, and VAT La Ni & ntilde;a using a linear first baroclinic mode reduced-gravity ocean model, and analyzed pycnocline depth extremes as the proxy for WA VAT MHWs and VAT La Ni & ntilde;a. We found that the tropical western and central equatorial Pacific wind stress forcing and related oceanic wave dynamics are dominant contributors to WA pycnocline depth variations compared to other oceanic regions. By applying idealized EWE-forced model experiments, we found both EWEs and VAT La Ni & ntilde;a typically precede WA VAT MHWs by about 4 months (2-6 months). Notably, 25% of WA VAT MHWs occurred about 4 months (2-6 months) after EWEs, without a preceding VAT La Ni & ntilde;a. Our findings reveal a new mechanism for WA VAT MHW generation, highlighting EWEs as key precursors independent of El Ni & ntilde;o-Southern Oscillation. This significantly advances our understanding of WA VAT MHW predictability, offering new insights beyond the established knowledge of VAT-La Ni & ntilde;a-associated drivers.

Mechanistic understanding of marine heatwaves (MHWs) requires a suitable definition for their detection, an approach to characterise their evolution, and an effective method to understand their causality. Much of our recent knowledge regarding MHWs has been achieved using a point-wise statistical definition that quantitatively defines MHWs as measurable warm ocean temperature extremes relative to a given threshold. While this commonly used definition is easy to use, with MHWs readily detectable and with near-global coverage from satellite sea surface temperature data, it does not quantify the spatial scale of events, their evolution in space and time, nor the association of that evolution with the key drivers. To overcome some of these limitations, more recent studies have investigated the evolution of MHWs as objects evolving in space and time to help broaden our understanding of MHWs. Our new approach represents an important step toward mechanistically characterising the space and time evolution of MHWs – it not only builds upon and extends object-based kinematic studies of MHWs but additionally connects these spatiotemporally evolving MHWs with their key drivers. Finally, we examine the potential predictability of these MHWs based on a linear inverse modelling approach.

There is growing concern among decision makers in a warming world that increasing frequency and severity of extreme events, such as marine heatwaves, is making historical information less representative and useful. To aid decision making in a rapidly warming ocean, researchers have developed new forecast tools that can predict marine heatwaves on sub-seasonal to seasonal timescales. As demand from marine stakeholders increases for actionable information, effective transfer is paramount for uptake and response. Here we consider three aspects critical to aid this information transfer. The model system must be useful – having the appropriate model type, resolution, and domain, with model skill verified against observations. The forecasts must be useable with skill on timescales relevant to decision making, interpretable by end users, and delivered in a practical format. The forecast information must be used, which requires clear communication at appropriate levels, strong user engagement, and consideration of industry agility and incentives. Feedback between these three aspects is needed to ensure continuous improvement in the level of forecast usefulness, useability, and uptake. Forecasts are a critical tool in the management of climate risk and the framework we present here will be important for maintaining profitable and sustainable marine industries into the future.

El Niño–Southern Oscillation (ENSO) affects sea surface temperatures (SSTs) around the world with varying degrees of influence. Dynamical understanding of global SST responses to ENSO has progressed substantially, but is far from complete. We propose a novel modelling approach, and use it to investigate why SSTs along Australia’s southeast coast are modulated by ENSO much less strongly than along Australia’s west coast. Combining tropical Pacific pacemaker ensemble simulations and ocean model perturbation experiments, this modelling approach identifies and compares the contributions of different mechanisms for the two regions. Off Australia’s west coast, the strong ENSO signature in SST variability is dominated by remote tropical Pacific ENSO-driven wind stress forcing via oceanic teleconnections, with smaller contributions from non-ENSO climate variability. However, off the southeast coast, the contribution from remote tropical Pacific ENSO-driven wind stress forcing is largely offset by the thermodynamic ENSO-driven buoyancy forcing locally within the Coral and Tasman Sea. This resultant weak ENSO signature in SST is further weakened by non-ENSO climate variability.

