Analysis of Marine Heatwaves in China’s Coastal Seas and Adjacent Offshore Waters

Abstract

Marine heatwaves (MHWs) are changing global ecosystems and bearing profound socio-economic impacts, yet our understanding of the spatial features, temporal evolution characteristics, and regional differences in China’s marginal seas remains insufficient. In this study, the spatio-temporal variation characteristics of the frequency, mean intensity, maximum intensity, cumulative intensity, duration and total days of MHWs are systematically analyzed based on daily sea surface temperature data from Operational Sea Surface Temperature and Ice Analysis (OSTIA) for the period of 1983–2020. The results show the following: The annual mean frequency of MHWs in China’s coastal seas is 1.4–4.6 counts per year and increases gradually from north to south. The annual mean of mean intensity, maximum intensity and cumulative intensity are, respectively, in the ranges of 0.3–2.9 °C, 0.4–3.7 °C and 3.9–41.7 °C days, all of which show a significant decreasing trend from north to south. The annual mean of duration and total days of MHWs are in the respective ranges 8.7–19.7 and 18.9–69.1 days. The annual mean of frequency, cumulative intensity, duration and total days all show a clear increasing trend, with respective linear increases of 1.03 counts, 3.57 °C days, and 1.98 and 17.58 days per decade. The annual means of the mean intensity and maximum intensity have no obvious upward trend, with the exceptions of the Bohai and Yellow Seas. Finally, MHWs were divided into the four categories of moderate, strong, severe and extreme, with the results showing that moderate ones account for more than 70% of all heatwave events, while strong, severe and extreme ones, respectively, account for about 25%, 2% and 0.02%.

1. Introduction

The sea surface temperature (SST) is one of the most important parameters reflecting the thermal state of the ocean surface. SST change will have an important impact on the marine environment and climate system [1]. MHWs are extreme heat events in which the sea surface temperature abnormally exceeds a certain threshold. Such events can last from several days to several months, and the coverage area can extend from several to thousands of km [2,3].

From the period of the industrial revolution, greenhouse gas emissions caused by human industrial production activities have led to continuous global warming [4], and most of the excess heat is stored in the ocean, which inevitably results in rising global ocean temperature. In recent years, due to an increasing abundance of SST observation data with high spatial and temporal resolution, a series of extreme warming events have been identified in the global ocean, causing widespread concern [5]. Continued MHWs are likely to threaten fragile marine ecosystem structure and function, manifesting in ways such as toxic algae surges and coral reef bleaching, marine biodiversity damage, and significant impact on aquaculture, fisheries and tourism. As early as 2003, an MHW that occurred in the western Mediterranean was studied by Sparnocchia [6] and Olita [7]. In 2011, a record-breaking warming event occurred along the southwestern coast of Western Australia. In this event, the SST was 3–5 °C higher than the normal climatology, which resulted in many deaths of fish and invertebrates, and the marine ecosystem was seriously damaged [8]. In 2012, a notable MHW struck most areas of the northwestern Atlantic, causing obvious changes in the spatial distribution and seasonal cycle of marine species [9]. In southeastern Australia, an extremely long MHW event lasting 251 days occurred from 2015 to 2016, which led to serious deaths of cultured shellfish and wild mollusks [10]. In addition, the Gulf of Maine (GoM), Middle Atlantic Bight (MAB) [11] and NE Pacific Ocean [12] have also been affected by marine heatwaves.

