西北太平洋近赤道热带气旋生成的特征分析

1979—2012年西北太平洋存在70个形成于0°～5°N的低纬度地区的热带气旋(TC),占TC总量的8%,其中达到台风等级的个数占64%。而针对此类缺少一定科氏力作用而形成的罕见TC生成的研究相对较少。本文利用JTWC的TC最佳观测资料、ERA-Interim再分析资料,以及NOAA-OISST海温资料,以西北太平洋近赤道TC为研究对象,统计诊断了其年际、年代际、季节分布特征,分析了其大尺度环境背景场,重点探讨了近赤道TC生成的影响因子。研究结果表明,近赤道TC具有明显的年际与年代际变化,并且近赤道TC具有与西北太平洋总TC恰好相反的季节变化。近赤道TC生成的大尺度环境背景场是东北冬季风与其在近赤道地区偏转形成的西北风之间的气旋性环流。对流层低层的绝对涡度动力项与对流层中层的湿度热量项是近赤道TC生成的主要贡献因子,并且相对于5°～10°N生成的TC,近赤道TC对对流层低层的正涡度与对流层中层的湿度条件的要求更高。

Analysis of near-equatorial tropical cyclone genesis in the western North Pacific

70 tropical cyclones(TCs) were formed in the belt within the range of 0°-5°N in the western North Pacific basin during the period of 1979 to 2012,accounting for 8% of the total TCs.In addition,64% of these were intensified to typhoons.However,studies regarding such near-equatorial TCs are rare.In this study,the best track from Joint Typhoon Warning Center,the reanalysis from European Centre for Medium-Range Weather Forecasts interim,and the National oceanic and atmospheric administration optimum interpolation sea surface temperature are used to diagnose the interannual,interdecadal and seasonal characteristics of the near-equatorial TCs.The large-scale circulation and cyclonegesis are mainly investigated.The results show that the near-equatorial TCs have obvious interannual and interdecadal variations and opposite seasonal variations contrast with the total TCs over the western North Pacific.The large-scale environmental fields of near-equatorial TC genesis are characterized by a cyclonic circulation within the northeasterly and northwesterly wind.The dynamic term of absolute vorticity in the lower troposphere and thermal term of relative humidity in the middle troposphere are the main contributors to the formation of the near-equatorial TCs.The higher positive vorticity in the lower troposphere and relative humidity in the middle troposphere are required compared with the TCs formed in the belt within the range of 5°-10°N.