【作者】 刘伯奇；

【导师】 何金海； 吴国雄；

【作者基本信息】 南京信息工程大学 ， 气象学， 2013， 博士

【摘要】 本文利用多套再分析资料和卫星观测资料,以春夏转换期(4-6月)南亚高压的活动特征为切入点,着眼于南亚高压的形成发展及其对亚洲夏季风爆发过程的影响,重点讨论了亚洲夏季风爆发前后高、低空环流的耦合过程及其相关的动力学机制,研究的落脚点是以南亚高压的形态变异为桥梁,将大气外强迫信号和内部动力过程相结合,进而建立亚洲夏季风爆发过程中各个子系统之间的联系。主要结论如下：(1)阐明了4-5月南亚高压的形成机理及其对孟加拉湾季风爆发性涡旋的影响。研究指出,在孟加拉湾夏季风爆发前约2周,菲律宾南部地区的对流迅速发展,其释放的凝结潜热能够在加热区北部的对流层高层制造一负涡度源。在该负涡度源的强迫下,南亚高压形成于南海上空。随后,中南半岛北部的局地降水令南亚高压西伸至孟加拉湾北部上空,造成了孟加拉湾南部的高层“喇叭口”状流场,对应明显的高空抽吸作用和上升运动,从而有利于孟加拉湾季风爆发性涡旋的形成。另一方面,孟加拉湾春季暖池为季风爆发性涡旋的形成提供了有利的下垫面条件和能量供应。因此,高层的抽吸作用和低层的暖池共同触发了孟加拉湾季风爆发性涡旋的形成,加强了孟加拉湾地区的表面西南气流。(2)揭示了5月初亚洲夏季风在孟加拉湾东部爆发后,无法直接向西传播至印度半岛和阿拉伯海的可能原因。这种亚洲夏季风无法从孟加拉湾直接西传至印度地区的现象称为亚洲夏季风的“爆发屏障”。孟加拉湾夏季风爆发后,季风对流释放的潜热造成低层气旋、高层反气旋的环流分布,在加热区以西的孟加拉湾西部至印度半岛东岸附近产生垂直南风切变,对应印度半岛上空的下沉气流,一方面加强了局地低空反气旋,不利于低层副高脊线的向南倾斜和西风气流的发展,进而减弱了印度地区的垂直东风切变,阻碍了夏季风的向西传播；另一方面该下沉气流增强了印度半岛的陆面感热加热,在地面气温纬向平流的作用下,孟加拉湾西部的印度沿岸地区形成表面气温增暖中心。此外,季风对流使孟加拉湾海表温度迅速下降,海表温度冷却中心和上述表面气温增暖中心的共同作用令孟加拉湾西部的大气柱从下表面失去大量热量,进一步抑制了孟加拉湾西部季风对流的形成发展,阻碍了亚洲夏季风的向西传播。(3)指出了强迫惯性不稳定在印度夏季风爆发过程中的重要作用。在印度夏季风爆发过程中,由于强烈的跨赤道气压梯度,对流层低层的绝对涡度零线(η=0)在阿拉伯海南部上空自赤道向北推进,从而在北半球近赤道区域形成负绝对涡度区,该区域表现出明显的惯性不稳定。当气流自南向北通过这一区域时,一方面位于其北侧的低层辐合中心逐渐北抬,另一方面在η=0线北侧,低空西风气流加速,造成明显的纬向动量平流。解析和诊断分析表明,该纬向动量平流可引起低空辐合中心向负绝对涡度区北部的偏东北方向漂移,说明除了边界层动力过程,阿拉伯海地区低层环流对强迫惯性不稳定的响应还取决于纬向非均匀的低空气流。此外,在印度夏季风爆发前,对流层高层的南亚高压东伸发展,将中纬度高位涡输送到阿拉伯海上空,形成局地“喇叭口”状流场,产生明显的抽吸作用,为夏季风的爆发推进提供了有利背景,而与强迫惯性不稳定有关的低空辐合中心北上则是夏季风爆发的触发因子。(4)讨论了ENSO事件对孟加拉湾和印度夏季风爆发期间高、低空环流耦合过程的影响及相关的动力机制。结果表明,在El Nino次年4月,印度洋和中东太平洋海温异常偏高,而西太平洋海温异常偏低。一方面,异常Walker环流的下沉支使西太平洋附近的赤道辐合带偏南,减弱了菲律宾南部的对流活动,对流被约束在近赤道的海洋大陆附近。①在对流层高层,南亚高压形成于1O°N以南的苏门答腊群岛北部,较气候态偏南,与不利于孟加拉湾南部高空辐散形势的形成。②在对流层低层,印度洋的暖海温异常与西太平洋的冷海温异常造成了孟加拉湾南部的异常低层东风,削弱了近赤道西风,令低空气旋式环流减弱,阻碍了季风爆发性涡旋的形成和发展。因此,El Nino事件能够通过减小孟加拉湾南部地区的高空辐散和低空气旋,抑制孟加拉湾季风对流的建立和发展,从而使得夏季风的爆发时间推迟。而在El Nino次年5月,受抑制的孟加拉湾季风对流不利于南亚高压西伸至阿拉伯海北部,减弱了阿拉伯海南部的高空辐散流场和抽吸效应。同时,由于阿拉伯半岛北部感热偏弱,不利于中层副热带反气旋的发展,减弱了阿拉伯海上空的南风输送和对称不稳定。此外,西印度洋的海温经向梯度减小了自南向北的跨赤道气压梯度,使强迫惯性不稳定偏弱,相应地索马里跨赤道气流和阿拉伯海上空的低层辐合也偏弱。可见,在与El Nino有关的高、中、低层流场的共同作用下,阿拉伯海地区上升运动偏弱,不利于季风对流的建立,印度夏季风爆发偏晚。La Nina事件的作用与El Nino相反。因此,ENSO事件能够通过影响高层南亚高压的形态、中层副热带反气旋的特征和低层惯性不稳定的强度,调控着印度夏季风的爆发早晚。更多还原

【Abstract】 Multiple reanalysis datasets and satellite observation were used to investigate the formation and pattern variation of South Asian High (SAH) from April to June and their influence on the onset processes of Asian Summer Monsoon (ASM). We started with the SAH evolution during the transition from spring to summer (April-June), and emphasized the vertical coupling of circulation in the upper and lower troposphere during the ASM onsetto reveal the related dynamical mechanisms. Thus the atmospheric external forcing and internal dynamics could be integrated together via the SAH pattern variation, which bridges the intrinsic associations among ASM subsystems finally. The primary conclusions are as follows.