Drivers of the unprecedented coastal marine heatwaves along Fujian and Zhejiang provinces, China

In recent decades, coastal marine heatwaves (MHWs) have intensified due to global warming, causing severe impacts on coastal marine ecosystems. In September and October 2021, significant reductions in seaweed production were observed along the coasts of Fujian (−19.23 %) and Zhejiang (−3.27 %) Provinces in China, which may be related to unprecedented coastal MHWs. This study utilizes three high-resolution satellite sea surface temperature (SST) datasets together with oceanic and atmospheric reanalysis products to examine the characteristics and mechanisms of these MHWs. Our analysis reveals that the record-breaking MHWs occurred primarily from September 21 to October 23, with an average intensity of up to 4 °C and lasting nearly the entire month of October. Mixed layer heat budget assessments indicate that net surface heat flux was the key factor influencing temperature changes (Corr = 0.71, p < 0.01). The positive net surface heat flux in September, driven by increased solar shortwave radiation, resulted from the intensification and westward expansion of the western Pacific subtropical high (WPSH). We emphasize that understanding the mechanisms behind the anomalous WPSH in autumn is vital for mitigating climate risks to seaweed production along the Fujian and Zhejiang coasts in the context of global warming.