Seasonal variation of Marine Heat Wave in the East China Sea

The satellite-based daily sea surface temperature was used to study the Marine Heat Wave (MHW) in the East China Sea. The seasonal variation of MHW is significant. In winter, the 200 m isobath on the shelf is identified as a section with high-frequency MHW. The duration of MHW is relatively long on the slope and deep basin, and mean intensity of MHW is obviously high in the central Taiwan Strait. In summer, the frequency of MHW is relatively high near the coast of the China Mainland. Meanwhile, the shelf section of the 200 m isobath is also high in frequency. Otherwise, the duration of MHW on the shelf is short. In the northern shelf, the mean intensity of MHW is obviously higher. The total days and maximum intensity of MHW are also discussed, aiming at giving near-fully description of MHW in the East China Sea.


Introduction
A Marine Heat Wave (MHW) is a sustained warm Sea Surface Temperature (SST) event [1,2,3,4,5].MHW has garnered significant attention due to its detrimental impact on the marine ecosystem [6,7].The occurrence o f Marine Heat Wave has become increasingly frequent in the context of global warming [8].Furthermore, MHW plays a crucial role in providing energy for the development of tropical cyclones [9].Consequently, the study of MHW holds significant importance in the fields of oceanography and climate change research [10,11,12,13].
Marine Heat Wave (MHW) is known to occur in the East China Sea (ECS), where studies have begun to address the attribution of MHW, including their frequency, duration, and intensity [14].Notably, several MHW hotspots have been identified, encompassing regions such as the western area adjacent to the Changjiang River, the near-coast of Zhejiang Province, and the near-coast of Jiangsu Province [14].Additionally, research efforts have investigated the phenomena and underlying mechanisms of an unprecedented MHW event in the ECS [15,16].One potential trigger of MHW in the ECS is the influence of relatively fresh water [17].Furthermore, numerical ocean models have been employed for MHW forecasting in this region [18].Seasonal variation of MHW has been shown to be important in marginal sea [19,20,21,22].Meanwhile, seasonal variation of SST is significant in the ECS [20].MHW trend is different between winter and summer in the ECS [21].The seasonal variation of MHW in the ECS is significant [22].In previous study, the duration was replaced by the total days of MHW [22], however, the duration is arguable more event-following.Seasonal variations of MHW in the ECS are not fully documented.In this study, we systematically studied the MHW characteristics of the four seasons in the ECS.
In the following sections, Section 2 introduces the data and methods, Section 3 describes the main characteristics of MHW in the ECS, and finally, Section 4 gives a summary.2.1.2.OSTIA SST OSTIA SST was used in this study [23].The temporal resolution is 1 day.The time period is from 1982/01/01 to 2021/12/31.The spatial resolution is 0.05 • ×0.05 • .The original satellite data was then projected to 0.25 • ×0.25 • resolution in the computation.

Data and method
2.2.Method 2.2.1.Definition of MHW MHW is defined as a SST event above 90% extreme value and the duration longer than 5 days [24,25].The time period 1982 to 2005 was used to determine the climatology and threshold.The mean and maximum intensity of MHW of all events are averaged [26].
The Figure 2 illustrates the SST thresholds for the four seasons.During winter (on January 15), the SST threshold near the coast is approximately 10°C, with the southeastern region being warmer than the northwest.Along the western coast of Taiwan, the SST threshold rises to about 20°C.Conversely, on a summer day (July 15), SST levels are noticeably higher compared to winter (January 15), with thresholds primarily exceeding 25°C, and in the deeper basins, surpassing 30°C.Spring and autumn serve as transitional seasons bridging the gap between winter and summer.

