Influence of the interdecadal variation of boreal summer intraseasonal oscillation on the abrupt decrease of multiple tropical cyclones in the western North Pacific
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摘要: 利用台风最佳路径、Bimodal intraseasonal oscillation（bimodal ISO）指数和全球逐日向外长波辐射数据，研究了北半球夏季季节内振荡（Boreal summer intraseasonal oscillation，BSISO）年代际变化造成1996/1997年后西北太平洋群发台风突变减少的可能机制。分析显示，仅有包含3个及以上个数台风成员的“MTC3”群发出现了突变减少，此类台风更倾向在传播速度较慢、低频对流维持时间较长的BSISO活跃位相内出现，对次季节信号强度的要求相对较低。1996/1997年后，BSISO东传范围减小、周期延长、对流活跃位相日数缩短，导致西北太平洋长时间连续维持对流抑制位相，低频对流在145°E以东海域的强度减弱。当偏西海域有先导台风活动时，它向东南侧激发的罗斯贝波频散波列在（5°—20°N，145°—165°E ）海域因没有低频对流耦合而快速消散，导致MTC3群发台风年代际突变减少。
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关键词:
- 北半球夏季季节内振荡 /
- 台风群发 /
- 年代际变化 /
- 突变减少
Abstract: Using best-track data of typhoon, the bimodal ISO index and global daily outgoing longwave radiation data, we study the possible mechanism of the interdecadal variation of boreal summer intraseasonal oscillation (BSISO) that has caused sudden decrease of multiple tropical cyclones (MTC) in the western North Pacific (WNP) after 1996/1997. The analysis shows that only the "MTC3" events, which contain 3 or more typhoons, abruptly decreased after 1996/1997. They are more likely to occur in the BSISO active phase with a slower propagation speed and longer low-frequency convection. Moreover, the MTC3 events have relatively low requirements for the BSISO amplitude. After 1996/1997, the eastward propagation range of the BSISO convection reduced, the period prolonged, and the days of BSISO active phase decreased, resulting in a significant weakening of low-frequency signals to the east of 140°E. Tropical WNP tended to remain in inactive phase for a long time during this period. When a typhoon pre-exists in the WNP, the southeastward-propagating Rossby dispersion wave train casused by it always dissipates without coupled low-frequency convection to the east of 140°E, which leads to the sudden decrease of MTC3 events after 1996/1997.-
Key words:
- BSISO /
- Multiple tropical cyclone events /
- Interdecadal variation /
- Abrupt decrease
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图 1 1979—2014年6—10月西北太平洋生成的STC （a）、MTC2 （b）、MTC3 （c） 和总台风 （d） 数量 （折线） 及 （5°—20°N，145°—165°E ） 区域内25—90 d滤波的OLR序列方差在OLR原始序列方差中的占比 （直方，单位：%）（虚线为折线1979—1996、1997—2014年的均值）
Figure 1. Total number of STC （a），MTC2 （b），MTC3 （c） and all the TCs （d） generated in the WNP from June to October，1979—2014 （solid lines）， and the proportion of the variance of 25—90-day filtered OLR in the original variance over （5—20° N，145—165° E）（bars，unit：%） （the dotted lines show the averages of the solid line from 1979 to 1996 and from 1997 to 2018）
图 2 1979—2014年6—10月STC、MTC2和MTC3台风逐月个数
Figure 2. Monthly number of STC，MTC2 and MTC3 typhoons from June to October，1979—2014
图 3 1979—2014年6—10月西北太平洋逐2.5°×2.5°区域生成的STC （a）、MTC2 （b） 和MTC3 （c） 台风数量
Figure 3. Numbers of STC （a）， MTC2 （b） and MTC3 （c） TCs generated in individual 2.5°×2.5° grids over the WNP from June to October，1979—2014
图 4 1979—2014年6—10月BSISO各位相中生成的STC、MTC2和MTC3台风数量
Figure 4. Numbers of STC，MTC2 and MTC3 TCs generated in each phase of the BSISO from June to October，1979—2014
图 5 STC生成或MTC2、MTC3群发事件开始时刻前后30 d，25—90 d滤波的OLR在120°—160°E （a、c、e）、以及5°—20°N区域内 （b、d、f） 的 Hovmöller图 （单位：W/m2；蓝色点划线表示STC生成或群发事件开始时刻，黑虚线表示群发事件最活跃的5°—20°N区域，括号中数字表示各类事件次数）
Figure 5. Hovmöller of 25—90-day filtered OLR over 120°—160°E （a，c，e） and 5°—20°N （b，d，f） within 30 days before and after the start of STC，MTC2 and MTC3 events （unit：W/m2；blue dotted lines indicate the moment of STC generation or the start of MTC events，black dotted lines indicate the 5°—20° N region where the MTC events are most active，numbers in parentheses indicate the occurrence times of each type of events）