Through late November and early December 2023, a severe category marine heatwave (MHW) was detected moving southwards off the east coast of Tasmania, Australia. The MHW was characterised by offshore sea surface temperature anomalies ~4oC above climatological values embedded within and around large anticyclonic eddies with warm anomalies to >1000m depth. Given the deleterious impacts from previous MHWs on marine ecosystems, fisheries, and aquaculture in the region, serious concerns were raised. To advise and prepare stakeholders, a series of online briefings was given by physical, biogeochemical, fisheries, and social scientists on the current and likely evolving environmental conditions associated with the MHW. So, how unusual was this event? Was it successfully forecast? Was it expected from our knowledge of large-scale modes of climate variability and their teleconnections? This presentation will discuss the characteristics, evolution – both forecast and projected – and emerging impacts of the November-December 2023 Tasman Sea MHW. It will be argued that the characteristics of this event mirror expectations from anthropogenic climate change, and that initialised seasonal SST forecasts were little different from expectations under climate change projections and trend persistence.

As the ocean has warmed, in recent decades marine heatwaves (MHWs) have emerged as a major threat to marine ecosystems and ecosystem services, presenting challenges for management of marine fisheries, aquaculture, tourism, and conservation, including for marine protected areas (MPAs). An MHW is a period of unusually high ocean temperatures, often defined as ocean temperatures that are warmer than 90% of the previous observations for a given time of year. MHWs along Australia’s coastal regions have led to mass coral bleaching on the Great Barrier Reef, damage to kelp forests and seagrass meadows in Western Australia, shifts in species, and fish and invertebrate mortality, all creating pressures on fisheries management (as reviewed in Smith et al., 2023). Understanding how climate change influences ocean extremes and impacts societal and natural values is key for evaluating future risks. Growing concerns around the effects of MHWs on marine industries, food security, ecosystem dynamics, and conservation efforts led to the development of MHW response plans for Tasmania and New South Wales during the summer of 2023/24 (Hobday et al., 2024).

Marine heatwaves are increasing in frequency, intensity, and duration as a result of climate change, and their biological impacts can in turn influence coastal communities. Despite advances in our knowledge of the physical drivers of marine heatwaves and their biological impacts, there has been limited work linking these extreme events to subsequent impacts on social systems. Describing risk to well-being in coastal communities from marine heatwaves requires the consideration of the severity of marine heatwaves, impacted systems, and social vulnerability. We compared potential risk to well-being, or quality of life, from 2012 to 2016 in coastal communities in the United States and Australia by considering marine heatwave total cumulative intensity, fishing dependence, and vulnerability indices. We extended a social indicators framework for the United States to develop vulnerability indices for coastal communities in Australia. Our approach revealed different spatial patterns in risk to well-being and its drivers between the two countries. Marine heatwaves as the hazard were a key driver of risk in both countries, and vulnerability for the United States and fishing employment for Australia were also influential. Our study demonstrates that risk does not necessarily equal the hazard, and there is non-transferability of risk results between countries despite similar physical oceanography and socioeconomic status. Identifying regions of high risk with our broad approach can help prioritize higher resolution community-level work to mitigate risk and develop adaptation pathways.