Frequency of MHW
Figure 3 display the frequency of MHW in the ECS.Similar to the results of previous studies [14], the frequency of MHW is relative high around the mouth of the Changjiang River.Specifically, the frequency of MHW is roughly 4.5 events per year in west of the mouth of Changjiang River, and near-coasts of Zhejiang and Jiangsu Provinces.There are two sections of high-frequency MHW on the shelf.The first section follows almost the meridional section of 122 • E, and the second section generally follows the 200 m isobath, beginning in the northwest of Taiwan Island and ending in the southeast of Japan.Meanwhile, the frequency of MHW near the Taiwan Island is also high.In the east of Taiwan Island, the frequency of MHW exceeds 5.5 events per year.Slope and deep basin (southeast of the research area) show low frequency.Figure 4 further shows the seasonal frequency of MHW in the ECS.In winter, the frequency of MHW is relatively low around the mouth of the Changjiang River and adjacent near-coast.The frequency of MHW is relatively high in the east and northeast of the Taiwan Island, The second section, which generally along 200 m isobath line, shows significantly higher MHW frequency than that on the northwest side (shallower shelf) and southeast side (deeper slope and basin).In addition, near slope and deep basin, the frequency of MHW is not high.
In spring, the frequency of MHW is relatively high around the mouth of the Changjiang River, also near the near-coast of the Zhejiang Province.Meanwhile, the second section of high frequency of MHW is further enhanced.Compared to winter, the width of second section further expands.The frequency of MHW around the Taiwan Island is also relatively high.
In summer, the frequency of MHW near the coast is extremely high.In the northeast of the mouth of the Changjiang River, the frequency of MHW reaches 1.6 events per year.The second high frequency section, which generally follows 200 m isobath, is reserved in this season, with a frequency of about 1.0 event per year.In the deep basin, the frequency of MHW is not high.
In autumn, the mouth of the Changjiang River is still a hotspot for high-frequency.In southwest of Japan, the frequency of MHW is relatively high.The frequency of MHW around the Taiwan Island decreases as compared with summer season.Meanwhile, in the deep ocean, the frequency of MHW remains relatively low.

Duration of MHW
Figure 5 shows the mean duration of MHW in the ECS.In the slope and deep basin, the duration of MHW is relatively long.Corresponding maximum duration of MHW exceeds 19 days.In the near-coast, the duration of MHW is relatively short, and the duration is nearly 11 days.It is worthy noted that, in the two specific sections of high MHW frequency, the MHW duration is not high, corresponding to approximately 11 days.In the northern shelf, where the water depth is nearly 80m, the duration of MHW is up to 16 days.Furthermore, Figure 6 displays seasonal variation of MHW duration.In winter, long duration of MHW is observed near the slope and deep basin, with a maximum duration of more than 25 days.On the northern shelf, where the water depth is about 50 to 80 m, the duration reaches 25 days.In the near-coast at northern boundary (33 • N, the duration is longer than 25 days.Otherwise, in the near-coast from the Taiwan Strait to the mouth of the Changjiang River, the duration is approximately 10 days.
In spring, the duration of MHW is relatively short.On the shelf, the duration is close to 10 days, and the duration in the slope and deep basin is still not high, and its duration is mainly 14 days.However, in some area like 125.5 • E,25 • N, the duration exceeds 20 days.
The duration of MHW in summer is relatively short compared to winter.Nevertheless, the duration of MHW is as long as 18 days on the slope (nearly 2000m), and about 16 days on the northern shelf (near 127 • E, 31.5 • N).
In autumn, the duration of MHW increases relative to summer.In the deep basin, the duration of MHW extends to 28 days.The duration in the near-coast also turns slightly longer than that in summer.At latitude 32 • N, on the northern shelf along 80 m isobath, the duration attains 20 days.

