大气波导是对流层中具有异常垂直大气折射率梯度的大气层结，对于预测电磁波传播和评估海上探测通信系统等具有重要的科学意义和应用价值。根据1998年南海季风试验期间，东沙岛、南沙岛探空站和“实验3号”、“科学1号”科考船在加密观测期间观测的1天4次高分辨率探空数据，统计分析了夏季风前后南海除了蒸发波导外的低空大气波导发生规律，结果表明夏季风爆发对于南海低空大气波导特征具有重要影响。总体上，大气波导发生概率在夏季风爆发前较高，爆发后较低；夏季风爆发前后，大气波导发生概率在南海北部变化较大(约12%)，在南海南部变化较小(约10%)。夏季风爆发抬升了南海北部大气波导层高度，降低了南海南部大气波导层高度，其幅度都小于300m；南海夏季风爆发后，除了南海北部大气波导强度略有增强外，南海低空大气波导厚度和强度都有所减少，其中南海南部减少较多，分别约为40m和1M单位。分析显示，湿度锐减是引发南海低空大气波导的直接原因，夏季风爆发向南海低层大气输送了大量暖湿水汽，形成了不同的大气边界层，造成了南海大气波导特征的南北差异。

The atmospheric duct in the troposphere comprises strong vertical refractivity gradient structures. It has great scientific significance and great values for propagations of radio waves and functions of surveillance equipments. Using the GPS sounding datasets obtained four times daily at Dongsha sounding station, Nansha sounding station, R/V“Kexue1”, and R/V“Shiyan3” during the South China Sea Monsoon Experiment (SCSMEX) in 1998, the characteristics of the lower atmospheric ducts were analyzed, excluding evaporation duct pre- and post-monsoon onset over the South China Sea (SCS). The results showed that the summer monsoon had an important influence on the lower atmospheric duct over the SCS. The occurrence probability of the atmospheric duct during the pre-monsoon period was larger than that in the active monsoon period and the amplitudes of the duct occurring probabilities pre- and post-monsoon onset were different spatially; they were about 12% over the northern South China Sea (SCS) and 10% over the southern SCS. Moreover, summer monsoon onset elevated the heights of the lower atmospheric duct layers over the northern SCS and reduced the heights of the lower atmospheric duct layers over the southern SCS, whose amplitudes of variation at given altitudes were all within 300 m. In addition, except that the duct strength in the northern SCS became a bit stronger during the active summer monsoon period, the strength in other parts of the SCS decreased, especial those in the southern SCS that were about 40m in thickness and 1M units in duct strength. The duct characteristics in the SCS were determined by the sharp decrease of water vapor with altitude. The summer monsoon brought much warm water vapor into the lower troposphere over the SCS where different atmospheric boundary layers formed, which resulted indifferent atmospheric duct characteristics from the northern to southern SCS.