The impact of boreal autumn SST anomalies over the South Pacific on boreal winter precipitation over East Asia

The possible mechanism behind the variability in the dipole pattern of boreal winter precipitation over East Asia is analyzed in this study. The results show that the SST anomalies(SSTAs) over the South Pacific Ocean(SPO) in boreal autumn are closely related to the variability in the dipole pattern of boreal winter precipitation over East Asia. The physical link between the boreal autumn SPO SSTAs and the boreal winter East Asian precipitation dipole pattern is shown to mainly be the seasonal persistence of the SPO SSTAs themselves. The seasonal persistence of the SPO SSTAs can memorize and transport the signal of the boreal autumn SSTAs to the following winter, and then stimulates a meridional teleconnection pattern from the SH to the NH, resulting in a meridional dipole pattern of atmospheric circulation over East Asia in boreal winter. As a major influencing factor, this dipole pattern of the atmospheric circulation can finally lead to the anomalous precipitation dipole pattern over East Asia in boreal winter. These observed physical processes are further confirmed in this study through numerical simulation. The evidence from this study, showing the impact of the SPO SSTAs in boreal autumn,not only deepens our understanding of the variability in East Asian boreal winter precipitation, but also provides a potentially useful predictor for precipitation in the region.     

东亚地区冬季地面气温延伸期概率预报研究

利用TIGGE资料中的ECMWF、NCEP、UKMO三个中心集合预报系统以及由此构成的多中心集合预报系统所提供的地面2 m气温10~15 d延伸期集合预报产品,建立贝叶斯模式平均(Bayesian Model Averaging,BMA)概率预报模型,对东亚地区冬季地面气温进行延伸期概率预报研究。采用距平相关系数、均方根误差、布莱尔评分、等级概率评分等指标分别对BMA确定性结果与概率预报进行评估。结果表明,BMA方法明显地改进了原始集合预报结果,预报技巧优于原始集合预报,且多中心BMA预报优于单中心BMA预报,最佳滑动训练期取35 d。BMA预报为气温的延伸期概率预报提供了更合理的概率分布,定量描述了预报的不确定性。     

Propagation and influence on tropical precipitation of intraseasonal variation over mid-latitude East Asia in boreal winter

本文研究冬季东亚中纬度地区9–29天季节内变化的传播特征和其对热带降水的影响。分析发现风场的季节内信号既可以向东传播,也可以向南传播。向东传播时,风场的季节内信号主要限于中纬度地区,表现为气旋和反气旋的向东移动。向南传播时,源于中纬度的经向风扰动可以深入到低纬度地区,甚至到达赤道附近。伴随的辐合辐散引起赤道附近地区降水异常,形成南海南部和中国东部-日本之间一个南北向偶极型降水异常。一个异常经向翻转环流在连接热带和中纬度季节内风场和降水变化中起着重要作用。     

Vertical tilt structure of East Asian trough and its interannual variation mechanism in boreal winter

Vertical tilt structure of the East Asian trough (EAT) and its interannual variation mechanism in boreal winter are studied using NCEP/NCAR, ERA40, and NCEP/DOE reanalyses. A vertical tilt index (VTI) is defined as the mean slope of vertical trough line on the longitude-height cross section to describe the tilting extent of the EAT, with high index indicating a more west-tilted trough and vice versa. The VTI series derived from the three reanalysis datasets are highly correlated with each other during the corresponding periods. A significant positive correlation is found between the VTI and the zonal range of the vertical trough line. Based on the close relation, a possible physical mechanism is proposed to explain the interannual variation of VTI. It demonstrates that positive (negative) temperature anomalies within the mean zonal range of the EAT result in expansion (contraction) of the zonal range and lead to high (low) VTI years. The composite analyses based on the three reanalysis datasets well support the proposed mechanism. Furthermore, the general relationship between the VTI and the zonal temperature gradient is discussed based on the proposed mechanism. It is revealed that the asymmetric change of temperature gradient on the western and eastern sides of the EAT plays an important role in the variation of VTI, which suggests that the tilting extent of the EAT is strongly affected by the two-order zonal change of temperature instead of the zonal temperature gradient (i.e., one-order change). Climate variability not only in the simultaneous winter but also in the following spring and summer over East Asia is closely related to the variation of the VTI. This study on the vertical tilting of the EAT may enrich knowledge of the East Asian winter monsoon and the climate variability over East Asia and may be helpful in improving the regional climate prediction in East Asia.     

Vertical structure,physical properties,and energy exchange of the East Asian trough in boreal winter

Climate Dynamics - The relation between the intensity of the East Asian trough and the strength of the East Asian winter monsoon is documented. A prominent dipole of the available potential energy...     

