【作者】 池艳珍；

【导师】 管兆勇； 何金海； 张福青；

【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1979-2010年NCEP的CFSR再分析资料、NCEP的CMAP降水资料、NOAA向外长波辐射资料(OLR)、降水资料(GPCP)、澳大利亚气象局MJO监测指数及日本“数字台风”网站数据等资料,采用合成分析及波动时空滤波等方法,分析了东亚副热带夏季风建立与撤退的气候学特征及年际变化,主要结论如下：1)气候平均东亚副热带夏季风建立或大气环流由冬季型向夏季型转换完成的时间为第24候,最显著的环流变化特征是东亚副热带区域对流层高层由南风转换为北风,与低层早已出现的南风配合,副热带地区出现强烈上升运动,南侧南海地区为下沉区,形成了局地季风经圈环流。因此将关键区平均的经向风垂直切变(925hPa减去200hPa)由负稳定转正的时间定义为东亚副热带夏季风的建立时间。进一步分析发现南亚高压西部中心在中南半岛的重建很可能是造成这种逆转的主要原因。2)气候平均东亚副热带夏季风撤退的时间为第52候。主要特征是：一是越赤道水汽输送的向南回撤,区域内水汽供应减少导致强降水迅速撤出；二是不断增强的大陆冷高压南压,改变海陆之间的热力对比,夏季海平面气压场上“东高西低”或温度场上“东冷西暖”分布转换为“西高东低”或“东暖西冷”,同时大陆高压前端分裂出的小冷高压南下,影响低层经向风；三是垂直对流运动迅速减弱南撤,低层的南风转为北风、高层则北风逐渐向南风过渡；四是热带气旋的形成、发展和移动对高低层经向风的转换起到促进作用。3)确定了1979～2010年东亚副热带夏季风建立(撤退)的时间序列。发现均具有显著的年际振荡现象,多年平均的建立(撤退)时间为22.3(51.3)候,标准差为2.8(3.0)候。最早建立(撤退)时间为15(46)候,最晚建立(撤退)时间为26(58)候。建立时间以19候、23候和24候的频次最大；撤退时间以51候和52候频次最大。4)揭示了建立与撤退过程异常年的环流差异。建立偏早年(偏晚年)高层南亚高压位于中南半岛东南端(西端),东亚副热带西风急流强劲(弱)且轴线偏南(偏北),垂直上升运动区域范围较小(大),经圈环流中心高度位于中(高)层。撤退偏早年(偏晚年),热带地区对流异常旺盛(相对较弱),东亚高层110°E处存在深(浅)槽区,区域内高层偏南风偏强(偏弱)、西风急流的强度弱(强)。5)揭示了热带MJ0对流活动可促发东亚副热带夏季风的建立。当热带MJO活动从西印度洋(40°E附近)传播到中东印度洋(80°E附近)时,深对流所释放的凝结潜热加热大气,通过"Matsuno-Gill"效应产生东传Kelvin波,导致对流层高层海洋大陆、中南半岛直至南海出现辐合,从而引起南亚高压在中南半岛南部的建立,而该区域的辐合作为正涡度源强迫其北侧的副热带地区产生高度场负距平,最终导致东亚副热带夏季风建立和大气环流从冬季型向夏季型转换的完成。因而将高低层风向转换、南亚高压在中南半岛的重建、热带MJO活动和副热带夏季风建立有机地联系在一起。6)揭示了中高纬冷高压和低纬热带气旋活动对副热带夏季风撤退的影响特征。冷高压改变海陆热力差异和低层风向而热带气旋通过改变高低层风向从而影响季风的撤退。有(无)热带气旋影响年季风撤退时热带地区对流异常旺盛(相对较弱),上升运动迅速(缓慢)退出副热带地区,副热带地区反向的季风经向环流中心高度较高(较低)；西太平洋副高西部脊不断北抬(脊线偏南),南半球越赤道西南气流较强(略弱)且向东能输送到西太平洋(在南海转向)；南亚高压西退(南压),东亚110°E的高空槽加深(偏浅),副热带区域位于高空槽前,盛行西南风(偏西风),经向风转换明显(不明显),高空西风急流强度略弱(强)。1991年副热带夏季风撤退过程的主要影响因子是热带气旋的发展和西移。更多还原

