Comparative tests and analyses on monsoon and regional precipitation

Summary Due to the strong variation of the Asian monsoon, many countries in Asia often suffer from serious natural disasters. Droughts and floods appear in East China frequently related to the large anomalies of the two branches of East Asian monsoon. Based on rainfall data recorded by 336 Chinese stations in the 1980s, two distinctly opposite rainfall types over East China in summer (JJA) are discovered. Correspondly, 850 hPa anomalous wind fields in the Eastern Hemisphere are also possessed by two types of converse patterns in spring (MAM) and summer (JJA). The onset time and intensity of the Somali jet and the two branches of Southeast and Indian monsoon are quite different. Furthermore, in the 500 hPa geopotential height anomaly fields in spring and summer, the variations of the previous general atmospheric circulation (in spring) are closely correlated to the two kinds of conversely distributed rainfall in summer. These two types of rainfall are also related to two types of conversely distributed sea surface temperature anomaly (SSTA) in the equatorial Pacific and Indian Oceans. To investigate model capabilities refelecting the above observed features, eight numerical experiments are carried out using the IAP 2-L AGCM, with observed monthly mean global SSTs as external forcing and observed atmospheric data on February 15 as initial conditions. The simulated distributions of rainfall anomalies over East China in summer are in good accordance with observations. With conversely distributed SSTAs in the equatorial Pacific, the simulated 850 hPa anomalous wind fields and the 500 hPa geopotential height anomalies are also conversely distributed, and are closely related to the two types of simulated rainfall anomalies over East China. The cross equatorial wind varies in strength, space and time. The simulated distributions of anomalous 500 hPa geopotential height in spring and the anomalous wind at 850 hPa in spring and summer are quite similar to observations.With 10 Figures     

ENSO对中国西北地区秋季异常降水的影响

以奇异值分解（SVD）方法为基础,用1960～1994年35年的资料,分析了ENSO对中国西北地区秋季降水异常的影响,然后用奇异向量的海温场时间系数与500hPa高度场和海平面气压场进行交叉相关,讨论了在ENSO年引起西北地区降水异常的环流特征。结果表明,赤道中东太平洋海表温度异常与西北秋季大范围的区域性降水异常有较好的对应关系,在ElNino年,西北地区秋季大部分地区降水异常偏少,其中青藏高原东北侧的青海东部、甘肃中东部、宁夏南部和陕西北部等西北主要的雨养农业区降水明显偏少。引起这种异常的环流特征表现为500hPa高度场出现PNA异常流型,强异常中心多分布在低纬度,印缅槽偏弱,西太平洋副高加强西伸,脊线南移,南海副高和北美副高加强;东亚中纬度冬季盛行的1波流型在秋季异常发展,新疆脊偏强,东亚大槽加深西移。海平面气压场上,乌拉尔山北部高压、欧洲南部低压及阿留申低压等较高纬度系统异常加强（加深）。LaNino年情况相反。     

东亚夏季风指数的年际变化与东亚大气环流

文中从夏季东亚热带、副热带环流系统特点出发 ,定义了能较好表征东亚夏季风环流年际变化的特征指数 ,并分析了东亚夏季风指数的年际变化与东亚大气环流及夏季中国东部降水的关系。文中定义的东亚夏季风指数既反映了夏季东亚大气环流风场的变化特征 ,也较好地反映了夏季中国东部降水的年际变化特征。此外 ,还探讨了东亚夏季风指数变化的先兆信号     

1998年夏季东北亚阻塞高压异常的大尺度环流特征及成因初探

分析了1998年夏季东北阻塞高压异常及大尺度环流特征，研究了1979－1998年夏季东北阻塞高压年际变化，环流背景特征以及东北亚阻塞高压异常的太平洋海温异常的关系，认为夏季东北亚500hPa平均高度偏高（低），东亚地区“＋－＋”（“－＋－”）的距平波列，是典型的东北亚阻塞高压强盛（稀少）的环流型，太平洋海温的异常，激发东焉地区的遥相关波列，致使东北亚500hPa高度场异常，可能是东北亚阻塞主压异常的原因之一，在亚少中高续高度场平均偏高的环流背景下，乌拉尔册地区的阻塞高压对东北亚阻塞高压的发展和建立起到了促进作用，1998年乌拉尔山阻塞高频繁也是东北亚阻塞高压异常频繁的可能原因之一。     

Lag-relationship between mid-tropospheric geopotential heights over the Northern Hemisphere and the Indian summer monsoon rainfall: Implications for forecasting