(1) The mechanism for SAH genesis from April to May is investigated to reveal the SAH effect on the Monsoon Onset Vortex (MOV) over the Bay of Bengal (BOB). Results show that the convection is developing rapidly over the southern Philippines on2weeks before the BOB summer monsoon onset. The released condensation heating could produce a negative vorticity source in the upper troposphere. Such negative vorticity source is steady before the BOB summer monsoon onset, and could force the SAH to form over the South China Sea (SCS), manifesting a Gill-type response. Subsequently, the local rainfall over the northern Indochina Peninsula leads the SAH to expand westwards to the northern BOB, inducing a trumpet-shaped flow field over the southeastern BOB. As a result, the evident divergence-pumping and strong atmospheric ascending is formed to favor the BOB MOV formation. Furthermore, the spring warm pool over the BOB provides energy to support the MOV formation. Therefore, both the upper pumping and lower warm pool trigger the BOB MOV together, and then the surface southwesterly is strengthened.(2) The ASM onset cannot propagate directly westwards to the India Peninsula and Arabian Sea in early May after it commences over the BOB. This phenomenon is named as the ASM "onset barrier", which is probably attributed to the atmospheric response and local land-air-sea interaction related to the latent heating realeased by BOB summer monsoon convection. After the BOB summer monsoon onset, the released latent heating brings on a cyclone in the lower but an anticyclone in the upper troposphere. Therefore, a vertical southerly shear appears near the western BOB and eastern coast of India Peninsula, accompanied by the local descending. When the downward flow arrives at the surface, the lower anticyclone is enhanced over the India to inhibit the ridgeline of subtropical anticyclone from titling southward. Thus the local vertical easterly shear is reduced to prevent the westward advancing of ASM onset. Moreover, the sinking flow over the India Peninsula could increase the land surface temperature to strengthen the land surface sensible heating. Then a warming center is formed over the west BOB under the influence of zonal advection of surface air temperature. Meanwhile the SST is decreased by monsoon convection to generate a cooling center over the BOB. The co-effect of warming air temperature and cooling SST makes the atmospheric column lose a lot of heat energy over the western BOB, suppressing the formation and development of monsoon convection to block the ASM onset to the west of BOB.(3) The forced inertial instability over the Arabian Sea plays an important role in the Indian summer monsoon onset. During the Indian summer monsoon onset, the strong crossing-equator gradient of surface pressure exists over the mid Arabian Sea, pushing the zero contour of absolute vorticity northwards to form an inertial instability region near the equator in the north hemisphere. As the air flow passing through this region, a lower convergence center to the north moves poleward gradually. Simultaneously the lower westerly is accellarating to form an evident zonal advection of momentum to the north of η=0. The analytic and diagnosing results point out that the zonal momentum advection could lead the lower convergence center to shift northeastward to the north of negative absolute vorticity area. It’s implied that the atmospheric response to the forced inertial instability over the Arabian Sea is determined by the zonal asymmetric