【作者】 高辉；

【导师】 何金海； 王会军； 薛峰；

【作者基本信息】 南京气象学院 ， 气象学， 2004， 博士

【摘要】 利用NCEP／NCAR再分析资料、南半球海冰密度资料及中国台站降水资料，本文系统分析了南半球大气环流的季节、季节内和年际变化特征及其对东亚夏季风的影响，主要结论归纳如下： (1)对南半球热带外大气环流而言，40°S和65°S是低层大气准半年振荡最为显著的两个纬带，在这两个纬带上，半年波的贡献都超过了70％，二者季节变化的反位相主要体现为半年波分量的反位相。据此，提出用40°S和65°S纬圈平均海平面气压差的半年周期分量的方差贡献作为准半年振荡的强度，该定义较前人的定义更为合理。其次，在南半球冬季，600—100hPa的南太平洋上空西风急流会出现分裂，其中副热带急流中心位于27.5°S，高纬度急流中心位于60°S，前者在200hPa层风速达到极大值，后者风速随高度增加而增加。另外还分析了南半球副热带高压低频振荡的纬向传播，指出马斯克林高压(马高)对澳大利亚高压(澳高)的影响主要通过准双周低频振荡的东传得以实现。 (2)越赤道气流的中心在低层位于925hPa而非850hPa，高层位于150hPa而非200hPa。东半球的越赤道气流是一种典型的季风型越赤道气流，而西半球越赤道气流则为信风型。就半球间空气质量输送而言，夏季输送强于冬季，东半球强于西半球。低层的索马里和南海越赤道气流对南海夏季风的爆发有至关重要的作用，在南海夏季风爆发前2候，索马里急流有一次迅速的增强，这一增强有利于加速孟加拉湾地区西风的向东扩展，并使控制在南海的西太平洋副高东撤；同时，南海越赤道气流的迅速增强也加速副高的北上，共同促使南海夏季风全面爆发。不仅如此，二者对季风爆发的早晚也有重要影响，当前期这两支南风气流建立偏早、强度偏强时，南海夏季风爆发易偏早；反之，当南风气流建立偏晚、强度偏弱时，南海夏季风爆发易偏晚。相关分析和合成分析的结果还显示，春季150hPa亚澳越赤道气流对东亚夏季风有重要影响：当春季该气流偏弱时，北半球夏季西太平洋副高强度偏强，位置偏南偏西，南亚高压强度也偏强，这样的环流背景使江淮流域6—7月降水偏多，华南华北降水偏少；反之，当春季该气流偏强时，夏季西太副高强度偏弱，位置偏北偏东，南亚高压强度也偏弱，江淮流域夏季降水偏少，华南华北降水偏多。 (3)南半球Ferrel环流圈的异常使绕极低压带和副热带高压带的强度出现了反位相的变化特征，即南极涛动。基于此，将标准化后的二者中心纬度(分别为3005和65’S)纬圈平均的海平面气压差的再次标准化值定义为南极涛动指数。研究发现，当夏季南极涛动偏强时，南极极涡和高纬度的西风急流强度都偏强，但中低纬度西风急流的强度偏弱;反之，当南极涛动偏弱时，南极极涡和高纬度西风急流强度将偏弱，中低纬度西风急流强度将偏强。南极涛动对东亚夏季风也有重要的超前影响，当4一5月南极涛动偏强时，夏季马高、澳高和索马里急流强度也将偏强，东亚夏季风强度偏弱:反之，当南极涛动偏弱时，马高、澳高和索马里急流强度偏弱，但东亚夏季风偏强。此外，南极涛动对我国江淮夏季降水尤其是梅雨有重要影响。当前春尤其是5月南极涛动偏强时，江淮地区梅雨量偏多，出梅偏晚，梅雨期长;而当南极涛动偏弱时，江淮地区梅雨量偏少，出梅偏早，梅雨期短。因此，南极涛动是一个能够对中国夏季降水尤其是梅雨异常产生重要影响的年际变化强信号，对江淮梅雨的预报也有重要意义。研究还发现，南极海冰密度变化对南极涛动有6个月的超前负相关。前期南极海冰密度异常偏高 (偏低)会使6个月后的南极涛动异常偏弱(偏强)。这为中国夏季降水尤其是梅雨的预报提供了一条可行的新途径。更多还原

