The amplified warming on the Tibetan Plateau (TA) is a distinctive characteristic of global climate change, leading to various climate responses with far-reaching implications. This study investigates the influence of interannual variation of TA on summer precipitation over East Asia (Pre_EA) using observational data and a Linear Baroclinic Model (LBM). When TA exceeds the Northern Hemisphere average, summer precipitation in the Yangtze River Valley significantly decreases, while it increases in North China and South China, resulting in a tripole Pre_EA pattern. Notably, the relationship between TA and Pre_EA is independent of the El Niño-Southern Oscillation (ENSO) and explains more variance in Pre_EA than ENSO. Our analysis reveals that TA enhances the tripole Pre_EA pattern by modulating moisture transport and vertical motion in the East Asia-North Pacific regions. Specifically, positive TA is linked to significant local tropospheric warming, which intensifies and eastward expands the South Asian High, creating a double-gyre meridional circulation over East Asia. Additionally, positive TA induces an eastward-propagating wave, reinforcing a midlatitude anomalous high-pressure belt over East Asia and the western North Pacific regions. These circulation changes weaken the East Asian subtropical jet, form a notable double jet configuration, and promote subsidence over mid-latitude East Asia. Moreover, anomalously warm sea surface temperatures in the Northwestern Pacific reinforce the TA-Pre_EA relationship by contributing to the mid-latitude East Asia-North Pacific high-pressure belt. Our LBM model experiments support these findings. Our study provides an in-depth understanding of the physical processes influencing summer precipitation variability in East Asia.

Based on observation data from 1958 to 2020, the current study explores the interdecadal modulation effects on moderate El Niño-Southern Oscillation (ENSO) episodes and East Asian (EA) winter surface air temperature (SAT) through the Pacific Decadal Oscillation (PDO). Strong and moderate ENSO episodes are classified by their amplitudes. The current work investigates the influence of moderate ENSO episodes on the EA winter SAT, especially moderate La Niña episodes, which show a close relationship with the EA winter SAT. To explore the PDO modulation effect on the influence of ENSO episodes, these ENSO episodes are further divided into two categories in terms of warm or cold PDO phases. The composite results show that in the warm phase of the PDO, the moderate La Niña signal is relatively strong and stable, with a profound impact on the EA winter SAT variability, whereas in the cold PDO phase, the relationship between the EA winter SAT and moderate La Niña episodes becomes ambiguous. Further studies show that the PDO modulates the moderate La Niña impacts on EA winter SAT primarily through varying the East Asian winter monsoon (EAWM). While moderate La Niña episodes take place in a warm PDO phase, positive and negative anomalies of sea level pressure (SLP) are observed in the Eurasian continent and mid–high-latitude North Pacific, respectively, favoring anomalous northerlies along the eastern coast of East Asia and therefore a colder-than-normal EA winter. In contrast, in a moderate La Niña winter during the cold PDO phase, the mid–high-latitude North Pacific is controlled by an anomalous high-pressure system with southerly anomalies along its western flank, and therefore, a weak warm pattern is observed for the EA winter SAT.

Considering the diverse operational need of Meiyu prediction, evaluation of multi-scale forecast skill of CFSv2 for Zhejiang Meiyu precipitation in 2018 was systematically conducted. Despite the forecast results on May 31, which is consistent with the observation, the initial forecast results from March 1 to May 31 could not accurately capture the trend of less precipitation in most regions of Zhejiang in June generally. On the extended-range（ER） scale, the total precipitation predicted by CFSv2 during Meiyu period exhibited approximately 30% less than observation. Based on the correlation coefficient, root mean square error and newly defined integrated forecast skill index, the forecast performance of CFSv2 on ER scale was comprehensively evaluated. The results showed that the forecast skills for total precipitation, daily regional-mean precipitation and daily stational precipitation are very limited during Meiyu period, and the forecast results for Meiyu region in Zhejiang were slightly better than for the entire Zhejiang. The evaluation results indicated that CFSv2 forecast products of Zhejiang show significant systematic bias. On the ER scale, with the decrease of forecast time scale, the improvement of forecast skill was not noticeable, and instead there is an optimal forecast time scale. Also, during the full forecasted period for each ensemble, the most skillful time-interval was commonly not the nearest time frame. To sum up, the CFSv2 forecast for Meiyu precipitation on the ER scale may not be sensitive to the initial field.

