长江中下游春季连阴雨的低纬环流及其低频振荡背景

本文应用OLR资料以及ECMWF风场资料分析了长江中下游春季连阴雨过程的热带和副热带环流系统的特征及与连晴过程的差异。同时,还进一步研究了春季东半球热带和副热带地区低频振荡的基本特征及传播规律,得出了可能产生长江中下游连阴雨的低频振荡的背景事实。     

春季热带地区OLR低频振荡及其与长江中下游连阴雨

本文应用NOAA卫星接收的1975—1983年向外长波辐射(OLR)资料研究了春季热带地区低频振荡的基本特征,得出东半球热带地区盛行周期为30天左右的低频振荡,这种振荡最显著的区域在0—10°S、70—90°E的印度洋地区。文章阐述了过渡季节这种低频振荡的经向和纬向传播特征。 本文还进一步指出,在热带低频振荡的不同阶段,ITCZ、北半球副热带以及西风带环流系统也呈现出显著的周期变化,并查证了低频振荡作为长江中下游连阴雨和连晴过程的背景事实。因此本研究工作也为长江中下游连阴雨和连晴天气的中、长期可预报性提供     

2007年秋季罕见的连阴雨天气成因分析

2007年秋季连阴雨是山东省有观测记录以来历时最长、危害程度最大的一次历史罕见的连阴雨天气,文章从大气环流背景、物理量特征等方面对其成因进行了详细的分析。结果表明,这次连阴雨是亚欧大尺度环流形势由纬向型向经向型转变的过程中发生的,是一次长波调整的过程。连阴雨期间乌拉尔山高压脊和西太平洋副热带高压较常年同期异常偏北、偏强,长时间稳定维持,同时副热带高压南侧东风带内热带气旋活动频繁,亚洲中纬度地区多短波槽活动,引导西西伯利亚冷空气东移南下与孟加拉湾和沿副热带高压外围北上的暖湿气流在黄河下游交绥,造成了山东持久的连阴雨天气。连阴雨期间物理量特征明显,在降水强度大、降水时间集中时段均有深厚的湿层、明显的水汽辐合及垂直上升运动,上升运动可达300～200 hPa。在降水间歇时段,700hPa以下维持下沉运动,相对湿度小于80%,且为一致的水汽辐散。     

鲁西北罕见的秋季连阴雨环流特征分析

利用常规天气图资料和NCEP提供的1°×1°FNL分析场资料等,分析了2007年9月26日至10月7日鲁西北秋季连阴雨期间大尺度环流及物理量特征.分析表明,连阴雨期间,亚欧上空大型环流相对稳定,乌拉尔山附近维持一稳定的阻塞高压,西太平洋副热带高压异常偏强,低纬地区热带气旋活动频繁,南亚高压和副热带急流位置明显偏北,亚洲中纬度多短波槽东移,700hPa鲁西北及其北侧多维持切变线,与副热带高压西北侧的西南暖湿气流在黄河中、下游一带持续相互作用,形成了长达12天的阴雨天气.     

2009年秋季沈丘县连阴雨天气过程分析

从环流形势、影响系统和水汽条件等方面分析了2009年9月7-20日沈丘县出现的历史罕见的秋季连阴雨天气过程的成因:副高脊线位于25°N以北、西伸脊点位于108°E,北部冷空气南下受副高阻挡,在40-50°N形成平直西风带锋区,锋区南侧偏西西北干冷气流与副高边缘西南暖湿气流交汇,切变系统频繁建立,是导致该区连阴雨天气的产生机制.亚洲两槽一脊型建立,蒙古东部低涡加强、东移南下,副高脊线南撤至20°N附近,预示了连阴雨天气结束.     

秋季连阴雨预报

我县是地瓜产区,10月至11月中旬的连阴雨天气,直接影响着地瓜的丰产丰收。秋季连阴雨是在大环流形势稳定的情况下产生的,一般在60—70°E及130°E附近为一暖脊(或阻高),两脊之间为一平直西风带,中纬度有短波槽带动小股冷空气频频东移。印缅低槽明显,暖湿气流源源不断地输向华北地区,从而造成我地连阴雨天气。     

长江中下游两次罕见连阴雨强降水的对比分析和模拟

利用中尺度数值模式MM5和实况资料,对比分析了分别发生在2008年秋季和2009年春季长江中下游历史罕见的强连阴雨天气过程.结果表明,欧亚地区500 hPa"两槽一脊"型是长江中下游春、秋季出现强连阴雨过程的典型环流形势.连阴雨过程低空急流异常活跃,存在风向、风速的辐合.比较该两次过程显示,2009年春季连阴雨过程低层...     

