【作者】 金琼；

【导师】 杨修群；

【作者基本信息】 南京大学 ， 气象学， 2011， 博士

【摘要】 本文首先利用1948-2007年共60年的NCEP/NCAR再分析资料以及1979-2007年的CMAP降水资料,对亚洲夏季风区的降水演变、大尺度环流的水平分布及其垂直结构进行了详细的分析,从而探讨了亚洲夏季风的建立及其推进过程。接着使用NCEP/NCAR冉分析资料倒算出大气视热源,通过研究对流层中高层的纬向海陆温度偏差以及大气非绝热加热的时空特征,寻找它们与亚洲夏季风环流之间的联系。最后,本文通过CCM3模式揭示了东亚大陆的潜热反馈对东亚副热带夏季风环流的建立与维持的贡献,以及它与亚洲热带夏季风环流的维持的内在联系,主要结论如下：1、东亚夏季风环流于气候平均态的第22候(4月16-20日),率先在副热带地区建立,主要表现为高低层经向风的季节反转、经向季风环流的形成以及位势高度场由冬季型的相当正压结构逆转为夏季型的斜压结构。与此同时,正的大气非绝热加热贯通了该地区的整个对流层。对流层高层风场、高度场的季节演变表明,副热带高压的中心在第22候突然山菲律宾以东地区跳至中南半岛上空,导致了东亚副热带地区受高压北侧的西北风控制,经向风由冬季型的南风逆转为夏季型的北风。加上该地区明显的降水,引起了局地的垂直上升运动,第22候,22.5°-30°N范围内的上升运动发展旺盛,达到了对流层项,导致经向Hadley环流的下沉支首先在该地区断裂,配合高低层的经向风的反转,构成了反Hadley的季风环流。位势高度的纬向偏差在冬季表现为东高西低,以120°E为界,海陆上空呈现相当正压结构；在夏季则表现为斜压结构,,中低层为西低东高,对应东亚大陆的季风低压与西太平洋副热带高压,高层为西高东低,对应南亚高压与中东太平洋槽。正好在第22候,纬向偏差环流的结构发生了冬、夏季位相的替换。这些季节演变特征表明东亚夏季风环流的三维结构于第22候,率先在副热带地区建立。此时,对流层中高层(500-200hPa)的纬向热力分布由大陆冷海洋暖的冬季位相过渡到大陆暖海洋冷的夏季位相,它是导致东亚副热带夏季风环流建立的重要机制。第14候开始,我国江南地区(22.5°-30°N,110°-120°E)出现了明显的持续性降水,伴随着显著的凝结潜热释放。凝结潜热由对流活动垂直向上输送,加热该地区的中高层,导致该地区上空的大气逐渐由热汇转为热源,第22候,正的非绝热加热率贯通了东亚副热带地区的整个对流层,这与上述夏季风环流、热力分布的建立时问十分吻合。2、潜热反馈是东亚副热带夏季风环流建立的一个推动力。如果关闭潜热反馈,将延迟该地区的纬向热力对比由冬至夏的季节反转、缩短夏季位相维持的时间。并且,该地区的上升运动很微弱,对流层高层的经向风常年维持南风,导致东亚副热带地区的局地经向季风环流不能闭合。东亚大陆的凝结潜热与副热带地区夏季型热力场的建立有着密切的联系。如果关闭潜热反馈,大大减弱、减慢了东亚大陆(包括青藏高原)由春至夏的增温效应,不但延迟了对流层中高层的纬向海陆热力对比从冬季型的大陆冷海洋暖过渡到夏季型的大陆暖海洋冷,还提前了它山夏季位相向冬季位相的过渡,换言之,缩短了夏季型热力分布维持的时问。在风场上,对流层高层的经向风发生了本质的变化,它几乎在气候态的全年都维持南风,并没有发生山南风转北风的季节转变。由于没有凝结潜热的反馈作用,副热带地区的非绝热加热表现为大范围的负值,即热汇区,该地区的上升运动十分微弱。总之,在气候态的全年,东亚副热带地区的局地季风环流不能够闭合。3、夏季平均而言,关闭东亚大陆的潜热反馈,将破坏局地的位势高度场纬向偏差的斜压结构,并且引起南亚高压的减弱、南退,以及分裂成两个中心带,使得高层盛行偏南风,不能形成闭合的局地经向环流。去潜热的敏感性试验表明,夏季平均状况下,东亚大陆的暖区范围变得狭窄,西北太平洋的冷区侵入我国东部地区,东西向温度梯度比控制试验减小了很多。虽然从宏观上来看,夏季平均的海陆对比表现为西暖冬冷,然而东亚副热带地区的局地位势高度纬向偏差呈现西低东高的分布,海、陆上空均表现为相当正压结构,斜压型受到破坏。关闭潜热反馈,使得对流层低层的副热带高压的位置偏西,东亚副热带地区出现了一个反气旋性环流的异常,从而削弱了该地区夏季风的强度：不仅如此,对流层高层的南亚高压的强度受到明显的减弱,并且具有南退的趋势,更重要的是,南亚高压在东亚副热带地区发生凹陷,分裂成两个中心,受其东部中心带的影响,东亚副热带地区高空盛行偏南风,与控制试验的北风形成鲜明的对比。从而使得经向环流圈的高低层都为南风,赤道以及低纬地区的上升气流在中纬度地区下沉,东亚副热带地区不能形成闭合的经向季风环流。由此可见,潜热反馈是维持东亚副热带夏季环流的必不可少的因子。4、亚洲热带夏季风的建立具有阶段性以及高层纬向风反转的一致性。伴随着索马里越赤道急流的建立,赤道西风逐渐加强北抬。处于印缅槽前的中南半岛、孟加拉湾地区的对流活动发展强盛,造成副热带高压首先在该地区断裂,经向温度对比反转,中南半岛夏季风于第26候爆发,孟加拉湾夏季风随后建立。当副热带高压完全撤离南海地区时,其西部边缘转向的西南风、印缅槽前的西南风,以及来自澳大利亚-印度尼西亚的越赤道气流汇合共同影响南海,意味着南海夏季风的爆发(第28候)。受到印缅槽后下沉运动影响的印度半岛,只有当经向温度偏差发生季节转型后,夏季风才能够建立(第31候)。通过分析200hPa的纬向风的季节演变,我们发现亚洲热带地区比较一致地在第26-27候由西风突变为东风,这主要是由南亚高压的重建引起的。第26候,原本位于菲律宾以东的洋面上的高压中心突然西跳至中南半岛的高空,15N以南,50°E以西的大范围的低纬地区处于高压南侧东风的控制之下,导致该地区的东风几乎同时建立。5、关闭东亚大陆的潜热反馈,减弱了夏季平均的南亚季风的强度,并且在南海地区的低层激发了气旋性环流,该地区对流活动得以发展。关闭东亚大陆的潜热反馈,使得高原附近的热源中心消失,并且大陆的暖区缩小、南移,强度变弱,因此夏季平均状况下,南亚地区的经向海陆热力差异受到显著的减弱。通过分析低层风场的水平特征,我们发现南亚低纬地区出现了大范围的东风异常,从而减弱了该地区夏季盛行的西风带,中南半岛甚至被东北风控制,这些特征预示了南亚季风的减弱,因此该地区的夏季平均降水具有减少的趋势。然而在南海地区,情景却有所不同,关闭潜热反馈在该地区的低层风场激发了一个气旋性环流,使得对流活动发展,降水增多,其释放的凝结潜热山对流活动垂直向上输送,加热中高层的大气,因此在500-200hPa平均的经向温度偏差场上出现了一个相对暖中心,增强了该地区的海陆热力对比,从而有利于夏季风的增强。更多还原

【Abstract】 In this study, NCEP/NCAR reanalysis data for the period1948-2007and CMAP data for1979-2007are employed to delineate the seasonal variation of precipitation, and the3-dimensional structure of atmospheric circulation over the Asian monsoon region, thus to investigate the onset and seasonal march of the summer monsoon therein. And then we analyze the spatial and temporal features of the zonal land-sea thermal contrast on the mid-upper troposphere and the diabatic heating calculated by wind and temperature