【作者】 向洋；

【导师】 杨修群；

【作者基本信息】 南京大学 ， 气象学， 2011， 博士

【摘要】 本文利用1958-2002年NCEP和ERA-40每日4个时次的再分析数据集,首先分析了夏季东亚高空西风急流(East Asian Subtropical Westerly Jet Stream, EAJS)位置与瞬变扰动(Transient Eddy, TE)气候态分布特征以及二者之间的空间配置关系,接着对EAJS位置年际变化与瞬变扰动通量异常之间的关系进行了观测分析,在此基础上,运用季节平均准地转位涡方程对TE异常对大气的影响进行了动力分析,从而揭示出TE异常在EAJS位置变化中的作用。主要结论如下：(1)气候平均而言,夏季EAJS与瞬变活动具有较好的空间配置关系。本文将急流轴的概念引入到EAJS位置变化的研究中,从而有效地将EAJS位置信号提取出来。气候平均而言,夏季EAJS主要位于40°N对流层高层200hPa附近,其轴线呈东西向分布。同时,统计分析表明,夏季瞬变活动也主要分布在40N附近对流层中高层,瞬变扰动最活跃的区域主要集中在沿EAJS轴线狭长的区域内。然而,统计上简单的空间配置关系并不能证明EAJS与瞬变扰动之间存在直接的因果联系。进一步运用简化后的季节平均纬向动量方程诊断发现,瞬变扰动经向动量通量的大值区域沿EAJS轴线分布,由于瞬变扰动动量通量与西风加速成正比关系,这表明,气候平均而言瞬变扰动有利于EAJS的维持,瞬变扰动与EAJS之间存在着直接的动力学联系。此外,观测分析发现夏季大气斜压性最强区域主要位于EAJS正下方和正上方,EAJS上方温度随纬度增加而增加,EAJS下方温度随纬度增加而减小,而中纬度200hPa大气斜压性相对较弱。(2)夏季EAJS位置年际变化特征及与之相伴随的典型异常环流型研究表明夏季EAJS位置的年际变率是夏季EAJS位置变化的强信号。通过对EAJS位置年际变化信号进行EOF分解发现,其主模态空间型反映了夏季EAJS位置具有整体一致南北振荡特征,不仅如此,该方法还揭示出EAJS位置南北振幅具有空间分布不均匀性,陆地上空EAJS振幅小于太平洋上空振幅。其时间系数表明EAJS具有显著的年际变化,我们将该时间序列PC1定义为EAJS位置指数。伴随EAJS南北移动,环流场也相应发生改变。将EAJS位置指数回归到风场上发现,当EAJS偏北(南)时,在气候平均EAJS区域附近伴随有一纬向伸长的反气旋式(气旋式)异常环流,该异常环流基本关于EAJS轴线对称；与之相对应,高度场异常主要表现为在气候平均EAJS区域附近伴随有一纬向伸长的位势高度正(负)变高。从异常环流的垂直结构来看,位势高度异常中心位于200hPa EAJS轴线附近,异常东、西风中心分列对流层高层EAJS的南、北两侧。异常风场和异常位势高度场均具有相当正压结构,它们之间较好地满足准地转关系。将PC1回归到大气斜压场,发现当EAJS偏北(南)时,EAJS北侧大气斜压性增强(减弱),其南侧大气斜压性减弱(增强)。(3)与夏季EAJS年际变化相伴随的瞬变扰动异常中纬度大气运动过程可以用准地转位涡方程来近似描述,在准地转位涡方程中,瞬变扰动与基本流之间的相互作用是通过基本流对瞬变涡度输送和瞬变热量输送的组织以及瞬变涡度输送和瞬变热量输送对基本流的反馈的方式来实现的。气候平均而言,瞬变扰动将瞬变涡度从EAJS主体区域向其两侧输送,从而在EAJS主体区域形成瞬变涡度通最辐散(负瞬变涡度强迫),在其两侧形成瞬变涡度通量辐合(正瞬变涡度强迫)；同时,瞬变扰动将瞬变热量从EAJS主体区域主要向其北侧输送,从而在EAJS主体区域形成瞬变热量通量辐散(负瞬变热力强迫),在其北侧形成瞬变热量通最辐合(正瞬变热力强迫),向南的瞬变热量输送并不明显,因此,瞬变热量输送使EAJS主体区域因失去热量而冷却,其北侧因得到热量而升温。瞬变涡度输送及瞬变热量输送均在主要集中在对流层中上层。当EAJS偏北时,瞬变扰动最活跃的区域也随之移至EAJS轴线北侧,瞬变活动异常一方面将使EAJS轴线北侧因瞬变扰动的增强而失去更多的涡度,从而在EAJS轴线偏北区域形成异常的负瞬变涡度强迫,其南侧和其北侧更远的地区因得到瞬变涡度而形成异常的正瞬变涡度强迫；另一方面,EAJS轴线北侧因异常瞬变扰动将更多的瞬变热量输送至其北侧更远的区域而形成冷异常,其北侧更远的区域因得到热量而形成暖异常。当EAJS偏南时,与上述情况相反。(4)基于简单的准地转模型,利用数值求解方法进一步分析瞬变扰动异常对基本流的反馈,从而揭示瞬变扰动异常在EAJS南北位置年际变化中的作用。大气系统内部瞬变扰动总是与大气内部涡度和热量自动重新分配相伴随,将这种由于瞬变扰而形成的瞬变涡度及瞬变热量源(汇)视为大气的内强迫因子,大气对其的响应即为瞬变扰动对基本流的反馈。当EAJS发生南北摆动时,瞬变扰动也随之发生变化,进而导致大气内部涡度和热量的空间分布也发生异常变化,从而在大气内部形成异常的强迫源。当EAJS偏北时,大气对关于气候平均EAJS轴线反对称分布的异常瞬变涡度强迫产生正压响应,主要表现为：在气候平均EAJS主体区域对流层整层为异常的东风所占据,在其两侧为异常的西风所占据,瞬变涡度所激发的异常风场与EAJS相伴随的异常风场相差π/2位相,这表明瞬变涡度强迫有让EAJS更加偏北的趋势；大气斜压场表现为EAJS北侧大气斜压性异常增强,EAJS正上(下)方,斜压性异常减弱,EAJS南侧斜压性异常增强,即瞬变涡度强迫有利于瞬变扰动在远离EAJS轴线的区域发生发展,不利于瞬变扰动在EAJS轴线附近发展。当EAJS偏南时,与上述情况相反。因此,瞬变涡度强迫在EAJS位置变化中起着正反馈作用,即瞬变扰动始终有时使EAJS位置更加偏北(南)的趋势。当EAJS偏北时,大气对瞬变异常热力强迫的响应主要表现为,在气候平均EAJS主体区域200hPa以下为异常的东风所占据,在其两侧为异常的西风所占据,而EAJS主体区域200hPa以上为东风异常；大气斜压场表现为EAJS正下方对流层中高层斜压性异常减弱,EAJS南侧斜压性异常增强。这表明瞬变热力强迫削弱了EAJS主体区域大气的垂直风切变,即大气的斜压性,最终将抑制EAJS主体区域及其北侧的瞬变扰动活动。当EAJS偏南时,与上述情况相反。因此,瞬变热力强迫在EAJS的南北移动中起负反馈作用。更多还原

【Abstract】 The climatological distributions of East Asian subtropical westerly jet stream (EAJS) and transient eddy (TE) activity, as well as their spatial matching relations, are firstly investigated using NCEP and ERA-404-times daily reanalysis data for the period of1958-2002. Then the spatio-temporal characters of the interannual meridional displacement of the EAJS and the TE activities associated with the EAJS are documented with the observed data. The feedbacks of the TE activities on the EAJS are examined finally by computing the budget of the relevant terms of the seasonal-mean form of the quasi-geostrophic potential vorticity(QGPV) equation. The roles, played by TE activities in the meridional displacement of the EAJS, are revealed finally. The main conclusions are as follows:(1) Climatologically, the spatial distribution of the EAJS matches the maximum of TE activities well during boreal summer.In boreal summer (including June, July and August), the EAJS is located at around40ON in the upper