南海西行北翘路径热带气旋合成分析

A COMPOSITE STUDY OF THE TROPICAL CYCLONES NORTHWARD TURNING MOTION OVER SOUTH CHINA SEA

合成对比分析了1979—2013年南海热带气旋(TC)发生西行北翘路径(RT)和西行不转向路径(ST)样本的环流形势及其差异。结果表明:RT样本对应的西风槽距离TC中心较近,南亚高压反气旋环流纬向跨度和经向跨度相对较小,样本中心南侧西南风加强,与副热带高压(副高)逐渐减弱东退南落,以及低层样本中心东南侧存在弱的反气旋环流有关;ST样本对应的南亚高压反气旋环流纬向跨度和经向跨度相对较大,副高呈东西向带状稳定少动。上述差异通过了置信度0.05的显著性检验。环流形势变化使得TC高层流出增强以及对流结构变化,引起与TC移向有关的位涡倾向1波正异常分布调整,是TC发生RT的重要原因。应用经验正交函数分解分析表明200 h Pa风场合成结果对于样本的高层环流形势具有良好的代表性。

This paper presents a composite study of the environmental flow patterns and differences around South China Sea tropical cyclones （TCs） undergoing recurvature-moving track （RT） and straight-moving track （ST） from 1979 to 2013. In the RT composite, an upper-level westerly trough is closer to the north of TC center, and this feature is associated with a less zonally and meridionally orientated South Asian High （SAH） anticyclone circulation. Additionally, a low-level southwesterly wind develops to the south of the TC center that is linked to the southward withdrawal of the subtropical high （SH）, and a weak anticyclone is located in the southeast of the TC center. In contrast to the RT samples, in tile ST composite, the SAH circulation is zonally and meridionally extensive, and a zonally-orientated SH anticyclone belt remains stable to the north of the TC center. Statistical tests show that these results are significant at the confidence level of 0.05. The modifications of the maximum wavenumber-1 potential vorticity tendency, which is mainly due to the changes of upper-level TC outflow patterns and convection structures, are dominantly for TC recurvature. By using scalar EOF analysis of the 200-hPa meridional wind around these two subsets, the 200-hPa wind composite results can represent upper-level environmental flow patterns of RT and ST cases properly.