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Heatwaves are important climatic extremes in atmospheric and oceanic systems that can have devastating and long-term impacts on ecosystems, with subsequent socioeconomic consequences. Recent prominent marine heatwaves have attracted considerable scientific and public interest. Despite this, a comprehensive assessment of how these ocean temperature extremes have been changing globally is missing. Using a range of ocean temperature data including global records of daily satellite observations, daily in situ measurements and gridded monthly in situ-based data sets, we identify significant increases in marine heatwaves over the past century. We find that from 1925 to 2016, global average marine heatwave frequency and duration increased by 34% and 17%, respectively, resulting in a 54% increase in annual marine heatwave days globally. Importantly, these trends can largely be explained by increases in mean ocean temperatures, suggesting that we can expect further increases in marine heatwave days under continued global warming.

Article type: Letter 41 42 One Sentence Summary: Marine heatwaves alter ecosystem structure and functioning at 43 global scales. 44 45 46 47 48 49 implications for marine ecosystems 1 . Concurrent with long-term persistent warming, 50 discrete periods of extreme regional ocean warming (marine heatwaves, 'MHWs') have 51 increased in frequency 2 . Here we quantify trends and attributes of MHWs across all 52 ocean basins and examine their biological impacts from species to ecosystems. Multiple 53 regions within the Pacific, Atlantic and Indian Oceans are particularly vulnerable to 54 MHW intensification, due to the co-existence of high levels of biodiversity, a prevalence 55 of species found at their warm range edges, or concurrent non-climatic human impacts. 56 The physical attributes of prominent MHWs varied considerably, but all had 57 deleterious impacts across a range of biological processes and taxa, including critical 58 foundation species (corals, seagrasses and kelps). MHWs, which will likely intensify 59 with anthropogenic climate change 3 , are rapidly emerging as forceful agents of 60 disturbance with the capacity to restructure entire ecosystems and disrupt the provision 61 of ecological goods and services in coming decades. 62 63 Anthropogenic climate change is driving the redistribution of species and reorganization of 64 natural systems and represents a major threat to global biodiversity 4,5 . The biosphere has 65 401 working group on marine heatwaves (www.marineheatwaves.org).

Several studies have documented fish populations changing in response to long-term warming. Over the past decade, sea surface temperatures in the Gulf of Maine increased faster than 99% of the global ocean. The warming, which was related to a northward shift in the Gulf Stream and to changes in the Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation, led to reduced recruitment and increased mortality in the region's Atlantic cod (Gadus morhua) stock. Failure to recognize the impact of warming on cod contributed to overfishing. Recovery of this fishery depends on sound management, but the size of the stock depends on future temperature conditions. The experience in the Gulf of Maine highlights the need to incorporate environmental factors into resource management.

Marine heatwaves (MHWs) can cause devastating impacts to marine life. Despite the serious consequences of MHWs, our understanding of their drivers is largely based on isolated case studies rather than any systematic unifying assessment. Here we provide the first global assessment under a consistent framework by combining a confidence assessment of the historical refereed literature from 1950 to February 2016, together with the analysis of MHWs determined from daily satellite sea surface temperatures from 1982–2016, to identify the important local processes, large-scale climate modes and teleconnections that are associated with MHWs regionally. Clear patterns emerge, including coherent relationships between enhanced or suppressed MHW occurrences with the dominant climate modes across most regions of the globe – an important exception being western boundary current regions where reports of MHW events are few and ocean-climate relationships are complex. These results provide a global baseline for future MHW process and prediction studies.

Prolonged high-temperature extreme events in the ocean, marine heatwaves, can have severe and long-lasting impacts on marine ecosystems, fisheries and associated services. This study applies a marine heatwave framework to analyse a global sea surface temperature product and identify the most extreme events, based on their intensity, duration and spatial extent. Many of these events have yet to be described in terms of their physical attributes, generation mechanisms, or ecological impacts. Our synthesis identifies commonalities between marine heatwave characteristics and seasonality, links to the El Niño-Southern Oscillation, triggering processes and impacts on ocean productivity. The most intense events preferentially occur in summer, when climatological oceanic mixed layers are shallow and winds are weak, but at a time preceding climatological maximum sea surface temperatures. Most subtropical extreme marine heatwaves were triggered by persistent atmospheric high-pressure systems and anomalously weak wind speeds, associated with increased insolation, and reduced ocean heat losses. Furthermore, the most extreme events tended to coincide with reduced chlorophyll-a concentration at low and mid-latitudes. Understanding the importance of the oceanic background state, local and remote drivers and the ocean productivity response from past events are critical steps toward improving predictions of future marine heatwaves and their impacts.

Extreme and large‐scale warming events in the ocean have been dubbed marine heatwaves, and these have been documented in both the Northern and Southern Hemispheres. This paper examines the intensity, duration, and frequency of positive sea surface temperature anomalies in the North Atlantic and North Pacific Oceans over the period 1950–2014 using an objective definition for marine heatwaves based on their probability of occurrence. Small‐area anomalies occur more frequently than large‐area anomalies, and this relationship can be characterized by a power law distribution. The relative frequency of large‐ versus small‐area anomalies, represented by the power law slope parameter, is modulated by basin‐scale modes of natural climate variability and anthropogenic warming. Findings suggest that the probability of marine heatwaves is a trade‐off between size, intensity, and duration and that region specific variability modulates the frequency of these events.

The reappearance of a northeast Pacific marine heatwave (MHW) sounded alarms in late summer 2019 for a warming event on par with the 2013-2016 MHW known as The Blob. Despite these two events having similar magnitudes in surface warming, differences in seasonality and salinity distinguish their evolutions. We compare and contrast the ocean's role in the evolution and persistence of the 2013-2016 and 2019-2020 MHWs using mapped temperature and salinity data from Argo floats. An unusual near-surface freshwater anomaly in the Gulf of Alaska during 2019 increased the stability of the water column, preventing the MHW from penetrating deep as strongly as the 2013-2016 event. This freshwater anomaly likely contributed to the intensification of the MHW by increasing the near-surface buoyancy. The gradual buildup of subsurface heat content throughout 2020 in the region suggests the potential for persistent ecological impacts. Plain Language Summary Surface marine heatwaves (MHWs) are periods of prolonged and extremely warm regional sea surface temperature that can negatively impact the health and productivity of marine ecosystems. Using surface and subsurface ocean observations, we compare and contrast two recent MHWs to show that salinity variations play an important role in the vertical distribution of temperature anomalies by changing the overall stability of the water column. During the 2019-2020 MHW, the near-surface waters in the Gulf of Alaska were fresher than normal, preventing warm sea surface temperatures from mixing as deeply into the subsurface as in the 2013-2016 MHW. The freshening in 2019 likely enhanced warming in the buoyant surface layer. As warmer temperatures gradually mix downward they can persist long after the surface MHW disappears, suggesting that the ocean can provide memory for long-lived MHWs. The subsurface persistence of MHWs has potential ramifications for long-lasting ecological impacts.