Understanding bottom and surface marine heatwaves along the continental shelf of China

Marine heatwaves (MHWs) have become longer and more frequent over the past century under anthropogenic climate change, with devastating impacts on marine ecosystems. Surface MHWs (SMHWs) and their drivers have been extensively studied using satellite sea surface temperature data, yet the mechanism and characteristics of subsurface MHWs, especially bottom MHWs (BMHWs) along continental shelves, remain unclear. Based on a high-resolution ocean reanalysis dataset, we compare SMHWs and BMHWs along the continental shelf of China and find that BMHWs are typically longer (0–16 d) and more intense (0 °C–50 °C days) than SMHWs. The categorizing of both the BMHW and SMHW shows that moderate and strong events commonly occur in most areas with relatively large spatial coverage, whereas severe and extreme events occur with relatively small spatial coverage. There is a clear negative relationship between the BMHW intensity and ocean depth along the continental shelf, while the BMHW annual days and ocean depth are positively correlated in the Bohai and East China Seas. Generally, BMHWs and SMHWs occur more frequently in shallow coastal regions where the mixed layer depth is more likely to extend to the seafloor, resulting in high BMHW and SMHW synchrony. In addition to spatial coherence, there is a good temporal correspondence between BMHWs and SMHWs across the continental shelf of China from 1993 to 2020.


Introduction
Extremely high ocean temperatures, known as marine heatwaves (MHWs), have increased in frequency and duration over the past century, causing devastating impacts on marine ecosystems (Frölicher et al 2018, Oliver et al 2018, 2021) MHWs have been extensively studied at the ocean surface due to the availability of gap-free satellite sea surface temperature (SST) datasets with high data quality and near-global coverage (Hu et al 2021, Fragkopoulou et al 2023).In fact, only half of the identified MHWs have continuous surface signals during their formation and dissipation, and approximately one-third are always in the subsurface ocean (Sun et al 2023).Therefore, gaining more insight into subsurface MHWs is crucial for better assessing the driving mechanisms of MHWs (Holbrook et al 2020).Despite the lack of continuous in suit subsurface data, previous studies have analyzed the statistical properties and vertical structures of the subsurface MHWs.For example, based on long-term mooring sites data, Schaeffer and Roughan (2017) demonstrated that subsurface MHWs are most intense at depth and regularly extend to the bottom during weak stratification.Using Argo float data, Scannell et al (2020) found that salinity variations play an important role in the vertical distribution of temperature anomalies in the Northeast Pacific Blob heatwave.Using global ocean temperature reanalysis data, Fragkopoulou et al (2023) showed that marine biodiversity is exposed to longer and more intense subsurface MHWs.Different categories of subsurface MHW vertical structures have also been investigated using in-situ mooring data, Argo floats, ocean models, and ocean temperature reanalysis datasets (Elzahaby and Schaeffer 2019, Elzahaby et al 2021, Schaeffer et al 2023, Sun et al 2023, Zhang et al 2023).
Despite progress in understanding the vertical extent, characteristics and potential drivers of subsurface MHWs, there has been little effort to characterize seafloor temperature extremes along the continental shelf (Amaya et al 2023).Intense seafloor temperature changes can uniquely and dramatically affect continental shelf biogeochemical processes.For instance, seafloor water temperature anomalies have shifted the distribution of demersal fish in the California Current System (Keller et al 2015), caused large-scale death of sea cucumbers in the Shandong Peninsula (Xu et al 2014), altered the recruitment dynamics of Atlantic cod (Fogarty et al 2008), resulted in the collapse of snow crabs in the eastern Bering Sea (Szuwalski et al 2023), and changed the air-water exchange of semi volatile organic compounds (Fan et al 2023).In addition, Amaya et al (2023) first demonstrated that bottom MHWs (BMHWs) are more intense and persist longer than surface MHWs, which can reach the seafloor within the mixed layer depth (MLD) along the continental shelves of North America.
In contrast to the recent advances in understanding BMHWs, the statistical characteristics of BMHWs along the continental shelves of China are mostly unknown.To fill this knowledge gap, this study aims to describe the statistical characteristics of BMHWs across the continental shelf of China (Bottom depth <400 m, including the Bohai Sea, East China Sea, northern South China Sea, and eastern Indo-China Peninsulas; figure S1) during 1993-2020 and the differences between BMHWs and surface MHWs (SMHWs), as well as their synchrony with MLD variations.This work provides a comprehensive understanding of the differences and connections between BMHWs and SMHWs through a highresolution ocean reanalysis (1/12 • ) along the continental shelf of China.