fields, in order to find their internal linkage with the Asian summer monsoon circulation. With a global climate CCM3, we reveal the contribution of the feedback of latent heating over East Asian continent to the establishment and maintenance of the subtropical East Asian summer monsoon circulation, as well as the summer mean of the tropical Asian summer monsoon circulation. The main conclusions are as follows:1. The general circulation over subtropical East Asia is clarified to transit to the summer phase on pentad22(from16th April to20th April), accompanied with the seasonal overturning of meridional winds both on the upper and lower troposphere, the establishment of meridional monsoon circulation and the transition of zonal geopotential height deviation from the winter quasi-barotropic structure to the summer baroclinic structure. At the same time, the positive diabatic heating penetrates through the whole troposphere.The seasonal march of the geopotential height on the upper troposphere shows that the center of subtropical high abruptly occurs over the Indo-China Peninsular around pentad22from the east of Philippines. These changes cause the northwesterly of the north part of the South Asian High (SAH) prevailing in the subtropical East Asia, and the transition of the meridional wind from winter southerly to summer northerly. In conjunction with the local vigorous ascending motion associated with the rainfall, the descent arm of Hadley circulation breaks up over the subtropical East Asia earliest, and a meridional monsoon circulation which is an anti-Hadley cell is found on pentad22. Furthermore, in winter, the zonal geopotential height deviation shows quasi-barotropic structure with the east higher than the west bordered along120°E; in summer, it performs baroclinic structure that the east is higher than the west on the mid-lower troposphere, according to the monsoon depression and the subtropical anticyclone, and the pattern is out of phase on the upper troposphere, according to the SAH and the mid-ocean trough. It is on the pentad22that the zonal geopotential height deviation shifts from the winter phase to summer. The aforementioned features are indicative that the earliest establishment of the summer monsoon circulation occurs over the subtropical East Asia on the pentad22. At the same time, we find the transition of the zonal thermal distribution over the subtropical East Asian on500-200hPa layer from winter to summer. Before pentad22, the East Asian continent is a cool zone, in contrast to the western North Pacific, but from pentad22on, the continent is under control of a relative warm zone. The zonal thermal contrast is the essential trigger of the subtropical East Asian summer monsoon circulation.It is found that the persistent rainfall occurs over Jiangnan zone (22.5°-30°N,110°-120°E) in south China from the14th pentad, and generates significant latent heating. The latent heating propagates upward accompanied with the convective activities, and warms up the air column on the mid-upper troposphere, leading to this area from heat sink to heat source. It appears an interesting story that it is just on the pentad22when the positive diabatic heating penetrates through the whole troposphere over the subtropical East Asia, and the period is consistent