东亚夏季风次季节变化研究进展

东亚夏季风次季节（10~90 d）变化是中国夏季持续性强降水、高温热浪等高影响天气事件的重要环流载体，处于天气预报上限和气候季节预测下限之间的预报过渡区。研究表明:东亚夏季风次季节变化是东亚夏季风的固有物理特征，它和季节进程之间的时间锁相关系是东亚夏季风次季节变化潜在可预报性的重要来源。东亚夏季风次季节变化与Madden-Julian振荡（MJO）存在显著差异，试图通过MJO来预测东亚夏季风次季节变化的不确定性较大。东亚夏季风次季节预测的另一重要来源是下垫面外强迫，包括欧亚大陆春季积雪、中国东部春季土壤湿度和厄尔尼诺-南方涛动（ENSO）事件。此外，去趋势偏-交叉相关分析统计方法能够分析东亚夏季风多因子和多时间尺度问题。目前，亟需解决的科学问题包括:东亚夏季风次季节模态的客观定量描述、造成东亚夏季风次季节模态年际变化的关键物理过程、不同外强迫因子对东亚夏季风次季节模态的共同影响。

Recent Advances on Sub-seasonal Variability of East Asian Summer Monsoon

The sub-seasonal (10-90 days) variability of East Asian summer monsoon (EASM) is crucial for extreme climate disasters (e.g., persistent heavy rainfall and heat waves) in China, which is a blind spot between the upper weather forecast and the seasonal prediction. Recent advances of EASM on sub-seasonal timescale are reported, including features of EASM sub-seasonal variation, influences of mid-latitudinal Eurasian soil moisture and snow cover, as well as the tropical air-sea interaction. Results show the potential predictability of EASM sub-seasonal variability depends on the phase-locking between the sub-seasonal variability and seasonal cycle of EASM. The sub-seasonal variation of EASM is the intrinsic physical mode, which is different from the Madden-Julian Oscillation. It is featured by the intra-seasonal interaction among the western Pacific subtropical high (WPSH), the South Asian High (SAH) and the Mongolian cyclone (MC), along with the alternation of sub-seasonal rain belt in China. The onset of South China Sea summer monsoon (SCSSM), the emergence of Meiyu over the Yangtze River and the starting of rainy season in North China are critical for both the seasonal and sub-seasonal prediction of summer rainfall in China. In mid-May, the eastward extension of SAH onto the South China Sea is vertically coupled with the retreat of WPSH, leading to the onset of SCSSM. Afterwards, the temporal evolution of sub-seasonal modes induced by WPSH, SAH and MC determines the beginning of rainy season over the Yangtze River and North China. Another predicting source of EASM sub-seasonal variation is the interaction between underlying forcing and atmospheric circulation. On one hand, the spring soil moisture over East China acts as an important precursor of summer monsoon onset and anomalous summer rainfall, and the spring snow cover over Eurasian continent could modulate the rainfall over South China. On the other hand, the relationship between tropical air-sea interaction and SCSSM onset shows evident interdecadal variation. The decaying rate of ENSO events and the mid-latitudinal wave activity in the upper troposphere can alter the sub-seasonal variation of EASM on interannual timescale. In addition, a new detrended DPCCA method is developed to investigate the interaction among multi-factors of EASM on multi-timescales. Unsolved questions about the sub-seasonal variation of EASM include objectively qualifying EASM sub-seasonal modes, the crucial process affecting year-by-year changes of EASM sub-seasonal modes, and co-effects of underlying factors on EASM sub-seasonal modes.