China’s marginal seas have not been exempt from ocean heatwaves. In 2018, the marine heatwave caused a large number of deaths of farmed sea cucumbers in Bohai Sea, resulting in the loss of 68,000 tons of production, with a direct economic loss of CNY 6.87 billion. The mass die-off of Zhangzi Island scallops in Bohai Sea in 2017 was also mainly caused by a marine heatwave event [13]. In addition, marine heatwaves caused an unprecedented coral bleaching event [14] in the northern South China Sea (nSCS) in 2022. With the frequent occurrence of extreme marine heatwave events and their devastating impacts on the marine environment and ecology, along with natural resources, heatwaves in China’s marginal seas have also attracted widespread attention from marine scientists. Over the past few decades, spatio-temporal variations of MHWs in China’s seas have been reported, and the trend of ocean heatwaves up to 2100 has been predicted [15]. The frequency of MHWs in China’s seas has increased by 1.13 counts per decade from 1982 to present, which is 2.5 times the global average [13]. The main reasons for the occurrence and maintenance of the MHWs in the Beibu Gulf from February to April 2019 are high temperature, large subtropical high area, and weak winter wind [16]. The total days, duration and frequency of MHWs in the South China Sea have a significant increasing trend during the period of 1982–2020, with the areas near the Nansha Islands and Beibu Gulf having higher annual mean total days and duration [17]. An unprecedented MHW event with peak intensity of about 20 °C was one of the main causes of record-breaking bleaching of the northern offshore reefs of the South China Sea in the summer of 2020 [18].

The occurrence of MHWs is affected by the synergistic effects of global climate change, regional climate characteristics, regional marine environmental factors, and meteorological conditions. In addition, MHWs in different regions also bear distinctive feature and spatio-temporal evolution characteristics [19]. Due to the complexity and uncertainty of MHWs, our understanding of their characteristics and mechanisms remains limited. Therefore, this study adopts the definition of MHWs indices commonly used in research [1] and analyzes their frequency, intensity, duration and total days in China’s coastal seas and adjacent offshore waters based on the high-resolution daily mean SST data from 1983 to 2020. Finally, the spatial and temporal distribution characteristics and changing trends of key indicators are statistically analyzed in 18 sub-regions within the study area, and the difference among marine heatwaves characteristics of Bohai Sea, Yellow Sea, East China Sea and South China Sea were discussed, for comprehensively and deeply understanding the spatio-temporal characteristics of offshore MHWs in China, and providing scientific support for future marine heatwave forecast and disaster prevention and reduction.

2. Materials and Methods

2.1. Study Area

The study area of this study is shown as Figure 1, and includes the Bohai, Yellow, East China and South China Seas and their adjacent waters. The study area is a continental margin sea in the western North Pacific Ocean, with a wide continental shelf area. The average water depths in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea are 18, 44, 370, and 1212 m, respectively. The upstream part of the Kuroshio current system flows through the East China Sea. The study area has experienced temperate, subtropical, and tropical regions from north to south, with significant climate differences. The bathymetry adopts NRL DBDB2 (Naval Research Laboratory Digital Bathymetry Data Base 2 min resolution) terrain. To better describe the regional characteristics, the entire study area was divided into 18 sections. The location of each sub region is shown in Figure 1, and the names are shown in

Table 1. Names of the 18 sub-regions of the study area.

No.	Name	No.	Name	No.	Name
1	Bohai Sea (BhS)	7	Northeastern East China Sea	13	Northwestern South China Sea
2	Northern Yellow Sea	8	Southeastern East China Sea	14	Northeastern South China Sea
3	Central Yellow Sea	9	Taiwan strait	15	Midwest South China Sea
4	Southern Yellow Sea	10	East of Taiwan	16	Mideast South China Sea
5	Northwestern East China Sea	11	Bashi Channel	17	Southwestern South China Sea
6	Southwestern East China Sea	12	Beibu Gulf	18	Southeastern South China Sea

.

2.2. Data Sources

The Operational Sea Surface Temperature and Ice Analysis (OSTIA) [20] is an analysis and inter-comparison product for sea surface temperature (SST) with global coverage, and was developed by the Met Office based on satellite and in situ data. Products include foundation SST obtained from the diurnal cycle grows, hourly skin SST and the Group for High Resolution SST (GHRSST) Multi-Product Ensemble (GMPE). The OSTIA global sea surface temperature reprocessed product from 1983 to 2020 is employed for marine heatwave analysis. The horizontal resolution of OSTIA is 1/20 degrees, and the time resolution is 1 day. The OSTIA product is available at https://ghrsst-pp.metoffice.gov.uk/ostia-website/index.html (accessed on 9 March 2022).