【Abstract】 Based on the NCEP/NCAR reanalysis products, the sea ice concentration data and the observation rainfall data in China, the seasonal, intraseasonal and interannual variations of the atmospheric circulation in the Southern Hemisphere (SH) are systematically analyzed together with their influences on East Asian Summer Monsoon (EASM). The major conclusions are summarized as follows:(1) In the lower level of the troposphere, the semi-annual oscillation (SAO) is most active along 40 S and 65 S in the extratropics of the SH. Over these latitudes, the variance percentages of SAO exceed 70%, and the anti-phase variation of the sea level pressure (SLP) between the two latitudes is primarily caused by their SAO components. A more reasonable SAO index is therefore defined as the SAO variance percentage of the difference of the zonally averaged SLP between 40°S and 65°S. In addition, in the austral winter, the westerly jet splits into two branches over the South Pacific from 600 to 100hPa, with the subtropical branch located at 27.5 S and the polar counterpart at 60 S. The maximal speed of the former is at 200hPa while the speed of the latter increases with height. Furthermore, the zonal propagation of the intraseasonal oscillation (ISO) of the SH subtropical high is also documented. Results show that the Australia high (AH) is influenced by the Mascarene high (MH) through the eastward propagation of the quasi-biweekly oscillation.(2)In the lower level of the troposphere, the center of the cross-equatorial flows (CEFs) is located at 925hPa other than 850hPa while it is at 150hPa instead of 200hPa in the upper level. The CEFs in the Eastern Hemisphere (EH) show monsoonal features while they are characterized by the trade wind in the Western Hemisphere. The mass transport between the Northern Hemisphere (NH) and the SH is stronger in boreal summer especially in the EH. The Somali jet and the South China Sea (SCS) CEF in the lower level play an important role in the onset of the South China Sea summer monsoon (SCSSM). The Somali jet is rapidly enhanced two pentads prior to the onset of SCSSM, thus accelerating the eastward extension of the westerly over the Bay of Bengal and leading to the eastward retreat of the western Pacific subtropical high (WPSH). At the same time, the rapid enhancement of SCS CEF can also result in the northward march of WPSH and the onset of SCSSM. The earlier establishment and stronger southerlies correspond to the earlier onset time of SCSSM, and vice versa. Both composite and correlation analyses show that the 150hPa Asia-AustraliaCEF (AACEF) in boreal spring has important influences on EASM. When AACEF is weaker, the summer WPSH tends to be stronger with a southwestward extension, and the South Asia high (SAH) will be stronger too. This circulation pattern will lead to more rainfall in the Yangtze and Huaihe River valley and less rainfall outside of this region. On the contrary, the opposite pattern occurs when the AACEF is stronger.(3) On interannual timescale, the changes in the SH Ferrel cell lead to the anti-phase variation of the subtropical high and the circumpolar low, i.e., the Antarctic oscillation (AAO). For this reason, the AAO index (AAOI) is defined as the normalized zonal mean SLP difference between 30 S and 65 S. The result indicates that when AAO is stronger in boreal summer, both the Antarctic vortex (AV) and the polar westerly jet (PWJ) are stronger while the subtropical westerly jet (STWJ) is weaker, and vice versa. Besides, AAO has a leading influence on EASM. When AAO is stronger in April and May, the MH and AH together with the Somali jet are stronger in summer, the intensity of EASM tends to be weaker, and vice versa. Corresponding to the stronger AAO in boreal spring, especially in May, there is more rainfall with a longer period of mei-yu along the Yangtze and Huaihe River valley. In contrast, there is less rainfall with a shorter period when AAO is weaker. AAO is therefore an important interannual signal influencing summer rainfall especially the mei更多还原