西北太平洋热带气旋活动的年际变化及其机理研究

【作者】 陈光华；

【导师】 黄荣辉；

【作者基本信息】 中国科学院研究生院（大气物理研究所） ， 气象学， 2007， 博士

【摘要】 鉴于西北太平洋暖池是全球海洋最暖海域,也是全球海-气相互作用最剧烈之处。因此,本论文以西北太平洋暖池次表层海温异常作为西太平洋冷暖状态的指标,来划分西太平洋热状态的差异,通过资料分析,动力诊断和数值模拟系统地研究了西北太平洋TC和台风活动的年际变化及其机理。得到如下主要结论:1.从1958～2003年西太平洋暖池次表层海温与WNP区域TC活动之间的关系分析表明:当西太平洋暖池处于热状态时,热带西太平洋季风槽位置偏向于西北侧,使得上升支向西北偏移,这使得西北太平洋TC多生成于偏西北海域,从而导致TC经常在我国东南沿海登陆;相反,当西太平洋暖池处于冷状态时,情况正好相反,季风槽区向WNP东南侧延伸,使得环流上升支以及TC平均生成位置向东南侧转移,此时生成的TC移动路径向西北移动时容易发生朝向东北的转向,从而易于影响日本及其偏东地区。2.本研究还把WNP划分4个不同的区域,以此深入研究热带西太平洋不同区域中TC生成的动力作用。分析结果表明:在WNP的西南区域,低层纬向风的纬向辐合对TC生成数量的年际变化起着重要的动力作用;而纬向风的经向切变对热带西太平洋的西北和东南区域TC的生成的动力作用显著;纬向风垂直切变与热带西太平洋的西南和东南区TC数量的年际异常有明显相关。当西太暖池处于暖状态时,可以激发出类似PJ-EAP沿东亚向北美传播的波列,波列在WNP西北侧出现气旋式环流异常,使得在此区域TC异常偏多;相反,当西太平洋暖池处于冷状态时,WNP西北侧则出现反气旋式异常环流,而异常气旋式环流出现在WNP东南侧,从而使得在WNP东南侧的TC异常偏多。3.大部分热带西太平洋偏低纬TC的形成与来自赤道中东太平洋3～10天的天气尺度波动存在密切关系。在中东太平洋赤道地区,西传天气尺度波动的主要模态为MRG波动,它可以在季风槽南侧的纬向风辐合区发生波长缩短与波能累积的物理过程,并且此波动还逐渐偏离赤道而转向西北,转变为对流和气旋环流中心紧密耦合的热带TD型扰动;这种扰动呈现出东北-西南倾斜的结构特征,可以通过与季风槽气旋环流的相互作用,获得动能和热能从而得到发展,有利于最终诱发TC的形成。此外,研究结果还表明两类波动转变的位置存在着年际变化,在西太暖池偏暖时,季风槽偏西,MRG波向TD型波动转变的平均位置偏西;而当西太暖池为冷状态年时,季风槽东扩,使得两类波动转变位置也向东偏移。这种波动转变位置的东西偏移,在一定程度上调制了TC生成的平均经向位置。4.热带太平洋上空大气的30～60天的低频振荡(MJO)对WNP区域TC生成起到调制作用。当WNP西侧为MJO的西风位相时,通过高低频正压能量的转换,使得在此区域生成的TC数量相对于东风位相要明显的偏多,而且随着西风位相中西风的加强(东风位相中东风的加强),TC的生成概率将得到增加(减少);并且分析结果也表明了在WNP东侧海域,MJO对TC活动的调制作用要减弱许多。此外,MJO经向的传播也可以使得WNP季风槽的槽线位置发生南北的偏移,从而对TC生成的平均位置产生调制。此外,在西太暖池处于暖年时,WNP西侧的MJO活动频繁,西风位相活跃,从而有利于此区域TC的生成,而冷年的情况正好相反。5.通过全球大气环流模式ECHAM5进行的控制试验和敏感性试验,结果表明,模式可以较好的模拟对海温响应的热带西太平洋季风槽的位置分布。虽然由于模式分辨率和物理过程等原因造成模拟TC的数量要明显偏少,但是其地理位置和季节分布比较合理。敏感试验也表明,对应于暖池偏暖(冷)状态,季风槽系统位置偏西(东),从而使得TC的平均生成位置偏西(东)。更多还原

