A tripole winter precipitation change pattern around the Tibetan Plateau in the late 1990s

Classical monsoon dynamics considers the winter/spring snow amount on the Tibetan Plateau (TP) as a major factor driving the East Asian summer monsoon (EASM) for its direct influence on the land–sea thermal contrast. Actually, the TP snow increased and decreased after the late 1970s and 1990s, respectively, accompanying the two major interdecadal changes in the EASM. Although studies have explored the possible mechanisms of the EASM interdecadal variations, and change in TP snow is considered as one of the major drivers, few studies have illustrated the underlying mechanisms of the interdecadal changes in the winter TP snow. This study reveals a tripole pattern of change, with decreased winter precipitation over the TP and an increase to its north and south after the late 1990s. Further analyses through numerical experiments demonstrate that the tropical Pacific SST changes in the late 1990s can robustly affect the winter TP precipitation through regulating the Walker and regional Hadley circulation. The cooling over the tropical central-eastern Pacific can enhance the Walker circulation cell over the Pacific and induce ascending motion anomalies over the Indo-Pacific region. These anomalies further drive descending motion anomalies over the TP and ascending motion anomalies to the north through regulating the regional Hadley circulation. Therefore, the positive–negative–positive winter precipitation anomalies around the TP are formed. This study improves the previously poor understanding of TP climate variation at interdecadal timescales.摘要在20世纪70年代和90年代末, 伴随着东亚夏季风的两次主要年代际变化, 高原积雪分别显著增加和减少. 尽管很多学者研究了东亚夏季风年代际变化的可能机制, 高原积雪变化也被认为是主要因素之一, 但是关于高原冬季积雪本身发生年代际变化的潜在机制尚鲜有研究. 本文揭示了20世纪90年代末高原及周边冬季降水的三极子变化特征: 高原主体上空主要为降水减少, 其南北两侧区域降水增加. 数值试验结果表明, 热带太平洋海温变化可以通过调节沃克环流和局地哈德莱环流, 对上述三极子降水变化型态产生显著影响.     

The increasing predominance of extreme precipitation in Southwest China since the late 1970s

Extreme precipitation events cause severe environmental and societal damage worldwide. Southwest China (SWC) is sensitive to such effects because of its overpopulation, underdevelopment, and fragile ecosystems. Using daily observations from 108 rain-gauge stations, the authors investigated the frequency of extreme precipitation events and their contribution to total precipitation in SWC since the late 1970s. Results indicate that total precipitation is decreasing insignificantly, but rainfall-events frequency is decreasing significantly, whereas the region is experiencing more frequent and intense extreme precipitation events. Note that although fewer stations are statistically significant, about 60% of the rain-gauge stations show an increasing trend in the frequency and intensity of extreme precipitation. Furthermore, there is an increasing trend in the contribution of total extreme precipitation to total precipitation, with extreme precipitation becoming dominant in the increasingly arid SWC region. The results carry important implications for policymakers, who should place greater emphasis on extreme precipitation and associated floods and landslides when drafting water-resource management policies.摘要本文分析了中国西南20世纪70年代末以来极端降水事件的频率, 强度及其对总降水的贡献. 结果表明, 该地区约60%的降水站点极端降水的频率和强度正在增加, 而大多数站点总降水频率明显减少. 同时极端降水总量对总降水量的贡献有显著增加的趋势, 极端降水在日益干旱的中国西南地区变得更具主导性. 研究结果提醒应更加重视极端降水及其可能引发的次生灾害, 如洪水, 山体滑坡等.     

An interdecadal decrease in extreme heat days in August over Northeast China around the early 1990s 1990年代初东北8月极端高温频次的年代际减少