【Abstract】 Based on the6-hour NCEP Climate Forecast System Reanalysis (CFSR) dataset, the daily NOAA Outgoing Longwave Radiation (OLR), the pentad NCEP Climate Precipitation Center Merged Analysis of Precipitation (CMAP), the pentad and daily NOAA precipitation (GPCP), the daily RMMs of MJO from Australian Meteorological Administration and the digital-typhoon dataset from1979to2010, onset-relative and withdrawal-relative composite analysis and the space-time wave filtering with respect to the composite anomalies are performed to examine the features of onset and withdrawal of East Asian subtropical summer monsoon (EASSM)(over110°-120°E,20°-30°N) and its interannual variations. The main results are as follows:1) It is shown that the climatological onset of EASSM or the earliest seasonal transition from winter pattern to summer pattern over East Asia occurs on the24th pentad with the most significant feature of the reversal of meridional wind in the upper troposphere and the formation of Ferrel-like monsoonal meridional flow. The reversal of the focus domain-averaged meridional wind shear (MWS) between925and200hPa from negative to persistent positive is thus used as the indice to best define the EASSM onset. Further analysis indicates that the vertical meridional flow reversal is most likely attributed to the reestablishment of the South Asian High (SAH) over southern Indo-China Peninsula after an abrupt "westward shift" in the upper troposphere.2) The climatological withdrawal of EASSM occurs on the52nd pentad with the distinguished features as following:firstly, the large amount rainfall retreats from focus domain due to the decreasing moisture transport according to the southward withdrawal of cross-equatorial vapor transport; secondly, the southward propagation of strengthening continental high (CH) changes the distribution of the thermal contrast between the continent and the ocean from warm in the west and cold in the east to the reversal one. Meanwhile, the spilt of the high from CH reaches over Shandong Peninsula and then changes the meridional wind at lower troposphere; thirdly, the reversal of meridional wind circulation averaged over110°-120°E accompanies with the southward shift and abrupt weakening of vertical; finally, the northwestward activities of tropical cyclones also favors to the circulation structure modulation especially in the meridional flow reversal.3) Slightly early than the climatological average, the mean onset (withdrawal) date for EASSM is the22.3(51.3) pentad with a standard variation of2.8(3.0) pentads and significant interannual variability in both time series based on the CFSR analysis from1979to2010. The earliest onset (withdrawal) date is on15th and46th pentad, while the latest one is on26th and58th pentad, respectively. The largest frequency is on19th as well as on23rd and24th pentad in onset cases, and the counterpart of withdrawal cases is on51st and52nd.4) Composite analysis shows that the SAH located over southern tip of Indo-China Peninsula in the early-onset years while over western tip in those late-onset years. In early-onset years, the westerly jet is stronger with southward axis and the ascending extension is narrower with lower center location of meridional cell, vice verse. As for the withdrawal situation, the robust convection exists over tropical areas, deep trough locates over110°E which favors the stronger southwesterly over domain in the upper troposphere, and the intensity of westerly jet is weaker in the early-withdrawal years, while vice versa.5) It is indicated that the onset of EASSM is strongly favored during the wet phases (phase1to3) of the tropical MJO when it propagates from the western Indian Ocean (IO) to central eastern10. The possible mechanism of the tropical MJO triggering the onset of EASSM is examined based on the space-time MJO-filtering to OLR, horizontal wind, geopotential height, and air temperature. It is suggested that the latent heating releasing by the MJO deep convection over central eastern10acts as the "Matsuno-Gill" effect of warm SST through the eastward propogation of Kelvin waves which contributes to the reestablishment of SAH over the southern Indo-China Peninsula in the upper troposphere. Thus the onset of EASSM is objectively connected with the reestablishment of SAH, the meridional wind reversal in the upper troposphere, and the tropical MJO activities.6) The three-dimension atmospheric features between the selected cyclone-influence and noncyclone-influence withdrawal years have been documented. The convection over tropical area is extremely strong (relatively weak) with fast (slow) southward shift of vertical ascending extension from subtropical area, and the Hadley-like cell has a slight higher (lower) center over subtropical area in the former (the latter) years. The western ridge of western Pacific subtropical high (WPSH) shifts northward and the strong cross-equatorial flow transports to the western Pacific in cyclone-influence years, while the western ridge of WPSH shifts southward and the weaker cross-equatorial flow is stalled over South China Sea in noncyclone-influence years. The SAH retreats westward (shifts southward), the trough over110°E deepens (shallows) its southern branch, thus the southwesterly (westerly) prevails over subtropical area as well as with weak (strong) westerly jet during the cyclone-influence (noncyclone-influence) withdrawal. Case analysis of the withdrawal in1991indicates that the main factor is the tropical cyclone activities.更多还原