Data
To characterize thet BMHWs/SMHWs along the continental shelf, we use the daily mean ocean bottom/surface temperatures from the Global Ocean Reanalysis and Simulations (GLORYS) 12V1  S1).

MHW definition
An MHW event was identified as a period when the surface/bottom temperature exceeded the 90th daily percentile for five or more consecutive days based on a fixed baseline climatology (here 1993-2020) (Hobday et al 2016).Two events separated by 2 d or less are considered to be part of the same event.The climatological mean and 90th percentile were calculated in two steps: first, the daily mean was calculated using an 11 day moving window (centered on the corresponding calendar day).Then, the climatology was smoothed using a 31 day moving average.We use the daily mean GLORYS data to construct a time series of temperature anomalies relative to the climatological period 1993-2020 at each grid cell.These anomalies are then linearly detrended to isolate transient BMHW and SMHW events from long-term warming signals.

MHW metrics and categories
Here, we used four metrics to describe the characteristics of the BMHW/SMHW: frequency, i.e. the number of BMHWs/SMHWs each year; annual days, i.e. the total number of BMHW/SMHW days each year; mean intensity, i.e. the mean magnitude of surface/bottom temperature anomalies occurring within all BMHWs/SMHWs each year; and cumulative intensity, i.e. the sum of daily surface/bottom temperature anomalies during BMHWs/SMHWs within a year.The detailed definitions and associated equations are given in table S1.

Spatial patterns and differences in the mean metrics of the BMHW and SMHW from 1993 to 2020
The multi-year mean annual days of the BMHW show relatively consistent high values over regions deeper than ∼50 m, with the highest values occurring in the East China Sea and Beibu Gulf, ranging from 33 to 38 d (figure 1(a)).In contrast, higher values of the multi-year mean annual days of SMHW appear in the South China Sea (figure 1(d)).Thus, there is a clear difference between the BMHW and SMHW along the continental shelf, with the BMHW being longer than the SMHW by approximately 4-16 d (figure 1(g)).
The multi-year mean intensity of the BMHW varies significantly across different locations, with the highest values observed in the north of Taiwan Island, ranging from 2.5 • C to 3.0 • C (figure 1(b)).However, the multi-year mean intensity of the SMHW is more spatially homogeneous (figure 1(e)) than that of the BMHW.Therefore, the BMHWs are warmer than the SMHWs in the southern East China Sea and the South China Sea, ranging from 0 The multi-year mean cumulative intensity of the BMHW also shows large spatial variations, with high values appear in the Bohai Sea, northern East China Sea, and Beibu Gulf, ranging from 60 • C to 75 • C days (figure 1(c)).In contrast, the multi-year mean cumulative intensity of the SMHW was less variable spatially (figure 1(f)).A notably difference of cumulative intensity is exhibited between the BMHW and SMHW along the continental shelf, particularly with high positive values occurring in the Bohai Sea, northern East China Sea, and Beibu Gulf, ranging from 25 • C to 50 • C days (figure 1(i)).In general, BMHWs are longer and larger than SMHWs in term of differences in annual days and cumulative intensity.

