Arctic Amplification of marine heatwaves under global warming

Domain: earth_science
Published: 2024-09-26
Editorial outcome: minor_revision

Abstract
Marine heatwaves (MHWs) and total heat exposures (THEs), extreme warming events occurring across the global oceans, seriously threaten marine ecosystems and coastal communities as the climate warms. However, future changes in MHWs and THEs in the Arctic Ocean, where unique marine ecosystems are present, are still unclear. Here, based on the latest CMIP6 climate simulations, we find that both MHWs and THEs in the Arctic Ocean are anticipated to intensify in a warming climate, mainly due to Arctic sea ice decline and long-term warming trend, respectively. Particularly striking is the projected rise in MHW mean intensity during the 21st century in the Arctic Ocean, surpassing the global average by more than sevenfold under the CMIP6 SSP585 scenario. This phenomenon, coined the ‘Arctic MHW Amplification’, underscores an impending and disproportionately elevated threat to the Arctic marine life, necessitating targeted conservation and adaptive strategies.

Introduction
Marine heatwaves (MHWs) are extreme ocean warming events that last for days to months1–4. They pose severe threats to marine ecosystems and coastal communities5–7. MHWs are widespread globally, including the Arctic Ocean located at the northernmost part of our planet8–13. Remarkably, over the past 40 years, the mean intensity of MHWs in the Arctic marginal seas has been comparable with that observed in other regions of the world’s oceans9.

Global warming has been increasing seawater temperature, dramatically impacting the structure and geographic distribution of marine species and ecosystems5,14–16. Confronted with thermal stress, some marine species have started to migrate poleward to cooler regions, including the Arctic Ocean17,18. Despite this migration, the future evolution of MHWs in the Arctic Ocean and the threats these events pose to Arctic marine species remain unclear.

While some marine species may adapt to the gradual, long-term ocean warming induced by climate change, others may face challenges in doing so14,19,20. Therefore, it is crucial to differentiate between extreme warming events relative to a shifting ocean temperature baseline (known as MHWs) and those relative to a fixed historical temperature baseline (referred to as total heat exposures, THEs) when investigating their future evolution in a warming world21,22. Providing insights into future changes in both MHWs and THEs can assist policymakers in assessing and proactively addressing the risks faced by various marine species.

In this study, we investigate historical (1985–2014) and future (2071–2100) MHWs and THEs in the Arctic Ocean using sea surface temperature (SST) and sea ice concentration (SIC) data from the Optimum Interaction Sea Surface Temperature v2.1 (OISST)23, as well as historical simulations and future projections in the Coupled Model Intercomparison Project phase 6 (CMIP6)24,25 (Supplementary Table 1). The projections used in this study are based on four Shared Socioeconomic Pathway (SSP) scenarios, including SSP126, SSP245, SSP370, and SSP585, representing the low, medium, high, and high-end future forcing pathways25,26, respectively. Here, the definitions of MHWs and THEs follow Amaya et al.22 (see “Methods”). We focus on four metrics to assess MHWs and THEs: mean intensity, annual total days, mean frequency, and mean duration. Mean intensity refers to the MHW/THE intensity averaged over MHW/THE events in a specific period. Annual total days represent the total number of days on which events occur each year. Mean frequency indicates the number of MHW/THE events per year averaged over a specific period. Mean duration indicates the duration of MHW/THE averaged over all events.

We reveal a much more prominent increase in MHW mean intensity in the Arctic in future warming climate when compared to the global ocean. Therefore, marine ecosystems in the Arctic may encounter increased threats from these extremes in the future, more so than their counterparts in lower-latitude oceans. We found that by the end of the 21st century, under the SSP585 warming scenario, the increase in MHW mean intensity in the Arctic is projected to be 7.6 times the global average, while the increase in THE mean intensity in the Arctic is expected to be 1.5 times the global average.

Results
MHWs and THEs in the past
The mean intensity of MHWs during the period of 1985–2014 derived from the OISST dataset is stronger in the Arctic marginal seas than in most ocean regions at mid and low latitudes, including coastal areas within economic zones (Fig. 1a, b). The multi-model mean (MMM) of CMIP6 successfully reproduces these observed features in MHW mean intensity in both the Arctic Ocean and lower latitude regions (Fig. 1e, f and Supplementary Fig. 1). The simulated magnitude of MHW mean intensity in the Arctic marginal seas closely aligns with observations, with typical values ranging from 2 to 3°C (Fig. 1b, f).

The annual total days of MHWs in the OISST data set typically exceed 20 days in mid and low latitudes, as well as in most of the Nordic Seas. In contrast, the duration is shorter in the Arctic marginal seas, with the exception of the Barents Sea (Fig. 1c, d). This pattern of fewer annual total days of MHWs in the Arctic is well reproduced by the CMIP6 MMM (Fig. 1g, h). The models slightly overestimate the annual total days of MHWs in both the Arctic marginal seas and lower latitude oceans (Fig. 1c, g and Supplementary Fig. 1), which is a typical model feature found before27.

Due to extensive sea ice cover, MHWs were largely absent in the Arctic deep basin area in the historical period, as consistently shown by the OISST and CMIP6 MMM results (Fig. 1). Overall, the CMIP6 MMM reasonably simulates the main characteristics of past MHWs evident in the OISST dataset, despite some model biases in the magnitudes of the MHW intensity and annual total days. These biases echo those previously identified in CMIP5 simulations28,29.

The spatial patterns of mean intensity and annual total days of THEs during 1985–2014, derived from both OISST and CMIP models, are quite similar to those of MHWs (Fig. 1 and Supplementary Fig. 2). Since THEs include contributions from long-term warming, their mean intensity is slightly larger than that of MHWs during the historical period.

MHWs and THEs in the future
MHWs and THEs over 2071–2100 under the SSP585 scenario25,26 of CMIP6 are analyzed to gain insights into how they will change in a future warming climate. Under the SSP585 warming scenario, MHWs with mean intensity surpassing 1°C and THEs with mean intensity exceeding 2°C are expected to become widespread across the majority of the global oceans over 2071–2100 (Fig. 2a, b, e, and f). Future MHWs are anticipated to have greater intensity in several regions, including the northwest Pacific and Atlantic oceans, the tropical Pacific Ocean, mid-latitude oceans in the Southern Hemisphere, and the Arctic Ocean (Fig. 2a). Similarly, future THEs are projected to be stronger in these regions compared to others (Fig. 2e). Notably, among the five global oceans, the Arctic Ocean is expected to experience the most intense MHWs and THEs in the future, with mean intensities of about 2°C and 3°C, respectively, in the period of 2071–2100 (Fig. 2a, b, e, and f). In the future, the intensity of MHWs and THEs in the Arctic Ocean will be larger than or at levels similar to hotspots of MHWs at mid and low latitudes (Supplementary Fig. 3).

Considering the global ocean, the most substantial increase in MHW mean intensity is projected to take place in the Arctic deep basin (Fig. 2c, d), while the MHW mean intensity in the Arctic marginal seas and in mid and low latitudes, where MHWs were prevalent in the past, is not anticipated to undergo marked changes in the future (Fig. 2c, d).

In regions outside the Arctic Ocean, the future changes in MHW mean intensity in the period 2071–2100 relative to the period 1985–2014 will generally remain below 0.4°C, including the coastal regions within economic zones, under the SSP585 scenario (Fig. 2c). This finding is in line with previous result based on dedicated numerical simulations21. Our results demonstrate that the MHW mean intensity in the deep basin area of the Arctic Ocean will exhibit a much more pronounced increase, by up to ~1.4°C (Fig. 2d). This distinctive increase in MHW mean intensity across the central Arctic can be attributed to the notable reduction in sea ice coverage resulting from a warming climate. The MHW mean intensity over the central Arctic was quite weak in the past (Fig. 1b, f) because SST is close to the freezing point under sea ice cover. However, the anticipated shift toward a seasonally ice-free condition (Supplementary Fig. 4) will facilitate the development of MHWs in this region.

In the future warming climate, THEs will experience the most pronounced intensification within the Arctic Ocean (Fig. 2e–h). The increase in THE mean intensity under the SSP585 scenario is projected to reach ~3°C in some areas of the Arctic Ocean, with the strongest increase expected in the Barents Sea, where it could amount to around 4°C (Fig. 2h). The MHW mean intensity in the Barents Sea is projected not to increase in the future (Fig. 2d), so the increase in THE mean intensity in the Barents Sea is mainly due to the long-term warming trend, which can be attributed to both enhanced Atlantic Water heat inflow and reduced local heat loss to the atmosphere30. In most of the Arctic shelf seas, the increase in THE mean intensity is also primarily driven by the long-term warming trend. For the central Arctic Ocean (approximately north of 75°N), our analysis indicates that the increase in MHW mean intensity accounts for about half of the increase in THE mean intensity, with the other half resulting from the direct effect of long-term warming.

The projected MHW annual total days during 2071–2100 resemble those of the historical period in terms of both their spatial pattern and magnitude across the global ocean, with the exception of the Arctic Ocean (Figs. 1g and 3a). The projected MHW annual total days in the Arctic Ocean (about 20 days) are shorter than those in the mid and low latitudes (about 30 days) (Fig. 3a, b). However, the most noteworthy increase in the annual total days of MHWs is expected within the Arctic deep basin (Fig. 3c, d), which corresponds to a substantial transition from almost negligible MHW days in the past to approximately 20 days in the future (Figs. 1h and 3b).

Due to the long-term warming trend, future changes in the annual total days of THEs will markedly contrast with those of MHWs. Projections indicate that in the period 2071–2100, THE annual total days are expected to exceed 300 days across about 80% of the global ocean expanse (Fig. 3e), whereas, in the central Arctic, they are projected to hover around 200 days (Fig. 3f). Accordingly, the future increase in THE annual total days in the Arctic Ocean is expected to be more modest compared to the global average (Fig. 3g, h). This outcome can be attributed to the presence of winter sea ice cover in the period 2071–2100 (Supplementary Fig. 4), a factor that constrains THE occurrence of the cold season in the Arctic Ocean (Supplementary Fig. 5).

Tests for statistical significance reveal that most of the models surpass the 95% confidence level for the projected future changes in mean intensity and annual total days of MHWs and THEs in the Arctic Ocean. However, the projected future changes in mean intensity and annual total days of MHWs in the mid and low latitudes are insignificant in most of the models (Supplementary Fig. 6).

The mean frequency of MHWs and THEs in the Arctic Ocean is projected to markedly increase in the future warming climate (Supplementary Fig. 7). In contrast, a decrease or no remarkable change in frequency is anticipated for the majority of other ocean regions.