with the establishment of the summer phase of the atmospheric circulation and zonal thermal contrast.2. The feedback of latent heating is a driving force of the establishment of the subtropical East Asian summer monsoon circulation. Removing its affection will delay the seasonal transition of the zonal thermal contrast from winter to summer, and decrease the period of summer phase. And a close monsoon circulation fails to establish, accompanied with weaker ascending motion and southerly on the upper troposphere.The zonal thermal contrast over the subtropical East Asia has a close linkage with the latent heating. If the feedback of latent heating is switched off, it will significantly slow down and weaken the warming process in East Asia landmass (including the Tibetan Plateau) from spring to summer. Thus it not only delays the seasonal transition of the zonal temperature deviation on mid-upper layer from the winter mode of cold land and warm sea to the summer mode of warm land and cold sea, but also makes the overturning from summer to winter earlier. In other words, the summer period is shortened.The prevailing wind on the upper troposphere maintains southerly all year around, without seasonal transition from winter southerly to summer northerly. Without the feedback of latent heating, it shows extensive negative diabatic heating (heat sink) over the subtropical region in the Northern Hemisphere, and weak ascending motion. Generally, a close monsoon circulation can not be found in that sector.3. In summer mean, removing the feedback of latent heating, the geopotential height deviation appears quasi-barotropic structure both over the ocean and continent, and the SAH will move southward and split into two centers with weaker intensity, which lead to the southerly prevailing in the upper troposphere over subtropical East Asia. Consequently, it can’t be found of a local monsoon circulation meridionally.The sensitive experiment without the latent heating feedback is indicative that the warm area in the East Asian continent is narrowed, with the cold zone in the western Pacific intruding to the eastern China, and the zonal temperature gradient is significantly declined, in summer mean. Although, the land-sea thermal contrast performs that the west is wanner while the east is colder in general, the geopotential height deviation performs quasi-barotropic structure, and negative deviation dominates on the mainland of East Asia, while positive value over the subtropical western North Pacific. The local baroclinic configuration fails to form over the subtropical East Asia.Without the feedback of latent heating, it will lead to the subtropical high on the low layer migrating westward and an anticyclonic anomaly over the subtropical East Asia, which imply weakened summer monsoon therein. On the upper layer, the SAH shifts southward with weaker intensity, and more importantly it breaks up to two centers, with the transition belt of the two centers over the subtropical East Asia. Therefore, the upper troposphere over the subtropical East Asia is always under control of southerly, in a sharp comparison with the northerly in the control experiment. In that case, the ascent of the air column over the tropic expels it outwards in the upper troposphere, and then descends over the