Marine heatwaves (MHWs) are prolonged anomalous warm seawater events that can severely impact marine ecosystems and have never been studied previously in the South China Sea. Here, we combine satellite sea surface temperature and reanalysis products to provide a systematic analysis of variations in summer MHWs in the South China Sea and potential influencing factors. The MHW days and duration, calculated as multiyear average, are mainly concentrated near the Nansha Islands and Beibu Gulf. Additionally, we find significant increasing trends in MHW days, duration, and frequency in the South China Sea during 1982–2020. The first empirical orthogonal function mode accounts for 60% of the total variance and exhibits a general mono‐sign pattern over the entire South China Sea. Increased solar radiation and released latent heat during summer MHWs contribute to high pressure in the western North Pacific. The anticyclone induced by the anomalous intensification of the western North Pacific subtropical high and its westward extension bring easterly wind anomalies to low latitudes, weakening the summer southwest monsoon south of 12°N in the South China Sea. The midwestern South China Sea upwelling weakens or even disappears due to a negative wind stress curl, causing basin‐wide severe summer MHWs. The paper also points out that more attention should be given to inter‐ocean interactions when monitoring the risks of unprecedented MHW‐related coral bleaching and marine ranching in the South China Sea.

Under the combined impacts of natural changes and human activities, the past, current, and future marine heatwaves (MHWs) in China's marginal seas and adjacent offshore waters (CMSOW) need a comprehensive understanding. This study provides a systematic analysis of the spatiotemporal variations using daily sea surface temperature data and simulates the future trend using 12 climate models. During 1982–2018, the mean annual total days, duration, frequency, and mean intensity of the MHWs in the CMSOW increased by 20–30 days/decade, 5–9 days/decade, 1–2 decade⁻¹, and 0.1–0.3°C/decade, respectively (p <0.01). The maximum sea surface temperature anomalies in the Bohai Sea was over 6–8°C, and the MHW's frequency, duration, and mean intensity were higher than twice the global average, which could have impacted fishery resources and occurrence of harmful algal blooms. The variations of the MHWs in the CMSOW result from the robust ocean surface warming, which is caused by increased solar radiation due to reduced cloud cover, reduced ocean heat loss from weaker wind speed, weakening but warmer Kuroshio, and strong El Niño. In the future, the areas with longer total days and duration will increase; the spatial pattern of frequency has a negative relationship with that of duration while that of mean intensity is mostly unchanged. Year 2040 is a key node for the future changes of MHW under different Representative Concentration Pathways. The trend of total days increases from fast to slow, and frequency shows an opposite trend; the duration and mean intensity rise faster after 2040.

Marine heatwave (MHW) characteristics, such as total days, average duration, and mean intensity, have increased in most global oceans. In contrast, marine cold‐spell (MCS) characteristics have a decreasing trend over most regions. However, the difference between MHW and MCS in the mean and trend in the present and future climate is unclear. Here, we provide a comparative global assessment of these changes based on satellite sea surface temperature for 1982–2020 and climate model projections for 2021–2070. Globally, there are clear differences in the mean and trend between MHW and MCS metrics. On seasonal timescales, MHWs mainly occur in summer and autumn, while MCSs generally occur in winter and spring. The occurrence area of MHWs shows an increasing trend during 1982–2020, while the occurrence area of MCSs shows an opposite trend. Furthermore, the increasing trend in MHW average duration led to a near‐permanent MHW status in the future projections. Upper‐ocean warming due to anthropogenic forcing is closely related to increase in MHWs and decrease in MCSs. In addition, the shallower ocean mixed layer depth in the future is favorable for the occurrence of MHWs. In contrast, the projected weakening of the Atlantic meridional overturning circulation facilitates the occurrence of MCSs in the subpolar North Atlantic. These results provide a global baseline for the difference and relationship between MHW and MCS under present climate and future projections.

Marine heatwaves (MHWs) are prevalent in the South China Sea (SCS) and are primarily caused by the anomalous western Pacific subtropical high (WPSH) which suppresses the SCS summer monsoon. Our study investigates future summer MHWs in the SCS in response to the enhanced WPSH. The results show that the WPSH will be strengthened in most models under the highest emission scenario. Total days of summer SCS MHWs will significantly increase with the enhancement of the WPSH and peak at the end of the 21st century. The high‐value area of total days of summer MHWs (>50 days) appears in the south‐central SCS. The intensity of MHWs exhibits little response to the strengthening WPSH. In addition, the weakening of the SCS cold filament due to the anomalous WPSH also favors summer MHWs. Furthermore, increased future severe summer SCS MHWs will create a higher bleaching risk for coral reefs.