Main drivers of changes in MHW and THE mean intensity
Future increases in Arctic MHW intensity are more pronounced under scenarios with higher forcing pathways (Fig. 4a–d). Under all warming scenarios, future changes in the mean intensity of Arctic MHWs are significantly anti-correlated with future changes in Arctic sea ice area across the models during both the warm half-year (Fig. 4e–h) and cold half-year (Fig. 4j–m). This linear relationship suggests that the reduction in sea ice cover is a key driver for the projected increase in Arctic MHW intensity, consistent with findings on changes in Arctic MHW intensity over the past decades31. The extended periods of open ocean resulting from sea ice retreat allow for a greater accumulation of seasonal surface heat flux in the ocean, thereby triggering stronger MHWs31.

The correlation coefficient between future changes in Arctic MHW mean intensity and Arctic sea ice area is lower in the SSP585 scenario (Fig. 4h, m) than in other scenarios. This is because the Arctic is nearly free of sea ice during most months in the last few decades of the SSP585 scenario. In this situation, while the accumulation of surface heat flux in the ocean and consequently the MHW intensity can further increase with stronger atmospheric warming, the sea ice area cannot further decrease during months when it is already ice-free. This weakens the correlation between changes in MHW intensity and changes in sea ice area. However, if we focus only on the period from March to May when sea ice is still present at the end of the 21st century in the SSP585 scenario, the correlation coefficient between future changes in MHW mean intensity and Arctic sea ice area is higher (Fig. 4i) compared to longer seasonal considerations (Fig. 4h, m). This reinforces the dominant role of sea ice decline in driving changes in MHW intensity as long as the Arctic retains some sea ice. In the SSP585 warming scenario, when the Arctic Ocean becomes free of sea ice throughout the year in the far future (Supplementary Fig. 8), MHWs in the Arctic Ocean will enter a stage where the changes in mean intensity and annual total days are relatively small (Supplementary Fig. 9), similar to the situation in the mid and low latitudes in the 21st century (Figs. 2c and 3c).

The seasonality of changes in Arctic MHW intensity is similar across different warming scenarios (Fig. 4a–d), featuring two peaks: one in summer (June and July) and another in December. The summer peak is associated with the seasonal maximum in solar radiation and net ocean surface heat flux32 (Supplementary Fig. 10). Even with the same extent of sea ice retreat, greater seasonal solar radiation can lead to more heat absorption in the ocean. The December peak is mainly linked to the seasonal maximum in sea ice decline (Fig. 4a–d), which increases open water where MHWs can occur. Thus, the seasonality of future changes in MHW intensity is jointly determined by the seasonality of solar radiation and sea ice decline. The seasonality of future changes in the SST variance mirror the seasonality of future changes in MHW intensity (Supplementary Fig. 11), indicating that SST variance is a good indicator of MHW intensity.

Future changes in the Arctic THE mean intensity is larger in summer (July–September) and smaller in winter (February–April) across all scenarios (Fig. 5a–d). This seasonality differs from that of changes in MHW mean intensity (Fig. 4a–d) and aligns more closely with changes in SST (Fig. 5a–d), indicating that the long-term warming trend is the primary driver of increases in THE intensity. This conclusion is further supported by the strong correlation between future changes in THE mean intensity and future changes in SST across the models in all warming scenarios (Fig. 5e–h).

The projected Arctic Amplification of MHWs
Figures 2 and 3 show that future increases in both the MHW mean intensity and the annual total days in the Arctic Ocean considerably surpass the global ocean average under the SSP585 scenario. We term this phenomenon ‘Arctic MHW Amplification’.

To quantitatively compare the future changes in MHWs and THEs between the Arctic Ocean and the global ocean, the results for all the warming scenarios are presented in Fig. 6 and Supplementary Table 2. The MMM difference of MHW mean intensity in the Arctic Ocean between 2071–2100 and 1985–2014 is 0.08, 0.18, 0.28, and 0.38°C under SSP126, SSP245, SSP370, and SSP585, respectively. In contrast, the global ocean averages are 0.00, 0.02, 0.06, and 0.05°C, respectively. Under the SSP585 scenario, the Arctic amplification factor of MHW mean intensity change is approximately 7.6. For THE mean intensity, the MMM difference is 0.16, 0.70, 1.17, and 1.82°C under SSP126, SSP245, SSP370, and SSP585, respectively, while the global ocean averages are 0.14, 0.41, 0.75, and 1.21°C, respectively. The Arctic amplification factor of THE mean intensity change is approximately 1.5 under the SSP585 scenario. Although THE amplification factor is relatively smaller, the changes in Arctic THE mean intensity are much larger than those of MHWs (Fig. 2). Therefore, it is crucial to emphasize the future changes of both Arctic MHWs and THEs.

For changes in the annual total days, Arctic amplification is found only for MHWs, not for THEs (Fig. 6b and d; Supplementary Table 2). Under the SSP585 scenario, the annual total days of MHWs are projected to increase by 9.45 days in the Arctic Ocean compared to 0.47 days in the global ocean. The THE annual total days are projected to increase by 223.58 days in the Arctic Ocean, less than the average (301.06 days) over the global ocean. In contrast, the increase in THE mean intensity in the Arctic is more pronounced than in the global ocean, as discussed above. This is because the mean intensity, per definition, is calculated by averaging over events. If there are no THE events in a period, this period does not contribute to lowering the calculated mean intensity.

For both changes in the mean frequency and mean duration, Arctic amplification is found for MHWs under all four scenarios, but not for THEs (Supplementary Figs. 12 and 13). In the Arctic Ocean, the future increase in the mean frequency of MHWs correlates with the severity of warming scenarios. However, the mean duration of MHWs does not show a similar trend. Consequently, the frequency increase of MHWs is the primary contributor to the projected rise in the annual total days of Arctic MHWs. Conversely, for Arctic THEs, the future increase in their mean duration correlates with the severity of warming scenarios, while their frequency does not. Therefore, the increase in the mean duration of THEs is the main factor contributing to the future rise in the mean annual total days of Arctic THEs.

Methods
Datasets
OISSTv2.123 used in this study is based on the Advanced Very High-Resolution Radiometer (AVHRR) infrared satellite SST data and observations from different platforms such as ships, buoys, and Argo floats. It has a spatial resolution of 0.25°×0.25°, available at https://www.ncei.noaa.gov/data/sea-surface-temperature-optimum-interpolation/v2.1/access/avhrr/. It has been widely used in MHW studies8,9,13,37,45,47. To determine whether a location is an open water, daily sea ice concentration (SIC) from OISSTv2.1 is also used.

The model data are from the CMIP6 repository24 (available at https://esgf-index1.ceda.ac.uk/projects/cmip6-ceda/). There are 18 CMIP6 models that have released daily SST and SIC datasets from their first realization of the historical experiment, 17 models under the SSP126, 16 models under SSP245, 15 models under SSP370, and 18 models under 585 scenarios25 (Supplementary Table 1). Historical simulations for the period of 1985–2014 are used for comparison with OISST. The SSP126, SSP245, SSP370, and SSP585 scenarios represent low, medium, high, and high-end future forcing pathways with an effective radiative forcing of 2.6, 4.5, 7.0, and 8.5 Wm−2 in 210025. The global mean surface air temperature anomalies for the period 2081–2100 under these four scenarios are 1.23, 2.14, 3.16, and 3.99°C relative to the period of 1995–201448.

Definitions and metrics of MHWs and THEs
MHWs refer to events of transient and extreme ocean warming relative to the climate condition of the period for which MHWs are considered. To distinguish the contributions from extreme ocean warming and long-term temperature trends, THE was proposed22. In this study, MHWs are calculated using detrended SST and a shifting baseline, while THEs are calculated using original SST and a fixed baseline (Supplementary Fig. 14). The baselines of MHWs for the historical and future periods are based on detrended SST in the historical and future periods, respectively. The baselines of THEs for the historical and future periods are the same, and both are based on the original SST in the historical period. So, the contributions from long-term warming are excluded in MHWs but included in THEs. That is, THEs represent the combination of long-term warming and MHWs. The analysis of MHWs and THEs, including the calculation of baselines, is performed for each model and each scenario separately before computing MMM results.

To detect MHWs and THEs, we followed the method of Hobday et al.2 and used the toolbox from https://github.com/ZijieZhaoMMHW/m_mhw1.0 but with some modifications. MHWs/THEs are detected when ocean temperature is higher than the 90th percentile threshold, and the situation is sustained for at least five consecutive days with gaps of less than three days. The baseline and threshold for a certain day were calculated using all SST daily data within an 11-day window centered on this day from all years of the 30-year period28. The obtained annual cycle of baseline and threshold values was then smoothed by applying a 31-day moving average28. This method is effective in ice-free regions2. For the Arctic Ocean, we have to pay attention to the impact of sea ice on SST in observations and model simulations. We notice that OISSTv2.1 sets SST in ice-covered oceans (SIC>35%) to freezing points of seawater to reduce bias and random errors23,49. We also used 35% SIC as a threshold for determining ice-free and ice-covered regions in this study. As the changes in the freezing point of seawater due to salinity changes are quite small, MHWs were considered to be absent under sea ice (where SIC>35%). If no MHW/THE event is detected at a location during the whole period, the metrics are set to 0 during MMM calculation.

In this study, we used several metrics to characterize MHWs and THEs, including mean intensity, annual total days, mean frequency, and duration, which are defined following Hobday et al.2 (Supplementary Fig. 14). MHW/THE mean intensity for each MHW/THE event is the mean temperature anomaly above the climatology (baseline) averaged over the event days (Supplementary Fig. 14). The mean intensity for a considered period is the average of the intensity for all the events in this period. Annual total days are defined as the total MHW/THE days per year. Mean frequency is defined as the annual number of occurrences of MHW/THE events averaged over the considered period. Duration is defined as the number of days between the start and end dates of an MHW/THE event (Supplementary Fig. 14), and mean duration indicates the average over all events during the considered period.

For seasonality analysis (Figs. 4, 5 and Supplementary Figs. 10, 11), MHW/THE monthly mean intensity cannot be calculated by averaging over events because an event may span calendar days of different calendar months. Therefore, we calculated the monthly mean intensity by averaging daily intensity on all event days in a calendar month. The future changes are defined as the difference between the period of 2071–2100 and the period of 1985–2014 if not otherwise stated. The Arctic Ocean is defined as the ocean area north of 65°N.

Data availability
All the data used in this research are freely available to the public and can be downloaded through the links detailed in the Methods section. The data generated in this study have been deposited at https://zenodo.org/records/13744458.