Northeast China (NEC) witnessed an interdecadal increase in summer extreme heat days (EHDs) around the mid-1990s. The current study reveals that this interdecadal increase only occurs in June and July, while August features a unique interdecadal decrease in EHDs around the early 1990s. Plausible reasons for the interdecadal decrease in EHDs in August are further investigated. Results show that the interdecadal decrease in EHDs in August is due to the deceased variability of daily maximum temperature (T_max). Overall, the variation of the T_max over NEC in August is modulated by the Eurasian teleconnection pattern, Silk Road pattern, and East Asia–Pacific pattern. However, the influence of the Silk Road pattern dramatically weakens after the early 1990s because the meridional wind variability along the westerly jet significantly decreases. The weakened influence of the Silk Road pattern contributes to the decreased T_max variability over NEC. Meanwhile, the convection over the western North Pacific, which accompanies the East Asia–Pacific pattern, presents a significant decrease in variance after the early 1990s, further decreasing the T_max variability over NEC.摘要东北夏季极端高温频次在1990年代中期出现年代际增多.本文指出该年代际增多只出现在6–7月, 而8月则呈现特殊性, 即在1990年代初出现年代际减少.进一步分析表明, 东北8月极端高温频次的年代际减少由日最高温度变率的年代际减小造成.东北日最高温度受到欧亚遥相关,丝绸之路遥相关和东亚-太平洋遥相关的共同调制.1990年代初之后, 西风急流上的经向风变率显著减小, 丝绸之路遥相关对下游的影响减弱, 导致东北日最高温度变率减小.同时, 西北太平洋热带对流的变率也在1990年代初出现年代际减小, 通过东亚-太平洋遥相关使东北日最高温度变率进一步减小.     

The impacts of the East Asian subtropical westerly jet on weather extremes over China in early and late summer

Summer weather extremes (e.g., heavy rainfall, heat waves) in China have been linked to anomalies of summer monsoon circulations. The East Asian subtropical westerly jet (EASWJ), an important component of the summer monsoon circulations, was investigated to elucidate the dynamical linkages between its intraseasonal variations and local weather extremes. Based on EOF analysis, the dominant mode of the EASWJ in early summer is characterized by anomalous westerlies centered over North China and anomalous easterlies centered over the south of Japan. This mode is conducive to the occurrence of precipitation extremes over Central and North China and humid heat extremes over most areas of China except Northwest and Northeast China. The centers of the dominant mode of the EASWJ in late summer extend more to the west and north than in early summer, and induce anomalous weather extremes in the corresponding areas. The dominant mode of the EASWJ in late summer is characterized by anomalous westerlies centered over the south of Lake Baikal and anomalous easterlies centered over Central China, which is favorable for the occurrence of precipitation extremes over northern and southern China and humid heat extremes over most areas of China except parts of southern China and northern Xinjiang Province. The variability of the EASWJ can influence precipitation and humid heat extremes by driving anomalous vertical motion and water vapor transport over the corresponding areas in early and late summer.摘要东亚副热带西风急流是影响中国极端天气的重要原因之一, 然而之前的研究主要关注整个夏季急流的变率, 对其早夏和晚夏变率的区别及其对极端天气的影响关注较少. 本文研究了早夏和晚夏东亚副热带西风急流季节内变化特征的区别, 以及这种区别带来的极端天气的差异及其可能的动力学机制. 研究结果表明, 相比于早夏, 晚夏急流季节内变化中心位置偏西偏北, 通过改变垂直运动和水汽输送可以影响极端降水和湿热浪在相应区域的发生概率.     

山东2009年10月降水气候背景及前期海温与大气环流特征

利用1951—2009年北半球500hPa高度、北太平洋海温、环流特征量、降水等资料,采用相关分析、合成分析、经验函数正交分解(EOF)、子波分析等多种统计技术,对影响山东2009年10月降水趋势的各种因素进行分析和研究。结果表明:山东10月降水大致存在3种降水分布型;在不同时间尺度的气候背景上,2009年10月山东基本处于一个少雨或由少雨向多雨转换的气候阶段;2009年春季加利福尼亚冷流的减弱,2009年6月开始的厄尔尼诺事件及6月起西太平洋副高持续的偏强、偏西、正常或偏南状态,各种指标均指示山东10月降水偏少的可能性大,预测与实况基本吻合。     