Seasonal and categorical differences between the BMHW and SMHW from 1993 to 2020
Clear differences in the multi-year mean seasonal days between the BMHW and SMHW exist from spring to winter, with the BMHW typically having longer days than the SMHW (figures S2(a)-(d)).Specifically, the seasonal days between the BMHW and SMHW in spring (MAM), autumn (SON), and winter (DJF) exhibit distinct differences across most of the northern shelf regions, BMHW typically lasting 0-5 d longer than the SMHW.Whereas in summer (JJA), the difference of seasonal averaged days between the BMHW and SMHW is relatively small, with BMHWs extending only 0-2.5 d longer than SMHWs.In contrast to the seasonal days, the differences in the seasonal mean intensity between the BMHW and SMHW exhibit diverse spatial patterns, which are likely shaped by complex interactions between ocean currents and bathymetry (figures S2(e)-(h)).For instance, the BMHW in summer shows a stronger intensity than SMHW on northern Taiwan Island, the Beibu Gulf, and the western coast of the Korean Peninsula, reaching 1 ), which may be related to the summer coastal warm current and Taiwan warm current.Corresponding, the BMHW intensity generally weaker than that of SMHW in the summer northern East China Sea, most likely due to cold water masses from higher latitudes (Wang et al 2014).
The category features of both the BMHW and SMHW showed that moderate and strong events occurred in most areas, while severe and extreme events occurred in very small areas (figure 2).In the moderate category, the multi-year mean annual days of BMHWs are longer than those of SMHWs on almost the entire continental shelf, reaching 5-15 d, while the frequency of BMHWs is lower than that of SMHWs, ranging from −0.75 to −1.5 counts in the Bohai Sea, the northern East China Sea, and the Beibu Gulf (figures 2(a) and (e)).In the strong category, the annual days between BMHWs and SMHWs is comparable and the difference distributions are widely dispersed, shown slight peak in the Bohai Sea and East China Sea for up to 10 d and no much difference in frequency (figures 2(b) and S2(f)).For the severe and extreme categories, the ecological impacts were limited due to the relatively small areas (figures 2(c), (d), (g) and (h).

Ocean bottom depth associated variations in BMHW intensity and annual days
To quantify how BMHW intensity and annual days vary with ocean depth, we evaluate the twodimensional histograms of BMHW mean intensity/annual days versus ocean bottom depth along the .Throughout the continental shelf, we also find that the BMHW mean intensity is negatively correlated with the ocean bottom depth (R = −0.55,p < 0.01), while the BMHW annual days are positively correlated with the ocean bottom depth (R = 0.37, p < 0.01) (figure S3).Notably, the positive (negative) correlation between the BMHW mean intensity (annual days) and ocean bottom depth is more likely at depths shallower than 100 m (figures 3 and S3).This suggests that there is a clear depth limit to the BMHW-depth relationship.However, the maximum depth of the Bohai Sea, an almost enclosed inland sea with an average depth of 18 m, is about 90 m.Thus, the BMHWs are limited to a depth of 100 m.
The shallow water MHWs, predominantly driven by the air-sea heat flux when the MLD is shallow, rapidly amplify surface warming (Elzahaby et al 2021, Schaeffer et al 2023).With the depth increase, the water temperature warms slowly, and the seasonal amplitude is usually smaller than the shallow water, and the temperature become relatively