东亚地区南北两个风暴源地中风暴的结构和动力学差异

基于欧洲中心中期天气预报再分析资料(ERA-40),使用涡旋追踪和合成技术、多尺度子空间变换以及局地多尺度能量分析方法研究了东亚地区南北两个风暴源地中风暴的差异。结果发现,南、北两个源地风暴在结构上和内部动力过程上均存在着显著不同。南支源地(40°N以南)风暴底层比高层强,与线性斜压模式中的最不稳定模态结构相似;而北支源地(40°N以北)风暴则正好相反,与下游发展理论所描述的斜压波结构相似。并且发现,南支源地风暴的非地转风场比北支源地风暴的强。能量学诊断结果显示,南支源地风暴的能量源除了斜压不稳定外,有很大一部分来自正压不稳定,而北支源地风暴中则是存在弱的动能逆尺度传输。此外,南支源地风暴的浮力转换和非绝热做功均比北支源地风暴的强,其主要原因是南支源地风暴的垂直运动更强,风暴中的水汽更加充足。      

Causes of mid-Pliocene strengthened summer and weakened winter monsoons over East Asia

The mid-Pliocene warm period was the most recent geological period in Earth's history that featured long-term warming.Both geological evidence and model results indicate that East Asian summer winds(EASWs) strengthened in monsoonal China, and that East Asian winter winds(EAWWs) weakened in northern monsoonal China during this period, as compared to the pre-industrial period. However, the corresponding mechanisms are still unclear. In this paper, the results of a set of numerical simulations are reported to analyze the effects of changed boundary conditions on the mid-Pliocene East Asian monsoon climate, based on PRISM3(Pliocene Research Interpretation and Synoptic Mapping) palaeoenvironmental reconstruction. The model results showed that the combined changes of sea surface temperatures, atmospheric CO2 concentration,and ice sheet extent were necessary to generate an overall warm climate on a large scale, and that these factors exerted the greatest effects on the strengthening of EASWs in monsoonal China. The orographic change produced significant local warming and had the greatest effect on the weakening of EAWWs in northern monsoonal China in the mid-Pliocene. Thus,these two factors both had important but different effects on the monsoon change. In comparison, the effects of vegetational change on the strengthened EASWs and weakened EAWWs were relatively weak. The changed monsoon winds can be explained by a reorganization of the meridional temperature gradient and zonal thermal contrast. Moreover, the effect of orbital parameters cannot be ignored. Results showed that changes in orbital parameters could have markedly affected the EASWs and EAWWs, and caused significant short-term oscillations in the mid-Pliocene monsoon climate in East Asia.     

A numerical study on the winter monsoon and cold surge over East Asia

By using the improved regional climate model （RegCM_NCC）, a numerical study has been undertaken for the East Asia region over a period of 5 years （1998-2002） in an effort to evaluate the model＇s ability to reproduce the winter monsoon conditions that were observed. The results showed that the model can successfully simulate the basic characteristics of the winter monsoon circulations, including the location and intensity of the cold-surface, high-pressure system, as well as the wind patterns and the intensity of the winter monsoon. The simulated occurrence frequency and regions of the cold surge were consistent with the observations. The simulated rainfall distribution over China was consistent with the observations collected in South China. The features of the simulated moisture transport were also in good agreement with the observations that were derived from the NCEP reanalysis data, indicating that moisture transport coming from the Bay of Bengal trough plays a crucial role in supplying moisture needed for precipitation in South China. In addition, the moisture transport coming from the near-equatorial west-Pacific was also important. These two branches of moisture transport converged in South China, as a prerequisite for occurrence of the precipitation that was observed there. Heat budgets have shown that the development of a heat sink over the East Asian continent was remarkable and its thermal contrast relative to the neighboring seas was the important forcing factor for the winter monsoon activity. The simulation also indicated that the significant differences in circulation patterns and rainfalls during the winters of 1997/98 and 1998/99 were affected by cold and warm ENSO events, respectively. The above analysis demonstrated the model＇s ability to simulate the East Asian winter monsoon.     

Characterizing the winter concurrent variation patterns of the subtropical and polar-front jets over East Asia

