MJO与西北太平洋热带气旋活动的关系及其年代际变化

THE MODULATION AND DECADAL CHANGE OF MADDEN-JULIAN OSCILLATION ON TROPICAL CYCLONES IN THE WESTERN NORTH PACIFIC OCEAN

利用澳大利亚气象局RMM指数(Real-time Multivariate MJO Index)和美国联合预警中心(Joint Typhoon Warming Center)的最佳台风路径资料以及NCEP/NCAR逐日和逐月平均的再分析资料,分析研究西北太平洋热带气旋(TC)的年代际变化及其与MJO之间的关系。结果表明,相比1979—1997年时段,1998—2010年时段的西北太平洋TC生成频数发生显著减少的改变,且TC频数减少主要出现在MJO活跃位相(4、5、6、7位相)中。通过对1979—1997年和1998—2010年两个时段的气候背景与MJO传播进程比较分析发现,1998—2010年时段TC频数显著降低,主要由于赤道太平洋海表温度La Ni?a型的冷却,导致该时段Walker环流增强,从而使得西北太平洋地区产生底层东风异常、负相对涡度增加、海平面气压升高、垂直风切变加大等抑制TC生成的大尺度环流条件。同时,在此气候背景下的MJO周期缩短、活跃持续日数减少、部分活跃位相低频风场出现东风异常和对流活跃面积收缩,也可能进一步导致TC频数在上述两个时段的较大差异。

This study focuses on the decadal variability of tropical cyclone （TC） on western North Pacific （WNP） and how this change is related to the Madden-Julian Oscillation （MJO）, based on a set of data including the RMM （Real-time Multivariate MJO） index from RMRC （bureau of meteorology） of the Centre for Australian Weather and Climate Research, the best tracks from the Navy Joint Typhoon Warming Center（JTWC）, and daily and monthly data of the NCEP/NCAR reanalysis. The result shows that TC frequency in WNP exhibited a statistically significant decrease during the period of 1998--2010, compared to the period of 1979--1997. The decrease on TC frequency in WNP mainly occurred during active MJO phases （phases 4, 5, 6, 7）. Comparisons of the climate background and the propagation differences of MJO between 1979--1997 and 1998--2010 concluded that the La Nina-like tropical sea surface temperature＇s cooling leads to a stronger Walker circulation and induces unfavorable atmosphere condition for TC in WNP, which includes positive low-level easterly anomaly, negative relative vorticity anomaly, increased sea level pressure and strong vertical wind sheer. Meanwhile, the shortened MJO cycle, declined active phase duration in WNP, appearance of easterly anomaly and shrunk convective area in part of MJO＇s active phases may also be causes for this decadal variation of TC.