2003年陕西秋季连阴雨降水特点及环流条件分析

文章分析了陕西省2003年秋季连阴雨降水的特征和形成秋季连阴雨天气的天气学、热力学特点。结果表明:2003年陕西秋季连阴雨降水的主要特征是降水的落区和时间相对集中, 降水强度大, 日数多且持续时间长, 是1954年以来陕西又一次极端连阴雨事件。在大气环流由夏季型向冬季型转变过程中, 对流层高层西风急流南压至40°N时, 青藏高原东北侧包括陕西在内的西北地区东部, 位于急流南侧的高空辐散区, 该地区对流层低层却保持着高温高湿的热力特征, 对流上升运动活跃, 形成了低层辐合高层辐散的垂直环流机制, 这时对流层中部500 hPa欧亚环流形势相对稳定, 乌拉尔山的长波脊和中亚低槽维持, 偏强、偏北、偏西的副热带高压外围偏南气流为该区域输送了大量的水汽, 从而形成了陕西及西北地区东部持续的连阴雨天气; 当西风带中纬度新疆高压脊建立, 副热带高压东移南退时, 陕西及西北地区东部的连阴雨天气结束。     

黄山地区冬季连阴雨气候特征及异常个例分析

利用常规气象观测资料和NCEP/NCAR再分析资料,对黄山地区1961—2019年冬季(上年12月至2月)连阴雨气候特征以及2019年异常个例进行分析.结果表明:1)黄山地区冬季连阴雨期间多年平均降水量184 mm,连阴雨日数28d,降水日数24 d,日照时数15 h;冬季连阴雨降水量、连阴雨日数和降水日数的年代际分布特征具有较好一致性;冬季连阴雨降水量西南部多、东北部少,连阴雨日数和降水日数中南部多、西北部及东部少,日照时数均较少.2)2019年冬季连阴雨过程的降水量、连阴雨日数和降水日数均为1962年以来最多,单站相关要素以2月7日—3月6日过程为最多,屯溪和歙县连阴雨日数破历史极值,各区县降水日数破历史极值;日照时数以2018年12月30日—2019年1月12日过程为最少,各区县破历史极值.黄山地区连阴雨平均强度指数为1962年以来第二高值,各区县均达冬季连阴雨的最强等级.3)乌拉尔山阻塞高压维持,欧亚中高纬度地区大气环流经向度大,不断南下的冷空气为连阴雨天气提供了温度条件;西太平洋副热带高压强度异常偏强,位置偏西、偏北,以及青藏高原南缘的南支低槽系统活跃,来自南海和孟加拉湾的暖湿气流为连阴雨天气提供了水汽条件.冷暖空气在长江中下游持续交绥,从而导致黄山地区2019年冬季多次出现异常连阴雨天气过程.     

2011年陕西省强秋淋天气分析

利用陕西省自动站实测的降水资料和美国国家环境预测中心提供的NECP/NCAR再分析资料,对2011年9月3—20日陕西省强秋淋天气的环流特征和动力热力学结构进行了诊断分析。结果表明：2011年的强秋淋天气具有持续时间长,累计雨量大,降水强度大,强降水日数多,气温持续偏低的特点。连阴雨期间副热带高压较常年同期强度偏强、位置偏西偏北,东亚大槽偏西,印缅槽偏强,即东亚大槽后部的偏北气流引导极地冷空气从贝湖南下至华北,同时印缅槽前和西太平洋副高西北侧的西南暖湿气流伸至115°E附近,陕西持续受冷暖空气交汇影响,出现近20d的强连阴雨天气。连阴雨期间,西太平洋副高的北抬南撤,急流在40°N附近维持和850hPa上南风的盛行都为连阴雨的形成提供了有利条件。另外,来自孟加拉湾和南海的水汽是这次连阴雨的主要水汽来源,假相当位温的高能区出现在105°E～110°E,能量锋区与此次连阴雨的雨带相对应,造成陕西省2011年强秋淋天气。     