In this study, the concurrent variation relationships between the East Asian subtropical jet (EASJ) and polar-front jet (PFJ) over the East Asian land mass in the winter season on different timescales are identified and the impacts of the jet concurrent variation patterns on the atmospheric circulation in mid–high latitude regions and climate anomalies in China are examined, using NCEP–NCAR reanalysis data and observational data. The major variability modes of the winter upper-level wind field on interannual timescales are characterized by the meridional shift of the PFJ and out-of-phase variation in the intensity of the subtropical jet and PFJ. On subseasonal and synoptic timescales, the concurrent variation relationships can be categorized into four configuration patterns: a strong (weak) subtropical jet accompanied by a weak (strong) PFJ, or a strong (weak) subtropical jet with a strong (weak) PFJ. The out-ofphase variation [i.e., a stronger (weaker) EASJ and weaker (stronger) PFJ] is found to be more common than the inphase variation [i.e., a stronger (weaker) EASJ and stronger (weaker) PFJ]. These concurrent variation relationships represent the integral structure and variation features of the atmospheric general circulation over East Asia, and have significant impacts on the weather and climate. The strong subtropical jet/weak PFJ (weak subtropical jet/strong PFJ) pattern leads to anomalous negative (positive) geopotential height in midlatitude regions and favors cold (warm) conditions, and positive (negative) rainfall anomalies in southern China. For both strong jet configurations, the geopotential height anomaly in the mid–high latitudes shows a northwest–southeast tilted dipole pattern, resulting in northern warm–southern cold temperature anomalies, and positive rainfall anomalies in southern China. For both weak jet situations, positive geopotential height anomalies dominate the East Asian area, and warm conditions occur over most areas in China, corresponding to less negative rainfall anomalies in southern China. The complicated rainfall and temperature anomaly patterns in China can be explained by the concurrent variation relationships between the two jets. A close relationship may exist between the synoptic-scale transient eddy activity (STEA) and the intensity of jet streams, especially for the PFJ. Significantly reduced (strengthened) STEA over the polar-front area is intimately associated with a decreased (increased) intensity of the PFJ.     

Regional dynamical downscaling with CCLM over East Asia

Inspired by the framework of the Coordinated Regional Climate Downscaling Experiment (CORDEX), the hindcast (1971–2000) and projection (2021–2050) simulations based on a resolution of $0.44^\circ$ over the East Asia domain are performed with the regional climate model COSMO-CLM (CCLM). The simulations are driven by ERA-40 reanalysis data and output of the global climate model ECHAM5. This is the first time that the CCLM is adapted and evaluated for the East Asia Monsoon region; the setup is considered a starting point for further improvements in this region by the CCLM community. The evaluation results show that the CCLM is able to reasonably capture the climate features in this region, especially the monsoon dynamics on small scales. However, total precipitation in the northern part of the domain, over the Tibetan Plateau, and over east Indonesia has a pronounced wet bias. The projected climate change under the A1B scenario indicates an overall annual surface temperature increase of 1–2 K, but no significant precipitation changes.     

辽宁冬季气温变化特征及与东亚冬季风的关系

采用1961-2012年辽宁省53站月平均气温资料及NCEP/NCAR再分析的月平均高度场、风场和海平面气压场资料,分析了近51 a来辽宁冬季气温的时空变化特征。在归纳并计算了前人定义的六种东亚冬季风指数的基础上,对比了辽宁冬季气温与这6种东亚冬季风指数的关系。结果表明：1961-2011年辽宁东部冬季气温均方差大于西部,全省绝大部分地区为显著的正趋势;辽宁冬季气温EOF第一模态表现为全省一致异常偏高或偏低的特征;时间系数显著上升,于1977年发生突变;并存在4.5 a、2 a和3 a的周期变化;朱艳峰定义的东亚冬季风指数是各指数中与辽宁冬季气温变化关系最紧密的指数,它无论从突变时间、周期特征、相关系数还是反位相对应关系上,都与辽宁冬季气温关系最密切,且850 hPa经向风场和海平面气压场也能说明二者有较好的对应关系。     

东亚冬季风寒温度的气候特征分析

基于ERA-Interim 1979—2019年的再分析资料,本文研究了东亚地区月平均、极端和强冷风寒温度空间分布。研究表明,在冬季3个月里,东亚地区风寒温度表现为自南向北递减,同时还具有较强的局地特征;最低极端风寒温度主要出现在内蒙古东北部、蒙古国东部和黑龙江地区;冬季3个月比较而言,平均风寒温度和极端风寒温度最低的月份为1月。强冷风寒温度发生频率最高的月份为2月。在成因方面,东亚大部分地区的风寒温度是由温度决定的,而中蒙交界区(蒙古和内蒙古东部地区)以及青藏高原北部地区,风速对风寒温度的影响最显著。此外,在所研究的几个区域中,京津冀城市群极端风寒温度与风速的负相关性最强。     

东亚、东南亚、南亚地区降水的年变化和年际变化

本文应用了1951—1980年4—9月月平均和1961—1970年4—9月旬平均降水资料,研究了东亚、南亚和东南亚降水的年变化和年际变化,分析了中国东部、西部降水差异及长江流域夏季旱涝的特征.结果指出,东亚、南亚和东南亚4—9月200mm以上大雨区分布的总趋势呈西南-东北向,它和西南季风的走向近于一致.在这条大雨带之中,包括了三种不同类型的降水.此外,中国江淮地区旬降水量在7—9月存在周期约为20天左右的准周期振荡现象,它和中国西部的降水分布完全不同.中国华南和华中的降水与El Nino现象具有相反的关系.统计结果表明,长江中下游夏季发生持续性多雨和持续性少雨的机率只有23％,大部分年份属于正常降水年份.最后也讨论了影响长江中下游地区夏季旱涝的环流因素.     