The CFSv2 forecast products have been widely used in climate prediction operation all over the world. Although the real-time forecast is able to basically capture large pattern of climate anomaly, there still exists obvious bias, which may have enormous impacts on predicted result and thus cannot be

2017年盛夏7-8月浙江省发生了大范围、持续性极端高温热浪事件,省内大部地区高温日数达到30 d以上,多数区域极端最高气温达到39℃以上、39站达40℃以上,多站平均气温、高温日数和极端最高气温破历史纪录,高温累计日数和有效积温仅次于2003年和2013年.要素特征和高温强度分析表明该年是有观测以来盛夏高温热浪最突出的年份之一.西太平洋副热带高压(西太副高)偏强偏西是造成此次极端事件的直接原因;对流层低层中国东南部—西北太平洋区域形成了强大的反气旋式风场异常,下沉增温与辐射增温共同推动浙江气温不断升高;全球变暖是极端高温热浪形成的重要气候背景;出梅偏早和影响台风偏少也有潜在贡献.西太副高位置和强度的变化与热带、副热带和中高纬度环流系统密切相关.热带海洋性大陆和南海—中南半岛区域潜热释放增强有利于通过经向环流加强副高;副热带30°N附近的北太平洋对流旺盛、与西太副高之间形成纬向热力梯度,激发出异常纬向垂直环流从而增强副高;中高纬度200 hPa西风急流轴略偏北、极涡明显偏向西半球削弱了中高纬地区对副热带系统的影响,有利于副高的稳定维持.最后提出了未来值得开展的极端高温研究工作.

极端最高气温是日最高气温变化的上限.基于浙江省66个常规气象站的长序列气温观测资料,分析了浙江区域1973-2019年极端最高气温的时空演变特点,发现全省平均极端最高气温和最大极端最高气温均表现出明显的上升趋势,二者之间存在显著相关,前者在上世纪80年代末～90年代初发生了突变;在空间分布上,多年平均极端最高气温和最大极端最高气温大致呈现自内陆向沿海逐步递减的特征;浙江全省极端最高气温多出现在浙南的丽水市,但从上世纪90年代中后期开始出现在浙北地区(主要是杭州市、绍兴市和宁波市)的年份明显增多;2005年之前全省极端最高气温主要出现在浙西和浙中地区,之后出现在浙东地区的年份显著增多;极端最高气温最易出现在盛夏7～8月特别是7月下旬和8月上旬.在此基础上,进一步探讨了全省极端最高气温出现在浙北、浙中和浙南地区对应的大尺度环流特征以及ENSO不同位相与盛夏极端最高气温出现月份之间的关联,并对拓展研究区域、开展长三角和华东区域极端最高气温研究提出了展望.

Based on various statistical indices, the abilities of multi-generation reanalyses, namely the NCEP / NCAR Reanalysis 1 (R1), the NCEP-DOE Reanalysis 2 (R2) and the NCEP Climate Forecast System Reanalysis (CFSR), to reproduce the spatiotemporal characteristics of precipitation over Zhejiang Province are comprehensively compared. The mean absolute bias percentages for three reanalyses are 20% (R1), 10% (R2) and 37% (CFSR). R2 (R1) gives the best (worst) general depiction of the spatial characteristics of the observed precipitation climatology, whereas a significant wet bias is noticed in the CFSR. All reanalyses reasonably reproduce the interannual variability with the correlation coefficients of 0.72 (R1), 0.72 (R2) and 0.84 (CFSR). All reanalyses well represent the first two modes of the observed precipitation through Empirical Orthogonal Function analysis, with CFSR giving the best capture of the principal components. The root-mean-square error (RMSE) is the largest (smallest) in the CFSR (R2). The large RMSE of CFSR in summer (especially in June) contributes mostly to its systematic wet bias. After 2001, the wet bias of CFSR substantially weakens, probably attributed to increasing observations assimilated in the CFSR. On a monthly basis, the percentage of neutral bias cases are similar for all reanalyses, while the ratio of positive (negative) bias cases for CFSR is distinctly larger (smaller) than that of R1 and R2. The proportions of negative bias cases for R1 and R2 begin to increase after 2001 while keeping stable for CFSR. On a daily basis, all reanalyses give good performances of reproducing light rain; however, the reflection of moderate rain and heavier rain by the CFSR is better than R1 and R2. Overall, despite being a third-generation reanalysis product, the CRSR does not exhibit comprehensive superiorities over R1 and R2 in all aspects on a regional scale.