2.3. Methods

2.3.1. Definitions of MHW Indices

As qualitatively defined by Hobday [1], a marine heatwave is a discrete, persistent and abnormal warm water event occurring within a given ocean area. Discrete means that an MHW is a recognizable event with clear start and end dates, while persistent means that it has at least five consecutive days of daily mean SST above its 90th percentile threshold. If the interval between two consecutive events is less than 2 days, then it is regarded as a continuous event. According to the definition of Hobday [1], a marine heatwave should be identified relative to the baseline climatology. In the present study, the climatology was calculated based on OSTIA data from 1983 to 2012, and the 90th percentile was calculated using daily SST during the climatology period.

Indicators including frequency, mean intensity, maximum intensity, cumulative intensity, duration and total days were used to describe the characteristics of the MHWs. Frequency is the annual number of marine heatwave events; duration is the number of days between the start and end dates; mean intensity is the average SST anomaly during a certain marine heatwave duration; maximum intensity is the maximum anomaly; cumulative intensity is the sum of daily intensities; and total days is the annual total MHW days. Formulas for calculation of MHWs characteristics are shown in Equations (1)–(4). The software used for calculating the parameters is available at https://github.com/ZijieZhaoMMHW/m_mhw1.0 (accessed on 15 February 2022).

$$D=t_e-t_s$$

(1)

$$i_{mean}=\overline{T(t)-T_m(j)}$$

(2)

$$i_{\max }=\max (T(t)-T_m(j))$$

(3)

$$i_{cum}={\displaystyle {\int }_{t_s}^{t_{e-1}}(T(t)-T_m(j))}dt$$

(4)

where D is the duration, t_e is the end date of an MHW, t_s is the start date of an MHW, i_mean is the mean intensity, T(t) is the temperature at time t, j is the day of the year, T_m is the climatological mean.

In addition, linear trend analysis, t-statistic of the two-sided hypothesis test, and linear correlation were used in this study.

2.3.2. Definitions of Marine Heatwave Categories

Different degrees of MHW events have vastly different effects on marine ecosystems. To describe the severity of marine heatwave events more intuitively, the categories of marine heatwaves are divided into four levels according to Hobday’s definition [2]:

$$N=\frac{\mathrm{sst}-T_m}{T_{90}-T_m}$$

(5)

where sst is the daily mean of sea surface temperature, T_m is the climatological mean, and T₉₀ is the 90th percentile of the climatological mean. When 1 < N ≤ 2, it is defined as a moderate MHW (Category I); when 2 < N ≤ 3, it is defined as a strong MHW (Category II); when 3 < N ≤ 4, it is defined as a severe MHW (Category III); and when N > 4, it is defined as an extreme MHW (Category Ⅳ).

3. Results

3.1. Spatial Distribution of MHWs during 1983–2020

The characteristics of the annual average MHW during the period of 1983–2020 are shown in Figure 2. According to the demarcation scope of the study area, sub-regional statistics are carried out on the features of the MHWs. The average frequency 1.4–4.6 counts per year. The maximum frequency occurs in the southwestern South China Sea, and the minimum frequency occurs in the central Yellow Sea. From the regional mean results of the annual average frequency, the mean value of the frequency is 2.3–3.4. The top three regions with the highest mean frequencies are the Beibu Gulf (3.4), northwestern South China Sea (3.2), and Taiwan Strait (3.1), while those with the lowest mean frequencies are the northern Yellow Sea (2.3), Bohai Sea (2.4) and central Yellow Sea (2.5). Overall, MHWs occur slightly less frequently in the Bohai and Yellow Seas, and more frequently in the coastal areas of the East and South China Seas and southern edge of the latter.

The mean intensity ranges from 0.3 to 2.9 °C, and basically shows a gradual decreasing trend from north to south. The maximum values and maximum means of intensity both occur in the Bohai and Yellow Seas, respectively, with mean value ranges of 2.0–2.2 °C. The mean of the intensity range in the East China Sea is 1.3–2.0 °C, and that in the South China Sea is 1.0–1.5 °C.