阿拉伯海热带气旋生成特征分析

利用印度气象局（India Meteorological Department,IMD）、国际气候管理最佳路径档案库（International Best Track Archive for Climate Stewardship,IBTrACS）提供的1982—2020年阿拉伯海热带气旋路径资料,美国国家环境预报中心（National Centers for Environmental Prediction,NCEP）再分析资料,对近39 a阿拉伯海热带气旋源地和路径特征、活跃区域、频数及气旋累积能量（accumulated cyclone energy,ACE）指数的季节特征和年际变化特征进行分析,并结合环境因素,说明其物理成因。结果表明：阿拉伯海热带气旋多发于10°~25°N,65°~75°E海域,5—6月、9—12月发生频数较高且强度较强,1—4月、7—8月发生频数较低且气旋近中心最大风速均小于35 kn;频数的季节变化主要受控于垂直风切变要素;阿拉伯海热带气旋发生频数和ACE近年有上升趋势,年际变化主要受控于海面温度（sea surface temperature,SST）和850 hPa相对湿度要素。     

The ENSO Events in the Tropical Pacific and Dipole Events in the Indian Ocean

A depth map (close to that of the thermocline as defined by 20℃) of climatically maximum seatemperature anomaly was created at the subsurface of the tropical Pacific and Indian Ocean, based on which the evolving sea-temperature anomaly at this depth map from 1960 to 2000 was statistically analyzed. It is noted that the evolving sea temperature anomaly at this depth map can be better analyzed than the evolving sea surface one. For example, during the ENSO event in the tropical Pacific, the seatemperature anomaly signals travel counter-clockwise within the range of 10°S-10°N, and while moving, the signals change in intensity or even type. If Dipole is used in the tropical Indian Ocean for analyzing the depth map of maximum sea-temperature anomaly, the sea-temperature anomalies of the eastern and western Indian Oceans would be negatively correlated in statistical sense (Dipole in real physical sense), which is unlike the sea surface temperature anomaly based analysis which demonstrates that the inter-annual positive and negative changes only occur on the gradients of the western and eastern temperature anomalies. Further analysis shows that the development of ENSO and Dipole has a time lag features statistically, with the sea-temperature anomaly in the eastern equatorial Pacific changing earlier (by three months or so). And the linkage between these two changes is a pair of coupled evolving Walker circulations that move reversely in the equatorial Pacific and Indian Oceans.     

Impact of atmospheric and oceanic conditions on the frequency and genesis location of tropical cyclones over the western North Pacific in 2004 and 2010

This study examines the impact of atmospheric and oceanic conditions during May–August of 2004 and 2010 on the frequency and genesis location of tropical cyclones over the western North Pacific. Using the WRF model, four numerical experiments were carried out based on different atmospheric conditions and SST forcing. The numerical experiments indicated that changes in atmospheric and oceanic conditions greatly affect tropical cyclone activity, and the roles of atmospheric conditions are slightly greater than oceanic conditions. Specifically, the total number of tropical cyclones was found to be mostly affected by atmospheric conditions, while the distribution of tropical cyclone genesis locations was mainly related to oceanic conditions, especially the distribution of SST. In 2010, a warmer SST occurred west of 140°E, with a colder SST east of 140°E. On the one hand, the easterly flow was enhanced through the effect of the increase in the zonal SST gradient.The strengthened easterly flow led to an anomalous boundary layer divergence over the region to the east of 140°E, which suppressed the formation of tropical cyclones over this region. On the other hand, the colder SST over the region to the east of 140°E led to a colder low-level air temperature, which resulted in decreased CAPE and static instability energy. The decrease in thermodynamic energy restricted the generation of tropical cyclones over the same region.     

西北太平洋热带气旋潜在生成指数的改进

热带气旋潜在生成指数(GPI,Genesis Potential Index)是热带气旋生成可能性大小的空间分布函数,利用大尺度环境场可以应用于热带气旋活动的季节预报,并且可以评估全球气候变化对热带气旋活动的影响。但是目前的GPI基本都是针对全球热带气旋活动构建的,没有考虑到热带气旋不同活动地区及其内部的差异。本研究考虑到南海和西北太平洋热带气旋生成的不同特点,分别构建了适用于南海(5～25°N,100～120°E)和西北太平洋(5～40°N,120～180°E)的热带气旋GPI。改进后的GPI对南海和西北太平洋区域热带气旋生成具有较好的模拟能力,不仅能很好地模拟南海和西北太平洋热带气旋生成频数空间分布的气候特征(相似系数为0.67),而且能够较好地模拟热带气旋生成在年际时间尺度上的空间分布特征。     