The extreme Arctic warm anomaly in November 2020

In November 2020, the eastern Arctic experienced an extensive extreme warm anomaly (i.e., the second strongest case since 1979), which was followed by extreme cold conditions over East Asia in early winter. The observed Arctic warm anomaly in November 2020 was able to extend upwards to the upper troposphere, characterized as a deep Arctic warm anomaly. In autumn 2020, substantial Arctic sea-ice loss that exceeded the record held since 1979, accompanied by increased upward turbulent heat flux, was able to strongly warm the Arctic. Furthermore, there was abundant northward moisture transport into the Arctic from the North Atlantic, which was the strongest in the past four decades. This extreme moisture intrusion was able to enhance the downward longwave radiation and strongly contribute to the warm conditions in the Arctic. Further analysis indicated that the remote moisture intrusion into the Arctic was promoted by the large-scale atmospheric circulation patterns, such as the wave train propagating from the midlatitude North Atlantic to the Arctic. This process may have been linked to the warmer sea surface temperature in the midlatitude North Atlantic.摘要2020年11月北极东部显著偏暖, 表面气温暖异常为1979年以来第二强, 且北极表层偏暖可以延伸至对流层上层. 本文进一步研究了此次北极极端偏暖的可能原因. 2020年秋季北极海冰大幅减少, 11月从北大西洋向北极的水汽输送显著增加, 且二者的变化幅度均超过了1979年以来的最高纪录, 进而导致北极出现极端暖异常. 此外, 从中纬度向北极的Rossby波传播有利于向极水汽输送增加, 且此过程可能与北大西洋中纬度海温异常有关.     

Strength of the North African monsoon in the Last Interglacial and under future warming

The authors explore the response of the Northern African (NAF) monsoon to orbital forcing in the Last Interglacial (LIG) compared with its response to greenhouses gas (GHG) forcing under the SSP5-8.5 scenario simulated in CMIP6. When the summer surface air temperature increases by 1 °C over the Northern Hemisphere, the NAF monsoon precipitation and its variability during the LIG increase by approximately 51% and 22%, respectively, which is much greater than under SSP5-8.5 (2.8% and 4.3%, respectively). GHG forcing enhances the NAF monsoon mainly by increasing the atmospheric moisture, while the LIG's orbital forcing intensifies the NAF monsoon by changing the monsoon circulation. During the LIG, models and data reconstructions indicate a salient hemispheric thermal contrast between the North and South Atlantic, strengthening the mean-state NAF monsoon precipitation. The interhemispheric temperature contrast enhances atmosphere–ocean interaction and the covariability of the northward sea surface temperature gradient and Saharan low, strengthening the NAF monsoon variability.摘要与人为强迫引起的全球变暖相比, 末次间冰期是轨道强迫引起的过去80万年来最暖的一个间冰期, 但鲜有人研究末次间冰期中北非季风的响应. 因此, 本文基于CMIP6多模式模拟结果对比研究了末次间冰期和SSP5–8.5情景下北非季风的响应, 发现末次间冰期下北非季风平均降水及其降水变率均远大于SSP5–8.5情景下的结果. 轨道强迫导致的北大西洋暖于南大西洋增加了北非季风环流和平均降水, 同时, 南北大西洋海温梯度变化通过增强热带北大西洋的海气相互作用增大了海温梯度和撒哈拉低压的变率, 从而增强了北非季风降水变率.     

Factors responsible for the increasing trend of mei-yu season rainfall during 1979–2020 over the western and eastern mei-yu domain

The mei-yu season (June–July) rainfall over the mei-yu monitoring domain (MMD) in the Yangtze–Huaihe Basin has shown an increasing trend in recent decades. This study examines the dominant factors responsible for this increasing trend for the period 1979–2020 based on station-observed rainfall and ERA5 reanalysis datasets from the perspective of changes in atmospheric circulation. Although significantly increasing trends exist in the mei-yu season rainfall over the entire MMD, the magnitude of the trends is slightly larger over the eastern MMD (EMMD) than over the western MMD (WMMD). Quantitative diagnoses demonstrate that the relative contributions of anomalous evaporation and moisture advection to the increasing rainfall trend are different between the EMMD and WMMD. The increasing rainfall trend over the WMMD (EMMD) is attributable to increased evaporation (enhanced vertical moisture advection), which is dependent on an anomalous cyclonic circulation in the middle-lower troposphere over the MMD. Such an anomalous cyclone on the northwestern side of the climatological western North Pacific subtropical high facilitates an increase in moisture divergence above 600 hPa over the EMMD, leading to enhanced vertical moisture advection in conjunction with strengthened moisture convergence at 850 hPa. By contrast, the anomalous cyclone favors increasing local evaporation over the WMMD.摘要近几十年来, 江淮流域梅雨监测区 (MMD) 的梅雨期 (6–7月) 降水呈增加趋势. 本文基于1979–2020年台站观测降水资料和ERA5再分析数据, 从大气环流变异的角度揭示了这种长期增加趋势的主要影响因素. 发现在MMD范围内, 梅雨期降水趋势的增幅东部大于西部. 水汽收支定量诊断表明, 异常的蒸发和水汽平流对MMD西部和东部降水增加趋势的相对贡献是不同的. MMD西部 (东部) 的降水趋势主要归咎于增强的局地蒸发 (增强的垂直水汽平流) , 后者又取决于MMD对流层中, 低层的异常气旋环流. 这种位于气候平均的西太平洋副热带高压西北侧的异常气旋有助于MMD东部600 hPa以上的水汽辐散增加, 伴随加强的850 hPa水汽辐合, 从而导致垂直水汽平流的增强. 相反, 该异常气旋则有利于增强MMD西部的局地蒸发.     