distribution of lower flow except for the dynamical process in the boundary layer. Besides, the South Asian High is extending eastward with high potential vorticity (PV) vortex transported to the Arabian Sea in the upper troposphere prior to the Indian summer monsoon onset. Thus a "trumpt-type" stream field is generated to cause the strong upper pumping effect, providing a favorate background to the onset and advancing of summer monsoon. While the northward movement of lower convergence due to the forced inertial instability triggers the Indian summer monsoon onset.(4) The ENSO effect on the vertical coupling of circulation in the upper and lower troposphere from April to June is discussed to understand the interannual variability of ASM onset process. In April ensuing the El Nino events, the sea surface temperature anomaly (SSTA) is warm over the Indian Ocean and central-eastern Pacific, but cold over the western Pacific. On the one hand, the anomalous sinking of Walker circulation is located over the western Pacific, leading the Inter-tropical convergence zone (ITCZ) to stay on south of its climatological position. Therefore, the convection is weakened the convection over the south Philippines to result in the SAH formation over the northern Sumatra. At this moment the SAH is situated to south of the climate-mean state, weakening the upper divergence over the southern BOB. On the other hand, an anomalous lower easterly is established over the southern BOB due to the warm SSTA over the Indian Ocean and cold SSTA over the western Pacific. Thus the near-equatorial westerly is weakened to decrease the lower cyclonic vorticty, precluding the BOB MOV from generating and developing. Accordingly, the El Nino events can suppress the BOB monsoon convection and postpone the onset time of BOB summer monsoon by diminishing the upper divergence and lower cyclone circulation over the southern BOB.While following El Nino events in May, the suppressed BOB monsoon convection inhibits the SAH from expanding westward to the northern Arabian Sea, decreasing the upper divergence-pumping effect over the southern Arabian Sea. Meanwhile the weakened land surface sensible heating over the north Arabian Peninsula is adverse to the subtropical anticyclone development in the middle troposphere. Furthermore, the declined meridional gradient of SST over the western Indian Ocean decreases the cross-equatorial gradient of surface pressure, weakening the inertial instability over the Arabian Sea. Then both the Somali cross-equatorial flow and the lower convergence are abated in the Indian summer monsoon region. After the El Nino events, the cooperation of abnormal circulation on different levels contribute to the anomalous descending over the Arabian Sea, inhibiting the formation of monsoon convection and delaying the Indian summer monsoon onset. The situation in the La Nina category is opposite. Consequently, the ENSO events can modulate the onset time of Indian summer monsoon via influencing the SAH pattern in the upper, the feature of subtropical anticyclone in the middle and the strengthen of inertial instability in the lower troposphere.更多还原