The influence of the Madden-Julian oscillation on high-latitude surface air temperature during boreal winter

By analyzing NCEP-NCAR reanalysis daily data for 1979–2016, the modulation by Madden-Julian Oscillation (MJO) of the wintertime surface air temperature (SAT) over high latitude is examined. The real-time multivariate MJO (RMM) index, which divides the MJO into eight phases, is used. It is found that a significantly negative SAT anomaly over the northern high latitude region of (180°–60 °W, 60°–90 °N) lags the MJO convection for 1∼2 weeks in phase 3, in which the enhanced convective activity exists over the Indian Ocean. While a significantly positive SAT anomaly appears over the same region following the MJO phase 7, as the tropical heating shows an opposite sign. Analysis of the anomalous circulation indicates that the observed SAT signal is probably a result of the northeastward propagating Rossby wave train triggered by MJO-related tropical forcing through Rossby wave energy dispersion. By using an anomalous atmospheric general circulation model (AGCM), the significant effect of tropical forcing on organizing the extratropical circulation anomaly is confirmed. Analysis of a temperature tendency equation further reveals that the intraseasonal SAT anomaly is primarily attributed to the advection of the mean temperature by the wind anomaly associated with the anomalous circulation of the MJO-related variability.     

热带西太平洋夏季风和南海夏季风之间的联系

根据热带西太平洋（１３０°－１６０°Ｅ,１０°－２０°Ｎ）上空对流的年际变化,对表面温度、向外长波幅射、８５０ ｈＰａ纬向风进行了合成分析。合成分析结果表明,热带西太平洋上空的弱（强）对流对应着前冬和春季厄尔尼诺（拉尼娜）型的海温异常。与以前的研究结果进行了比较,说明上述海温异常的时空分布也与热带西太平洋和南海季风的爆发早晚相关联。合成分析结果还表明,热带西太平洋上空的弱（强）对流对应着从热带西太平洋向西伸展到盂加拉湾的东风（西风）异常。数值模拟也得到类似的结果。此外,在对流弱（强）的夏季,热带西太平洋上空的对流和南海低层纬向风均表现出弱（强）的季节演变特征。     

华西秋雨趋势变化的年代际转折及其成因分析

华西地区（25°N～35°N,100°E～110°E）是中国秋季降水主要地区之一。本文根据华西地区72站月平均降水资料、NCEP/NCAR再分析资料和哈德莱中心海温及海冰资料,利用相关和回归等分析方法研究了1961～2014年华西地区秋雨的年代际变率及其与大气环流和海温的关系。华西秋季降水年代际变率分解为呈现显著下降趋势的P1时段（1964～1998年）和呈现上升趋势的P2时段（1998～2014年）发现,对应P1时段降水下降趋势的华西区域大气位势高度异常场具有西正东负结构,大尺度环流场显示为从大西洋东传经北极巴伦支—喀拉海区至东亚的准纬向波列,该波列体现了上游负位相NAO（North Atlantic Oscillation）的调制作用。对于P2时段的降水上升趋势,其位势高度场配置与P1时段相反,而大尺度波列结构在欧亚大陆的部分呈西北—东南走向,且整体偏西,体现了上游正位相NAO的调制作用。这种环流结构导致华西区域西北侧形成负异常中心,有利于西南暖湿气流进入研究区域。影响华西秋雨趋势转折的海温关键区位于热带中东太平洋和热带印度洋。在P1时段,华西秋雨降水趋势与同期热带中东太平洋和印度洋海温呈显著正相关关系。而在P2时段,华西秋雨与前冬热带中东太平洋和印度洋海温存在显著负相关,前冬西北太平洋海温正异常也同时影响了华西秋雨的上升趋势。     

A study on the decreasing trend in tropical easterly jet stream (TEJ) and its impact on Indian summer monsoon rainfall