Marine heatwaves (MHWs), prolonged periods of unusually high ocean temperatures, significantly impact global ecosystems. However, there is ongoing debate regarding the definition of these extreme events, which is crucial for effective research and communication among marine scientists, decision-makers, and the broader public. Fundamental to all MHW analyses is a clearly defined background oceanic climate – i.e., a temperature ‘baseline’ against which the MHW is defined. While a single approach to implementing a baseline may not be suitable for all MHW research applications, the choice of a baseline for analysing MHWs must be intentional as it affects research outcomes.This perspective examines baseline choices and discuss their implications for marine organism and ecosystem risks, and their relevance in communicating MHW characteristics and metrics to stakeholders, policymakers, and the public. In particular we analyses five different baseline approaches for computing MHW statistics, assesses their technical differences, and discusses their ecological implications. Different baselines suggest widely different trends in MHW characteristics in a warming world. This would, for example, imply differences in future risk, reflective of marine organisms with different adaptive potential, thereby affecting recommendations for management strategies. We also examine the consequences of different baseline choices on ease of implementation and communication with wider audiences. Our analyses highlight the need to clearly specify a chosen baseline in MHW studies, and to be mindful of its implications for MHW statistics, practical considerations, and interpretations concerning the adaptive capacities of marine organisms, ecosystems and human systems. The challenges and implications of different MHW baselines highlighted here have similar relevance in research and communication for other branches of climate extremes.

As marine heatwaves (MHWs) become more intense and longer lasting due to global warming, understanding the drivers and impacts of these events is crucial for effective marine resource management. This study investigates the influence of El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), Southern Annular Mode (SAM), Sub-Tropical Ridge High (STRH), and Madden Julian Oscillation (MJO) on sea surface temperature (SST) anomalies and MHWs around Australia. The aim of this research is to improve our understanding of the drivers of MHWs on sub-seasonal to seasonal (S2S) timescales, which bridges the gap between short-term weather and interannual to long-term climate variability. By analysing SST anomalies and MHWs characteristics during specific driver phases, a simple MHW hazard index is developed. Our findings support previous research indicating that La Niña plays a role in driving MHWs off the coast of Western Australia and reveals a previously unrecognised connection between ocean warming off Queensland and Tasman Sea low-pressure systems associated with the negative phase of the STRH. Our research emphasizes the importance of considering multiple drivers and their compounding effects on MHWs by showing significant changes to typical La Niña MHW patterns with the additional influence of the MJO. By considering drivers acting in the S2S timescale, forecasts can more accurately capture the timing, intensity, and spatial extent of MHW events within a season. These improved forecasts can enhance the ability of marine managers to adapt and allocate resources based on evolving climate conditions, enabling effective implementation of harm minimisation strategies.

Abstract Australia’s record-breaking 2019/20 Black Summer fire weather resulted from a combination of natural and anthropogenic climate factors, but the full range of natural variability in fire weather is unknown. We reconstruct southeast Australian fire weather over the Common Era based on an East Antarctic ice core sea-salt aerosol record. This record reflects the Southern Ocean synoptic-scale weather patterns and Antarctic stratospheric polar vortex strength that pre-condition elevated fire danger over southeast Australia. We show that the (a) intensity of the 2019/20 fire weather was unprecedented since 1950 and (b) frequency of above average fire weather seasons from 2010–2020 has only occurred once since 1950 (over 1977–1987), but there are analogues for similar extreme fire danger caused by natural variability in the 2000-year reconstruction. This highlights the need for fire risk mitigation that considers the full range of plausible natural variability in Australia’s fire weather as well as anthropogenic forcing.

Large-scale marine heatwaves in the Northeast Pacific (NEP), identified here and previously as 'warm blobs', have devastating impacts on regional ecosystems. An anomalous atmospheric ridge over the NEP is known to be crucial for maintaining these warm blobs, also causing abnormally cold temperatures over North America during the cold season. Previous studies linked this ridge to teleconnections from tropical sea surface temperature anomalies. However, it was unclear whether teleconnections from the extratropics could also contribute to the ridge. Here we show that planetary wave trains, triggered by increased rainfall and latent heat release over the Mediterranean Sea accompanied by decreased rainfall over the North Atlantic, can transport wave energy to the NEP, guided by the westerly jet, and induce a quasi-barotropic ridge there. Our findings provide insights into extratropical teleconnections sustaining the NEP ridge, offering a source of potential predictability for the warm blobs and temperature fluctuations over North America.