Summary In this study the relationship between mid-tropospheric geopotential heights over the Northern Hemisphere (20° N to 90° N, around the globe) and Indian summer monsoon rainfall (ISMR: June to September total rainfall) have been examined. For this purpose, the monthly 500 hPa geopotential heights in a 2.5° lat./lon. grid over the Northern Hemisphere and the ISMR data for the period 1958 to 2003 have been used.The analysis demonstrates a dipole structure in the correlation pattern over the East Pacific Ocean in the month of January which intensifies in February and weakens in March.The average 500 hPa geopotential height over the eastern tropical Pacific Ocean during February (index one), has a significant positive relationship (r = 0.72) with the ISMR. In addition, the surface air temperature (SAT) anomaly over North-west Eurasia during January (index two) is found to be strongly related with the subsequent summer monsoon rainfall. These relationships are found to be consistent and robust during the period of analysis and these indices are found to be independent of each other.Hence, using index one and index two, a multiple linear regression model is developed for the prediction of the ISMR and the empirical relationships are verified on independent data. The forecast of the ISMR, using the above model, is found to be satisfactory.The dipole structure in the correlation pattern over the East Pacific region during February weakens once the ENSO (El-Nino and Southern Oscillation) events are excluded from the analysis. This suggests that the dipole type relationship between mid-tropospheric geopotential heights over the East Pacific Ocean and the ISMR may be a manifestation of the ENSO cycle.     

Boreal winter rainfall anomaly over the tropical indo-pacific and its effect on northern hemisphere atmospheric circulation in CMIP5 models

Experimental outputs of 11 Atmospheric Model Intercomparison Project （AMIP） models from phase 5 of the Coupled Model Intercomparison Project （CMIP5） are analyzed to assess the atmospheric circulation anomaly over Northern Hemisphere induced by the anomalous rainfall over tropical Pacific and Indian Ocean during boreal winter.The analysis shows that the main features of the interannual variation of tropical rainfall anomalies,especially over the Central Pacific （CP） （5°S-5°N,175°E-135°W） and Indo-western Pacific （IWP） （20°S-20°N,110°-150°E） are well captured in all the CMIP5/AMIP models.For the IWP and western Indian Ocean （WIO） （10°S-10°N,45°-75°E）,the anomalous rainfall is weaker in the 11 CMIP5/AMIP models than in the observation.During El Ni（n）o/La Ni（n）a mature phases in boreal winter,consistent with observations,there are geopotential height anomalies known as the Pacific North American （PNA） pattern and Indo-western Pacific and East Asia （IWPEA） pattern in the upper troposphere,and the northwestern Pacific anticyclone （cyclone） （NWPA） in the lower troposphere in the models.Comparison between the models and observations shows that the ability to simulate the PNA and NWPA pattern depends on the ability to simulate the anomalous rainfall over the CP,while the ability to simulate the IWPEA pattern is related to the ability to simulate the rainfall anomaly in the IWP and WIO,as the SST anomaly is same in AMIP experiments.It is found that the tropical rainfall anomaly is important in modeling the impact of the tropical Indo-Pacific Ocean on the extratropical atmospheric circulation anomaly.     

Relationship Between Soil Temperature in May over Northwest China and the East Asian Summer Monsoon Precipitation

This study investigates the relationship between the soil temperature in May and the East Asian summer monsoon （EASM） precipitation in June and July using station observed soil temperature data over Northwest China from 1971 to 2000.It is found that the memory of the soil temperature at 80-cm depth can persist for at least 2 months,and the soil temperature in May is closely linked to the EASM precipitation in June and July.When the soil temperature is warmer in May over Northwest China,less rainfall occurs over the Yangtze and Huaihe River valley but more rainfall occurs over South China in June and July.It is proposed that positive anomalous soil temperature in May over Northwest China corresponds to higher geopotential heights over the most parts of the mainland of East Asia,which tend to weaken the ensuing EASM.Moreover,in June and July,a cyclonic circulation anomaly occurs over Southeast China and Northwest Pacific and an anticyclonic anomaly appears in the Yangtze and Huaihe River valley at 850 hPa.All the above tend to suppress the precipitation in the Yangtze and Huaihe River valley.The results also indicate that the soil temperature in May over Northwest China is closely related to the East Asia/Pacific （EAP） teleconnection pattern,and it may be employed as a useful predictor for the East Asian summer monsoon rainfall.     

