辐散风和瞬变的涡度通量的异常对定常波年际异常的强迫作用

研究了１９８２／１９８３年１２—２月对流层上层由辐散风和瞬变的涡度通量所产生的涡度源异常对定常波异常的强迫作用。结果表明，瞬变的涡度通量辐合的异常有抵消或耗散辐散风产生的涡度源异常的趋势；定常波的异常是在这两种强迫力的共同作用下维持的。同时还表明，对１９８２／１９８３年的情况，热带和赤道中东太平洋上的涡度源异常以及中纬度北太平洋上的涡度源异常都对太平洋／北美地区的大气环流异常起了重要的作用。     

MAINTENANCE MECHANISMS OF THE STATIONARY WAVE ANOMALY TELECONNECTION PATTERNS IN THE UPPER TROPOSPHERE

The spatial structure and temporal variations of the stationary wave anomalies in the uppertroposphere during the Northern Hemisphere winter are studied with the ECMWF data of 1980—1988 and PCA(principal component analysis)method.The vorticity source anomalies of the mainteleconnection patterns are analyzed,and their roles in the maintenance of the stationary waveanomaly teleconnection patterns are investigated with the harotropic model.The results show thatthe anomalies of transient vorticity flux convergences tend to cancel the anomaly of vorticitysources generated by divergent winds.The stationary wave anomaly patterns caused by divergentwind anomalies only are very different from observations.However,if the anomalous transientvorticity flux convergences are also included in the vorticity sources,the simulated patterns will begreatly improved.     

对流层上层定常波异常遥相关型的维持机制

用ＥＣＭＷＦ１９８０－１９８８年的资料和主成份分析方法研究了北半球冬季定常波月平均异常的空间结构和时间变化，分析了对应主要异常型的由辐散风和瞬变涡度通量的异常引起的涡度源异常，并用正压模式研究了涡度源异常在这些异常型维持中的作用。结果表明，瞬变的涡度通量辐合的异常有抵消辐散风所产生的涡度源异常的趋势；单独考虑辐散风异常作为强迫所产生的环流异常与遥相关型的结构和强度相差很大，但如果将瞬变涡度通量辐合的异常也包含到强迫源中，则强迫出来的环流异常会有很大改善     

辐散风和瞬变的涡度通量对定常波的强迫

本文从理论分析、资料诊断和正压模式数值试验三个方面研究了瞬变的涡度通量的辐合与辐散风产生的涡度源之间的关系及它们在对流层上层定常波维持中的作用。结果表明， 瞬变涡度通量的辐合对辐散风产生的涡度源有抵消或耗散的趋势；定常波是在两种强迫力的共同作用下维持的。同时还研究了不同地区散度场对定常波的作用， 结果表明，中纬度东亚地区的辐合场对维持东亚大槽是重要的，并且这一辐合场与位于印度尼西亚地区的辐散场是维持西太平洋上副热带反气旋系统的主要强迫源。     

Variability patterns of Rossby wave source

Rossby waves (RW) propagation due to a local forcing is one of the mechanisms responsible for wave trains configurations known as teleconnections. The term teleconnection refers to anomalies patterns that are correlated in several regions of the world, causing large-scale changes in atmospheric waves patterns and temperature and precipitation regimes. The aim of teleconnections studies is to provide a better understanding of atmospheric variability and their mechanisms of action in order to identify patterns that can be tracked and predicted. The purpose of this study was to characterize seasonal and spatial variability of atmospheric RW sources. The RW source at 200?hPa was calculated for the four seasons with reanalysis data of zonal and meridional wind. In the Northern Hemisphere (NH), the RW sources were located on East Asia, North America, North Atlantic and Pacific. The main RW sources in the Southern Hemisphere (SH) were located over Intertropical, South Pacific, South Atlantic and South Indian Convergence Zones. Extratropical regions were also identified, mainly to the south of Australia. The vortex stretching term (S1) and the advection of absolute vorticity by the divergent wind (S2) were analyzed to discuss the physical mechanisms for RW generation. In the NH, the source at East Asia in DJF changed to a sink in JJA, related to the dominance of S1 term in DJF and S2 term in JJA. In the SH, the vortex stretching term had the dominant contribution for RW source located to the south of Australia. The main forcing for RW sources at east of Australia was the advection of absolute vorticity by divergent flow. Over South America, both terms contributed to the source in DJF. The main modes of RW source variability were discussed by using empirical orthogonal functions analysis. RW variability was characterized by wave trains configurations in both hemispheres over regions of jet streams and storm tracks, associated with favorable and unfavorable areas for RW generation.     