Abstract Local meteorology over the Great Barrier Reef (GBR) can significantly influence ocean temperatures, which in turn impacts coral ecosystems. While El Niño–Southern Oscillation (ENSO) provides insight into the expected synoptic states, it lacks details of anticipated sub‐seasonal weather variability at local scales. This study explores the influence of the Madden‐Julian oscillation (MJO) on Australian tropical climate, both independently and in combination with ENSO, focusing on GBR impacts. We find that during El Niño periods, including the summer of 2009/10, faster propagating MJO patterns can disrupt background warm, dry conditions, and potentially provide cooling relief via increased cloud cover and stronger winds. In La Niña periods, such as the summer of 2021/22, the MJO tends to be prevented from passing the Maritime continent, forcing it to remain in a standing pattern in the Indian Ocean. This leads to decreased cloud cover and weaker winds over the GBR, generating warm ocean anomalies.

AbstractMarine heatwaves have profoundly impacted marine ecosystems over large areas of the world oceans, calling for improved understanding of their dynamics and predictability. Here, we critically review the recent substantial advances in this active area of research, including the exploration of the three-dimensional structure and evolution of these extremes, their drivers, their connection with other extremes in the ocean and over land, future projections, and assessment of their predictability and current prediction skill. To make progress on predicting and projecting marine heatwaves and their impacts, a more complete mechanistic understanding of these extremes over the full ocean depth and at the relevant spatial and temporal scales is needed, together with models that can realistically capture the leading mechanisms at those scales. Sustained observing systems, as well as measuring platforms that can be rapidly deployed, are essential to achieve comprehensive event characterizations while also chronicling the evolving nature of these extremes and their impacts in our changing climate.

AbstractMarine heatwaves (MHWs) have caused devasting ecological and socioeconomic impacts worldwide. Understanding the connection of regional events to large‐scale climatic drivers is key for enhancing predictability and mitigating MHW impacts. Despite the reported connection between MHWs globally and El Niño–Southern Oscillation (ENSO), establishing statistically significant links between different types of ENSO events and MHWs remains challenging due to the limited duration of observational data. Here, we use 10,000 years of simulations from a Linear Inverse Model (LIM) to address this issue. Our findings reveal distinct connections between MHWs and ENSO, with diverging influences from different flavors of El Niño and La Niña events. In addition, under long‐lasting El Niño conditions, the likelihood of MHWs increases by up to 12‐fold in the Indian and Pacific Oceans. This study highlights the global connections between ENSO diversity and variations in MHW events.

Marine heatwaves pose an increasing threat to fisheries and aquaculture around the world under climate change. However, the threat has not been estimated for the coming decades in a form that meets the needs of these industries. Tasmanian fisheries and aquaculture in southeast Australia have been severely impacted by marine heatwaves in recent years, especially the oyster, abalone, and salmon industries. In a series of semi-structured interviews with key Tasmanian fishery and aquaculture stakeholders, information was gathered about the following: (i) the impacts they have experienced to date from marine heatwaves, (ii) their planning for future marine heatwaves, and (iii) the information that would be most useful to aid planning. Using CMIP6 historical and future simulations of sea surface temperatures around Tasmania, we developed a marine heatwave hazard index guided by these stakeholder conversations. The region experienced a severe marine heatwave during the austral summer of 2015/16, which has been used here as a reference point to define the index. Our marine heatwave hazard index shows that conditions like those experienced in 2015/16 are projected to occur approximately 1-in-5 years by the 2050s under a low emissions scenario (SSP1-2.6) or 1-in-2 years under a high emissions scenario (SSP5-8.5). Increased frequency of marine heatwaves will likely reduce productivity by both direct (mortality) and in-direct (ecosystem change, greater incidence of disease) impacts on target species. The illustrative hazard index is one step towards a marine heatwave risk index, which would also need to consider aspects of exposure and vulnerability to be of greater utility to stakeholders.