Total days of MHW
Total days gives integral description of MHW duration.Figure 7 shows the mean state of total days.On 40-year average, the annual total days is more than 70 days in the east of Taiwan Island.It is also noted that, in this area, the long total days is contributed from high frequency (Figure 3), while the duration is relatively short (Figure 5).Similar results are observed for two sections with high frequency, where the first section roughly follows 122 • E, and the second section nearly along 200-m isobath.In contract, in the latitude band of 22 • to 29 • N, the total days is considerably low in the near-coast of the China Mainland.In the deep basin, the total days of MHW also reaches 50 days, where the duration is high (Figure 5) but frequency is low (Figure 3).
Figure 8 displays the seasonal variation of total days.In winter, the total days is high in east of Taiwan Island, however, the total days is nearly zero in the near-coast of the China Mainland.In the depth range between 50 to 80 m, the duration is relatively high on the shelf around (123 • E, 29 • N).In the northern shelf, where the water depth is greater than 50 m, the total days is also relatively high (nearly 5 days per month).Meanwhile, at water depth greater than 200 m, the total days is relatively high in the latitude band of 26 • to 30 • N, roughly overlapping the pathway of the Kuroshio (Figure 1).In the deep basin (127 • E, 24.5 • N), the total days attains 5.5 days per month.
In spring, the total days is also high in some hotspots, which at-least include west of the mouth of the Changjiang River, the north of Taiwan Island, the east of Taiwan Island, near the 200 m shelf (123 • E, 26 • N), and the area near 29 • N with water depth ranges in 200 to 2000 m.
In summer, the total days is high in the northern shelf at latitude 33 • N, north and east of Taiwan Island, and on the shelf near the Kuroshio pathway.
In autumn, the northern shelf around (123 • E, 33 • N) is high in the total days (6 days per month).Meanwhile, on the shelf of [29 • N, 32 • N] latitude, with the water depth in the range of 50 to 200 m, the total days is also high.In the deep basin around [129 • , 131 • ]E×[24.5 • 26 • ]N, total days is as high as 6 days per month, while this area is always low in total days during other three season.

Mean intensity of MHW
Figure 9 shows the spatial distribution of the mean intensity of MHW.In northeast of the mouth of the Changjiang River, the mean intensity of MHW is relatively high.The corresponding mean intensity is about 2.3 • C per event.In the central Taiwan Strait, the intensity of MHW is considerably high as 1.8 • C per event.On slope, the intensity of MHW is as low as 1.4 • C per event.The mean intensity in deep basin is slightly stronger than that on slope.Seasonal variation of mean intensity is also significant.In winter, high intensity of MHW is observed in the central Taiwan Strait, with a mean intensity of more than 3 • C per event.In north of Taiwan Island, where the water depth ranges in 50 to 200 m, the mean intensity is also relatively high.The corresponding mean intensity of MHW reaches 1.9 • C per event.In the northeast of the mouth of the Changjiang River, the mean intensity of MHW is higher than adjacent area.Another area with high intensity locates in southwest of Japan, and the mean intensity of MHW attains 1.7 • C per event.
In spring, the MHW occurs high mean intensity on the main shelf.In the northeast of the mouth of the Changjiang River, where the water depth is less than 50 m, the mean intensity of MHW is as high as 2.7 • C per event.On the shelf of 123 • E where water depth ranges in 50 to 80 m, the mean intensity of MHW is considerably high.Meanwhile, at west of the Japan near (129 • E, 31 • N), the mean intensity of MHW is relatively high, with a maximum of nearly 2.1 • C per event.In deep basin, the intensity also attains 2.0 • C event.
In summer, the northern shelf, which locates at northeast of the mouth of Changjiang River, has highest mean intensity, and the mean intensity reaches 2. The mean intensity on the slope is relative low.Otherwise, the deep basin also exhibits an intensity of 1.8 • C per event.
In autumn, the seasonal intensity of MHW in research area is low, although there is a spatial difference between the near-coast and deep basin.Around the mouth of the Changjiang River, the mean intensity of MHW attains 1.8 • C per event.