利用浙江省66个基本气象站1979—2010年的日平均气温数据,系统评估了三套再分析资料R1、R2和CFSR对浙江省气温的刻画能力.结果表明:三套再分析资料的气候平均态与观测均存在一定差异,其中R1、R2的空间分布型与观测较为接近,CFSR与观测差异较大;三套再分析资料均存在系统性冷偏差且这一偏差在32年中稳定存在,其中CFSR的冷偏差更显著,浙南地区是其冷偏差的重要来源.三套资料的均方根误差均存在季节变化:冬季(特别是1月)误差较小而夏季(特别是7-8月)误差较大,R1和R2的季节差异强于CFSR.CFSR对浙江省气温变率的把握能力优于R1和R2,其距平场EOF分解前三模态的空间型态和时间系数与观测更为接近.系统误差订正后,三套再分析资料的可信度得到显著改善,CFSR的改善效果最明显,说明系统性误差是三套再分析资料偏差的重要来源.改善后三套再分析资料的均方根误差和空间相关系数大体相当.CFSR网格点气温插值到观测站点时因海拔差异导致的误差以及CFSR在浙江省的模式地形偏高可能是其有较大冷偏差的重要原因.

This study assesses the impact of the Pacific Decadal Oscillation (PDO) on the severity of autumn droughts in North China (ADNC) during the period of 1962-2013. The ADNC are defined using the Palmer drought severity index (PDSI). The results show that the ADNC are more severe during negative PDO phases than during positive PDO phases. During the negative PDO phase, both the autumn precipitation and the Penman-Monteith reference evaporation (PET_pm) play roles in the significant severity of the ADNC. In contrast, during positive PDO phases, the droughts over North China are impacted to a greater degree by the climate conditions in previous seasons. Regression analysis shows that the atmospheric circulation anomalies associated with ADNC are quite different over North China under different PDO phases. Specifically, during negative PDO phases, anomalous low-pressure systems are observed over the East Asia-western Japan area, indicating a strengthened East Asian trough (EAT). Anomalous northerly winds bring cold and dry air from high-latitude inland regions to North China, leading to severe ADNC during this period. Meanwhile, significant descending motion is observed in the upstream region of the EAT and the area of North China, accompanied by reduced cloudiness and reduced precipitation over these regions. In contrast, during positive PDO phases, the ADNC-associated atmospheric circulation anomalies over North China are quite weak. Further analysis shows that during the negative phase of PDO years, the El Nino Southern Oscillation can impact the severity of ADNC through the centre of the Pacific-East Asian teleconnection pattern over northeastern Asia.

Changes in the relationship between interannual variation of spring (March–April–May) surface air temperature (SAT) over eastern Eurasia (SAT_EA) and Eurasian snow during 1972–2009 are investigated. The results show that the interannual variation in SAT_EA is significantly correlated with Eurasian snow cover anomalies (SCA) over two key regions, which include the western (SCAW) and eastern (SCAE) parts of the Eurasian continent. A pronounced climate shift of SAT_EA is observed around the late 1980s, and therefore, the data are divided into two subperiods, that is, 1972–1987 (P1) and 1988–2009 (P2). Examinations show that the relationship between SAT_EA and SCAW is impacted by the Arctic Oscillation. In contrast, the relationship between SCAE and the SAT_EA is Arctic Oscillation independent and is significantly increased from P1 to P2. Further analysis shows that the increased correlation between SAT_EA and SCAE from P1 to P2 cannot be well explained by the snow‐related surface heat flux. A wave train‐like anomalous circulation plays an important role in enhancing their relationship through intensified wind‐induced heat advection in P2, which causes reduced snow and increased SAT_EA. A lead‐lag regression analysis of the SCAE and the circulation anomalies indicates that the changes of the SCAE and SAT_EA are mainly responses to atmospheric circulation anomalies that occur 2 months before spring. Numerical experiments that use a linear baroclinic model indicate that the snow anomalies around Lake Baikal may impact the variation of SAT_EA through modulating atmospheric circulation over the East Asia‐North Pacific area.

This study investigates the interdecadal changes in the relationship between the Eurasian snow cover extent (SCE) and spring precipitation over southern China. The results show that the spring precipitation over southern China has experienced an obvious change from an active to a quiet period since the late 1980s. The entire data set is divided into two subperiods, that is, 1972–1987 (P1) and 1988–2009 (P2). The interannual variation of the spring precipitation of southern China is found to be significantly related to Eurasian SCE anomalies around northwest of Lake Baikal. Less‐than‐normal SCE northwest of Lake Baikal corresponds to more‐than‐normal spring precipitation over southern China. The result of a numerical experiment performed using a baroclinic atmospheric model indicates that the Eurasian snow anomalies around northwest of Lake Baikal can impact the variation of spring precipitation over southern China by modulating the atmospheric circulations over eastern Asia and the western North Pacific region. Further examination shows that the SCE anomalies around Lake Baikal contribute differently to the variation of spring precipitation over southern China before and after the late 1980s. During P1, the SCE anomalies around Lake Baikal contribute to the variation of spring precipitation over China in the form of a mono‐sign pattern, which is similar to the first empirical orthogonal function mode of spring precipitation, while in P2, the SCE anomalies around Lake Baikal favor a north‐south dipole mode of the spring precipitation variation, which bears considerable similarity to the second empirical orthogonal function mode of spring precipitation over China.