BMHW and SMHW synchrony
There is a distinct spatial pattern of synchrony between BMHWs and SMHWs along the continental shelf, with both types of events co-occurring more often in shallow coastal regions (figure 4(a)).In addition, the synchrony of BMHWs and SMHWs is highly correlated with the MLD/bathymetry ratio (R = 0.79, p < 0.01), i.e.where the MLD/bathymetry ratio is high, BMHWs and SMHWs co-occur more frequently (figure 4(b)).Generally, the MLD can extend to the seafloor in shallow waters, allowing well-mixed seawater to conduct heat fluxes received from the atmosphere in the surface layer to the bottom waters, facilitating the co-occurrence of the BMHW and SMHW (Amaya et al 2023).
Particularly, in shallow coastal regions associated with the highest co-occurrence of BMHW and SMHW, the MLD may usually extend to the seafloor.In this situation, the distinction between SMHW and BMHW may be meaningless.Therefore, in future studies, it is necessary to exclude areas where the MLD can frequently extend to ocean bottom, and then it is worth to separate BMHWs from SMHWs.
Given that seafloor depths on most regions of the continental shelf of China are typically less than 100 m (figure S1), more temporal consistency between BMHWs and SMHWs can be expected on the continental shelf of China.Therefore, the variations in the spatial extent of BMHWs and SMHWs over time are examined in figure 5. Evidently, there is good temporal correspondence between the BMHW and SMHW area fractions across the continental shelf from 1993 to 2020 (Corr = 0.64, p < 0.01) (figure 5(a)).For example, widespread and persistent BMHWs and SMHWs with a maximum mean intensity of 2.7 • C occurred in 1997/98 (gray shading in figure 5).During the peak periods, the BMHWs beginning in 1998 encompassed 70% of the continental shelf area in the South China Sea with a maximum mean intensity of 1.8 • C; the SMHWs beginning several months earlier than the BMHWs in late 1997, encompassed nearly 100% of the area with a maximum mean intensity of 2.4 • C (figure 5(d)).These widespread and prolonged BMHWs and SMHWs are known to be caused by super El Niño events and had substantial ecological

Conclusion and discussion
Using state-of-the-art reanalysis data, we analyzed the statistical characteristics of BMHWs and SMHWs along the continental shelf of China from 1993 to 2020.It can be concluded that BMHWs are longer (0-16 d) and more intense (0 • C-50 • C days) than SMHWs in terms of the multi-year mean annual days and cumulative intensity, especially in the southern East China Sea and the South China Sea, with a range of 0 • C-1 • C in mean intensity.BMHWs have longer days than SMHWs in most seasons.Meanwhile, topography play an important role in the distributions of mean seasonal intensity between BMHWs and SMHWs.The categories of the BMHW and SMHW showed that Moderate and Strong events occurred in most areas, while Severe and Extreme events occurred in relatively small areas.Furthermore, in the Moderate category, the multi-year mean annual days of BMHWs are longer than those of SMHWs on almost the entire continental shelf, reaching 5-15 d.The relationship of BMHW and ocean depth are also investigated, and shows that BMHW mean intensity varies inversely with the ocean depth along the continental shelf of China, while the BMHW annual days and ocean depth were positively correlated in the Bohai (R = 0.84, p < 0.01) and East China Seas (R = 0.49, p < 0.01), and no clear correlation was found in the South China Sea.A positive (negative) correlation between the BMHW mean intensity (annual days) and ocean depth is more likely in areas with water depths shallower than 100 m.Finally, BMHWs and SMHWs are more common in shallow coastal regions where the MLD is more likely to extend to the seafloor, resulting in high BMHW and SMHW synchrony.In addition, there is a good temporal correspondence between BMHWs and SMHWs across the continental shelf from 1993 to 2020.
The opposite sign of the correlation coefficients of the BMHW-depth relationship for the mean intensity and annual days may be related to the complicated bathymetric and oceanographic features of the three continental shelves of the China Seas.For example, the Bohai Sea is a semi-enclosed shallow inland sea with an average depth of 18 m (Zhang et al . Studies have shown that severe and persistent MHWs can have significant biological and socioecological impacts globally, such as coral bleaching (Hughes et al 2018, Yao and Wang 2021, Marzonie et al 2023), changes in primary productivity (McCabe et al 2016, Noh et al 2022, Zhan et al 2023), rapid shifts in species distributions (Jacox et al 2020, He et al 2023), and mass mortality of marine organisms (Jones et al 2018, Konsta et al 2022, Manahan 2023, Szuwalski et al 2023).Driven by anthropogenic climate change and ocean warming, MHW intensity and annual days are projected to increase in the future, resulting in many parts of the ocean entering a near-permanent MHW state by the late 21st century (Oliver et al 2019).

