Low frequency oscillation characteristics of 12-30 d persistent heavy rainfall over South China

Based on daily rainfall data collected at observational stations in China, NCEP/NCAR reanalysis data, NOAA daily outgoing long wave radiation (OLR) and sea surface temperature (SST) data from May to August during 1982-2011, the basic characteristics of 12-30 d persistent heavy rainfall are analyzed by selecting low frequency rain events. The corresponding low frequency atmospheric circulation pattern, and the source and propagation of low frequency signals are investigated using the phase-composite method. The impacts of the low frequency air-sea interaction on the persistent heavy rainfall are also studied. Results show that summer rainfall over South China has a significant characteristic of low frequency (12-30 d) oscillation, and persistent heavy rainfall event occurs most frequently in June. The rain band spreads from the southeast to the northwest of South China during the rainy period. Accompanied with the persistent heavy rainfall, a strong low frequency cyclone controls South China and adjacent sea area, leading to strong ascending motions; meanwhile, a low frequency anticyclone is located over the region from the South China Sea (SCS) to the Philippines Sea (PS) in the lower atmosphere. The strong southwesterly winds continuously bring water vapor from the SCS to South China, leading to water vapor convergence and ascending motions there. The low frequency signal in the lower atmosphere originates from the northwestward propagation of the low frequency oscillation over the SCS-PS and the tropical western Pacific, which is associated with the westward extension and eastward retreat of the western Pacific subtropical high. In the upper atmosphere, a strong low frequency cyclone centered over north of South China around (22°-45°N, 95°-130°E) is couple with a low frequency anticyclone located over the Bay of Bengal to SCS, which provides a strong divergent environment over South China that is favorable for the development of convergence in the lower atmosphere and ascending motion. As a result, persistent heavy rainfall occurs over South China. The low frequency signal in the upper atmosphere comes from the southeastward propagation of the low frequency Rossby wave train. The low frequency sea surface temperature anomaly (SSTA) can be modulated via the process of cloud radiation and heat fluxes driven by the low frequency circulation anomaly, while the low frequency SSTA can significantly influence the atmosphere in turn by changing the stability in the lower atmosphere. Such an air-sea interaction process on low frequency time scale favors the propagation of the low frequency signal in the lower atmosphere, thereby affects the development and termination of persistent heavy rainfall.