Detection of the anthropogenic signal and urbanization effects in extreme temperature changes in eastern China

With its rapid rise in temperatures and accelerated urbanization in recent decades, eastern China may be affected by both global warming and the urban heat island effect. To investigate the influence of anthropogenic forcing and urbanization on extreme temperature, the authors conducted detection and attribution analyses on 16 extreme indices using extended observational data during 1958–2020 and the models that participated in CMIP5 and CMIP6. The extended observational data till 2020 show continued warming in extreme temperatures in recent years. Most of the indices display an increase in warm extremes and decrease in cold extremes. Both CMIP5 and CMIP6 models are able to reflect these warming features, albeit the models can over- or underestimate some extreme indices. The two-signal detection with anthropogenic and urbanization effects jointly considered showed that the anthropogenic and urban signals can be simultaneously detected and separated only in two frequency indices, i.e., the frequency of warm and cold nights. The anthropogenic forcing explains about two-thirds of the warming, while URB contributes about one-third for these two indices. For most of the other indices, only the anthropogenic signal can be detected. This indicates that the urban signal is distinct from the natural variability mainly for the nighttime frequency indices but not for the other extreme temperature indies. Given the important influence of nighttime extremes on human health, this suggests an urgent need for cities to adapt to both global warming and urbanization.摘要作为中国经济最发达的地区, 中国东部受到城市热岛效应和温室气体排放等人类活动的明显影响. 本文利用最新的观测和全球气候模式资料, 对极端温度强度, 频率和持续时间等16个极端温度指数进行了检测归因分析, 研究了人为强迫和城市化效应对中国东部极端温度变化的影响. 结果表明, 近年来极端温度持续增暖, 极端暖事件增加, 极端冷事件减少. 新一代全球气候模式能够合理地反映这些变暖特征, 但是部分模式可能高估或低估了观测到的变化. 基于最优指纹方法的双信号检测表明, 人为信号和城市化效应只能在暖夜和冷夜两个频率指标上同时被检测并分离, 其变化约三分之二可归因于人类活动, 剩余的三分之一可归因于城市化效应. 而在极端温度其他指数的变化中, 只有人类活动的影响能够被检测到.     

Characteristics of diurnal variations of warm-season precipitation over Xinjiang Province in China

降水日变化受大气热力,动力过程以及复杂地形影响,演变特征复杂且区域差异显著.本文采用中国气象局发布的中国地面与CMORPH融合逐小时降水产品(2008-2019年),分析了新疆省暖季降水日变化特征.研究结果表明:(1)新疆大部分地区降水主峰值发生在清晨;(2)持续时间超过三小时的降水事件是新疆地区主要降水事件,贡献了南...     

Decadal–centennial hydroclimate variability over eastern China during the last millennium: Results from the product of Paleo Hydrodynamics Data Assimilation: 过去千年中国东部年代际-百年尺度干湿变化特征:基于古水文动力同化数据