Figure 2 .
Figure 2. The multi-year mean category differences in the (a)-(d) number of days and (e)-(h) frequency between BMHWs and SMHWs along the continental shelf of China during 1993-2020.(a), (e) Moderate, (b), (f) Strong, (c), (g) Severe, and (d), (h) Extreme.Sea boundaries are marked with pink dashed lines.All differences exceeding the 99% confidence level.

Figure 3 .
Figure 3. BMHW mean (a)-(c) intensity and (d)-(f) annual days variations with ocean bottom depth in the (a), (d) Bohai Sea, (b), (e) East China Sea, and (c), (f) South China Sea.Shading indicates the probability that a grid cell (with a bottom depth <400 m) falls within a given intensity-depth interval.The Spearman correlation between the BMHW mean intensity/annual days and bottom depth across all grid cells is shown in the upper right of each panel.Gray dots indicate the average BMHW mean intensity and annual days over regular depth intervals of 50 m.The position of each dot along the x-axis represents the center of the depth interval used for averaging.

Figure 4 .
Figure 4. (a) BMHWs and SMHWs temporal synchrony as measured by the fraction of months from 1993 to 2020 along the continental shelf of China.A value of 1 indicates that BMHWs and SMHWs are 100% co-occurrence.(b) BMHWs and SMHWs synchrony versus the ratio of mixed layer depth (MLD) to bathymetric depth composited at each grid cell.Shading indicates the probability that a grid cell (with bottom depth <400 m) falls within a given synchrony-MLD/bathymetry interval.The Spearman correlation between the BMHW and SMHW synchrony and MLD/bathymetry ratios across all grid cells is shown in the bottom right of the panel.Sea boundaries are marked with pink dashed lines.

Figure 5 .
Figure 5.The fraction of area experiencing SMHWs (upper panel) and BMHWs (lower panel) for each month from 1993 to 2020 (a) along the continental shelf of China, (b) the Bohai Sea, (c) the East China Sea, and (d) the South China Sea.Colors indicates averaged SMHW and BMHW mean intensity in a given month.Gray shadings represent the periods of the 1997/98 and 2015/16 El Niño events.Horizontal dotted lines mark areal extent of 1.Only grid cells with bottom depths <400 m were used to calculate area percentages and mean intensity.The area fraction correlation coefficient between SMHWs and BMHWs is shown in the center of each panel.

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Cao et al 2018), which leads to a strong correlation between the BMHW and ocean bottom depth; the East China Sea is one of the world's largest continental shelf seas, impacted by the intrusion of the Kuroshio Current (Oey et al 2013, Wu et al 2017) and the input of a large amount of fresh water from the Yangtze River (Yan et al 2008), which may reduce the BMHW-depth relationship; and the water depth changes rapidly in the narrow continental shelf of the South China Sea, which may lead to the correlation between the BMHW duration and ocean depth not being positive as in other areas.Previous studies have revealed that MHWs are typically more intense in the subsurface at 50-300 m, and their duration increases with depth (Hu et al 2021, Fragkopoulou et al 2023).Similar to subsurface MHWs, BMHWs are typically more intense than local SMHWs.Moreover, half of MHWs have persistent surface signals during their lifetime, and nearly one-third remain hidden below the surface without any imprint on SSTs at depths of 0-200 m (Sun et al 2023).The upper limit of this depth range is deeper than the bottom depth of most areas on the continental shelf of China.This may indicate a possible small discrepancy between the local SMHWs and BMHWs.For instance, more than one-third of MHWs (∼36%) extend into the deeper water column (at a maximum depth of 50 m) off Australian coastal water; these MHWs are called extended MHWs (Schaeffer et al 2023).In addition, nearly one-third (∼29%) of MHWs extend below the surface MLD in the eastern Pacific (Köhn et al 2024).Consequently, we can find frequent coexistence of BMHWs and SMHWs in the continental shelves of China (figure 5).Therefore, subsurface MHWs, including BMHWs, have the potential to be detected by SMHWs (Köhn et al 2024), especially in shallow coastal waters such as the Bohai Sea (Corr = 0.70, p < 0.01) (figure 5(b)).