Influence of the Boreal Summer Intraseasonal Oscillation on Extreme Temperature Events in the Northern Hemisphere

The impact of the boreal summer intraseasonal oscillation (BSISO) on extreme hot and cool events was investigated, by analyzing the observed and reanalysis data for the period from 1983 to 2012. It is found that the frequency of the extreme events in middle and high latitudes is significantly modulated by the BSISO convection in the tropics, with a 3–9-day lag. During phases 1 and 2 when the BSISO positive rainfall anomaly is primarily located over a northwest–southeast oriented belt extending from India to Maritime Continent and a negative rainfall anomaly appears in western North Pacific, the frequency of extreme hot events is 40% more than the frequency of non-extreme hot events. Most noticeable increase appears in midlatitude North Pacific (north of 40°N) and higher-latitude polar region.Two physical mechanisms are primarily responsible for the change of the extreme frequency. First, an upper-tropospheric Rossby wave train (due to the wave energy propagation) is generated in response to a negative heating anomaly over tropical western North Pacific in phases 1 and 2. This wave train consists of a strong high pressure anomaly center northeast of Japan, a weak low pressure anomaly center over Alaska, and a strong high pressure anomaly center over the western coast of United States. Easterly anomalies to the south of the two strong midlatitude high pressure centers weaken the climatological subtropical jet along 40°N, which is accompanied by anomalous subsidence and warming in North Pacific north of 40°N. Second, an enhanced monsoonal heating over South Asia and East Asia sets up a transverse monsoonal overturning circulation, with large-scale ascending (descending) anomalies over tropical Indian (Pacific) Ocean. Both the processes favor more frequent extreme hot events in higher-latitude Northern Hemisphere. An anomalous atmospheric general circulation model is used to confirm the tropical heating effect.     

一个海气耦合模式对东亚夏季气候预测潜力的评估

利用一个具有较高分辨率的海气耦合模式SINTEX-F(Scale Interaction Experiment-Frontier Research Center for Global Change coupled GCM)的多年回报结果,评估了该海气耦合模式对东亚区域,尤其是中国地区气候异常的预测潜力.与观测实况的比较结果表明:SINTEX-F模式对夏季降水、500 hPa高度场和地表气温都有一定的预测技巧,但是相比而言降水与高度场的回报技巧要高于地表气温;而且耦合模式对东亚地区气候异常的主要空间分布和年际变化特征也有较好的预测潜力,对500 hPa高度场效果较好;对降水异常的年际变化也有一定的预测潜力,尤其是我国中部地区效果较好,但是模式预测的降水异常的幅值较观测相对偏弱;此外对我国西部的极端气候也有一定的预测潜力.     

北方雨季中国东部降水异常模态的环流特征及成因分析

根据1958～2011年中国东部（105°E以东）316站逐日降水资料及NCEP/NCAR逐日再分析资料,利用统计分析、物理量诊断等方法,探讨北方雨季（7月11日至8月31日）中国东部降水异常模态及同期、前期的大气环流特征。分析发现,北方雨季中国东部降水异常表现为三个相互独立的降水模态:第一模态为偏西型,当其时间系数为正（负）时,河套地区降水偏多（少）,江淮流域上游降水偏少（多）,南方大部降水偏多（少）;第二模态为北方一致型,当其时间系数为正（负）时,北方降水一致偏多（少）,长江流域降水偏少（多）;第三模态为偏东型,当其时间系数为正（负）时,东北南部至长江中游降水偏多（少）,华东沿海降水偏少（多）。研究发现,造成北方雨季三个降水异常模态的环流特征各不相同:偏西型降水主要受西亚高空副热带西风急流位置南北偏移影响;北方一致型降水主要由东亚-太平洋遥相关波列导致;偏东型降水主要与海陆气压异常对比造成的东亚夏季风变化有关。此外,三个模态与前期环流异常有密切联系。第一模态的正（负）异常由7月上旬200 hPa来自北大西洋的异常波列造成乌拉尔山位势高度负（正）异常和巴尔喀什湖以南位势高度正（负）异常引起。第二模态的正（负）异常与前期7月上旬200 hPa北大西洋上位势高度负（正）异常产生的沿中纬度（高纬度）路径向下游传播的波列有关。第三模态的正（负）异常由春季3月份低层蒙古上空异常的气旋（反气旋）持续至同期造成。     

Impacts of the leading modes of tropical Indian Ocean sea surface temperature anomaly on sub-seasonal evolution of the circulation and rainfall over East Asia during boreal spring and summer