【Abstract】 Considering the warm pool (WP) in western North Pacific (WNP) is the place with not only the warmest water but also the strongest air-sea interaction in the globe, the subsurface temperature anomaly in the WP is identified as index to quantitatively define thermal state of the WNP. The interannual variation in tropical cyclones (TCs) and its mechanism is systematically explored in this dissertation using data analysis, dynamic diagnosis and numerical simulation. The follows are the main results:1. The relationship between subsurface temperature in the WP and TCs activity for the period 1959~2003 show that, during the warm state years in the WP, monsoon trough location shifts northwest which is accompanied by the shift of anomalous convection and cyclonic circulation center, hence more TCs tend to form at higher latitudes and further westward while the subsidence branch is located in the southeast quadrant to suppress TC genesis there. The consequences in cold years are reversed. Anomalous westerly and monsoon trough are likely to penetrate eastward and southward, so that more TCs form at that quadrant and tend to recurve northeast.2. To assess dynamic impacts of monsoonal circulation on TCs formation in different areas over the WNP, the WNP is divided into four domains. In the southwestern part of the WNP, the interannual variation in TCs number is attributed to the convergence of zonal wind in low troposphere; the latitudinal shear of zonal wind plays an important role in cyclogenesis at the northwestern and southeast quadrants over the WNP; the vertical shear of zonal wind is well correlated with annual TCs number in the southwest and southeast of the WNP. During the warm state in the WP, the wave train like PJ-EAP is induced to propagate from the East Asian to North America, accompanied by anomalous cyclonic circulation lying in the northwest of the WNP, which is favorable for cyclogenesis in this domain. On the contrary, during the cold state years, the circulation pattern is characterized by anticyclonic anomaly in the northwest of the WNP and cyclonic anomaly in the southeast of the WNP, which leads to more TCs formation at the southeast quadrant of the WNP.3. A large number of TCs forming in low latitude over the WNP is associated with 3~10 day filtered disturbances with synoptic scale. The main mode represents Mixed Rossby-gravity waves (MRG waves) in the middle and eastern Pacific equatorial area. MRG waves can experience the physical process with wavelength reduction and energy accumulation under the influences of convergence of zonal wind to the south of monsoon trough. Meanwhile, MRG waves gradually deviate from the equator and propagate northwestward, evolving into tropical-depression-type disturbances (TD-type disturbances) with cyclonic center tightly coupled with convection. The disturbances possess northeast-southwest tilt structure, which is conducive to obtain kinetic energy and heat energy to develop into TCs through the interaction with monsoon trough flow. In addition, it is found that the regional variation of low tropospheric wave transition is well correlated with the thermal states in the WP. When the WP is in the warm phase, monsoon trough lie in the west of the WNP, which induces the westward shift of the waves transition point along the equator. On the contrary, during the cold phase of the WP, the active convection and monsoon trough extend eastward further to the central Pacific, which produce eastward displacement of wave transition location. The interannual variation in TCs formation location is partly ascribed to regional variation of transition of tropical waves.4. The Madden-Julian Oscillation (MJO) can modulate the TCs activity over the WNP. When MJO westerly prevails in the west of WNP, TCs is likely to form in that area through the transition of barotropic energy in the high and low frequency waves, compared with MJO easterly. The probability of cyclogenesis should increase (reduce) with enhanced MJO westerly (easterly). However, the effects of modulation of MJO would be reduced in the eastern part of the WNP. Besides, the latitudinal shift of MJO could change the meridional location of monsoon trough, and modulate TCs mean location. In addition, during the warm years of the WP, MJO event is frequent and westerly is active in the west of the WNP, which benefits to TCs formation, while the situation is reversed in the cold years of the WP.5. The atmospheric model ECHAM5 is used to conduct the control and sensitivity simulations, and the results suggest that model has good skill in simulating geographic distribution of monsoon trough over the WNP responding to sea surface temperature. Despite the reduction of simulated TCs number due to coarse resolution and deficiency in parameterization process, the TCs geographic and seasonal distribution is reasonable. The sensitivity simulations show that the shift westward (eastward) of monsoon trough, which corresponds to the warm (cold) state in the WP, can cause the mean location of TCs genesis westward (eastward).更多还原