Decadal–centennial hydroclimate variability over eastern China during the last millennium is investigated using the product of Paleo Hydrodynamics Data Assimilation (PHYDA). Results reveal that the PHYDA depicts a more homogeneous temporal pattern during the early part of the Little Ice Age with other reconstructions than those during the other periods, and could also identify the droughts of 1352–90 AD, 1445–98 AD, 1580–94 AD, and 1626–65 AD during this period. On centennial time scales, the PHYDA shows that the linkage between the Palmer drought severity index over eastern China and the Atlantic Multidecadal Oscillation (AMO) index is more marked than that with the El Niño–Southern Oscillation and the location of the intertropical convergence zone over the Asian–Australian monsoon area during the period after the 1350s. For the decadal droughts, the PHYDA suggests most of the drought events during the last millennium were linked to the El Niño–like mean states and the negative AMO states.摘要利用古水文动力同化数据 (PHYDA) 研究了过去千年中国东部年代际-百年尺度干湿变化特征.结果表明, 对比其它重建数据PHYDA在百年尺度上对小冰期前期中国东部干湿变化的再现能力最好, 其对这一时期发生的年代际干旱事件包括1352–90年,1445–98年,1580–94年和1626–65年干旱事件的再现能力也最强.通过与强迫因子的对比和回归分析, 发现1350年后中国东部百年尺度干事变化主要受北大西洋年代际振荡影响, 而年代际干旱事件的主导因子则是厄尔尼诺和负位相的北大西洋年代际振荡.     

Response of extreme precipitation to increasing extratropical cyclonic vortex frequency

Since the 2000s, extratropical extremes have been more frequent, which are closely related to anomalies of planetary-scale and synoptic-scale systems. This study focuses on a key synoptic system, the extratropical cyclonic vortex (ECV) over land, to investigate its relations with extreme precipitation. It was found that ECVs have been more active post-2000, which has induced more extreme precipitation, and such variation is projected to persist along with increasing temperature within 1.5°C of global warming. An enhanced quasi-stationary vortex (QSV) primarily contributes to the ECV, rather than inactive synoptic-scale transient eddies (STEs). Inactive STEs respond to a decline in baroclinicity due to the tendency of the homogeneous temperature gradient. However, such conditions are helpful to widening the westerly jet belt, favoring strong dynamic processes of quasi-resonant amplification and interaction of STEs with the quasi-stationary wave, and the result favors an increasing frequency and persistence of QSVs, contributing to extreme precipitation.摘要自21世纪以来, 热带外极端降水频次增加. 随着中高纬度的显著增温, 经向温度梯度减弱导致低层大气斜压性减小, 由此产生的气旋型瞬变涡天气系统等活动减弱. 然而, 热力分布导致西风急流带变宽, 经向环流加大, 有助于行星尺度波动相关的涡旋异常增加, 如东北冷涡, 中亚涡, 东欧-地中海涡, 北美涡等, 进而增加了气旋涡影响范围的极端降水频次. 在未来变暖背景和1.5°C增温的目标内, 热带外气旋涡增强会进一步促进极端降水发生.     

Key regions where land surface processes shape the East Asian climate

Land–atmosphere interaction, as one of the key processes affecting the atmosphere and climate over East Asia, has drawn increasing attention during the past few decades. However, the current level of understanding regarding the mechanisms through which land surface processes impact the East Asian climate needs to be improved. Based on existing studies, six key regions where land surface processes affect the East Asian climate are proposed in this study, which can provide a valuable reference for future research into land–atmosphere interaction in East Asia.摘要陆气相互作用是影响东亚大气环流和气候的一个关键过程, 受到了越来越多的关注. 然而, 关于陆面过程影响东亚气候的相关机理的理解还有待提升. 在已有研究基础上, 提出了陆面过程影响东亚气候研究值得关注的青藏高原, 欧亚中高纬地区, 中国东部季风区, 中南半岛, 中亚中纬度区域, 西亚等6个关键区, 期待为加强陆面过程与东亚气候研究提供一定参考.     

Synoptic-scale reversal of dipole surface temperature anomalies over East Asia and Central Siberia in November 2021

In early-to-mid November 2021, a pronounced reversal of surface air temperature (SAT) anomalies (SATAs) occurred over East Asia and Central Siberia, with extreme SATAs that reached up to about 10 °C. Such a synoptic-scale reversal of SATAs was characterized by the alternate emergence of the “colder Central Siberia–warmer East Asia” pattern and the “warmer Central Siberia–colder East Asia” pattern in November 2021. Coinciding with the reversals of the meridional dipole SATAs, large-scale atmospheric circulation anomalies experienced reversed changes. The development of the anomalous cyclonic (anticyclonic) flow over East Aisa (Central Siberia) was crucial for the occurrence of the “warmer Central Siberia–colder East Asia” pattern. Moreover, as the leading mode of daily SAT variability in approximately 56% of the Novembers during 1979–2021, the meridional dipole pattern of warmer (colder) anomalies over Central Siberia and colder (warmer) anomalies over East Asia may be one of the dominant modes of November SAT variability over Eurasia on the synoptic scale.摘要2021年11月, 东亚与中西伯利亚经历了相反的冷暖异常转换, 表现为“中西伯利亚偏冷, 东亚偏暖”与“中西伯利亚偏暖, 东亚偏冷”的交替出现. 该偶极型气温异常的天气尺度反转伴随着大尺度大气环流异常的反转. 进一步分析表明, 东亚与中西伯利亚的偶极型气温异常反转是1979–2021年期间11月欧亚气温日变化的主导模态之一(发生概率超过56%).     

