Characteristics and Mechanism of the 10-20-Day Oscillation of Spring Rainfall over Southern China

The intraseasonal oscillations (ISOs) of southern China spring rainfall (SCSR) are examined based on daily rain gauge rainfall data and NCEP/Department of Energy Reanalysis 2 (NCEP-2) products for the period 1980-2008. The objective of this study is to reveal the structure and propagation of the dominant ISO of SCSR as well as its driving mechanisms, thereby gaining an understanding of the causes of extreme wet and dry SCSR. The EOF analysis and power spectrum analysis show that the 10-20-day oscillation is a predominant ISO of SCSR in most years. Composite analyses and wave-activity propagation diagnosis demonstrate that the 10-20-day oscillation of SCSR is characterized by an alternate occurrence of a huge anomalous anticyclone (cyclone) encircling the Tibetan Plateau in the lower troposphere, with anomalous low-level northeasterly (southwesterly) winds prevailing over southern China, producing lower-tropospheric divergence (convergence). In the middle and upper troposphere, the oscillation appears as a southeastward propagating coherent wave train made up of a series of anomalous cyclones and anticyclones, which are aligned in a northwest-southeast direction. This whole wave train also drifts eastward, with strong upper-tropospheric convergence (divergence) alternately superimposed over the lower-tropospheric divergence (convergence) within and south of the Yangtze basin, resulting in deficient (excessive) rainfall in southern China. The thermal structure of the 10-20-day ISO of SCSR and its association with the mechanical-thermal forcing of the Tibetan Plateau are also explored.