Code availability
All codes used to produce the figures are available at https://zenodo.org/records/13744458.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review on the manuscript “Arctic Amplification of Marine Heatwaves under global warming” by Yan He et.al. The study is devoted to marine heat waves, one of the recent physical characteristics of marine environment. The paper analyzes observational data and the results of three-dimensional numerical modeling of the climate system under a scenario of anthropogenic impact. The characteristics of marine heat waves generated by the ensemble of models at the present time and at the end of the 21st century are discussed. The work is of interest, but some comments have arisen. 1.The work does not clearly define the characteristics used. I would also recommend providing the corresponding formulas to make the presentation of the method more clear. Marine Heat Waves. The determination of MMT can be based on initial data and data that has previously been detrended. For example, to analyze MHWs in the Arctic seas, two approaches were used in the work https://doi.org/10.3390/rs13214436 . This was done in order to highlight areas where MVs are a consequence of a general increase in temperature, and areas where extreme events occur against the background of a general increase. It is worth clarifying how baseline temperature was assessed in the present study. Total Heat Exposures. From my point of view, you can use the temperature of another period as a base temperature, and call it THEs, if this period is close enough. At the moment I do not have a clear understanding of what the authors obtained by analyzing THEs. In this work, the historical period is far away, and we do not know if there may have been more extreme events in the previous period relative to the same period (if we care about marine life and their ability to adapt). . 2.There is a discussion of the results without reference to the methods and data that are given at the end of the work. Terms that characterize MHWs and THE appear in the text, but no explanation is given for them. This raises questions and makes the work difficult to read. Apparently, the structure of the article, where sections of materials and methods are described at the end of the work, was determined by the editors. But in this case, the authors should provide at least a minimal explanation of the terms used or include active links to these sections. 3.p.58-68. I would recommend giving at least a minimal definition of SSP5-8.5 scenario. 4.pp.72«The Optimum Interaction Sea Surface Temperature v2.1 (OISST)24 data set indicates that the mean intensity of MHWs during the period of 1985–2014 was stronger in the…..» The sentence is formulated in such a way that it is unclear how information about MHW appeared in OISST data. 5.206-207. SST beneath sea ice in the Arctic is close to seawater freezing point, exhibiting minor variations29, primarily influenced by changes in sea surface salinity. This conclusion is unclear. 6. pp. 277-278. “MHWs and THEs are terms suggested by Amaya et al21 to distinguish different definitions of extreme warming events in the ocean. “ Amaya et al21 were not the first who suggested the term MHWs. Please, rephrase. 7. pp.287-296.Please explain, how the multi-model-mean (MMM) was calculated in your study given that every model has different ice concentration.", "reviewer_2": "Comments on “Arctic Amplification of Marine Heatwaves under global warming” by He et al., 2023 This study explores the changes in MHWs and THEs in the Arctic Ocean in a future warming climate. The authors show an increase in mean intensity and annual total days of MHWs and THEs in the Arctic Ocean between 1985-2014 and 2071-2100 using the CMIP6 climate simulations. The increase in MHW mean intensity in the Arctic Ocean is projected to be 4 times the global average by the end of the 21st century under the SSP5-8.5 warming scenario, while the increase in THE mean intensity in the Arctic Ocean is expected to be only 1.6 times the global average. The increase in annual total days of MHWs in the Arctic Ocean is projected to be approximately 9 times the global average increase. However, the increase in the annual total days of THEs in the Arctic Ocean is projected to be smaller than the global average increase. The increase in both the MHW mean intensity and annual total days in the Arctic Ocean that greatly exceeds the global average is called ‘Arctic MHW Amplification’. The authors suggest that the increase in the mean intensity and annual total days of MHWs and THEs is mainly attributed to the decrease in sea ice coverage due to global warming. However, the authors seem not to fully understand the differences between the definitions of MHWs and THEs, as well as fail to elucidate the differences in their future changes and the associated drivers. The currently explained physical mechanisms for the increase in the mean intensity and annual total days of MHWs and THEs in the Arctic Ocean in a future warming climate are certainly reasonable but may not be surprising to the scientific community. Several issues in this study are highlighted below: 1) The authors detect MHWs for the two periods of 1984-2014 and 2071-2100, respectively, using these two periods as their respective climatological reference baseline. It is well known that the global/Arctic sea surface has shown a significant warming trend over the past decades. In the future, the global/Arctic sea surface is projected to continue warming. I am wondering how the authors considered the effect of long-term warming in the periods 1985-2014 and 2071-2100 on MHWs. According to the Method proposed in this study, the characteristics of MHWs for the period of 1985-2014 could be the same as the THEs for the period of 1985-2014, but this is clearly not the case. Although the authors followed the suggestions of Amaya et al (2023) and tried to distinguish the differences between MHW and THE. According to suggestions of Amaya et al (2023), MHWs and THEs should distinguish between the effects of long-term warming trends, but it is apparent that the authors did not consider the effects of long-term warming trends on MHWs over a 30-year period. The mean state of MHWs during 1985-2014 and 2071-2100 shown in Figs 1a-b, 2a-b, and 3a-b are not MHWs as defined by Amaya et al (2023). The authors should consider removing the long-term warming trend from the calculation of MHWs in order to truly distinguish between MHWs and THEs. 2) Statistically significant tests for differences between two periods (Figs 2-4) are missing, but important to show. 3) The authors used 35% SIC as a threshold for determining ice-free and ice-covered regions and considered that there to be no MHWs when sea ice is present (where SIC>35%). There is no doubt that there will be more and more MHWs in the Arctic Ocean with diminishing sea ice, regardless of whether its climatological mean changes or not. In other words, the projected Arctic Amplification of MHWs is sensitive to changes in sea ice in the future. If the change in sea ice is projected to become weaker and slower in the low-emission scenario, the Arctic Amplification of MHWs will also become weaker. Alternatively, when sea ice changes stabilize, the Arctic Amplification of MHWs will disappear. Arctic MHW Amplification should be further explored based on different emission scenarios or numerical models. 4) Given the seasonality of sea ice cover in the Arctic Ocean by the end of the 21st century in the SSP5-8.5, the authors could also analyze seasonal changes in MHWs and THEs in the Arctic Ocean. 5) The authors show both future changes in MHWs and THEs, but the characteristic differences between them seem unclear. The authors present the mean state and change characteristics of MHW mean intensity in the Arctic Ocean in the future in 3 paragraphs (lines 102-130), but present the mean state and change characteristics of THEs in the Arctic Ocean in only 1 sentence (lines 133-136). Since MHWs and THEs are proposed by different definitions and have different impacts on ecosystems, it is important to elaborate on the differences between them. 6) The authors identify the causes of the increases in mean intensity and annual total days of MHWs and THEs in the Arctic Ocean in the Abstract (lines 29-30) and in the main text (lines 136-140, 163-165, and 236-240), but do not provide sufficient evidence to support a causal relationship between them. Importantly, the authors conflate the causes of increases in MHWs and THEs, which are extreme events under different definitions, leading to a lack of logical clarity in the explanations and discussions about this part. Moreover, it is also important to quantify the contribution of different impact factors to the increases in MHWs and THEs, respectively, and to show the similarities and differences in the impact factors. 7) Not only changes in sea ice, but also changes in mean temperature and temperature variance can be provided. This information helps to understand the causes and physical mechanism of increases in mean intensity and annual total days of MHWs and THEs, as well as the differences between them. 8) The comparisons of mean state and changes of MHWs and THEs in the Arctic Ocean with that in lower latitude regions in the section ‘MHWs and THEs in the future’ are vague and unfocused. The figures in the text also do not well represent the differences between the Arctic Ocean MHWs and other regions. Moreover, the authors compare the changes of MHWs and THEs in the Arctic Ocean with the global average, however, we know that MHWs have large spatial variations with stronger mean intensity in the western boundary current extension regions, the central and eastern equatorial Pacific Ocean, and eastern boundary current regions. It would be more meaningful to compare the differences in MHW changes in the Arctic Ocean with regions where MHWs are stronger and longer lasting. 9) Lines 66-69, although the mean intensity of MHWs in the Arctic Ocean is projected to be four times stronger than the global average by the end of the 21st century, while the mean intensity of THEs in the Arctic Ocean is projected to be 1.6 times stronger than the global average. However, in fact, the mean intensity of THEs in the Arctic Ocean is projected to be stronger than the mean intensity of MHWs by the end of the 21st century, and the change in THE mean intensity in the Arctic Ocean in the future period relative to the historical period is also projected to be stronger than the change in MHW mean intensity. Thus, the comparison of MHWs and THEs in the Arctic Ocean with the global average is actually somewhat misleading. 10) The authors state that MHWs and THEs have different impacts on marine life in the Arctic Ocean. Please specify what organisms are affected differently. Minor comments: 1) Surge means a sudden large increase. I don’t think the increase in mean intensity and annual total days of MHWs during the 21st century will be sudden. 2) Line 68, reference 23 is not appropriate to cite here. 3) Lines 77-79, ‘…this observed contrast in MHW mean intensity between the Arctic Ocean and regions of lower latitudes…’ The MHW mean intensity in some regions of lower latitudes is also strong, comparable to the MHW mean intensity in the Arctic Ocean. 4) Line 81, annual aggregate of MHW days=> annual total days of MHWs 5) Line 82, this figure? 6) Line 83, The annual total days of MHWs in the Greenland Sea seems to be comparable to that in the Barents Sea. 7) Lines 79-80 and 85-87, please show the map of differences of global MHWs between OISST and CMIP6 to more clearly demonstrate the differences in global MHWs between observations and models. 8) Lines 108-110, Supplementary Fig 4 can be cited here. 9) Lines 111-112 and 124-130, Both these two parts describe the future change of MHW mean intensity in the Arctic Ocean and are therefore recommended to be combined. The mechanisms associated with the future changes could be considered as a separate paragraph. 10) Line 133, what’s the meaning of ‘echoing the pattern observed for the MHW mean intensity’? The pattern of MHW mean intensity in future period is not the same as the pattern in the historical period. 