Subseasonal predictability of the transition of the stratospheric polar vortex: A case study in winter 1987/88

The stratospheric polar vortex (SPV), which is an important factor in subseasonal-to-seasonal climate variability and climateprediction, exhibited a remarkable transition from weak in early winter to strong in late winter in 1987/88 (most significant on the interannual timescale during 1979–2019). Therefore, in this study, the subseasonal predictability of this transition SPV case in 1987/88 was investigated using the hindcasts from a selected model (that of the Japan Meteorological Agency) in the Subseasonal-to-Seasonal Prediction project database. Results indicated that the predictability of both weak and strong SPV stages in winter 1987/88, especially near their peak dates, exhibited large sensitivity to the initial condition, which derived mainly from the sensitivity in capturing the 100-hPa eddy heat flux anomalies. Meanwhile, the key tropospheric precursory systems with respect to the occurrence and predictability of this transition SPV case were investigated. The Eurasian teleconnection wave trains might have been a key precursor for the weak SPV stage, while significant tropospheric precursors for the strong SPV stage were not found in this study. In addition, positive correlation (r = 0.41) existed between the forecast biases of the SPV and the NAO in winter 1987/88, which indicates that reducing the forecast biases of the SPV might help to improve the forecasting of the NAO and tropospheric weather.摘要平流层极涡作为冬季次季节尺度上一个重要的可预测性来源, 其强度在1987/88年冬季表现为1979–2019年最显著的转折, 即在前 (后) 冬极端偏弱 (强). 因此在本文中选取这一个例研究了该年冬季平流层极涡在次季节尺度上的可预测性. 结果表明弱极涡和强极涡事件的预测与模式能否准确预测上传行星波的强度紧密相关. 同时, 发现前期对流层欧亚遥相关波列可能是弱极涡事件发生的关键预兆信号. 此外, 模式对平流层极涡强度和北大西洋涛动预测误差之间存在显著正相关关系, 表明模式减少平流层极涡的预测误差可能可以提高北大西洋涛动及相关对流层气候预测.     

Evaluating the effects of topographical factors on the precipitation simulated by kilometer-scale versus quarter-degree dynamical downscaling models in eastern China

Topography as well as its attributes are fundamental factors during precipitation generation. Various models with different complexity have been established to interpret the topography–precipitation relationship. In this study, the topography–precipitation relationships simulated by two dynamical downscaling models (DDMs) at the kilometer-scale and traditional quarter-degree resolution in eastern China are evaluated by utilizing multi-scale geographically weighted regression with station precipitation observations as reference. The precipitation simulated by the kilometer-scale DDM had a higher agreement with observations than the quarter-degree simulation. For the effects of topography on precipitation, observations revealed a dominant role played by the topographical relief in the precipitation distribution at most stations in the study region. The kilometer-scale DDM generally reflected this dominant role of topographical relief. However, the quarter-degree DDM showed an excessive dependency of the precipitation distribution on the topographical elevation. This research highlights the key role of underground sub-grid variations on the precipitation in eastern China, which implies a potential way forward for precipitation simulation improvements.摘要与传统的1/4度 (≈25-30 km) 动力降尺度模拟相比, 公里尺度模拟的降水空间分布与观测结果更为接近. 为了研究这一差异原因, 本研究以华东地区为例, 探究了地形因子在观测和模拟的降水中的作用. 为了更好地体现地形因子对降水分布非均匀性的影响, 以及不同地形因子作用的尺度差异, 本研究采用多尺度地理加权回归模型, 对五个主要地形因子与公里尺度和1/4度分辨率模拟的降水的关系进行了评估. 基于观测数据的研究结果显示地形起伏度, 地形高程和离海岸线距离对华东地区降水分布的非均匀性都有重要影响, 其中地形起伏度在研究区大部分站点降水分布中起主导作用; 公里尺度模拟结果基本反映了地形起伏度的主导作用; 而1 / 4度模拟结果表现出降水对地形高程的过度依赖. 本研究揭示了公里尺度地形分布对中国东部降水的非均匀分布的关键作用, 研究结果可以为改进降水模拟提供新的思路.     