The two leading modes of the interannual variability of the tropical Indian Ocean (TIO) sea surface temperature (SST) anomaly are the Indian Ocean basin mode (IOBM) and the Indian Ocean dipole mode (IODM) from March to August. In this paper, the relationship between the TIO SST anomaly and the sub-seasonal evolution of the circulation and rainfall over East Asia during boreal spring and summer is investigated by using correlation analysis and composite analysis based on multi-source observation data from 1979 to 2013, together with numerical simulations from an atmospheric general circulation model. The results indicate that the impacts of the IOBM on the circulation and rainfall over East Asia vary remarkably from spring to summer. The anomalous anticyclone over the tropical Northwest Pacific induced by the warm IOBM is closely linked with the Pacific–Japan or East Asia–Pacific teleconnection pattern, which persists from March to August. In the upper troposphere over East Asia, the warm phase of the IOBM generates a significant anticyclonic response from March to May. In June and July, however, the circulation response is characterized by enhanced subtropical westerly flow. A distinct anomalous cyclone is found in August. Overall, the IOBM can exert significant influence on the western North Pacific subtropical high, the South Asian high, and the East Asian jet, which collectively modulate the precipitation anomaly over East Asia. In contrast, the effects of the IODM on the climate anomaly over East Asia are relatively weak in boreal spring and summer. Therefore, studying the impacts of the TIO SST anomaly on the climate anomaly in East Asia should take full account of the different sub-seasonal response during boreal spring and summer.     

亚洲—太平洋地区冬季海平面气压异常偶极模与我国冬季气候异常的关系

利用全球月平均海平面气压资料和各标准等压面的位势场、风场,以及我国160站的降水和温度资料,研究了亚洲—太平洋（20°N~70°N,40°E~120°W）地区（简称亚太地区）冬季（12~2月）海平面气压异常偶极型振荡（偶极模）的时间变化特征,以及该异常偶极模与我国冬季气候异常的关系。结果表明：近半个世纪以来,亚太地区冬季海平面气压场异常以经向型（ME）偶极模负异常和纬向型（ZO）偶极模正异常为主,其中又以ZO正异常发生的频数较高。对应ME模,其各空间层的高度场异常形势和高度场与MEI（经向型指数）的相关系数分布形势十分类似;对应ZO模则主要表现为与阿留申地区高相关中心的一致性特征,说明阿留申低压的变化对其起着重要作用。ME模与我国冬季东部的降水异常和气温异常关系密切,ZO模仅对东北地区的降水异常有影响,因为ZO模的异常对我国上空大气环流异常的影响相对较小;这一结果同我们传统上在冬季比较关注纬向气压差的变化（即ZO模）有些相左,提示我们在冬季更要关注ME模的情况。大气环流形势的分析表明, MEI正异常时,即西伯利亚—蒙古冷高压的北半部气压异常偏高,副热带西北太平洋地区气压偏低时,利于东亚大槽的加深加强,东亚冬季风因而也会偏强;但是由于此时我国东部主要为下沉气流控制,暖湿气流难于源源不断地输送到我国境内,从而造成此时冬季我国东部尤其是长江中下游地区降水偏少。反之,当MEI负位相时,大气环流异常利于造成我国东部大范围的冬季降水。     

青藏高原冬春季雪盖对东亚夏季大气环流影响的数值试验

利用ＩＡＰ２－ＬＡＧＣＭ进行了青藏高原冬春季雪盖异常对东亚夏季大气环流、加热场和降水影响的数值试验。结果表明，该影响十分显著，持续性很强。当高原冬春季雪盖异常增厚、范围扩大时，夏季（ＪＪＡ）高原地区及我国北方５００ｈＰａ位势高度降低，南方变高，西太平洋副高减弱。大气对雪盖异常的响应呈明显的波列特征。我国北方大部地区土壤温度降低，南方土壤温度升高。夏季各月降水异常分布形势并不完全一致，但与同期５００ｈＰａ高度场异常分布形势有关。     

西北太平洋冷海温对东亚初夏大气环流影响的数值研究

利用改进的NCAR CCM3气候模式, 研究了1992年西北太平洋持续冷海温对东亚初夏季节大气环流的影响。西北太平洋冷海温不利于初夏东亚南支西风急流季节性北移, 引起亚洲东部沿海低槽明显加深, 东亚大槽平均高度场降低了4.66 dagpm, 从而也不利于西太平洋副热带高压的西伸加强。西北太平洋冷海温还不利于我国大陆初夏温度场回升, 特别是引起我国东北地区近地面温度下降2~5 ℃, 是影响东北冷夏现象的重要原因之一。模拟结果表明, 1992年初夏江淮入梅期较常年偏晚, 降水异常偏少, 与紧邻东亚大陆的西北太平洋持续冷海温有关。     