Winter climate anomalies in Europe and their associated circulation at 500 hPa

Regional anomalies of the surface climate over Europe are defined by a simultanous EOF-analysis of the normalized monthly mean sea level pressure, temperature and precipitation fields of 100 winters (December–February, 1887–1986) at 40 stations. The monthly amplitudes of the first EOF (about 25% of the total variance) are used as an index for the monthly winter climate anomaly. They characterize a high (low) pressure cell over central Europe associated with a positive (negative) temperature and precipitation anomaly over northern (central-southern) Europe as indicated by a northward (southward) shift of the tail end of the cross-Atlantic cyclone track. These patterns resemble the phenomenological anticyclonic (cyclonic) Grosswetter classification and the European blocking (enhanced zonal flow) regime. The second EOF is of similar magnitude and gives latitudinal corrections to these two basic flow regimes. The joint probability distribution of both amplitudes shows a weak bimodality mainly associated with the first EOF. Further insight into the underlying physical processes of the climate anomaly patterns in Europe is obtained from the extended Eliassen-Palm flux diagnostics of the barotropic transient eddy-mean flow interaction (Hoskins et al. 1983) and the stationary wave propagation (Plumb 1985). The diagnostics confined to the barotropic components and applied to the regression and the composite anomaly fields of the transient and stationary eddy flows of the 500 hPa geopotential (1946–87, north of 20°N) leads to the following results: (1) The bandpass filtered transient eddy variances of the 500 hPa geopotential show a shift of the cross-Atlantic storm track: In high (low) pressure situations over Europe the cross-Atlantic storm track intensity is enhanced (reduced) and its tail end is shifted northward (remains zonal); the North Pacific storm track extends further (less) eastward and thus closer to the west coast of North America. (2) The extreme high pressure system over Europe tends to be supported by an anomalous transient eddy forcing of the mean flow stream-function: it enhances the zonal wind to its north and generates anticyclonic vorticity about 10° upstream from its center. In the low pressure composite the anomalous cyclonic vorticity is generated reducing the zonal flow to its north. (3) The occurrence (lack) of a strong eastward stationary wave activity flux over the Atlantic is associated with the high (low) pressure situations over Europe. Finally, a positive feedback is conjectured between the stationary wavetrain modifying the tail end of the cross-Atlantic storm track and the transient eddies intensifying this anomaly.     

2015/2016厄尔尼诺事件的衰减位相及其对应的西北太平洋环流异常

利用季节平均Had ISST海温、CMAP降水及NCEP风场数据,分析了2015/2016超级厄尔尼诺衰减期的特征及其对应的西北太平洋大气环流异常。结果表明:2015/2016厄尔尼诺事件除了成熟位相冬季强度大以外,还具有在随后春季衰减快,到夏季就消亡的特征。伴随着厄尔尼诺的迅速衰减,西北太平洋有较强的反气旋环流异常维持。厄尔尼诺衰减位相与西太反气旋异常存在相互作用。一方面,由于此次厄尔尼诺事件强度强,衰减期热带印度洋有显著的暖海温异常从冬季一直维持到夏季,有利于西太反气旋的增强和维持。另一方面,西太反气旋环流异常的维持及其南侧东风异常的发展使得中东太平洋正海温异常减弱,令厄尔尼诺事件快速衰退。此外,通过与1982/1983和1997/1998年比较发现,这三次超强厄尔尼诺事件虽强度相当,但衰减位相及与之相联系的西太反气旋异常都不尽相同。1982/1983事件衰减慢,维持时间长,对应的西太反气旋强度弱。而1997/1998事件衰减快,维持时间短,对应的西太反气旋强度在春季和夏季都强盛维持。2015/2016事件的衰减速度明显快于1982/1983事件,对应的西太反气旋也强于1982/1983事件,由于2015/2016事件增暖中心偏向于中东太平洋,而这里是厄尔尼诺衰减过程中负海温异常最先出现的区域,因此尽管2015/2016事件中西太反气旋异常的强度弱于1997/1998事件,但衰减速度及衰减位相维持时间与1997/1998相当。本文研究结果表明厄尔尼诺衰减与西太反气旋异常之间的关系较为复杂,需进一步研究。     

Western Pacific Jet Stream Anomalies at 200 hPa in Winter Associated with Oceanic Surface Heating and Transient Eddy Activity