Response of a westerly-trough rainfall episode to multi-scale topographic control in southwestern China

This study aims to quantify the response of a westerly-trough rainfall episode that occurred in summer 2020 to multi-scale topographic control in southwestern China, based on observations and numerical simulations. The multi-scale topography is composed of the Tibetan Plateau, Hengduan Cordillera (HC), and Sichuan Basin (SB). The westerly trough was characterized by southeastward deepening together with an in-phase propagating rainfall episode. By utilizing the results of numerical experiments, how the multi-scale topography impacted this westerly trough rainfall episode is explored. It is found that HC was the pivotal topographic factor affecting the southeastward extension of the trough and related rainfall, while SB accerelated the eastward movement of the westerly trough and changed the tilting direction of the trough line, thus further changing the location and orientation of precipitation. For extreme rainfall with intensity exceeding 10 mm h^?1, a roughly threefold rise in the cover ratio (from 1.8% to 7.2%) and fourfold increase in the areal rainfall amount per hour occurred by removing the HC barrier, due to the strongest vorticity and long-distance transport capacity to potential vorticy mass accompanying the southeast-stretching trough. Our results quantitatively reveal a strong response of westerly trough rainfall to multi-scale topographic control in southwestern China, therefore serving as an important reference for future decision making and effective model improvement.摘要中国西南部地形复杂, 降水频发, 地形对降水的影响至关重要. 本文基于观测和数值模拟, 定量揭示了青藏高原, 横断山脉和四川盆地多尺度地形对该地区西风槽降水的影响. 发现横断山脉是影响槽东南伸展, 降水传播的关键地形要素, 而四川盆地可加速西风槽东移, 改变槽线倾斜方向, 进而改变降水的位置和方向. 对于极端降水事件, 移除横断山脉屏障后, 降水覆盖率约增加3倍 (从1.8%增至7.2%), 小时面雨量增强4倍. 这些研究, 可为地形复杂地区降水的未来预报决策和有效模式改进提供参考.     

A deep learning–based U-Net model for ENSO-related precipitation responses to sea surface temperature anomalies over the tropical Pacific

SST–precipitation feedback plays an important role in ENSO evolution over the tropical Pacific and thus it is critically important to realistically represent precipitation-induced feedback for accurate simulations and predictions of ENSO. Typically, in hybrid coupled modeling for ENSO predictions, statistical atmospheric models are adopted to determine linear precipitation responses to interannual SST anomalies. However, in current coupled climate models, the observed precipitation–SST relationship is not well represented. In this study, a data-driven deep learning-based U-Net model was used to construct a nonlinear response model of interannual precipitation variability to SST anomalies. It was found that the U-Net model outperformed the traditional EOF-based method in calculating the precipitation variability. Particularly over the western-central tropical Pacific, the mean-square error (MSE) of the precipitation estimates in the U-Net model was smaller than that in the EOF model. The performance of the U-Net model was further improved when additional tendency information on SST and precipitation variability was also introduced as input variables, leading to a pronounced MSE reduction over the ITCZ.摘要SST–降水反馈过程在热带太平洋ENSO演变过程中起着重要作用, 能否真实地在数值模式中表征SST–降水年际异常之间的关系及相关反馈过程, 对于准确模拟和预测ENSO至关重要. 例如, 在一些模拟ENSO的混合型耦合模式中, 通常采用大气统计模型 (如经验正交函数; EOF) 来表征降水 (海气界面淡水通量的一个重要分量) 对SST年际异常的线性响应. 然而在当前的耦合模式中, 真实观测到的降水–SST统计关系还不能被很好地再现出来, 从而引起 ENSO模拟误差和不确定性. 在本研究中, 使用基于深度学习的U-Net模型来构建热带太平洋降水异常场对SST年际异常的非线性响应模型. 研究发现: U-Net模型的性能优于传统的基于EOF方法的模型. 特别是在热带西太平洋海区, U-Net模型估算的降水误差远小于EOF模型的模拟. 此外, 当SST和降水异常的趋势信息作为输入变量也被同时引入以进一步约束模式训练时, U-Net模型的性能可以进一步提高, 如能使热带辐合带区域的误差显著降低.     