图 6 1979—1996 （a）和1997—2014年 （b） 6—10月25—90 d滤波的OLR序列方差在OLR原始序列方差中的占比以及 （c） P2和P1时段的百分比 （单位：%；散点为各时段MTC3群发台风生成位置，黑色框线表示群发台风密集生成的区域；反斜杠表示负差值显著区域，正斜杠表示正差值显著区域，显著性水平90%）
Figure 6. Proportions of the variance of 25—90-day filtered OLR in the original variance from June to October，1979—1996 （a） and 1997—2014 （b），and （c）the percentage of P2 to P1 （unit：%；scattered dots are the genesis positions of MTC3 TCs in each period，the black box indicates the area of the region where the MTC events are most active；the backslashed area indicates the area with significant negative differences，and the forward slashed area indicates the area with significant positive differences，the confidence level is 90%）
图 7 BSISO第6位相出现开始时刻前后30 d，25—90 d滤波的OLR在120°—160°E （a、b） 以及5°—20°N区域内 （c、d） 的 Hovmöller图 （单位：W/m2；蓝色点划线表示第6位相的第1天，黑虚线表示群发事件最活跃的5°—20°N区域，黄虚线指示OLR≤−8 W/m2区域的东边界，黄色箭头表示对流主体衰减的方向）
Figure 7. Hovmöller of 25—90-day filtered OLR over 120°—160°E （a，b） and 5°—20°N （c，d） within 30 days before and after the start of the BSISO Phase 6 （unit：W/m2；blue dotted lines indicate the first day of phase 6，black dotted lines indicate the 5°—20° N region where the MTC events are most active，yellow dotted lines indicate the eastern boundary of the region where OLR≤−8 W/m2，yellow arrow indicates the direction in which the convection declines most obviously）
图 8 1979—1996年 （a） 和1997—2014年 （b） 6—10月MTC3群发事件开始前后各30 d的25—90 d滤波的OLR在120°—160°E区域内的Hovmöller图 （单位：W/m2；蓝色点划线为MTC3群发事件的第1天，黑虚线为群发密集区南北边界，括号中数字表示该时段MTC3群发事件的次数，散点表示群发中第1—5及后续台风的生成位置及它们各自与本次群发事件中第1个台风的生成天数间隔）
Figure 8. Hovmöller of 25—90-day filtered OLR over 120°—160°E within 30 days before and after the start of MTC3 events （unit：W/m2） from June to October in 1979—1996 （a） and 1997—2014 （b） （blue dotted lines indicate the first day of MTC3 events；black dotted line is the north-south boundaries of the area where the MTC events are most active；the number in parentheses indicate the number of MTC3 group events in each period；scattered dots indicate the genesis locations of the 1st—5th and subsequent TCs，and the intervals of the days between them and the 1st TC）
图 9 研究区域划分 （a）（红色框线表示和MTC3最相关的BSISO活动区域） 及25—90 d的OLR方差占总方差比值序列与MTC3群发台风序列的相关系数和t检验值 （b）（直方为相关系数；折线为t检验值；红虚线为90%显著性水平）
Figure 9. Division of the study region （a）（the red box indicates the area where the BSISO is most related to MTC3 events）， correlation coefficients and t-test values between the ratio of 25—90-day filtered OLR variance to original OLR variance series and the MTC3 TCs series （b） （the bars show the correlation coefficients，the line shows the t-test value，the red dotted line is the 90% confidence level）
图 10 1979—1996 （a） 和1997—2014年 （b） 6—10月的3—8 d的E矢量 （单位：m2/s2）及逐2.5°×2.5°区域中STC和MTC2生成数
Figure 10. 3—8-day E-vectors （unit：m2/s2） from June to October，1979—1996 （a） and 1997—2014 （b）and the number of STC and MTC2 TCs generated in 2.5°×2.5° grids
表 1 1979—1996、1997—2014年6—10月BSISO各位相出现日数及其差值（单位：d）
Table 1. Number of days in each phase of the BSISO and its difference between 1979—1996 and 1997—2014 （unit：d）
1 2 3 4 5 6 7 8 1979—1996年 305 289 374 360 343 333 386 364 1997—2014年 314 344 380 360 329 335 362 330 差值 −9 −55 −6 0 14 −2 24 34 
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表 2 MTC3事件中台风与当次事件中第1个台风生成日之间的平均间隔天数 （括号内为对应的台风数）
Table 2. Intervals of days between the subsequent TCs and the 1st in MTC3 events （number of TCs in parentheses）
1979—1996年 1997—2014年 第2个 2.2（46） 2.4（46） 第3个 5.0（25） 5.2（25） 第4个 9.6（18） 7.9（13） 第5个及以上 20.9（21） 19.0（6）
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表 3 （5°—20°N、135°—170°E）内逐5个经度区域平均的25—90 d的OLR方差占比序列和逐年MTC3群发台风数序列的同期相关系数
研究区域 同期相关系数 显著性水平 135°—140°E 0.060 \ 140°—145°E 0.125 50% 145°—150°E 0.325 90% 150°—155°E 0.304 90% 155°—160°E 0.329 90% 160°—165°E 0.300 90% 165°—170°E 0.057 \
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