冬春季东亚气溶胶流出通量年际变率的相关环流异常分析

本文使用戈达德对地观测系统（Goddard Earth Observing System,GEOS）全球三维大气化学传输模式GEOS-Chem模拟了气象场驱动下1986~2006年冬春季东亚到太平洋区域气溶胶的流出通量,分析了流出通量的年际变率及其相关的环流异常。结果表明,偏多（少）的东亚气溶胶流出对应东北亚-西太平洋区域（Northeast Asia-western Pacific,NAWP）500 hPa定常波负值中心强度变强（弱）。NAWP区域500 hPa位势高度场负（正）异常还可能造成气溶胶流出路径的变化,即更多（少）的气溶胶相对于气候态偏北5~10个纬度的路径向太平洋区域传输。这种位势高度场上的异常伴随着对流层中低层西风和大气斜压性异常,从而引起东亚到太平洋区域气溶胶流出通量及其路径的年际变化。     

冬季各月北太平洋涛动的年际变化及其与我国降水异常的联系

利用ERA-Interim的海平面气压(Sea Level Pressure,SLP)再分析资料和中国160站的降水观测资料,分析了冬季各月(当年12月、次年1月和2月)北太平洋涛动(North Pacific Oscillation,NPO)的年际变化特征,及其与我国同期降水异常之间的联系。结果表明:1)冬季各月NPO指数的年际变化较为显著,但各月NPO指数年际变化之间的相关性较差,1979—2012年冬季12月与1月NPO指数年际变化之间的相关系数为0.09,而1月与2月NPO指数的相关系数仅为-0.003,均没有通过信度检验。2)1月和12月NPO指数年际变化与同期我国黄淮流域降水异常之间存在明显的正相关,而2月NPO指数年际变化与同期我国华北降水异常之间为明显的负相关。3)当1月(12月)NPO指数增加1个标准差时,我国黄淮流域降水量比多年平均值增加约50%(40%);而当2月NPO指数增加1个标准差时,我国华北降水量比多年平均值减少约30%。     

1993年北半球环流特征及其对我国天气气候的影响

１９９３年北半球主要环流特征表现为：５００ｈＰａ副产持续加强西伸，位置偏南；亚洲中纬度春，夏经向环流发展，东亚太平洋遥相关型稳定；西藏高原５００ｈＰａ位势高度持续偏高，印缅槽持续偏弱；１００ｈＰａ位势高度场低纬持续偏高，中，高纬持续偏低，夏季南亚高压位置异常偏高。热带海洋出现明显异常，一次新的厄尔尼诺现象发生。热带海洋和北半球大气环流的异常对我国天气气候产生明显影响。     

Impact of the western North Pacific subtropical high on the East Asian monsoon precipitation and the Indian Ocean precipitation in the boreal summertime

The western North Pacific subtropical high (WNPSH) is a crucial component of the East Asian summer monsoon (EASM) system and significantly influences the precipitation in East Asia. In this study, distinguished role of WNPSH on the EASM and Indian Ocean monsoon (IOM) are investigated. Based on the boreal summer mean field of 850-hPa geopotential height and its interannual variability, the WNPSH index (WNPSHI) is defined by the areaaveraged geopotential height over the region [110°–150°E, 15°–30°N]. The WNPSHI is significantly related to the precipitation over the East Asian monsoon (EAM) region [105°–150°E, 30°–40°N] and IOM region [70°–105°E, 5°–15°N]. Rainfalls over these two regions have good correlation with WNPSH developments and the geopotential height fields at 850 hPa related to the EAM precipitation and IOM precipitation have remarkably different teleconnection patterns in boreal summer. These features exhibit that EAM and IOM precipitations have different type of development processes associated with different type of WNPSH each other. Focusing on the relationships among the EAM precipitation, IOM precipitation, and the WNPSH variabilities, we assume that WNPSH and EAM precipitation are usually fluctuated simultaneously through the sea surface temperature (SST)-subtropical ridge-monsoon rainfall feedback, whereas the IOM precipitation varies through the different process. To clarify the relationships among WNPSH, EAM, and IOM, two cases are selected. The first one is the case that all of WNPSH, EAM, and IOM are in phase (WE(+)I(+)), and the second one is the case that WNPSH and EAM are in phase and WNPSH/EAM and IOM is out of phase (WE(+)I(?)). These two cases are connected to the thermal forcing associated with SST anomalies over the eastern Pacific and Indian Ocean. This different thermal forcing induces the change in circulation fields, and then anomalous circulation fields influence the moisture convergence over Asian monsoon regions interactively. Therefore, the monsoon rainfall may be changed according to the thermal conditions over the tropics.     

1997年中国夏季旱涝和北半球平流层平均环流分析

通过分析对应我国夏季三类降水型的北半球50 hPa平均环流场特征,表明1997年我国夏季降水异常的50 hPa环流特征,主要是从冬到夏大范围高度场偏高,极地涡旋冬季弱春季强,太平洋高压冬春异常加强。