11) Line 235, ‘the Barents Sea the location’=>’ the Barents Sea’"}, "author_response": "Response to Reviewer #1 (Note: Reviewer comments in black and our point-to-point replies in blue) Review on the manuscript “Arctic Amplification of Marine Heatwaves under global warming” by Yan He et.al. The study is devoted to marine heat waves, one of the recent physical characteristics of marine environment. The paper analyzes observational data and the results of three-dimensional numerical modeling of the climate system under a scenario of anthropogenic impact. The characteristics of marine heat waves generated by the ensemble of models at the present time and at the end of the 21st century are discussed. The work is of interest, but some comments have arisen. Reply: We would like to express our sincere thanks to you for all valuable comments and suggestions, which helped to make the manuscript more accurate and easier to understand. We have tried our best to refine the manuscript following the suggestions. The manuscript is much improved through considering these comments. 1.The work does not clearly define the characteristics used. I would also recommend providing the corresponding formulas to make the presentation of the method more clear. Reply: Following your suggestion, we added the formular for the mean intensity of MHW/THE. Furthermore, we improved the text in the Method section to better explain the definitions, especially to avoid confusion about MHWs and THEs. Marine Heat Waves. The determination of MMT can be based on initial data and data that has previously been detrended. For example, to analyze MHWs in the Arctic seas, two approaches were used in the work https://doi.org/10.3390/rs13214436 . This was done in order to highlight areas where MVs are a consequence of a general increase in temperature, and areas where extreme events occur against the background of a general increase. It is worth clarifying how baseline temperature was assessed in the present study. Reply: We agree with you. The paper of Golubeva et al., 2021 (https://doi.org/10.3390/rs13214436) investigated marine heatwaves in the Siberian Arctic Seas and adjacent region from 2000 to 2020, using initial and detrended SSTs to find out the effects of warming trends on the Arctic MHW. This helps us understand that there is an obvious difference between MHWs identified by the initial SST and the detrended SST in the same period. We think this paper was already attempting to distinguish between MHW and THE. Therefore, it is necessary to discuss from two aspects: MHWs that are events of transient and extreme ocean warming relative to the climate condition of the period for which MHWs are considered with detrended temperature data, and THEs which include the warming trend and MHWs. At the time of 2021 when this paper was published, THE had not yet been proposed as a term to describe the extreme warming events. Oliver et al (2019) reviewed that scientists from different disciplines and for different research objectives were using different thresholds to define extreme warming events in the ocean. However, each definition results in different interpretations of frequency, intensity, duration and spatial extent of marine heatwaves (Amaya et al., 2023). So three terms were encouraged by Amaya et al (2023) to discuss and clearly differentiate various events and impacts. Following your suggestion, we revised the text: ‘That is, the MHWs in the historical period are warming events relative to the ocean temperature in the historical period, and the MHWs at the end of the 21st century are warming events relative to the ocean temperature at the end of the 21st century. On the contrary, THEs refer to warming events relative to the historical period.’ to ‘In this study, MHWs are calculated using detrended SST and a shifting baseline, while THEs are calculated using original SST and a fixed baseline. The base lines of MHWs for the historical and future periods are based on detrended SST in the historical and future periods, respectively. The baselines of THEs for the historical and future periods are the same, and both are based on the original SST in the historical period. So the contributions from the long-term warming are excluded in MHWs, but included in THEs. That is, THEs represent the combination of long-term warming and MHWs.’ to clarify how the baselines of MHW and THE were assessed in revised manuscript. References: Oliver, E. et al. Marine heatwaves. Annu. Rev.Mar. Sci. 13, 313–342 (2021). Amaya, D. et al. Marine heatwaves need clear definitions so coastal communities can adapt. Nature 616, 29-32 (2023) Total Heat Exposures. From my point of view, you can use the temperature of another period as a base temperature, and call it THEs, if this period is close enough. At the moment I do not have a clear understanding of what the authors obtained by analyzing THEs. In this work, the historical period is far away, and we do not know if there may have been more extreme events in the previous period relative to the same period (if we care about marine life and their ability to adapt). Reply: According to the definitions by Amaya et al (2023), THE is the combination of long-term warming and MHW. THE is calculated using a fixed baseline, and the baseline can be far away. When we consider future impacts of MHW and THE on marine ecosystem and diversity, the historical baseline that can reflect the current state is a usable baseline. When we care about marine life and their ability to adapt, we think both types of ocean temperature extremes, i.e. MHW and THE, should be studied. In the revised paper we calculated THEs for both the historical period and future period. Reference: Amaya, D. et al. Marine heatwaves need clear definitions so coastal communities can adapt. Nature 616, 29-32 (2023) 2.There is a discussion of the results without reference to the methods and data that are given at the end of the work. Terms that characterize MHWs and THE appear in the text, but no explanation is given for them. This raises questions and makes the work difficult to read. Apparently, the structure of the article, where sections of materials and methods are described at the end of the work, was determined by the editors. But in this case, the authors should provide at least a minimal explanation of the terms used or include active links to these sections. Reply: We agree with you. Following your suggestions, the brief introduction of methods and datasets, the definitions of MHW/THE, the explanation of these terms, and active links were added in the main text section, as follows. “In this study, we investigate historical and future MHWs and THEs in the Arctic Ocean using sea surface temperature (SST) and sea ice concentration (SIC) data from the Optimum Interaction Sea Surface Temperature v2.1 (OISST)23, as well as historical simulations and future projections in the Coupled Model Intercomparison Project phase 6 (CMIP6)25 (Supplementary Table 1). The projections used in this study are based on four Shared Socioeconomic Pathway (SSP) scenarios, including SSP126, SSP245, SSP370 and SSP585, representing the low, medium, high and high-end future forcing pathways25, 26, respectively. Here, the definitions of MHWs and THEs follow Amaya et al.22 (see ‘Materials and Methods’). We use three metrics to assess MHWs and THEs: mean intensity, annual total days, and mean frequency. Mean intensity refers to the MHW/THE intensity averaged over a specific period. Annual total days represent the total number of days events occur each year. Mean frequency indicates the number of MHW/THE occurrences per year.” 3.p.58-68. I would recommend giving at least a minimal definition of SSP5-8.5 scenario. Reply: We accepted your suggestion and added ‘The projections used in this study are based on four Shared Socioeconomic Pathway (SSP) scenarios, including SSP126, SSP245, SSP370 and SSP585, representing the low, medium, high and high-end future forcing pathways25, 26, respectively.’ in Text section and ‘The SSP126, SSP245, SSP370 and SSP585 scenarios represent low, medium, high and high-end future forcing pathways with effective radiative forcing of 2.6, 4.5, 7.0 and 8.5 W m−2 in 210025. The global mean surface air temperature anomalies for the period 2081–2100 under these four scenarios are 1.23°C, 2.14°C, 3.16°C, and 3.99°C relative to the period of 1995–201447.’ in Datasets section in revised manuscript. 4.pp.72«The Optimum Interaction Sea Surface Temperature v2.1 (OISST)24 data set indicates that the mean intensity of MHWs during the period of 1985–2014 was stronger in the…..» The sentence is formulated in such a way that it is unclear how information about MHW appeared in OISST data. Reply: We revised the sentence to ‘The mean intensity of MHWs during the period of 1985–2014 derived from the OISST dataset is stronger in the Arctic marginal seas than in most ocean regions at mid and low latitudes, including coastal areas within economic zones (Fig 1a and b).’ in revised manuscript. 5.206-207. SST beneath sea ice in the Arctic is close to seawater freezing point, exhibiting minor variations29, primarily influenced by changes in sea surface salinity. This conclusion is unclear. Reply: We accepted your suggestion and revised the sentence to ‘Ocean temperature beneath sea ice in the Arctic is close to seawater freezing point which is primarily influenced by changes in sea surface salinity, exhibiting minor variations33.’ in revised manuscript. 6. pp. 277-278. “MHWs and THEs are terms suggested by Amaya et al21 to distinguish different definitions of extreme warming events in the ocean. “ Amaya et al21 were not the first who suggested the term MHWs. Please, rephrase. Reply: We accepted your suggestion and revised the sentence to ‘In the past two decades, MHWs have been widely studied1-4. However, different definitions result in different interpretations of frequency, intensity, duration and spatial extent of MHWs. To distinguish the contributions from extreme ocean warming and long-term temperature trend, THE was proposed22.’ in revised manuscript. 7. pp.287-296.Please explain, how the multi-model-mean (MMM) was calculated in your study given that every model has different ice concentration. Reply: Sea ice concentration data were only used for event detection. To reduce bias and random errors, SST beneath sea ice cover of 35% SIC is excluded in the detection of extremes. With the selected SST, we identified events and calculated 30-year mean metrics of MHW and THE for each model. Then we interpolated these metrics onto the same set of grids. Finally we did the multi-model mean calculation. We added “If no MHW/THE event is detected at a location during the whole period, the metrics are set to 0 during MMM calculation.” in the section of Definitions and metrics of MHWs and THEs. Response to Reviewer #2 (Note: Reviewer comments in black and our point-to-point replies in blue) Comments on “Arctic Amplification of Marine Heatwaves under global warming” by He et al., 2023 This study explores the changes in MHWs and THEs in the Arctic Ocean in a future warming climate. The authors show an increase in mean intensity and annual total days of MHWs and THEs in the Arctic Ocean between 1985-2014 and 2071-2100 using the CMIP6 climate simulations. The increase in MHW mean intensity in the Arctic Ocean is projected to be 4 times the global average by the end of the 21st century under the SSP5-8.5 warming scenario, while the increase in THE mean intensity in the Arctic Ocean is expected to be only 1.6 times the global average. The increase in annual total days of MHWs in the Arctic Ocean is projected to be approximately 9 times the global average increase. However, the increase in the annual total days of THEs in the Arctic Ocean is projected to be smaller than the global average increase. The increase in both the MHW mean intensity and annual total days in the Arctic Ocean that greatly exceeds the global average is called ‘Arctic MHW Amplification’. The authors suggest that the increase in the mean intensity and annual total days of MHWs and THEs is mainly attributed to the decrease in sea ice coverage due to global warming. Reply: We would like to express our sincere thanks to you for all valuable comments and suggestions, which helped to make the manuscript more accurate and easier to understand. We have tried our best to refine the manuscript following the suggestions. The manuscript is much improved through considering these comments. However, the authors seem not to fully understand the differences between the definitions of MHWs and THEs, as well as fail to elucidate the differences in their future changes and the associated drivers. The currently explained physical