Using the NCEP/NCAR reanalysis wind and temperature data (1948–2011) and India Meteorological Department (IMD) rainfall data, a long-term trend in the tropical easterly jet stream and its effect on Indian summer monsoon rainfall has been explained in the present study. A decreasing trend in zonal wind speed at 100 mb (maximum decrease), 150 mb, and 200 mb (minimum) is observed. The upper-level (100, 150, and 200 mb) zonal wind speed has been correlated with the surface air temperature anomaly index (ATAI) in the month of May, which is taken as the difference in temperature anomaly over land (22.5°N–27.5°N, 80°E–90°E) and Ocean (5°S–0°S, 75°E–85°E). Significant high correlation is observed between May ATAI and tropical easterly jet stream (TEJ) which suggests that the decreasing land–sea temperature contrast could be one major reason behind the decreasing trend in TEJ. The analysis of spatial distribution of rainfall over India shows a decreasing trend in rainfall over Jammu and Kashmir, Arunachal Pradesh, central Indian region, and western coast of India. Increasing trend in rainfall is observed over south peninsular and northeastern part of India. From the spatial correlation analysis of zonal wind with gridded rainfall, it is observed that the correlation of rainfall is found to be high with the TEJ speed over the regions where the decreasing trend in rainfall is observed. Similarly, from the analysis of spatial correlation between rainfall and May ATAI, positive spatial correlation is observed between May ATAI and summer monsoon rainfall over the regions such as south peninsular India where the rainfall trend is positive, and negative correlation is observed over the places such as Jammu and Kashmir where negative rainfall trend is observed. The decreased land–sea temperature contrast in the pre-monsoon month could be one major reason behind the decreased trend in TEJ as well as the observed spatial variation in the summer monsoon rainfall trend. Thus, the study explained the long-term trend in TEJ and its relation with May month temperature over the Indian Ocean and land region and its effect on the trend and spatial distribution of Indian summer monsoon rainfall.     

Mechanisms of tropical Pacific interannual-to-decadal variability in the ARPEGE/ORCA global coupled model

Spatial and temporal structures of interannual-to-decadal variability in the tropical Pacific Ocean are investigated using results from a global atmosphere–ocean coupled general circulation model. The model produces quite realistic mean state characteristics, despite a sea surface temperature cold bias and a thermocline that is shallower than observations in the western Pacific. The periodicity and spatial patterns of the modelled El Niño Southern Oscillations (ENSO) compare well with those observed over the last 100 years, although the quasi-biennial timescale is dominant. Lag-regression analysis between the mean zonal wind stress and the 20°C isotherm depth suggests that the recently proposed recharge-oscillator paradigm is operating in the model. Decadal thermocline variability is characterized by enhanced variance over the western tropical South Pacific (~7°S). The associated subsurface temperature variability is primarily due to adiabatic displacements of the thermocline as a whole, arising from Ekman pumping anomalies located in the central Pacific, south of the equator. Related wind anomalies appear to be caused by SST anomalies in the eastern equatorial Pacific. This quasi-decadal variability has a timescale between 8 years and 20 years. The relationship between this decadal tropical mode and the low-frequency modulation of ENSO variance is also discussed. Results question the commonly accepted hypothesis that the low-frequency modulation of ENSO is due to decadal changes of the mean state characteristics.     

Recent climatic trends in the tropical Atlantic

A homogeneous monthly data set of sea surface temperature (SST) and pseudo wind stress based on in situ observations is used to investigate the climatic trends over the tropical Atlantic during the last five decades (1964–2012). After a decrease of SST by about 1 °C during 1964–1975, most apparent in the northern tropical region, the entire tropical basin warmed up. That warming was the most substantial (>1 °C) in the eastern tropical ocean and in the longitudinal band of the intertropical convergence zone. Surprisingly, the trade wind system also strengthened over the peirod 1964–2012. Complementary information extracted from other observational data sources confirms the simultaneity of SST warming and the strengthening of the surface winds. Examining data sets of surface heat flux during the last few decades for the same region, we find that the SST warming was not a consequence of atmospheric heat flux forcing. Conversely, we suggest that long-term SST warming drives changes in atmosphere parameters at the sea surface, most notably an increase in latent heat flux, and that an acceleration of the hydrological cycle induces a strengthening of the trade winds and an acceleration of the Hadley circulation. These trends are also accompanied by rising sea levels and upper ocean heat content over similar multi-decadal time scales in the tropical Atlantic. Though more work is needed to fully understand these long term trends, especially what happens from the mid-1970’s, it is likely that changes in ocean circulation involving some combination of the Atlantic meridional overtuning circulation and the subtropical cells are required to explain the observations.