Extremely long-lived marine heatwave in South China Sea during summer 2020: Combined effects of the seasonal and intraseasonal variations

South China Sea (SCS) experienced an exceptionally long-lived marine heatwave (MHW) event in summer 2020, which broke the historical records in terms of its time duration since 1982. This long-lasting MHW was primarily attributed to the combined effects of the seasonal cycle (SC) and 30–60-day intraseasonal oscillation (ISO) components of anomalous atmospheric circulations. During the SC developing phase, the superimposed anticyclonic circulation regimes of the SC and 30–60-day ISO jointly contributed to the first warming sea surface temperature (SST) peak over the central SCS in July, via the combined effects of net downward shortwave radiation (SWR) and latent heat flux (LHF) anomalies. In contrast, during the SC decaying phase, the SCS MHW-related warming SST tendency was mostly due to 30–60-day ISO signals. During the growth phase of ISO, the joint warming effect of the SWR and LHF anomalies was the dominant factor maintaining the SCS MHW. In the decaying phase of 30–60-day ISO, however, the second MHW-related warming peak around Beibu Gulf in September was mainly contributed by the sharply reduced LHF loss due to strong southwesterly anomalies over the northern SCS on the 30–60-day timescale. The varying SC component plays a crucial role in the occurrence and maintenance of MHWs, while the 30–60-day ISO can effectively regulate the development and pattern of SCS MHWs. These results suggest that, besides the vital role of the SC component, the 30–60-day ISO should also be considered for fully understanding the extreme MHW events.