The relationships between the 200-hPa westerly jet stream anomalies over the East Asian coastal water- western Pacific(WPJS),and the oceanic surface heating and synoptic-scale transient eddy(STE)activity anomalies over the North Pacific in wintertime are examined by using ERA-40 and NCEP/NCAR reanalysis data.The analysis demonstrates that the surface heating and the STE anomalies have different patterns, corresponding to the three WPJS anomalous modes,respectively.In the first WPJS anomalous mode,the WPJS main part shows no robust anomaly.The anomalous westerly wind,occurring over the mid-latitude central-eastern Pacific past the date line is associated with the anomalous heating presenting both in the tropical central-eastern Pacific past the date line and the center of the North Pacific basin.Meanwhile,the STE anomaly appears around the region of the anomalous zonal wind.The fluctuation in jet strength shown in the second WPJS mode is strongly related to the heating anomaly in the Kuroshio Current region and the STE anomaly in the jet exit region.The third mode demonstrates a northward/southward shift of the WPJS,which has a statistical connection with a south-north dipolar pattern of the heating anomaly in the western North Pacific separated at 35°N.Meanwhile,the STE spatial displacement is in conjunction with jet shifts in the same direction.The heating anomaly has a close connection with the atmospheric circulation, and thus changes the mid-latitude baroclinicity,leading to the STE anomaly,which then reinforces the WPJS anomaly via internal atmospheric dynamics.     

1982年夏季热带大气环流异常与ElNio事件发展机制的探讨

本文利用1980—1983年四个夏季(以7月平均为代表)的ECMRWF资料,分析了1982—1983年El Nino事件强烈发展前的1982年夏季热带大气环流的异常.从一系列显著异常的观测事实分析中得出:1982年夏季异常强的东亚高空东风急流是1982～1983年El Nino事件发展的一个可能促发机制;1982年澳大利亚冬季冷空气的异常堆积导致澳大利亚至东太平洋纬向热力对比的增强,是赤道太平洋偏东信风迅速减弱的一个重要原因.     

Connection between the Silk Road Pattern in July and the following January temperature over East Asia

This study investigates a cross-seasonal influence of the Silk Road Pattern (SRP) in July and discusses the related mechanism. Both the reanalysis and observational datasets indicate that the July SRP is closely related to the following January temperature over East Asia during 1958/59–2001/02. Linear regression results reveal that, following a higher-than-normal SRP index in July, the Siberian high, Aleutian low, Urals high, East Asian trough, and meridional shear of the East Asian jet intensify significantly in January. Such atmospheric circulation anomalies are favorable for northerly wind anomalies over East Asia, leading to more southward advection of cold air and causing a decrease in temperature. Further analysis indicates that the North Pacific sea surface temperature anomalies (SSTAs) might play a critical role in storing the anomalous signal of the July SRP. The significant SSTAs related to the July SRP weaken in October and November, re-emerge in December, and strengthen in the following January. Such an SSTA pattern in January can induce a surface anomalous cyclone over North Pacific and lead to dominant convergence anomalies over northwestern Pacific. Correspondingly, significant divergence anomalies appear, collocated in the upper-level troposphere in situ. Due to the advection of vorticity by divergent wind, which can be regarded as a wave source, a stationary Rossby wave originates from North Pacific and propagates eastward to East Asia, leading to temperature anomalies through its influence on the large-scale atmospheric circulation.     

RESEARCH ON THE INTERANNUAL AND INTERDECADAL VARIABILITIES OF THE MONSOON TROUGH AND THEIR IMPACTS ON TROPICAL CYCLONE GENESIS OVER THE WESTERN NORTH PACIFIC OCEAN

In this paper, we mainly summarize and review the progresses in recent climatological studies (by CMSR, IAP/CAS and some associated domestic and international institutions) on the interannual and interdecadal variabilities of monsoon troughs and their impacts on tropical cyclones and typhoons (TCs) geneses over the western North Pacific Ocean. The climatological characteristics of monsoon troughs and four types of circulation patterns favorable to TCs genesis over the western North Pacific Ocean in summer and autumn are given in this paper. It is also shown in this paper that the monsoon trough over the western North Pacific Ocean has obvious interannual and interdecadal variabilities. Especially, it is revealed in this paper that the interannual and interdecadal variabilities of the monsoon trough over the western North Pacific Ocean influence the TCs genesis not only through the impact on distributions of the vorticity in the lower troposphere and the divergence in the upper troposphere, the water vapor in the mid- and lower troposphere and the vertical shear of wind fields between the upper and lower troposphere over the western North Pacific Ocean, but also through the dynamical effects of the transition between convectively coupled tropical waves and providing disturbance energy. Besides, some climatological problems associated with TCs activity over the western North Pacific Ocean that need to be studied further are also pointed out in this paper.     