The maximum intensity range of MHWs is 0.4–3.7 °C, and the spatial distribution is almost exactly the same as that of the mean intensity. The maximum value occurs in the Bohai Sea, and the mean of maximum intensity gradually decreased from 2.7 to 1.3 °C from the Bohai Sea to the South China Sea.

The cumulative intensity of MHWs is 3.9–41.7 °C days, and the spatial distribution is similar to those of the mean intensity and maximum intensity. The largest mean of 25.5 °C days occurs in the northern Yellow Sea. Overall, the cumulative intensities of the Bohai and Yellow Sea are greater than those of the East and South China Seas.

The duration of MHWs ranges from 8.7 to 19.7 days, and the longest duration occurs to the east of Taiwan. From the spatial distribution of the mean results, the areas with longer mean durations are the Yellow Sea, Beibu Gulf, the east of Taiwan, and the central and western South China Sea.

The total days of MHWs is 18.9–69.1, and the highest total days is in the southwestern South China Sea, while the lowest is in the Northern Yellow Sea. In addition, the total days throughout the Beibu Gulf is also very high, next only to the southwestern South China Sea.

In general, from the perspective of spatial distribution, MHWs in the Bohai and Yellow Seas have the lowest frequency, but their intensities are significantly greater and their durations longer. Compared with the Bohai and Yellow Seas, MHWs in the East and South China Seas occur slightly more frequently and last longer in total days, yet their intensities are significantly lower.

3.2. Inter-Annual Variations of MHW Indicators

To provide more details regarding the differences for each indicator among the 18 sub-regions within the study area, the inter-annual variations of regions with the highest and lowest mean values are shown in Figure 3, and the inter-annual mean of the spatial averaged MHW indicators statistics are shown in

Table 2. Inter-annual mean of MHW indicators.

No.	Name	Frequency (Counts/Year)	Intensity (°C)	Max_Intensity (°C)	Cumulative Intensity (°C Days)	Duration (Days)	Total Days (Days)
1	Bohai Sea	2.4	2.2	2.7	23.6	10.9	30.1
2	Northern Yellow Sea	2.3	2.1	2.7	25.5	11.8	30.0
3	Central Yellow Sea	2.5	2.0	2.6	24.5	11.9	33.5
4	Southern Yellow Sea	2.6	2.0	2.5	23.1	11.2	32.8
5	Northwestern East China Sea	2.6	2.0	2.4	21.4	10.7	31.6
6	Southwestern East Sea	3	1.6	2.0	16.8	10.2	33.2
7	Northeastern East China Sea	2.7	1.7	2.1	18.5	10.6	32.1
8	Southeastern East China Sea	2.9	1.4	1.8	15.5	10.7	35.5
9	Taiwan strait	3.1	1.5	1.9	15.6	9.9	32.9
10	East of Taiwan	2.8	1.3	1.7	16.5	11.8	37.6
11	Bashi Channel	2.9	1.2	1.6	13.5	10.7	35.8
12	Beibu Gulf	3.4	1.4	1.9	17.4	11.8	44.8
13	Northwestern South China Sea	3.2	1.4	1.7	15.0	10.6	37.4
14	Northeastern South China Sea	3	1.5	1.8	15.7	10.4	35.3
15	Midwest South China Sea	3	1.3	1.6	14.9	11.2	39.3
16	Mideast South China Sea	2.7	1.3	1.6	14.1	10.8	32.7
17	Southwestern South China Sea	3	1.1	1.4	13.2	11.3	38.1
18	Southeastern South China Sea	3	1.0	1.3	11.5	10.7	35.6
	Mean	2.8	1.6	2.0	17.6	11.0	34.9

.

The annual mean of frequency throughout the study area is 2.8 counts per year. Overall, the mean frequency of MHWs shows an upward trend from 1983 to 2020, with a maximum value of 6.6 counts in 2019, and a minimum value of 2 counts in 1984. The Beibu Gulf has an annual mean frequency of 3.4 counts per year, which is the highest among the 18 sub-regions, while its maximum frequency is 7.4 counts in 2019, and its minimum frequency is 0.5 counts in 1989. The northern Yellow Sea has an annual mean frequency of 2.3 counts per year, which is the lowest among the 18 sub-regions, while its maximum frequency is 5.9 counts in 2017, and its minimum frequency is 0.04 counts in 1987.

