63 / 2023-07-25 07:39:24

Multi-Scale Factors Contributing to Increased Non-Tropical Cyclone Rainfall in the South China Sea in Autumn under Different ENSO Phases

ENSO,the South China Sea,non-tropical cyclone rainfall,ITCZ

The rainfall in the South China Sea (SCS) usually peaks in autumn (September and October), with approximately 58% attributed to non-tropical cyclone (non-TC) rainfall. El Niño-Southern Oscillation (ENSO) is an important air-sea interaction phenomenon in tropical regions. Under specific phases, it can lead to an increase in non-TC rainfall in the SCS, and the mechanisms are required further exploration. This study investigated the multi-scale factors contributing to increased autumn non-TC rainfall in the SCS by using reanalysis, satellite, and station data. The results reveal that during La Niña years, the Intertropical Convergence Zone (ITCZ) can strengthen and accelerate northward in autumn, bringing heavy rainfall to the northern SCS. The stalled Madden-Julian Oscillation in the Indian Ocean-SCS region triggers a Gill response, strengthening the intensity of the ITCZ. Additionally, enhanced equatorial airflow, eastward-moving Continental High, and upper-level energy divergence all promote the northward movement of the ITCZ. However, during La Niña years, the anomalous strengthening, westward and southward extension of the Western Pacific Subtropical High (WPSH) can hinder the northward movement of the ITCZ. At this time, the northern SCS is predominantly controlled by the WPSH, with anomalous sea surface warming. Nevertheless, rainfall can still occur in the Northern SCS when there is wind shear in the lower levels. Surprisingly, strong El Niño years are also associated with increased rainfall. Positive surface latent heat flux anomaly and anomalous cyclone near the Beibu Gulf, combined with negative surface latent heat flux anomaly and anomalous anticyclone around the Bashi Strait, establish an anomalous zonal vertical circulation in the northern SCS. The anomalous circulation enhances the upward motion near the Hainan region, ultimately increasing the rainfall. This study further deepens the understanding of autumn rainfall in the SCS, aiming to provide a theoretical basis for rainfall prediction under climate change.