mechanisms for the increase in the mean intensity and annual total days of MHWs and THEs in the Arctic Ocean in a future warming climate are certainly reasonable but may not be surprising to the scientific community. Several issues in this study are highlighted below: Reply: Thank you very much for your suggestions that help deepen our understanding of MHWs and THEs and the reasons why they were projected to increase so remarkably in the Arctic Ocean. We provided more specific explanations for the definitions and characteristics of MHWs and THEs in the revised manuscript. Based on the definitions of MHWs and THEs by Amaya et al (2023), we identified MHWs, events that are the transient and extremely warm relative to the expected conditions for a given place and time, based on a shifting baseline using detrended SST to remove long-term warning trend. Meanwhile, THEs as the combinations of long-term warming and MHWs are detected using original SST based on a fixed baseline. To elucidate the mechanisms of increasing MHW and THE in the Arctic Ocean, we added the analysis of the effects of sea ice area and SST under four various future scenarios (SSP126, SSP245, SSP370, and SSP585) respectively. We found that there are significant correlations between the decreases in sea ice area and the increases in MHW mean intensity, while the increases in THE mean intensity showed significant correlations with the increases of SST under various scenarios. Reference: Amaya, D. et al. Marine heatwaves need clear definitions so coastal communities can adapt. Nature 616, 29-32 (2023) 1) The authors detect MHWs for the two periods of 1984-2014 and 2071-2100, respectively, using these two periods as their respective climatological reference baseline. It is well known that the global/Arctic sea surface has shown a significant warming trend over the past decades. In the future, the global/Arctic sea surface is projected to continue warming. I am wondering how the authors considered the effect of long-term warming in the periods 1985-2014 and 2071-2100 on MHWs. According to the Method proposed in this study, the characteristics of MHWs for the period of 1985-2014 could be the same as the THEs for the period of 1985-2014, but this is clearly not the case. Although the authors followed the suggestions of Amaya et al (2023) and tried to distinguish the differences between MHW and THE. According to suggestions of Amaya et al (2023), MHWs and THEs should distinguish between the effects of long-term warming trends, but it is apparent that the authors did not consider the effects of long-term warming trends on MHWs over a 30-year period. The mean state of MHWs during 1985-2014 and 2071-2100 shown in Figs 1a-b, 2a-b, and 3a-b are not MHWs as defined by Amaya et al (2023). The authors should consider removing the long-term warming trend from the calculation of MHWs in order to truly distinguish between MHWs and THEs. Reply: We agree with you. We used detrended SST and a shifting baseline to detect MHWs to exclude long-term warming, while used initial SST and a fixed baseline to detect THE. So MHW and THE in the period of 1985-2014 is different, but the difference is relatively small (comparing Fig. 1 and Supplementary Fig 2). To clarify the definitions, we added ‘In this study, MHWs are calculated using detrended SST and a shifting baseline, while THEs are calculated using original SST and a fixed baseline. The base lines of MHWs for the historical and future periods are based on detrended SST in the historical and future periods, respectively. The baselines of THEs for the historical and future periods are the same, and both are based on the original SST in the historical period. So the contributions from the long-term warming are excluded in MHWs, but included in THEs. That is, THEs represent the combination of long-term warming and MHWs.’ in the revised manuscript. Besides, we revised all the figures (including Figs.1–3) about MHWs in the revised manuscript. 2) Statistically significant tests for differences between two periods (Figs 2-4) are missing, but important to show. Reply: We accepted your suggestion and conducted statistical significance tests for the difference between two periods in Figs. 2–4. For Figs. 2–3, we did t-test for each model we used. We added ‘Statistically significance tests reveal that most of the models surpass the 95% confidence level for the projected future changes in mean intensity and annual total days of MHWs and THEs in the Arctic Ocean. However, the projected future changes in mean intensity and annual total days of MHWs in the mid and low latitudes are insignificant in most of the models (Supplementary Fig. 5).’ in revised manuscript. Moreover, we changed the original Figure 4 into the form of boxplots as below (Figure 6 now) so that it could show more statistical characteristics. Supplementary Fig. 5 Statistical significance tests for the changes of MHW/THE mean intensity and annual total days in Figures 2 and 3. The shading is the percentage of models that exceed the 95% confidence level. Fig. 6 Comparison of future MHW/THE changes between the Arctic Ocean and the global ocean. (a) and (c) The mean intensity and (b) and (d) annual total days of MHWs and THEs averaged over the Arctic Ocean and the global ocean in four warming scenarios. The red and blue box plots depict the results for the Arctic Ocean and the global ocean, respectively. The changes are the results of 2071–2100 relative to 1985–2014. 3) The authors used 35% SIC as a threshold for determining ice-free and ice-covered regions and considered that there to be no MHWs when sea ice is present (where SIC>35%). There is no doubt that there will be more and more MHWs in the Arctic Ocean with diminishing sea ice, regardless of whether its climatological mean changes or not. In other words, the projected Arctic Amplification of MHWs is sensitive to changes in sea ice in the future. If the change in sea ice is projected to become weaker and slower in the low-emission scenario, the Arctic Amplification of MHWs will also become weaker. Alternatively, when sea ice changes stabilize, the Arctic Amplification of MHWs will disappear. Arctic MHW Amplification should be further explored based on different emission scenarios or numerical models. Reply: We agree with you. We accepted your suggestion and added SSP126 (low emission pathway), SSP245 (medium emission pathway) and SSP370 (high emission pathway but not that high as SSP585), together with SSP585, to conduct a more comprehensive analysis of Arctic MHW Amplification. Figure 6 was redesigned as follows and showed that even under SSP126, the low forcing scenario, Arctic MHW Amplification could still be found. We added ‘To quantitatively compare the future changes in MHWs and THEs between the Arctic Ocean and the global ocean, the results for all the warming scenarios are presented in Figure 6 and Supplementary Table 2.’, ‘The MMM difference of MHW mean intensity in the Arctic Ocean between 2071–2100 and 1985–2014 is 0.08°C, 0.18°C, 0.28°C, and 0.38°C under SSP126, SSP245, SSP370, and SSP585, respectively. In contrast, the global ocean averages are 0.00°C, 0.02°C, 0.06°C, and 0.05°C, respectively. Under the SSP585 scenario, Arctic amplification factor of MHW mean intensity change is approximately 7.6. For THE mean intensity, the MMM difference is 0.16°C, 0.70°C, 1.17°C, and 1.82°C under SSP126, SSP245, SSP370, and SSP585, respectively, while the global ocean averages are 0.14°C, 0.41°C, 0.75°C, and 1.21°C, respectively. The Arctic amplification factor of THE mean intensity change is approximately 1.5 under the SSP585 scenario. Although the THE amplification factor is relatively smaller, the changes in Arctic THE mean intensity are much larger than those of MHWs (Fig. 2). Therefore, it is crucial to emphasize the future changes of both Arctic MHWs and THEs.’, ‘For changes in the annual total days, Arctic amplification is found only for MHWs, not for THEs (Fig. 6b and d; Supplementary Table 2). Under the SSP585 scenario, the annual total days of MHWs are projected to increase by 9.45 days in the Arctic Ocean compared to 0.47 days in the global ocean. In contrast, the THE annual total days are projected to increase by 223.58 days in the Arctic Ocean and 301.06 days in the global ocean.’ in ‘Results’ section of the revised manuscript. 4) Given the seasonality of sea ice cover in the Arctic Ocean by the end of the 21st century in the SSP5-8.5, the authors could also analyze seasonal changes in MHWs and THEs in the Arctic Ocean. Reply: We accepted your suggestion and analyzed seasonal changes in MHWs and THEs under various SSPs. We added figures as follows and added ‘The seasonality of changes in Arctic MHW intensity is similar across different warming scenarios (Fig. 4a-d), featuring two peaks: one in June and another in December. The June peak is associated with the seasonal maximum in solar radiation32. With the same extent of sea ice retreat, greater seasonal solar radiation can lead to more heat absorption in the ocean. The December peak is linked to the seasonal maximum in sea ice decline occurring in fall. Although sea ice decline is greatest in fall, the December peak in MHW intensity increase is lower than the June peak because solar radiation is lower during the cold season. Thus, the seasonality of future changes in MHW intensity is jointly determined by the seasonality of solar radiation and sea ice decline. The seasonality of future changes in the SST variance mirrors the seasonality of future changes in MHW intensity (Supplementary Fig. 7), indicating that SST variance is a good indicator of MHW intensity.’ and ‘Future changes in THE intensity are larger in summer (July–September) and smaller in winter (February–April) across all scenarios (Fig. 5a–d). This seasonality differs from that of changes in MHW intensity (Fig. 4a–d) and aligns more closely with changes in SST (Fig. 5a–d), indicating that the long-term warming trend is the primary driver of increases in THE intensity.’ in ‘Results’ section of the revised manuscript. Fig. 4 Relationship between changes in MHW mean intensity and sea ice area in the Arctic Ocean in different warming scenarios. (a)–(d) Monthly changes in MHW mean intensity (red box plots) and sea ice area (blue lines) in the Arctic Ocean. Correlations between MHW mean intensity changes and sea ice area changes across CMIP6 models during (e–h) warm season (March–August) and (j–m) cold season (September–February). (i) Correlation in the period from March to May under the SSP585 scenario. In the box plots in (a)–(d), the marks inside the boxes indicate the median, and the bottom and top edges of the boxes indicate the 25th percentile and 75th percentile, respectively. The red vertical lines extend to the most extreme data points not considered outliers. The black '+' symbols denote outliers. The changes are the results of 2071–2100 relative to 1985–2014. Fig. 5 Relationship between changes in THE mean intensity and SST in the Arctic Ocean in different warming scenarios. (a)–(d) Monthly changes in THE mean intensity (red box plots) and sea ice area (blue lines). (e)–(h) Correlations between annual MHW mean intensity changes and annual sea ice area changes across CMIP6 models. The changes are the results of 2071–2100 relative to 1985–2014. 5) The authors show both future changes in MHWs and THEs, but the characteristic differences between them seem unclear. The authors present the mean state and change characteristics of MHW mean intensity in the Arctic Ocean in the future in 3 paragraphs (lines 102-130), but present the mean state and change characteristics of THEs in the Arctic Ocean in only 1 sentence (lines 133-136). Since MHWs and THEs are proposed by different definitions and have different impacts on ecosystems, it is important to elaborate on the differences between them. Reply: We accepted your suggestion and added ‘Similarly, future THEs are projected to be stronger in these regions compared to others (Fig. 2e).’ in ‘Results’ section of the revised manuscript. We revised ‘Notably, among the five global oceans, the Arctic Ocean is expected to experience the most intense MHWs in the future, with a mean intensity of about 2°C in the period of 2071–2100 (Fig. 2a and b).’ to ‘Notably, among the five global oceans, the Arctic Ocean is expected to experience the most intense MHWs and THEs in the future, with mean intensities of about 2°C and 3°C, respectively, in the period of 2071–2100 (Fig. 2a, b, e, and f). In the future, the intensity of MHWs and THEs in the Arctic Ocean will be larger than or at levels similar to hotspots of MHWs at mid and low-latitudes (Supplementary Fig. 3).’ and revised ‘In the future warming climate, both the mean intensity of MHWs and the extent of their increase will exhibit most pronounced magnitudes within the Arctic Ocean (Fig. 2a–d), echoing the pattern observed for the MHW mean intensity (Fig. 1a–d).’ to ‘In the future warming climate, THEs will experience the most pronounced increase within the Arctic Ocean (Fig. 2e–h). The increase in the THE mean intensity under the SSP585 scenario is projected to reach approximately 3°C in some areas of the Arctic Ocean, with the strongest increase expected in the Barents Sea, where it could amount to around 4°C (Fig. 2h). The MHW mean intensity in the Barents Sea is projected not to increase in the future (Fig. 2d), so the increase in the THE mean intensity in the Barents Sea is mainly due to the long-term warming trend, which can be attributed to both enhanced Atlantic Water heat inflow and reduced local heat loss to the atmosphere30.’ in ‘Results’ section of the revised manuscript. 6) The authors identify the causes of the increases in mean intensity and annual total days of MHWs and THEs in the Arctic Ocean in the Abstract (lines 29-30) and in the main text (lines 136-140, 163-165, and 236-240), but do not provide sufficient evidence to support a causal relationship between them. Importantly, the authors conflate the causes of increases in MHWs and THEs, which are extreme events under different definitions, leading to a lack of logical clarity in the explanations and discussions about this part. Moreover, it is also important to quantify the contribution of different impact factors to the increases in MHWs and THEs, respectively, and to show the similarities and differences in the impact factors. Reply: Following your suggestion we added a section about driving factors. We added Figs. 4 and 5 (in our reply to Comment 4) and Supplementary Fig. 7 (as follow) and added the analysis associated with Arctic sea ice area, mean SST and SST variance under various SSPs. We added ‘Future increases in Arctic MHW intensity are more pronounced under scenarios with higher forcing pathways (Fig. 4a–d). Under all warming scenarios, future changes in the mean intensity of Arctic MHWs are significantly anticorrelated with future changes in Arctic sea ice area across the models during both the warm half year (Fig. 4e–h) and cold half year (Fig. 4j–m). This linear relationship suggests that the reduction in sea ice cover is a key driver for the projected increase in Arctic MHW intensity, consistent with findings on changes in Arctic MHW intensity over the past decades31. The extended periods of open ocean resulting from sea ice retreat allow for greater accumulation of seasonal surface heat flux in the ocean, thereby triggering stronger MHWs31.’, ‘The correlation coefficient between future changes in MHW mean intensity and Arctic sea ice area is lower in the SSP585 scenario (Fig. 4h and m) than in other scenarios. This is because the Arctic is nearly free of sea ice during most months in the last few decades of the SSP585 scenario. In this situation, while the accumulation of surface heat flux in the ocean and consequently the MHW intensity can further increase with stronger atmospheric warming, the sea ice area cannot further decrease during months when it is already ice-fee. This weakens the correlation between changes in MHW intensity and changes in sea ice area. However, if we focus only on the period from March to May, when sea ice is still present at the end of the 21st century in the SSP585 scenario, the correlation coefficient between future changes in MHW mean intensity and Arctic sea ice area is higher (Fig. 4i) compared to longer seasonal considerations (Fig. 4h and m). This reinforces the dominant role of sea ice decline in driving changes in MHW intensity as long as the Arctic retains some sea ice.’, ‘The seasonality of changes in Arctic MHW intensity is similar across different warming scenarios (Fig. 4a-d), featuring two peaks: one in June and another in December. The June peak is associated with the seasonal maximum in solar radiation32. With the same extent of sea ice retreat, greater seasonal solar radiation can lead to more heat absorption in the ocean. The December peak is linked to the seasonal maximum in sea ice decline occurring in fall. Although sea ice decline is greatest in fall, the December peak in MHW intensity increase is lower than the June peak because solar radiation is lower during the cold season. Thus, the seasonality of future changes in MHW intensity is jointly determined by the seasonality of solar radiation and sea ice decline. The seasonality of future changes in the SST variance mirrors the seasonality of future changes in MHW intensity (Supplementary Fig. 7), indicating that SST variance is a good indicator of MHW intensity.’ and ‘Future changes in THE intensity are larger in summer (July–September) and smaller in winter (February–April) across all scenarios (Fig. 5a–d). This seasonality differs from that of changes in MHW intensity (Fig. 4a–d) and aligns more closely with changes in SST (Fig. 5a–d), indicating that the long-term warming trend is the primary driver of increases in THE intensity. This conclusion is further supported by the strong correlation between future changes in THE mean intensity and future changes in SST across the models in all warming scenarios (Fig. 5e–h). Additionally, the future increase in THE mean intensity is much larger than the increase in MHW mean intensity (Fig. 2d and h), underscoring the dominant influence of the long-term warming on THE intensity.’ in ‘Results’ section of the revised manuscript. Supplementary Fig. 7 Seasonality of changes in MHW mean intensity (red boxes and lines) and SST variance (blue lines) in the Arctic Ocean. The explanations for boxplots are the same as in Fig. 4. The changes are the results of 2071–2100 relative to 1985–2014. 7) Not only changes in sea ice, but also changes in mean temperature and temperature variance can be provided. This information helps to understand the causes and physical mechanism of increases in mean intensity and annual total days of MHWs and THEs, as well as the differences between them. Reply: We accepted your suggestion and provided changes in SST and its variance in Fig. 5 and Supplementary Fig. 7 to analyze the different mechanisms of MHW and THE increases. See more details of analysis and figures in our reply to Comment 6. 8) The comparisons of mean state and changes of MHWs and THEs in the Arctic Ocean with that in lower latitude regions in the section ‘MHWs and THEs in the future’ are vague and unfocused. The figures in the text also do not well represent the differences between the Arctic Ocean MHWs and other regions. Moreover, the authors compare the changes of MHWs and THEs in the Arctic Ocean with the global average, however, we know that MHWs have large spatial variations with stronger mean intensity in the western boundary current extension regions, the central and eastern equatorial Pacific Ocean, and eastern boundary current regions. It would be more meaningful to compare the differences in MHW changes in the Arctic Ocean with regions where MHWs are stronger and longer lasting. Reply: In the ‘MHWs and THEs in the future’ section, we compared the Arctic Ocean with other oceans at middle and low latitudes and found that MHW and THE and their changes in the Arctic Ocean were getting stronger in the future while in other oceans the strong signals could only be found in several hotspots. Next, we compared the MHW/THE mean intensity in the Arctic Ocean and these hotspots in the ‘Discussion’ section and hoped to address the issue of MHW/THE in future Arctic Ocean. We accepted your suggestion and added western boundary current extensions in northwestern Pacific and Atlantic Oceans to our study since we already had three typical regions including (1) NE PO representing eastern boundary current region (2) ET PO representing the central and eastern equatorial Pacific Ocean, (3) CS PO. In these three regions, notably severe MHWs were observed in the past. We updated the figure as below. The boxplots were added to show that the future mean intensity in the Arctic Ocean is stronger than or at the same level as these typical regions. We revised as ‘Not only is the increase in the mean intensity of MHWs and THEs in the Arctic Ocean expected to surpass the global average, but also the mean intensity itself in the Arctic Ocean is projected be prominent by the end of the 21st century (Fig. 2). In the past, several notably severe MHWs have been observed, causing ecological damages and potentially devastating consequences4, 34. Examples include the well-known 2014/15 ‘Blob’ in the northeast Pacific35, the 1997/98 El Niño event in the tropical Pacific36, and the warming event associated with the 2009 El Niño in the central south Pacific37. Some western boundary current extension regions have also shown large temperature variability which could bring both unanticipated challenges and opportunities to fisheries38. These typical regions will still need to be closely monitored in the future because strong MHWs and THEs are projected to occur. The future mean intensity of MHWs and THEs in the Arctic Ocean is anticipated to be stronger than or at the same level as these typical regions (Supplementary Fig. 3).’ in ‘Discussion’ section of the revised manuscript. Supplementary Fig. 3 Comparison of mean intensity of MHWs and THEs under SSP585 in 2071–2100 between the Arctic Ocean and five hotspot regions in the other Oceans. (a) and (b) The Distributions of CMIP6 MMM MHW and THE mean intensity in global oceans. The black boxes indicate the locations of the five hotspot regions. (c) and (d) The red and blue box plots are the results for the Arctic Ocean and the hotspot regions, respectively. Strong MHWs were observed in these five regions in the past (see the main text). The five regions are: 40°N–50°N, 150°W–135°W in the northeast Pacific (NE PO); 5°S–5°N, 180°W–85°W in the eastern tropical Pacific (ET PO); 55°S–35°S, 160°W–110°W in the central south Pacific (CS PO); 36°N–50°N, 145°E–190°E around the western boundary current and its extension in the northwest Pacific (NW PO); 37°N–52°N, 20°W–55°W around the western boundary current and its extension in the northwest Atlantic (NW AT). 9) Lines 66-69, although the mean intensity of MHWs in the Arctic Ocean is projected to be four times stronger than the global average by the end of the 21st century, while the mean intensity of THEs in the Arctic Ocean is projected to be 1.6 times stronger than the global average. However, in fact, the mean intensity of THEs in the Arctic Ocean is projected to be stronger than the mean intensity of MHWs by the end of the 21st century, and the change in THE mean intensity in the Arctic Ocean in the future period relative to the historical period is also projected to be stronger than the change in MHW mean intensity. Thus, the comparison of MHWs and THEs in the Arctic Ocean with the global average is actually somewhat misleading. Reply: We totally agree with you. We compared changes in MHW and THE mean intensity in the Arctic Ocean and global ocean because we found that the most prominent feature in Figs. 2c and 3c is the increases in mean intensity and annual total days of MHW in the Arctic Ocean. For the global ocean, the changes in mean intensity and annual total days would not be much more prominent than those in the history. So we needed an amplification factor to measure how significant the MHW changed in the Arctic Ocean comparing with the global ocean. We revised as ‘Figures 2 and 3 show that the future increases in both the MHW mean intensity and the annual total days in the Arctic Ocean considerably surpass the global ocean average under the SSP585 scenario. We term this phenomenon ‘Arctic MHW Amplification’.’, ‘To quantitatively compare the future changes in MHWs and THEs between the Arctic Ocean and the global ocean, the results for all the warming scenarios are presented in Figure 6 and Supplementary Table 2.’ and ‘The MMM difference of MHW mean intensity in the Arctic Ocean between 2071–2100 and 1985–2014 is 0.08°C, 0.18°C, 0.28°C, and 0.38°C under SSP126, SSP245, SSP370, and SSP585, respectively. In contrast, the global ocean averages are 0.00°C, 0.02°C, 0.06°C, and 0.05°C, respectively. Under the SSP585 scenario, Arctic amplification factor of MHW mean intensity change is approximately 7.6. For THE mean intensity, the MMM difference is 0.16°C, 0.70°C, 1.17°C, and 1.82°C under SSP126, SSP245, SSP370, and SSP585, respectively, while the global ocean averages are 0.14°C, 0.41°C, 0.75°C, and 1.21°C, respectively. The Arctic amplification factor of THE mean intensity change is approximately 1.5 under the SSP585 scenario. Although the THE amplification factor is relatively smaller, the changes in Arctic THE mean intensity are much larger than those of MHWs (Fig. 2). Therefore, it is crucial to emphasize the future changes of both Arctic MHWs and THEs.’ in ‘Results’ section in the revised manuscript. 10) The authors state that MHWs and THEs have different impacts on marine life in the Arctic Ocean. Please specify what organisms are affected differently. Reply: We accepted your suggestion and added ‘An increase in MHW/THE intensity and annual total days can trigger extensive and enduring consequences for marine life41. MHWs and THEs have different impacts on marine life in low and mid latitudes22, and they might also have different impacts in the Arctic Ocean in the future. Climate warming is inducing structural change over large spatial scales at high latitudes, leading to a borealization of fish communities in the Arctic42. In the case of phytoplankton, even strongly increased temperatures can stimulate community productivity, whereas their response to different kinds of heatwaves varies43.’ in ‘Discussion’ section of the revised manuscript. Minor comments: 1) Surge means a sudden large increase. I don’t think the increase in mean intensity and annual total days of MHWs during the 21st century will be sudden. Reply: we revised it to ‘rise’ or ‘increase’ in revised manuscript. 2) Line 68, reference 23 is not appropriate to cite here. Reply: We accepted your suggestion and deleted reference 23 in line 68 in revised manuscript. 3) Lines 77-79, ‘…this observed contrast in MHW mean intensity between the Arctic Ocean and regions of lower latitudes…’ The MHW mean intensity in some regions of lower latitudes is also strong, comparable to the MHW mean intensity in the Arctic Ocean. Reply: We accepted your suggestion and revised to ‘The multi-model mean (MMM) of CMIP6 successfully reproduces these observed features in MHW mean intensity in both the Arctic Ocean and lower latitude regions (Fig. 1e, 1f and Supplementary Fig. 1).’ in revised manuscript. 4) Line 81, annual aggregate of MHW days=> annual total days of MHWs Reply: changed. 5) Line 82, this figure? Reply: Because we had already indicated the figure numbers at the end of this sentence, we deleted ‘this figure’ in revised manuscript. 6) Line 83, The annual total days of MHWs in the Greenland Sea seems to be comparable to that in the Barents Sea. Reply: We accepted your suggestion and added ‘, as well as in most of the Nordic Seas.’ in revised manuscript. 7) Lines 79-80 and 85-87, please show the map of differences of global MHWs between OISST and CMIP6 to more clearly demonstrate the differences in global MHWs between observations and models. Reply: We accepted your suggestion, so we added the map of differences between CMIP6 MMM and OISST in mean intensity and annual total days of MHWs during 1985–2014 as Supplementary Figure 1 and revised ‘(Fig. 1e, and f)’ to ‘(Fig. 1e, 1f and Supplementary Fig. 1)’ in revised manuscript. 8) Lines 108-110, Supplementary Fig 4 can be cited here. Reply: We accepted your suggestion. According to other reviewer’s comment on Supplementary Fig 4, we had updated it to a new figure that showed comparisons of more typical regions under more SSPs with a new figure number of Supplementary Fig 3. So we revised ‘(Fig. 2a and b)’ to ‘(Fig. 2a, 2e and Supplementary Fig 3)’ in revised manuscript. 9) Lines 111-112 and 124-130, Both these two parts describe the future change of MHW mean intensity in the Arctic Ocean and are therefore recommended to be combined. The mechanisms associated with the future changes could be considered as a separate paragraph. Reply: We accepted your suggestion and combined these two parts into ‘The most substantial increase in MHW mean intensity is also projected to take place in the Arctic deep basin (Fig. 2c and d), while the MHW mean intensity in the Arctic marginal seas, where MHWs were prevalent in the past, is not anticipated to undergo marked changes in the future (Fig. 2c and d).’ as a paragraph in revised manuscript. 10) Line 133, what’s the meaning of ‘echoing the pattern observed for the MHW mean intensity’? The pattern of MHW mean intensity in future period is not the same as the pattern in the historical period. Reply: We revised the sentence to ‘In the future warming climate, THEs will experience the most pronounced increase within the Arctic Ocean (Fig. 2e–h).’ in revised manuscript. 11) Line 235, ‘the Barents Sea the location’=>’ the Barents Sea’ Reply: changed."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Review 2 on the manuscript “Arctic Amplification of Marine Heatwaves under global warming” by Yan He et.al. The study is devoted to marine heat waves, one of the recent physical characteristics of marine environment. The paper analyzes observational data and the results of three-dimensional numerical modeling of the climate system under a scenario of anthropogenic impact. The characteristics of marine heat waves generated by the ensemble of models at the present time and at the end of the 21st century are discussed. I find the work interesting. It is interesting both in terms of estimating possible future changes in the Earth's climate system on average and in terms of representing these processes by individual models. I think it will be referenced. I can recommend it for publication, but I still have minor comments. 1. I can't say I'm completely satisfied with the response to my comment about the presentation of method of analysis. The authors do not provide a clear description so that it can be repeated. For example, how was the baseline temperature calculated from several models output for MHW and for THE ? what does the line on top in the formula (row 419) denote? 2.Reference 13 on row 47 should be removed as this paper does not provide sufficient information on the impact of marine heat waves on marine species. Instead, it could be included in row 373. 3.In Supplement: column Realization in Table1 should be excluded, as it is unclear to most readers what it means and the authors do not explain it.", "reviewer_2": "I would like to thank the authors for their work and for addressing my concerns and presenting new results and detailed explanations. The revised manuscript is improved over the initial submission and provides missing details or explanations pointed out by the reviewers. Nonetheless, I still have concerns about the following issues. 1) The seasonality of changes in MHW mean intensity does not coincide with that of changes in sea ice. The authors suggest that the solar radiation, which reaches its maximum in June and is weaker during the cold season, contributes to the seasonality of changes in MHW mean intensity, but did not provide sufficient evidence to support this conclusion. As shown in Supplementary Figure 7, the seasonality of changes in MHW mean intensity is in good agreement with that of changes in SST variance. Thus, it is important to clearly illustrate the physical processes that modulate the seasonality of changes in SST variance. Currently, sea ice may be only one of the causes of SST variance, and other important physical processes need to be elucidated (possibly including shortwave radiation or net air-sea heat fluxes). 2) Does the change in SST variance contribute to the change in THE mean intensity? If so, how much does the change in mean SST and the change in SST variance contribute, respectively? 3) The change in THE mean intensity of the Arctic Ocean is stronger than the change in THE mean intensity of the global ocean, but the change in annual THE total days of the Arctic Ocean is smaller than the change in annual THE total days of the global ocean. How to understand this situation? 4) Lines 211-215, when the Arctic Ocean is free of sea ice in the future, will the MHWs in the Arctic Ocean enter another stage where the mean intensity and annual total days will not change? Will changes in SST be dominated by changes in mean SST rather than SST variance? 5) In the Abstract, ‘both MHWs and THEs in the Arctic Ocean are anticipated to intensify and extend in duration in a warming climate, mainly due to Arctic sea ice decline and amplified ocean warming in the region’. This statement is not rigorous. First of all, the annual total days and the duration of MHWs and THEs are not the same, and the authors only show changes in the annual total days and frequency and not in the duration. The changes in annual total days of MHWs and THEs by both frequency and duration. How does the duration of MHWs and THEs change in the Arctic Ocean? Which has a larger effect on the annual total days, frequency or duration? Second, the factors that dominate the changes in MHWs and THEs are not the same. Minor comments: 1) Line 91, In contrast, whereas… 2) Lines 140-142, add some references? 3) Lines 324-325, according to the definition of MHWs in this study, there appears to be no change in the mean intensity of MHWs in these typical regions (the northeast Pacific, the tropical Pacific, and the central south Pacific). 4) Lines 387-390, Could the authors please elaborate on the years of the historical and future periods?"}, "author_response": "Response to Reviewer #1 (Note: Reviewer comments in black and our point-to-point replies in blue) on the manuscript “Arctic Amplification of Marine Heatwaves under global warming” by Yan He et.al. The study is devoted to marine heat waves, one of the recent physical characteristics of marine environment. The paper analyzes observational data and the results of three-dimensional numerical modeling of the climate system under a scenario of anthropogenic impact. The characteristics of marine heat waves generated by the ensemble of models at the present time and at the end of the 21st century are discussed. I find the work interesting. It is interesting both in terms of estimating possible future changes in the Earth's climate system on average and in terms of representing these processes by individual models. I think it will be referenced. I can recommend it for publication, but I still have minor comments. Dear reviewer, Thank you very much for your thoughtful evaluation and kind recommendation of this study. We greatly appreciate your attention to marine heatwaves, total heat exposures and their potential impacts on the Earth's climate system. Your comments are important and greatly improved our work. 1. I can't say I'm completely satisfied with the response to my comment about the presentation of method of analysis. The authors do not provide a clear description so that it can be repeated. For example, how was the baseline temperature calculated from several models output for MHW and for THE ? what does the line on top in the formula (row 419) denote? Reply: In order to clearly describe the method, we provided a schematic of MHWs and THEs in Supplementary Fig. 14, as also shown below. Accordingly, the definition and metrics of MHWs and THEs in the Method section are revised. We also added \"The analysis of MHWs and THEs, including the calculation of baselines, is performed for each model and each scenario separately before computing MMM results\". In the revised version we removed the formula as the schematic together with the revised text can better explain the definition. Supplementary Fig. 14 Schematic of (a) MHWs, (b) THEs, and associated metrics used in this study. The black, green and blue lines in (a) are the detrended SST, climatology of detrended SST, and the 90th percentile of detrended SST. The black, green and blue lines in (b) are the original SST, climatology of historical original SST, and the 90th percentile of historical original SST. This figure is adapted from http://www.marineheatwaves.org/. 2.Reference 13 on row 47 should be removed as this paper does not provide sufficient information on the impact of marine heat waves on marine species. Instead, it could be included in row 373. Reply: We accepted your suggestion and revised the reference 13 to the right place. 