A global overview of marine heatwaves in a changing climate

Communications Earth and Environment A global overview of marine heatwaves in a changing climate

Marine 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.

In recent decades, episodes of warm ocean temperature extremes have been associated with more intense and frequent impacts on marine organisms, ecosystems and reliant human industries around the world. By analogy with their atmospheric counterpart, these extreme ocean temperature events have been termed "marine heatwaves." Some of the most prominent events, together with the unprecedented warming during the boreal summer of twenty twenty-three are presented in Box one. Marine heatwaves influence regional climate phenomena and often drive substantial impacts on the marine environment. For example, marine heatwaves in the Indian Ocean have been found to modulate the monsoon winds and rains over the Indian subcontinent, impacting water and food security over the region. Marine heatwaves interact with and intensify tropical cyclones, making them more destructive. Biological marine heatwave impacts include mass mortality events in invertebrates, fish, birds and marine mammals, coral bleaching, declines in foundation species, and entire ecosystem restructuring, with far-reaching socioeconomic impacts.

Recent reviews and perspectives have outlined major steps forward in understanding marine heatwave characteristics, drivers, and predictability, along with the economic impacts they cause. However, in this rapidly evolving field, more recent research has provided new insights into marine heatwaves, while generating important new questions and research avenues.

Although marine heatwave research has primarily considered temperature extremes at the ocean surface, subsurface temperature extremes may be more intense and longer-lasting than their surface counterparts. Given the prevalence of life throughout the water column, subsurface marine heatwaves need to be closely observed, mechanistically understood, and skillfully predicted. In addition, while the physical characterization of marine heatwaves has mainly focused on large-scale events (Box one), marine heatwaves are now also studied in more localized coastal areas, marginal seas, and fjords, where they are negatively impacting the local ecology and coastal communities. Marine heatwaves are also increasingly being examined along with other extreme conditions, like high acidity or low-oxygen, sea level extremes, floods, droughts, severe weather events, or even terrestrial heat waves over the adjacent land. These "compound events" act as multiple stressors for marine life and societies.

The ability to predict marine heatwaves and compound events from days to seasons in advance is key for stakeholder preparation and mitigation efforts. Skillful forecasts require enhanced understanding of marine heatwave drivers to assess their predictability, and prediction systems that realistically capture the processes underpinning that predictability. While progress has been made in prediction activities, additional improvements could be achieved through a deepened understanding of the relative roles of different

Box one | Historical marine heatwaves and the unprecedented summer of twenty twenty-three

Recent decades have witnessed the occurrence of marine heatwaves that were particularly intense, long-lasting and impactful (top panel of Box figure, showing SST anomalies above one degree Celsius at the peak month of each marine heatwave). These most prominent marine heatwaves generally occurred in different regions at different times. However, the boreal summer of twenty twenty-three recorded global monthly-mean SSTs at record high since the beginning of the instrumental record, with a large fraction of the ocean experiencing extreme conditions, as illustrated by the widespread SST anomalies above the ninetieth percentile (nineteen eighty-two-two thousand eleven baseline) during July twenty twenty-three (bottom panel of Box figure). In particular, average North Atlantic (zero to sixty degrees North, zero to eighty degrees West) temperatures reached levels of warming that exceeded four standard deviations of the nineteen eighty to two thousand eleven period during parts of July and September twenty twenty-three, with an annual average approximately zero point two three degrees Celsius higher than in twenty twenty-three.

What caused this unprecedented global extreme? The developing El Niño in twenty twenty-three can be expected to have caused an increase in radiative heating due to the influence of the El Niño SST pattern on atmospheric static stability and low-level clouds. In addition, El Niño can alter the atmospheric circulation and cause the development of SST anomalies in different regions of the world, like the northeast Pacific and the tropical North Atlantic, although warming in the tropical North Atlantic usually occurs after the peak of an El Niño event rather than during its development phase. The pattern of Atlantic warming is consistent with the negative phase of the North Atlantic Oscillation, which was indeed strongly negative from mid-April to mid-May and most of July twenty twenty-three. The concentration of the twenty twenty-three warming in near-surface waters suggests that upper ocean stratification, possibly modulated by large-scale climate modes, may have played an important role in preventing the excess heat absorbed by the ocean from being effectively distributed downward, resulting in enhanced surface warming. Other hypotheses regarding the unprecedented twenty twenty-three warming include a decreased transport of Saharan dust to the western Atlantic, and a reduction of ship emissions following a twenty twenty international agreement, leading to an increase in radiative forcing, although the influence of these factors on Atlantic warming has yet to be demonstrated. Another proposed hypothesis pertains to the aftermath of the January twenty twenty-two Hunga Tonga-Hunga Ha'apai volcanic eruption in Tonga. This eruption emitted aerosols, which had cooling effects, while simultaneously releasing stratospheric water vapor, which had warming effects. However, these factors are estimated to explain, at most, a marginal net cooling of a few hundredths of a degree, rather than a warming. In addition to these mostly natural drivers, the ocean is estimated to have absorbed about ninety percent of the excess heat associated with global warming, causing an average warming of the upper two thousand meters of the global ocean of approximately six point six times ten to the twenty-one joules per year over nineteen fifty-eight to twenty twenty-three. Thus, it is very likely that climate change has contributed to the intensity and widespread coverage of the twenty twenty-three marine heatwaves.

Marine heatwave drivers, and dynamical model improvements, which include an assessment of the sensitivity of marine heatwave forecasts to model resolution.

As the oceans continue to warm with anthropogenic climate change, defining marine heatwaves under non-stationary conditions becomes increasingly challenging, as commonly used definitions will lead to a permanent marine heatwave state in areas experiencing sufficient warming (Fig. one). In addition, separating the processes internal to the climate system from those of anthropogenic origin is key to the mechanistic understanding of the nature of marine heatwaves and the assessment of their predictability and their future changes.

This article extends previous reviews by highlighting the new emerging areas in MHW research outlined above, including: a critical reevaluation of MHW definitions and their detection, both at the surface and in the subsurface, in the presence of climate change; observational needs and new emerging "observing" strategies; advances in the understanding of both surface and subsurface MHW drivers to aid prediction efforts; compound events and their prediction; and investigations to assess future MHW projections using empirical approaches and state-of-the-art modeling systems. This review also provides a perspective on new and promising avenues for advancing our understanding and prediction capabilities of ocean extremes in the context of our changing climate.