The inter-annual mean values of the mean intensity in the 18 sub-regions differ significantly. The maximum is 2.2 °C in the Bohai Sea, mean is 1.6 °C and minimum is 1.0 °C in the southeastern South China Sea. Figure 3b shows that the mean inter-annual variation of the mean intensity is low, and there is no significant upward trend over time. The maximum of mean intensity in the Bohai Sea is 3.0 °C in 2012, and the minimum is 1.7 °C in 1983. The maximum of mean intensity in the southeastern South China Sea is 1.3 °C in 1983, and the minimum is 0.9 °C in 2002.

The time variation trend of the maximum intensity closely resembles that of the mean intensity, except that the value of the corresponding year in the same sub-region is greater. The maximum of maximum intensity in the Bohai Sea is 3.7 °C in 2012, and the minimum is 2.0 °C in 1983. Finally, the maximum of maximum intensity in the southeastern South China Sea is 1.7 °C in 1983, and the minimum is 1.0 °C in 2002.

The inter-annual mean values of the cumulative intensity in the 18 sub-regions is 17.6 °C. The region with the highest inter-annual mean of cumulative intensity is the northern Yellow Sea, at 25.49 °C, while the maximum value of 62.82 °C appeared in 2020, and the minimum value of 12.7 °C appeared in 1993. The region with the smallest inter-annual mean of cumulative intensity is the southeastern South China Sea, at 11.54 °C. The maximum value of 27 °C appeared in 2020, and the minimum value of 5.5 °C in 1985.

The region with the highest inter-annual mean of duration is the central Yellow Sea, at 11.9 days, while the maximum value of 46.87 days occurred in 2020, and the minimum value of 5.9 days occurred in 1984. The region with the smallest inter-annual mean duration is the Taiwan Strait, at 9.9 days, while the maximum value of 15.3 days occurred in 2018, and the minimum value of 6.3 days occurred in 1985.

The region with the highest inter-annual mean of total days is the Beibu Gulf, at 44.85 days, the maximum value of 188.0 days occurred in 2019, and the minimum value of 4.1 days occurred in 1989. The region with the lowest inter-annual mean of total days is the northern Yellow Sea, at 30.0 days, while the maximum value of 123.7 days occurred in 2019, and the minimum value of 0.28 days occurred in 1987.

3.3. Time Variation Trend of MHWs during 1983–2020

To explore the variation trend of MHWs with time, the linear trends of each index of MHWs per decade during the period of 1983–2020 are calculated (Figure 4), and the regional statistics are shown in

Table 3. Statistics of the features of linear increase in MHWs per decade during 1983–2020.

No.	Name	Frequency (Counts/Decade)	Intensity (°C/Decade)	Max_Intensity (°C/Decade)	Cumulative Intensity (°C Days/Decade)	Duration (Days/Decade)	Total Days (Days/Decade)
1	Bohai Sea	0.96	0.08	0.14	5.31	2.02	17.51
2	Northern Yellow Sea	0.72	0.06	0.12	5.78	2.37	14.36
3	Central Yellow Sea	0.94	0.06	0.14	8.25	3.71	20.28
4	Southern Yellow Sea	1.15	0.05	0.12	4.84	2.17	19.44
5	Northwestern East China Sea	1.08	0.02	0.07	3.52	1.69	16.18
6	Southwestern East Sea	1.03	0.03	0.07	2.71	1.44	15.51
7	Northeastern East China Sea	1.03	0.00	0.04	3.06	1.83	15.97
8	Southeastern East China Sea	1.08	0.02	0.05	2.82	1.74	17.26
9	Taiwan strait	1.00	0.00	0.04	2.30	1.39	14.81
10	East of Taiwan	0.81	0.01	0.04	4.58	2.95	17.24
11	Bashi Channel	1.15	0.00	0.01	2.05	1.62	17.32
12	Beibu Gulf	1.16	0.06	0.11	5.42	2.87	26.30
13	Northwestern South China Sea	1.13	0.02	0.05	2.99	1.82	19.59
14	Northeastern South China Sea	1.17	0.01	0.02	3.14	1.88	18.12
15	Midwest South China Sea	1.11	0.02	0.05	2.98	2.00	19.92
16	Mideast South China Sea	0.80	−0.01	0.00	1.77	1.42	12.37
17	Southwestern South China Sea	1.14	0.01	0.03	1.89	1.55	18.35
18	Southeastern South China Sea	1.02	−0.03	−0.03	0.85	1.20	15.91
	mean	1.03	0.02	0.06	3.57	1.98	17.58