3.In Supplement: column Realization in Table1 should be excluded, as it is unclear to most readers what it means and the authors do not explain it. Reply: We accepted your suggestion and excluded column Realization in Table1 in Supplement documentation. Response to Reviewer #2 (Note: Reviewer comments in black and our point-to-point replies in blue) I would like to thank the authors for their work and for addressing my concerns and presenting new results and detailed explanations. The revised manuscript is improved over the initial submission and provides missing details or explanations pointed out by the reviewers. Nonetheless, I still have concerns about the following issues. Dear reviewer, Thank you very much for reviewing our manuscript again and providing us with the valuable comments and suggestions. Accordingly, we carried out necessary analysis and investigated the comprehensive mechanism of MHW/THE changes, the differences between MHWs and THEs, and MHW changes in the ice-free Arctic Ocean. Your comments are important to us and the manuscript has been improved with your help. 1) The seasonality of changes in MHW mean intensity does not coincide with that of changes in sea ice. The authors suggest that the solar radiation, which reaches its maximum in June and is weaker during the cold season, contributes to the seasonality of changes in MHW mean intensity, but did not provide sufficient evidence to support this conclusion. As shown in Supplementary Figure 7, the seasonality of changes in MHW mean intensity is in good agreement with that of changes in SST variance. Thus, it is important to clearly illustrate the physical processes that modulate the seasonality of changes in SST variance. Currently, sea ice may be only one of the causes of SST variance, and other important physical processes need to be elucidated (possibly including shortwave radiation or net air-sea heat fluxes). Reply: Following your suggestion, we downloaded data and analyzed the seasonality of changes in net shortwave radiation flux and net sea surface total heat fluxes into the Arctic Ocean (Supplementary Figure 10, as below) to explain the causes of the seasonality of changes in Arctic MHW mean intensity in the revised manuscript. This figure supports our previous speculation: ‘The summer peak is associated with the seasonal maximum in solar radiation and net ocean surface heat flux (Supplementary Fig. 10).’, ‘The December peak is mainly linked to the seasonal maximum in sea ice decline (Fig. 4a–d), which increases open water where MHWs can occur.’ Supplementary Fig. 10 Relationship between changes in MHW mean intensity and net shortwave radiation flux and net sea surface total heat flux in the Arctic Ocean in different warming scenarios. (a)–(d) Monthly changes in MHW mean intensity (red box plots) and net shortwave flux into the Arctic Ocean (blue lines). (e)–(h) Monthly changes in MHW mean intensity (red box plots) and net sea surface heat flux into the Arctic Ocean (blue lines). The explanations for boxplots are the same as in Figure 4. The changes are the results of 2071–2100 relative to 1985–2014. 2) Does the change in SST variance contribute to the change in THE mean intensity? If so, how much does the change in mean SST and the change in SST variance contribute, respectively? Reply: THE is the combination of MHW and long-term warming. MHW is the extreme warming event in the ocean, and the change of SST variance contributes to the change in MHW mean intensity. So, the change of SST variance should also contribute to the change in THE mean intensity. But it is difficult to directly calculate its contributions, because variance includes not only extremes. However, we can assess the contributions from MHWs and long-term warming to THEs. We calculated the area-weighted mean value of the change of MHW mean intensity in the central Arctic Ocean (north of 75°N in Fig. 2d) and the area-weighted mean value of the change of THE mean intensity (Fig. 2h) in the same region. The changes of MHW mean intensity and THE mean intensity in the central Arctic Ocean are 1.27 °C and 2.28 °C, respectively. We added ‘For the central Arctic Ocean (approximately north of 75°N), our analysis indicates that the increase in MHW mean intensity accounts for about half of the increase in THE mean intensity, with the other half resulting from the direct effect of long-term warming.’ in the revised manuscript. 3) The change in THE mean intensity of the Arctic Ocean is stronger than the change in THE mean intensity of the global ocean, but the change in annual THE total days of the Arctic Ocean is smaller than the change in annual THE total days of the global ocean. How to understand this situation? Reply: Our results show that the change in THE total days in the Arctic Ocean is smaller than the change in annual THE total days of the global ocean. The main reason is that there is still winter sea ice in the Arctic Ocean in the period 2071-2100 (Supplementary Fig. 4), which limits the number of THE days. The new Supplementary Fig. 5 shows that the changes in the number of days with THEs in each month have stronger seasonality in the Arctic Ocean under various SSPs, in comparison with the global ocean. We added ‘In contrast, the change in THE mean intensity in the Arctic is more pronounced than in the global ocean, as discussed above. This is because the mean intensity, per definition, is calculated by averaging over events. If there are no THE events in a period, this period does not lower the calculated mean intensity.’ in the revised manuscript. Supplementary Fig. 5 The changes in the number of days with THEs in each month. (a)–(d) The changes averaged over the Arctic Ocean. (e)–(h) The changes averaged over the global ocean. The explanations for boxplots are the same as in Figure 4. The changes are the results of 2071–2100 relative to 1985–2014. 4) Lines 211-215, when the Arctic Ocean is free of sea ice in the future, will the MHWs in the Arctic Ocean enter another stage where the mean intensity and annual total days will not change? Will changes in SST be dominated by changes in mean SST rather than SST variance? Reply: Winter Arctic Ocean is not ice-free before 2100 even under SSP585. Three models, including ACCESS-CM2, IPSL-CM6A-LR, and MRI-ESM2-0, have released daily sea surface temperature and sea ice concentration data till 2300. So, we only used these three models to investigate the Arctic Ocean MHWs when the Arctic Ocean is free of sea ice. Supplementary Figure 8 shows that the Arctic is projected to be ice-free (<1.0×106 km2) in March after 2150. Two periods of 2171–2200 and 2271–2300 are used to study the changes of MHW mean intensity and annual total days. Supplementary Figure 9 shows that when the Arctic Ocean is free of sea ice, the changes of Arctic Ocean MHW mean intensity and annual total days is quite small, and we could conclude that the change of SST will be mainly dominated by mean SST. We added ‘In the SSP585 warming scenario, when the Arctic Ocean becomes free of sea ice throughout the year in the far future (Supplementary Fig. 8), MHWs in the Arctic Ocean will enter a stage where the changes of mean intensity and annual total days are very small (Supplementary Fig. 9), similar to the situation in the mid-low latitudes in the 21st century (Fig. 2c and Fig. 3c).’ in revised manuscript. Supplementary Fig. 8 Arctic sea ice area in March during 2000–2300. Arctic sea ice area in March during 2000–2300 based on simulations and projections under SSP585 in ACCESS-CM2, IPSL-CM6A-LR, and MRI-ESM2-0. Black dashed line represents the ice–free threshold. Supplementary Fig. 9 Arctic MHW mean intensity and annual total days during periods of 2171–2200 and 2271–2300. (a) and (b) MHW mean intensity during periods of 2171–2200 and 2271–2300. (c) The changes of MHW mean intensity in 2271–2300 relative to 2171–2200. (d) and (e) MHW annual total days during periods of 2171–2200 and 2271–2300. (f) The changes of MHW annual total days in 2271–2300 relative to 2171–2200. The colorbar ranges are the same as those used in Figs. 2 and 3 for an easy comparison. 5) In the Abstract, ‘both MHWs and THEs in the Arctic Ocean are anticipated to intensify and extend in duration in a warming climate, mainly due to Arctic sea ice decline and amplified ocean warming in the region’. This statement is not rigorous. First of all, the annual total days and the duration of MHWs and THEs are not the same, and the authors only show changes in the annual total days and frequency and not in the duration. The changes in annual total days of MHWs and THEs by both frequency and duration. How does the duration of MHWs and THEs change in the Arctic Ocean? Which has a larger effect on the annual total days, frequency or duration? Second, the factors that dominate the changes in MHWs and THEs are not the same. Reply: We agree with you. Following your suggestion, we modified the abstract and also analyzed the mean duration changes in MHWs and THEs. Based on the results of frequency and mean duration (Supplementary Fig. 12-13), we add ‘For both changes in the mean frequency and mean duration, Arctic amplification is found for MHWs under all the four scenarios, but not for THEs (Supplementary Fig. 12 and 13). In the Arctic Ocean, the future increase in the frequency of MHWs correlates with the severity of warming scenarios. However, the mean duration of MHWs does not show a similar trend. Consequently, the frequency increase of MHWs is the primary contributor to the projected rise in the annual total days of Arctic MHWs. Conversely, for Arctic THEs, the future increase in their mean duration correlates with the severity of warming scenarios, while their frequency does not. Therefore, the increase in the mean duration of THEs is the main factor contributing to the future rise in the annual total days of Arctic THEs.' In the Abstract, the sentence of ‘both MHWs and THEs in the Arctic Ocean are anticipated to intensify and extend in duration in a warming climate, mainly due to Arctic sea ice decline and amplified ocean warming in the region’ was revised to ‘both MHWs and THEs in the Arctic Ocean are anticipated to intensify in a warming climate, mainly due to Arctic sea ice decline and long-term warming trend, respectively’. Supplementary Fig. 13 Comparison of future changes in the mean duration of MHWs and THEs between the Arctic Ocean (red) and the global ocean (blue). (a) The future changes in the mean duration of MHWs. (b) The future changes in the mean duration of THEs. The explanations for boxplots are the same as in Figure 4. The changes are the results of 2071–2100 relative to 1985–2014. Minor comments: 1) Line 91, In contrast, whereas… Reply: ‘whereas’ was deleted in the revised manuscript. 2) Lines 140-142, add some references? Reply: Our finding is consistent with the reference 21. We do not know other previous studies for this comparison. We changed \"results\" to \"result\" in the revision. 21. Guo, X. et al. Threat by marine heatwaves to adaptive large marine ecosystems in an eddy-resolving model. Nat. Clim. Chang. 12, 179–186 (2022). 3) Lines 324-325, according to the definition of MHWs in this study, there appears to be no change in the mean intensity of MHWs in these typical regions (the northeast Pacific, the tropical Pacific, and the central south Pacific). Reply: We added this information in the revision. \"These regions appear to have no notable changes in MHW mean intensity during the 21st century, and the future mean intensity of MHWs and THEs in the Arctic Ocean is anticipated to be stronger than or at the same level as these typical regions (Supplementary Fig. 3).\" 4) Lines 387-390, Could the authors please elaborate on the years of the historical and future periods? Reply: In the introduction section, the sentence ‘In this study, we investigate historical and future MHWs and THEs in the Arctic Ocean using…’ was changed to ‘In this study, we investigate historical (1985–2014) and future (2071–2100) MHWs and THEs in the Arctic Ocean using…’ in the revised manuscript. In the Method section, we mention that ‘The future changes are defined as the difference between the period of 2071–2100 and the period of 1985–2014 if not otherwise stated.’ REVIEWERS' COMMENTS Reviewer #2 (Remarks to the Author): I would like to thank the authors for carefully considering my comments and revising their manuscript accordingly. I think the current manuscript is much improved and would recommend it for publication."}]

Decision letter