Moisture Sources and Sinks of Low-Level Jets in South China: A Backward and Forward Trace Model Approach

Abstract Low-level jets (LLJs) play a critical role as carriers of moisture, significantly influencing heavy rainfall. In South China, three distinct LLJ branches are identified: the boundary layer jets in the Beibu Gulf (BLJ-BG) and in the northern South China Sea (BLJ-SCS), as well as the synoptic-system-related LLJs in South China (SLLJ-SC). This study investigates their varying moisture transport, sources, and sinks utilizing the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT), employing both backward and forward trace simulations. As the presummer rainy season progresses, the primary moisture sources of LLJs gradually shift southwestward from the South China Sea to the Indian Ocean. BLJ-BG predominantly directs moisture trajectories to cross the Indochina Peninsula, facilitating more moisture transport from the Indian Ocean (∼42.7% after the onset of the South China Sea summer monsoon) and strengthening monsoonal moisture transport compared to non-BLJ-BG events. Conversely, BLJ-SCS consistently draws moisture from the South China Sea (∼44.5%), with trajectories circumventing the Indochina Peninsula. Notably, BLJ-SCS transports even less moisture from the Indian Ocean than nonevent periods. After the monsoon onset, SLLJ-SC shares a common southwestern moisture channel with BLJ-BG and exhibits a pronounced tendency to transport moisture from the northern side (∼10.3% moisture trajectories) compared to other LLJs, though still less than the non-SLLJ-SC events. Moisture transported by BLJ-BG (BLJ-SCS) is released in northeastern Guangxi (central Guangdong), coinciding with intensified inland maximum rainfall centers. SLLJ-SC transports more moisture further north into northern Jiangxi, aligning with the formation of rainbands further north. Significance Statement Different types of low-level jets (LLJs), including boundary layer jets and synoptic-weather-related LLJs occur frequently in South China during the presummer rainy season. This study distinguishes their respective roles in moisture transport from the perspectives of moisture paths, sources, and sinks. These LLJs do not uniformly enhance the southwesterly monsoonal moisture transport from distant oceanic regions. In particular, the boundary layer jets in the northern South China Sea predominantly draw moisture nearby. Moisture sinks associated with these LLJs align with areas where precipitation is intensified. This study provides valuable insights for forecasting extreme precipitation events in the region and establishes a connection between the origins and culmination of moisture in rainfall through the perspectives of LLJs as a crucial moisture carrier.