. The trend of frequency has a linear increase of −0.27–2.98 counts per decade during 1983–2020 within the 18 sub-regions, as well as a linear increase of 1.03 counts per decade throughout the study area (p < 0.01), which is more than two times higher than the global averaged MHW frequency, which has an increased trend of 0.45 annual events per decade over 1982–2016 [3]. The northeastern South China Sea has the highest mean value of a linear increase of 1.17 counts per decade, followed by the Beibu Gulf at 1.16, and the regions with the third largest mean linear increase of 1.15 are the Bashi Channel and southern Yellow Sea (p < 0.01).

The linear increase in mean intensity ranges from −0.15 to 0.55 °C per decade during 1983–2020, while the average value of linear increase in mean intensity in the 18 sub-regions are all below 0.1 °C per decade (p < 0.01), which is almost equal with the linear trend in global average MHW intensity of +0.085 °C per decade [2]. The mean intensity of the MHWs changes little with time and shows no significant increase. The maximum intensity of the MHWs has a linear increase of −0.19–0.79 °C per decade (p < 0.01). Similar to mean intensity, the rising trend of the maximum intensity is not obvious. The average values of the maximum intensity’s linear increase are all below 0.1 °C per decade, except for the Bohai and Yellow Seas.

The cumulative intensity of the MHWs increased slightly over time, with a linear increase of −4.61–21.28 °C days per decade (p < 0.01). The highest mean of increase was in the central Yellow Sea, with an increase of 8.25 °C days per decade, followed by the northern Yellow Sea, at 5.78 °C days per decade, while the smallest increase was in the southeastern South China Sea, at 0.85 °C per decade (p < 0.01).

The duration of MHWs increased linearly by −4.61–21.28 days per decade (p < 0.01). The mean value of linear increase is 1.2–3.7 days per decade and shows an upward trend of 1.98 days per decade throughout the study area (p < 0.01), while the mean MHW duration has increased by 1.3 days per decade since 1982 [2], which is much slower than that in the study area. The areas with the highest mean values of increase are the central Yellow Sea, east of Taiwan, and the Beibu Gulf.

The total days of MHW occurrence has an obvious upward trend, with a linear increase of 0.18–59.94 days per decade (p < 0.01). The smallest increase in total days is 12.37 days per decade, which occurs in the central South China Sea, while the largest mean increase is 26.3 days per decade (p < 0.01), in the Beibu Gulf.

4. Discussion

4.1. Comparison of Spatial Mean of Frequency in China’s Seas

Based on the MHW frequency during 1983–2020, the monthly frequency was statistically analyzed within the Bohai, Yellow, East China and South China Seas, as shown in Figure 5. Overall, the frequency of MHWs increased gradually from north to south in China’s coastal seas, with the Bohai Sea having the lowest mean of 0.16 counts per month, followed by the Yellow Sea at 0.20, while the East and South China Seas both had means of 0.23 counts per month. On an inter-monthly scale, there are visible monthly differences of frequency in the Bohai and Yellow Seas, which signifies that the MHW occurrences there bear obvious seasonality. In these two seas, the monthly frequency difference is more obvious than in the East and South China Seas. As for the most frequent months, the maximum frequency in the Bohai, Yellow and East China Seas basically occurs in July and August [14], while that in the South China Sea occurs in May and June. It is worth noting that the frequency in the Yellow Sea in August is 0.37 counts, the highest in all of offshore China. Among China’s four coastal areas, the lowest frequency occurs in the Bohai Sea, at 0.07 counts in December. In addition, the lowest frequency values of the Yellow, East China and South China Seas all occurred in February.