冬季北半球大气对秋冬季巴伦支海海冰异常的敏感性研究

基于ERA-Interim再分析资料,借助大气模式CAM4,分析了北半球冬季不同月份的平均大气对巴伦支海不同振幅及不同季节海冰扰动的敏感性,并考察了中高纬度典型大气模态的分布变化情况.结果表明,冬季巴伦支海海冰的减少,会导致湍流热通量异常向上、局地异常变暖及水汽含量的异常升高,且相关异常的强度和范围随着海冰减少幅度的减...     

Interaction between anomalous winter monsoon in East Asia and El Nino events

Based on a series of data analyses, the intimate relations between anomalous winter monsoon in East Asia and El Nino are studied in this paper.Anomalistic circulation in the Northern Hemisphere caused by El Nino event can lead to enhancing the Ferrel cell and the westerlies in the mid-latitudes as the Hadley cell and result in the location of the front zone in East Asia to the north. These are unfavourable for the cold wave breaking out southward in East Asia. Therefore, there are warmer weather and weaker winter monsoon in East Asia in El Nino winter.There are stronger and frequent cold waves in East Asia during the wintertime prior to the occurrence of El Nino event. They will induce stronger winter monsoon in East Asia. Thus, the weakened trade wind and enhanced cumulus convection in the equatorial middle-western Pacific area caused by the stronger winter monsoon will play an impor-tant role in the occurrence of El Nino event. Therefore, the anomalously strong winter monsoon in East Asia during wintertime might be an important mechanism to cause El Nino event.     

Northern poleward edge of regional Hadley cell over western Pacific during boreal winter: year-to-year variability,influence factors and associated winter climate anomalies

Compared to the zonal-mean Hadley cell (HC), our knowledge of the characteristics, influence factors and associated climate anomalies of the regional HC remains quite limited. Here, we examine interannual variability of the northern poleward HC edge over western Pacific (WPHCE) during boreal winter. Results suggest that interannual variability of the WPHCE is impacted by the El Niño-Southern Oscillation (ENSO) Modoki, North Pacific Oscillation (NPO) and North Atlantic Oscillation (NAO). The WPHCE tends to shift poleward during negative phase of the ENSO Modoki, and positive phases of the NPO and NAO, which highlights not merely the tropical forcing but also the extratropical signals that modulate the WPHCE. ENSO modoki, NPO and NAO modulate the WPHCE via inducing atmospheric anomalies over the western North Pacific. We further investigate the climatic impacts of the WPHCE on East Asia. The poleward shift of the northern descending branch of the WPHC results in anomalous upward (downward) motions and upper-level divergence (convergence) anomalies over south-central China (northern East-Asia), leading to increased (decreased) rainfall there. Moreover, pronounced cold surface air temperature anomalies appear over south-central China when the sinking branch of the WPHC moves poleward. Based on the temperature diagnostic analysis, negative surface temperature tendency anomalies over central China are mostly attributable to the cold zonal temperature advection and ascent-induced adiabatic cooling, while the negative anomalies over South China are largely due to the cold meridional temperature advection. These findings could improve our knowledge of the WPHCE variability and enrich the knowledge of forcing factors for East Asian winter climate.

     

Characteristics of Atmospheric Circulation Associated with Cold Surge Occurrences in East Asia： A Case Study During 2005/06 Winter

The characteristics of the upper-level circulation and thermodynamical properties for the period when two distinct cold surges broke out over East Asia during the 2005/06 winter are investigated. From early December 2005 to early January 2006, exceptionally cold weather lasted for approximately one month due to two successive cold surges that took place on 2 December 2005 and 2 January 2006, respectively. This study reveals that both involve the upper-tropospheric circulation, which induces the amplification and expansion of the surface Siberian high toward East Asia, but arose from different causes： the former is caused by the upper-level blocking originated from the North Pacific and the latter is caused by the upper-level wave train across the Eurasian Continent. In particular, it is suggested that the lower-tropospheric anomalous wind caused by upper-level circulation anomalies and a steep meridional temperature gradient amplified by phase-locked annual cycle combined to induce very strong cold advection in East Asia, which resulted in exceptionally cold weather that lasted for several weeks. The present results emphasize that the characteristics of the upper-tropospheric circulation can be considered as important precursors to cold surge occurrences in East Asia.