The Westerly Anomalies over the Tropical Pacific and Their Dynamical Effect on the ENSO Cycles during 1980～1994

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The westerly anomalies over the tropical pacific and their dynamical effect on the enso cycles during 1980–1994

In this paper, the zonal wind anomalies in the lower troposphere over the tropical Pacific during 1980–1994 are analyzed by using the observed data. The results show that during the formation of the 1982/83, 1986/87 and 1991 / 92 ENSO events, there were the larger westerly anomalies in the lower troposphere over the equatorial Pacific. Moreover, it is explained by using the correlation analyses that the westerly anomalies over the equatorial Pacific could cause the warm episodes of the equatorial central and eastern Pacific. A simple air-sea coupled model is used to discuss theoretically the dynamical effect of the observed westerly anomalies of wind stress near the sea surface of the equatorial Pacific on the ENSO cycle occurred in the period of 1981–1983. It is shown by using the theoretical calculations of the equatorial oceanic Kelvin wave and Rossby waves responding to the forcing of the observed anomalies of zonal wind stress near the sea surface of the equatorial Pacific that the westerly anomalies of wind stress near the sea surface of the equatorial Pacific make significant dynamical effect on the ENSO cycles occurred in the period of 1982–1983.     

SOME STATISTICAL CHARACTERISTICS AND THE APPLICATION OF JAPANESE GMS LOW CLOUD WINDS

A statistical study of GMS low cloud winds in January,April,July and October,1983 shows that on an average,there exist 223.5 low cloud winds over western North Pacific each synoptic time.The low cloud winds have a diurnal change with more low cloud winds at 12 Z than at 00 Z. The wind fields at 850 hPa over western North Pacific,with and without application of low cloud winds, have been analyzed by a successive correction scheme.The results indicate that the flow patterns without low cloud winds are considerably distorted from short of wind data over the oceanic region.On the contrary,with application of low cloud winds,the flow patterns get much more improved over the oceanic region.     

IMPACT OF LARGE-SCALE CIRCULATION ON THE INTERDECADAL VARIATIONS OF THE WESTERN NORTH PACIFIC TROPICAL CYCLONE

Based on the annual frequency data of tropical cyclones from 1960 to 2005 and by the polynomial fit and statistical analysis, this work has discovered that TC activity in the 46a exhibits significant decadal- scale variability. It has two high frequency periods (HFP) and two low frequency periods (LFP). Significant differences in the number of TCs between HFP and LFP are found in active TC seasons from July to October. Differences of large-scale circulation during HFP and LFP have been investigated with NCEP/NOAA data for the season. In HFP, the condition includes not only higher sea surface temperature,lower sea level pressure, larger divergence of upper air, larger relative vorticity at low levels and smaller vertical shear, but also 500-hPa wind vector being more available for TC activity and moving to western North Pacific, the position of the subtropical anticyclone over the western Pacific shifting more northward,and South Asian Anticyclone at 100-hPa being much smaller than that in LFP. The precipitation of western North Pacific has no clear influence on TC activity.     

The dynamical effects of divergent wind on the intraseasonal variability of the East Asian circulation

In this paper, the dynamical effects of divergent wind on the intraseasonal variability of atmospheric circulation over East Asia are analyzed by using the function of Rossby-wave source and the energy exchanging function be-tween divergent component and rotational component of the flow.The results analyzed from the observed data show that the advection of vorticity by divergent wind caused by the heating due to the monsoon rainfall in the south to the Yangtze River and the strong convective activities around the Philippines may play an important role in the northward jump of westerly jet stream during the seasonal transition from spring circulation to summer circulation over East Asia. Due to the northward movement of the advection of vorticity by the divergent wind, the energy transformation from divergent component into rotational component can be caused over the Yellow River basin and Northwest China and will cause the intensification of the zonal flow there. Thus, the jet stream abruptly shifts northward to North China.Moreover, the analysed results also show that the advection of vorticity by divergent wind caused by the heating due to the strong convective activities around the Philippines also plays an important role in the intraseasonal varia-bility of the circulation over East Asia during the seasonal transition from summer to winter. With the southward movement of the advection of vorticity by the divergent wind, the energy transformation from divergent component into rotational component can be caused over East Asia, especially over the Yangtze-Huaihe River basin. Therefore, the jet stream gradually moves southward from North China to the Yangtze River basin.     

正压大气中基态定常波对强迫响应的影响

本文利用正压涡度方程模式研究了热带不同经度位置异常强迫激发的中纬响应类型与北半球冬季基本气流定常波结构的关系。发现对热带印度洋和太平洋的大部分地区,中纬的响应类型主要由基态定常波的结构决定,并且基态定常波通过向扰动转换动能维持了中纬的响应。通过这些结果解释了GCM研究中发现的中综异常响应类型对El Nino成熟期海表面温度异常位置变化的不敏感性。此外还讨论了观测和GCM中出现的热带印度洋——太平洋地区低频尺度上的纬向偶极强迫源对GCM响应结果的作用。