levels of atmosphere, with its axis orientating in west-east direction. Meanwhile, the TE activities are prosperous within an elongated belt in the midlatitudes, and the maximum of the TE activities concentrates in a narrow area along the axis of the EAJS. This coherent spatial distribution could indicated that a close relationship might exist between the EAJS and TE activities. Hence, their relationship is further analysed with the seasonal-mean simplified zonal momentum equation, in which the TE activities may expressed in terms of TE momentum fluxes and the acceleration of time-mean zonal wind is proportional to the convergence of the horizontal momentum fluxes. It is indicated that the distribution of the meridional TE momentum fluxes oriented west-east along the climatological axis of the EAJS. Thus, the the meridional TE momentum fluxes act to accelerate the zonal component of wind.In addition, it is found that the strongest baroclinicity of atmosphere mainly lies in the levels above and under the EAJS, respectively. Among all the levels, at the level near200hPa the baroclinicity of atmosphere is weaker.(2) The interannual meridional displacement of the EAJS in boreal summer and the associated circulations.It is found that the interannual variability of the meridional displacement is the strong signal of the EAJS location variation. By introducing the approach of defining the axis of jet to depict the position of EAJS, the meridional displacement of the EASJ is defined as anomalous deviation from its climatological axis in latitude. A2-8yr band-pass filter is applied to extract its interannual variation. Performing the EOF analysis on the filtered anomalies of axes in latitude then leads to the spatial and temporal distribution of the variation of the EASJ. The results exhibit that in summertime the EASJ axis features a significant longitudinally-unanimous interannual meridional displacement, and the amplitude of the EASJ over the Pacific is larger than that over the mainland. The corresponding principal component time series is defined as the EAJS location index (PC1).Atmospheric anomalous circulation pattern associated with EASJ interannual variations can be identified by regressing the geopotential height and zonal wind against the normalized PC1. It is shown that accompanying the meridional wandering of the EASJ, the regressed geopotential height anomalies with equivalent barotropic structure are found to be in the vicinity of the climatological EASJ axis, and the zonal wind, however, exhibits a dipole pattern asymmetric about the axis. These patterns suggest that a positive geopotential height anomaly near the climatological EASJ axis can result in a westerly acceleration (deceleration) on the poleward (equatorward) side of the climatological EASJ axis, thus leading to a poleward shift of the EASJ; and vice versa. Certainly, the geopotential height and zonal wind anomalies satisfy the quasi-geostrophic relation.Also, atmospheric baroclinicity to the south (north) of the climatological axis of EAJS strengthens (weakens) accompanied by poleward (equatorward) shift of the EAJS.