Maximum intensity of MHW
We further investigate the maximum intensity of MHW in the ECS, as shown in Figure 11.Compared to the mean intensity (Figure 9), the pattern is similar.High maximum intensity is observed in the northeast of the mouth of the Changjiang River.The maximum intensity is also high in the central Taiwan Strait (about 2.3 • C per event).Meanwhile, on the shelf with a water depth of 50 to 80 m and a latitude spans from 26.2 • to 29 • N, the maximum intensity is comparable with that in the central Taiwan Strait.The maximum intensity of MHW is relatively weak on the slope as well as in the deep basin.It is also noted that the maximum intensity of MHW is higher than the mean intensity.In the northeast of the mouth of the Changjiang River, the maximum intensity of MHW is as high as 2.9 • C per event, while the mean intensity is roughly 2.3 • C per event.
Seasonal variation of maximum intensity of MHW is as also similar to that of mean intensity (Figure 12).Winter is characterized by high values in the central Taiwan Strait.In spring, the In summer, the area with a high frequency of MHW shifts to the near-coast of the China Mainland.Compared to winter, MHW durations on the shelf are shorter.However, the northern shelf (northeast of the mouth of the Changjiang River) experiences significantly higher mean MHW intensities.
Spring and autumn serve as transitional seasons between winter and summer.In spring, the southwest-to-northeast shelf section along the 200 m isobath exhibits high MHW frequency, with shorter durations compared to winter.The mean MHW intensity increases on the main shelf.In autumn, MHW frequency is elevated around the Changjiang River, while the deep basin experiences lower MHW frequency.The MHW duration in the deep basin is longer than in summer, and the mean MHW intensity near the coast is slightly stronger than in the slope and deep basin.
We also investigate the total days of MHW, which combines frequency and duration.In areas such as the east of the Taiwan Island and the two high-frequency MHW sections, total day values are high due to the frequent occurrence of MHW.Meanwhile, the deep basin exhibits high total days values because of the extended durations.This pattern highlights the importance of the east of the Taiwan Island in both winter and summer.In autumn, total days values are notably high, especially in the northern shallower shelf (water depth less than 50 m) and the deep basin.
The maximum intensity of MHW generally aligns with the mean intensity pattern, with the maximum intensity being slightly higher.In winter, the maximum MHW intensity reaches 3.0°C per event in the northeast of the mouth of the Changjiang River.The central Taiwan Strait

Figure 3 .
Figure 3. Frequency of Marine Heat Wave in the East China Sea (units: events per year).The magenta solid lines represent isobaths of 50, 80,200 and 2000 m.

Figure 4 .
Figure 4. Seasonal frequency of Marine Heat Wave in the East China Sea (units: events per year).(a) Winter, (b) spring (c) summer and (d) autumn.The magenta solid lines represent isobaths of 50, 80, 200 and 2000 m.

Figure 5 .
Figure 5. Duration of Marine Heat Wave in the East China Sea (units: days).The magenta solid lines represent isobaths of 50, 80, 200 and 2000 m.

Figure 6 .
Figure 6.Seasonal duration of Marine Heat Wave in the East China Sea (units: days).(a) Winter, (b) spring (c) summer and (d) autumn.The magenta solid lines represent isobaths of 50, 80, 200 and 2000 m.

Figure 7 .
Figure 7.Total days of Marine Heat Wave in the East China Sea (units: days per year).The magenta solid lines represent isobaths of 50, 80, 200 and 2000 m.

Figure 8 .
Figure 8. Seasonal total days of Marine Heat Wave in the East China Sea (units: days per month).(a) Winter, (b) spring (c) summer and (d) autumn.The magenta solid lines represent isobaths of 50, 80, 200 and 2000 m.

Figure 9 .
Figure 9. Mean intensity of Marine Heat Wave in the East China Sea (units: • C per event).The magenta solid lines represent isobaths of 50, 80, 200 and 2000 m.

Figure 11 .
Figure 11.Maximum intensity of Marine Heat Wave in the East China Sea (units: • C per event).The magenta solid lines represent isobaths of 50, 80, 200 and 2000 m.

Figure 12 .
Figure 12.Seasonal maximum intensity of Marine Heat Wave in the East China Sea (units: • C per event).(a) Winter, (b) spring (c) summer and (d) autumn.The magenta solid lines represent isobaths of 50, 80, 200 and 2000 m.