mid-latitude region. In summer mean, the meridional circulation doesn’t show a close mosoon circulation over the subtropical East Asia. Thus the feedback of latent heating is a necessity for the maintenance of the subtropical East Asian summer monsoon circulation.4、The tropical Asian summer monsoon establishes by stages, while the seasonal overturning of zonal wind in the upper troposphere occurs all together in the four subsystems.The equatorial westerly (?)nces and migrates northward gradually, accompanying with the establishment of Somalia Jet. Since the Indo-China Peninsular and Bay of Bengal are located in front of the Indo-Burma trough, the convection is active and leads to the subtropical high breaking up in that sector earliest and the transition of meridional thermal contrast. The onset of Indo-China Peninsular summer monsoon occurs on pentad26, and propagates to the Bay of Bengal later. Once the subtropical high moves out from the South China Sea (SCS), the SCS is under influences of the southwesterly in the west flank of the subtropical high, in conjunction with the southwesterly in the front of the Indo-Burma trough and the cross-equatorial current emanating from Australia, which is indicative of the onset of the SCS summer monsoon. Since the Indian Peninsular is under control of the descent motion of the Indo-Burma trough, the summer monsoon outbursts only when the transition of the meridional thermal contrast occurs on pentad31.Based on the analysis of the seasonal variation of the zonal wind on the200hPa layer, the easterly is found on pe(?)d26-27in the tropical Asia, which is a consequence of the re-establishment of SAH. The subtropical high abruptly occurs over the Indo-China Peninsular around pentad22from the east of Philippines. In that case, the low latitudes south of15°N, east of50°E is under control of the easterly in the south flank of the subtropical high, and the seasonal transition of the zonal wind occurs all together in the four subsystems.5、Removing the feedback of latent heating, it will decrease the magnitude of the summer mean of South Asian monsoon, and generate a cyclonic circulation on the low layer over SCS, which is favor of convective activities.Without the affection of latent heating, the heating center over the Tibetan Plateau can not be found, and the warm area on the continent is narrowed with southward moving. In summer mean, the land-sea thermal contrast is significantly weakened in the South Asian region. An extensive easterly anomaly occurs on the low layer of the low latitudes in South Asia, which diminishes the prevailing westerly belt in summer. Even the Indo-China Peninsular is under influence of northeasterly. These changes are indicative of the weakening of the South Asian summer monsoon, and decreasing of the monsoon rainfall. But the story is different over SCS. A cyclonic is forced in the low layer of SCS, which leads to strong convection and more rainfall. The latent heating associated with the rainfall propagates upward, and warms up the air column on the mid-upper troposphere. As a result, a warm center over SCS is found in the field of meridional temperature deviation, averaged from500to200hPa, and thus intensifies the land-sea thermal contrast. In general, the SCS summer monsoon is stronger to some extent.更多还原