The Bohai and Yellow Seas are located in the temperate zone, with significant changes in sea surface temperature throughout the four seasons. The East China Sea spans both temperate and subtropical regions, and the South China Sea is located in the tropics, whose sea surface temperature are both significantly higher than the corresponding periods of that in the Bohai and Yellow Seas, and the seasonal changes are not as significant as those of the Bohai and Yellow Seas [21]. Therefore, the mean frequency of marine heatwaves in the East China Sea and South China Sea is higher than those in the Bohai and Yellow Seas, but the seasonal changes are not as significant as those in the Bohai and Yellow Seas. In addition, the winter sea surface temperature in the Bohai Sea is the lowest among the four sea areas, so the frequency in the Bohai Sea is the lowest in winter.

4.2. Comparison of Spatial Mean of Mean Intensity and Maximum Intensity in China’s Seas

The intensity of marine heatwaves differs greatly between various sea areas and months, and the mean intensity distribution results are gained based on the MHWs mean intensity during 1983–2020 (Figure 6). The mean value of mean intensity decreased gradually from north to south in China’s coastal seas, among which the respective values of the Bohai, Yellow, East China and South China Seas were 2.17, 2.05, 1.56 and 1.23 °C [22]. In the Bohai and Yellow Seas, the percentage of mean intensity in the range of 1.5–2.5 °C is the largest, at about 62% [23]. The East and South China Seas’ respective percentages of mean intensity in the range of 0.5–1.5 °C are the highest, at 52.7 and 81.5%, and the percentage of mean intensity greater than 2.5 is significantly lower than that in the Bohai and Yellow Sea.

Like the mean intensity, the maximum intensity distributions are statistically analyzed, and the results are shown in Figure 7. The mean value of maximum intensity decreased gradually from north to south in the coastal seas of China, among which the respective values of the Bohai, Yellow, East China and South China Seas were 2.75, 2.57, 1.95 and 1.57 °C. In the first three of these seas, the highest percentage of the maximum intensity is in the range of 1.5–2.5 °C, while that in the South China Sea is 0.5–1.5 °C. The distributions of the maximum intensity in the Bohai and Yellow Seas are very similar, and the percentage greater than 3.5 is significantly higher than in the East and South China Seas.

The Bohai Sea is a near closed shallow sea that penetrates into Chinese Mainland, with an average depth of only 18 m. The East China Sea is a semi-enclosed bay located entirely on the continental shelf, with an average depth of 44 m. The East China Sea is a marginal sea in the Western Pacific Ocean, characterized by both shallow and deep seas, with an average depth of 370 m. The South China Sea belongs to the deep sea, with an average depth of 4200 m. Considering the influence of water depth on MHWs, the correlation coefficient between water depth and MHW indicators throughout the study area was statistically analyzed, as shown in

Table 4. Correlation coefficient between bathymetry and MHW indicators.

	Mean_int	Max_int	Frequency	Duration
R	−0.642	−0.650	0.133	0.090
P	0.004	0.004	0.598	0.722

. The mean intensity and maximum intensity of MHWs are significantly negatively related to water depth. In other words, the shallower the water depth, the greater the intensity of the MHW [14]. This explains that both the mean intensity and maximum intensity decrease sequentially in the following order: the Bohai Sea, Yellow Sea, East China Sea and the South China Sea. Contrastingly, the frequency and duration of MHWs are not significantly related to water depth.

