Marine heatwaves (MHWs), which are the unusually warm periods in the ocean, are becoming more frequent and lasting longer in the northwest European Shelf (NWES), particularly near the coast, from 1993 to 2023. However, thermal stratification is weakening, implying that the sea surface warming caused by MHWs is insufficient to counteract the overall stratification decline due to global warming. Moreover, the varying salinity has a notable impact on the trend of density stratification change.