(3) The anomalous activities of the TE associated with the meridional displacement of the EAJS.The atmospheric movement in the midlatitudes can be described by the quasi-geotrophic potential vorticity equation (QGPV). Under the framework of the QGPV equation, the interaction between the TE activities and the time-mean flow in the manner that the time-mean flow organized the TE activities and the TE impacted on the time-mean flow by redistribution of the TE fluxes. Climatologically, the TE transports vorticity from the climatological axis to the poleward and equatorward sides of the EAJS, consequently resulting in a divergence of TE vorticity near the EAJS and a convergence of TE vorticity far away from EAJS. Meanwhile, the TE transports the TE heat from the area near the EAJS to the north side of the EAJS, which leads to a divergence of TE heat near the EAJS and a. convergence of TE heat on the northern side of the EAJS.As the EAJS shifts poleward, the most prosperous area of TE activities moves northward. On the one hand, the anomalous TE activities transport much more TE vorticity from the area near the climatological EAJS’axis to its both sides far away, hence leading to meridional dipole structure asymmetric about the climatological EASJ axis; on the other hand, the anomalous TE activities transport much more TE heat from the area near the climatological EAJS’axis to poleward of the EAJS, consequently resulting in a anomalous cooling in the vicinity of the EAJS, and a anomalous warming to the northern side of the EAJS.(4) Based on the QGPV equation, the feedback of the anomalous TE activities on the EAJS may be examined by solving the numerical solution of the QGPV equation, and the role, played by the TE in the movement of the EAJS, is revealed eventually.The effect of TE on time-mean flow may be approached by treating the convergences of TE fluxes as virtual sources or sinks of heat and vorticity and then seeking for the responses to these sources or sinks. In this study, we examine initial atmospheric responses to the TE forcing anomalies by solving QGPV equation together with appropriate boundary conditions.The geopotential height tendencies induced by TE vorticity forcing anomalies are characterized by a dipole pattern with distinct equivalent barotropic structure. Consequently, under the geostrophic balance, the zonal wind tendencies exhibit a sandwich-like pattern. These patterns indicate that, in accordance with a positive PC1(i.e., a northward displacement of the EASJ), there is a significant positive geopotential height tendency north of the climatological EASJ axis which produces a negative zonal wind tendency (decelerating zonal wind) near the climatological axis and a positive zonal wind tendency (accelerating zonal wind) far north of the axis. By comparing the anomalous circulation regressed on PC1with the initial tendency forced by TE vorticity, it can be easily found that the geopotential height and zonal wind tendencies induced by the TE vorticity forcing possess a phase that meridionally leads the geopotential height and zonal wind anomalies themselves roughly by π/2phase. This suggests that the TE vorticity forcing anomalies act to reinforce further northward progression of the EASJ as it moves northward; and vice versa. Thus, the TE vorticity forcing tends to play a positive feedback role in the interaction between the TE and the time-mean flow.The TE thermal forcing tends to give rise to a baroclinic response. There are negative geopotential tendencies on the poleward side under the axis of the EASJ, and smaller positive geopotential tendencies on the equatorward side beneath the axis of the EASJ. Resultantly, the zonal geostrophic wind tendencies are characterized by westerly acceleration beneath the axis but deceleration above the axis, especially north of the climatological EASJ axis. This suggests that the TE thermal forcing anomalies act to reduce the vertical shear of zonal wind and thus reduce the atmospheric baroclinicity, which eventually suppresses the TE activity, as the EASJ moves northward. Thus, the TE thermal forcing tends to play a negative feedback role in the interaction between the TE and the time-mean flow.更多还原