4.3. Comparison of Categories in China’s Seas

MHW events of different intensities have different impacts over varying degrees on marine ecology. For example, the greater the intensity of marine heatwave events, the greater the harm to marine ecology, which is even irreversible. To measure the degree of danger of marine heatwaves in each month, the distribution of MHW events categorized as moderate, strong, severe and extreme have been analyzed, and the results are shown in Figure 8. As can be seen from the figure, in all four sea areas from January to December, the highest percentage is moderate MHWs, at about 70–75%, followed by strong MHWs, at about 23–27%, and the percentage of severe MHWs is very small, at only about 2–3% [22]. The percentages of MHWs in the Bohai Sea, Yellow Sea and East China Seas were all only about 0.1%, while that in the South China Sea was 0.3%, which is significantly higher. In terms of monthly distribution, the categories of MHWs in the Bohai and Yellow Seas has more obvious seasonal differences, with the percentage of MHWs categorized as moderate, strong and severe being higher from May to September. In contrast, seasonal differences in the East and South China Seas are smaller than in the Bohai and Yellow Seas. In the former, the peak value appeared in August, with 9.7% of moderate MHWs, 5.7% of strong MHWs, and 0.7% of severe MHWs. In the Yellow Sea, the peak value also occurred in August, with the respective percentages of MHWs categorized as moderate, strong and severe at 11.7, 3.7 and 0.3%. In the East China Sea, the peak value appeared in July, and the respective percentages of MHWs categorized as moderate, strong and severe were 7.8, 3.3 and 0.2%. In the South China Sea, the peak value was in May, with the respective percentages of MHWs categorized as moderate, strong and severe at 7.8, 3.9 and 0.5%.

In the four sea areas, the seasonal variation trend of moderate and strong marine heatwaves, which account for the vast majority of the proportion, are basically consistent with the seasonal variation trend of frequency, which is also related to the climate characteristics of the four sea areas. The seasonal variation of SST in the Bohai and Yellow Seas is more significant than that in the East China Sea and South China Sea, so the seasonal variation of moderate and strong marine heatwaves in the Bohai and Yellow Seas is also more significant than that in the East China Sea and South China Sea.

4.4. Comparison of Duration in China Seas

The distribution of duration of MHWs during 1983–2020 was calculated within the four sea areas, shown in Figure 9. The statistical range ranges from 5 to 60 days, with each interval increasing by 5 days. The percentage of MHWs lasting longer than 60 days is too low to be shown here. On the whole, although the mean of duration is slightly different in the four China seas, the distribution of duration in the four sea areas was similar, which indicates that different climate and terrain characteristics have little impact on the duration of marine heatwaves. The duration ranges from 5 to 10 have the largest percentage, which account for about 60% [23]. When the duration increases from range 10 to 15 days to 30 to 35 days, the percentage of MHWs is 20%, 9%, 5%, 3%, and 2%, respectively.

5. Conclusions

In this study, the spatial distribution and temporal variation of MHWs in offshore China and its adjacent waters during 1983–2020 are analyzed based on OSTIA data, and the comparison of MHWs indicators within China’s four seas is discussed. The major conclusions are summarized as follows:

(1)

For spatial distribution, the annual mean frequency of MHWs increased gradually from north to south in China’s coastal seas. The Bohai Sea has the lowest annual mean frequency, followed in order from low to high by the Yellow, East China and South China Seas. As for temporal variation trend, the annual mean frequency of MHWs has a linear increase of 1.03 counts per decade throughout the study area. In terms of the months with the highest frequency, the maximum frequencies in the Bohai, Yellow and East China Seas basically occur in July and August, while that in the South China Sea occurs in May and June.

(2)

Contrary to frequency, the annual mean of mean intensity and maximum intensity and cumulative intensity of MHWs show a significant decreasing trend in the order from north to south in the Bohai, Yellow, East China and South China Seas, and were significantly negatively related to water depth. The linear increases in mean intensity and maximum intensity are not obvious. The cumulative intensity increased slightly over time, with a mean linear increase of 3.57 °C per decade.

(3)

Among the four categories of MHWs in China’s four offshore sea areas, the moderate MHWs account for the vast majority of events, with a percentage of about 70–75%, followed by strong ones at about 23–27%, while the severe ones account for only about 2–3%, and extreme ones occurred rarely.

(4)

The duration shows an upward trend of 1.98 days per decade throughout the study area. The distributions of duration in the four sea areas were similar, and the duration range of 5–10 days has the highest percentage, accounting for about 60%.