Diverse marine heatwave intensity trends in the marginal seas of China

In the past few decades, both the frequency and intensity of marine heatwaves (MHWs) have increased globally, which has been attributed to ocean surface warming. This study investigates the trend in the MHW intensity in the marginal seas of China and contributions of seasonal mean sea surface temperature (SST) trend and subseasonal (time periods less than 90 days) SST variance trend. The MHW intensity trend displays spatially non-uniform features and seasonal differences. It is found that both seasonal mean SST trend and subseasonal SST variance trend are required to account for the MHW mean intensity trend. In most of the South China Sea, subseasonal SST variance displays a decreasing trend, indicating a reduction in day-to-day SST fluctuations. This results in weak or negative trend in the MHW mean intensity though seasonal mean SST displays a prominent rising trend. In the Gulf of Tonkin in winter and the eastern Yellow Sea in summer where seasonal mean SST trend is relatively small, subseasonal SST variance trend induces an increasing trend in the MHW mean intensity. In the East China Sea in winter, seasonal mean SST increasing trend has a main contribution and the increased subseasonal SST variance has a supplementary contribution to the increase in the MHW mean intensity. Further analysis reveals that the subseasonal SST variance is regulated by the seasonal mean SST gradient. A decrease (an increase) in mean SST gradient leads to reduced (enhanced) subseasonal SST variance.