Observed decadal shifts and trends in global tropical cyclone activities from 1980 to 2021

This study investigates the variability of annual tropical cyclone (TC) frequency and intensity over six major ocean basins from 1980 to 2021. Statistical change-point and trend analyses were performed on the TC time series to detect significant decadal variation in TC activities. In the middle of the last decade of the 20th century, the frequency of TC genesis in the North Atlantic basin (NA) and North Indian Ocean (NIO) increased dramatically. In contrast, the frequency in the western North Pacific (WNP) decreased significantly at the end of the century. The other three basins—the East Pacific, southern Indian, and South Pacific—all experienced a declining trend in annual TC frequency. Over recent decades, the average TC intensity has decreased in the East Pacific and the NA, whereas it has risen in the other ocean basins. Specifically, from 2013 to 2021, the average peak TC intensity in the NIO has enhanced significantly. The magnitude of the Genesis Potential Index exhibits fluctuation that is consistent with large-scale parameters in the NIO, NA, and WNP, emphasizing the enhancing and declining trends in TCs. In addition, a trend and correlation analysis of the averaged large-scale characteristics with TCs revealed significant associations between the vertical wind shear and TC frequency over the NIO, NA, and WNP. Therefore, global TC trends and decadal variations associated with environmental parameters deserve further investigation in the future, mainly linked to the significant climate modes.摘要研究发现在1980–2021期间全球6个海域每年热带气旋的发生频次和强度具有显著年代际变化规律, 最近几十年, 北大西洋和北印度洋的热带气旋发生频次明显增加, 但西北太平洋的热带气旋却显著下降. 另外三个海域, 东太平洋, 南印度洋和南太平洋发现所生成的热带气旋有减少趋势. 但在过去十几年, 平均热带气旋的强度除了在东太平洋和北大西洋有所减弱但在其他几个海域有所加强, 特别是在 2013–2021期间, 北印度洋的平均热带气旋的强度增强明显. 热带气旋的潜在生成指数 (GPI) 增加或减少趋势变化与北印度洋, 北大西洋和西太平洋热带气旋变化相关的大尺度环流一致. 另外, 北印度洋, 北大西洋和西太平洋上空的垂直风切变是影响其区域热带气旋发生频次变化的主要因子, 不同的气候模态也可能对全球热带气旋的趋势变化和年代际变化有影响, 值得进一步研究.     

Detection of the synoptic southeastward-extending Siberian cold high during 1978–2017

To better understand the relationship between anticyclones in Siberia and cold-air activities and temperature changes in East Asia, this study proposes a 2D anticyclone identification method based on a deep-learning model, Mask R-CNN, which can reliably detect the changes in the morphological characteristics of anticyclones. Using the new method, the authors identified the southeastward-extending Siberian cold high (SEESCH), which greatly affects wintertime temperatures in China. This type of cold high is one of the main synoptic systems (45.7%) emerging from Siberia in winter. Cold air carried by SEESCH has a significant negative correlation with the temperature changes in the downstream area, and 52% of SEESCHs are accompanied by cold-air accumulation in North and East China, which has a significant impact on regional cooling. These results provide clues for studying the interconnection between SEESCHs and extreme cold events.摘要为了更好地研究西伯利亚地区反气旋与冷空气活动,东亚地区气温变化之间的关联, 本文提出一种基于Mask R-CNN的反气旋识别方法, 能够较为准确地刻画反气旋形态特征变化. 使用该方法能够识别对中国冬季气温具有较大影响的东南延伸型西伯利亚冷高压(SEESCH), 这种冷高压是冬季出现在西伯利亚地区的主要天气系统之一(45.7%). SEESCH携带的冷空气与下游地区温度变化呈显著负相关, 52%的SEESCH伴随着华北华东地区冷空气聚集, 对区域降温有显著影响. 这些结果为研究 SEESCH 与极端寒冷事件之间的联系提供线索.