西北太平洋和南海不同时段生成热带气旋频数及其水汽条件的分类

对1961—2010年南海和西北太平洋不同时段生成热带气旋(tropical cyclone,TC)频数的时空分布及水汽条件对其产生的影响进行了分类研究。结果表明,可以将TC活动划分为活跃期(6—11月)和平静期(上年12—当年5月)两个时段。在TC活跃期和平静期,南海和西北太平洋上TC频数的EOF第一特征向量都表现为一致的增加或减少。活跃期EOF的第二特征向量表现为南海与西北太平洋中西部的TC频数存在相反的变化趋势,平静期EOF的第二特征向量则表现为130°E以西海域的TC频数与130~150°E范围内生成热带气旋存在相反的变化趋势。活跃期和平静期西北太平洋TC的生成频数与水汽通量散度均存在显著的负相关;而在活跃期南海TC频数与水汽通量散度仅在南海中北部有弱的负相关,在平静期南海东部到菲律宾附近海域有显著的负相关。因此,水汽条件的影响使得在活跃期南海和西北太平洋TC高频年中,南海北部和西北太平洋中东部TC频数明显偏多,而平静期高频年中,南海东部以及西北太平洋中西部TC频数明显偏多。

Analysis on the frequency and water vapor condition for the formation of tropical cyclones over South China Sea and Northwest Pacific Ocean during different periods

Classification studies were conducted in the paper on the temporal and spatial variations as well as the effect of water vapor on the tropical cyclone(TC) frequency over the South China Sea(SCS) and Northwest Pacific Ocean(WNP) in different periods during 1961-2010.The major results are summarized as follows:The whole year can be divided into an active period(June-November) and a calm period(December-May) for TC.The first eigenvectors during both periods show a uniform enhancement or reduction of the TC frequency over the SCS and WNP,while the second eigenvector in the active period shows that the TC frequency over the WNP is in opposite phase with that over the SCS,and the second eigenvector in the calm period shows that the TC frequency to the west of 130°E is in opposite phase with that between 130°E and 150°E.Correlation and composite analysis indicates that the divergence of water vapor flux shows a significant negative correlation with the TC frequency over the WNP in both periods and over the eastern SCS and the vicinity of Philippines in the calm period,but they show only a weak negative correlation over the central and northern SCS in the active period.Therefore,the water vapor conditions may lead to higher TC frequency over the northern(eastern) SCS and the central and eastern(western) parts of the WNP during the active(calm) period.