ABSTRACTThe modulation of the Pacific Decadal Oscillation (PDO) to the relationship between the El Niño‐Southern Oscillation (ENSO) events and the winter climate over North America (NA) was investigated using both observational data and a simple general circulation model. ENSO events are divided into strong and moderate events according to their amplitudes. These ENSO events are further categorized into two groups according to the phases of the PDO. The present study focuses on the impact of moderate ENSO events on the wintertime climate over NA. The results show that a moderate El Niño signal is strong and stable and can significantly influence the wintertime climate variability over NA during a cold PDO phase, whereas climate anomalies are weak and are not significantly associated with moderate El Niño events during a positive PDO phase. Possible mechanisms accounting for the modulation of the impacts of moderate El Niño events on the wintertime climate over NA are analysed. Two factors may contribute to the different impacts of moderate El Niño events during warm and cold PDO phases. First, the different characteristics of the tropical Pacific sea surface temperature anomalies (SSTAs) associated with moderate El Niño events. Second, the difference in the climatological mean state between the warm and cold phases of PDO years. Our numerical experiments suggest that the cold PDO mean state is more favourable to a wavetrain‐like atmospheric response to an El Niño‐like tropical Pacific forcing over the North American sector than the warm PDO mean state is.

The current study investigates the interdecadal changes in the relationship between the winter precipitation anomalies in southeastern China, El Nino-Southern Oscillation (ENSO), and the East Asian winter monsoon (EAWM) at the end of the twentieth century. It appears that the relationships between the interannual variability of the southeastern China winter precipitation and ENSO as well as EAWM are obviously weakened after 1998/99. The possible mechanisms accounting for this interdecadal change in the relationship have been examined by dividing the data into two subperiods [1980-98 (P1) and 1999-2015 (P2)]. The results indicate that, without the linear contribution of EAWM, ENSO only play a limited role in the variability of winter precipitation in southeastern China in both subperiods. In contrast, in P1, corresponding to an ENSO-independent weaker-than-normal EAWM, anomalous southerlies along coastal southeastern China associated with an anticyclone over the northwestern Pacific transport water vapor to China. However, in P2 the impact of EAWM on winter precipitation in southeastern China is weakened because of the regime shift of EAWM. The EAWM-related positive SLP anomalies over the North Pacific move eastward in P2, causing an eastward migration of the associated anomalous southerlies along its western flank and therefore cannot significantly contribute to the positive winter precipitation anomalies in southeastern China.

ABSTRACTThe diverse influences of the El Niño Southern Oscillation (ENSO) on wintertime precipitation over the Maritime Continent are herein investigated using observational data. The ENSO events are divided into four categories—strong El Niño, strong La Niña, moderate El Niño and moderate La Niña—according to their amplitudes. Significant negative Maritime Continent precipitation anomalies are observed during strong El Niño events, whereas wet conditions occur during strong La Niña events. In the case of moderate La Niña events, the Maritime Continent rainfall anomalies are similar in spatial structure and comparable in amplitude to strong La Niña events, whereas almost no significant rainfall anomalies are observed in moderate El Niño years. Further analysis shows that strong El Niño (strong/moderate La Niña) events are associated with a significant anticyclone (cyclone) surface response over the western tropical Pacific and related to a significant descending (ascending) branch of the Walker circulation around the Philippines. However, during moderate El Niño events, both the central‐eastern and the western tropical sea surface temperature anomalies (SSTA) are weak and cannot significantly change the Walker circulation over the tropical Pacific. Therefore, the descending branch anomalies of the Walker circulation associated with moderate El Niño events around the Philippines are quite weak and cannot cause significant negative precipitation anomalies in that location. In addition, there is large case‐to‐case variance in the tropical Pacific SSTA associated with moderate El Niño events, which also contributes to the weak Maritime Continent rainfall response in the composite analysis.

The interdecadal change of the leading mode of the mean winter precipitation over China has been investigated using observational data for the period from 1960 to 2012. The leading empirical orthogonal function (EOF) mode (EOF1) of the winter precipitation over China displays a mono-sign pattern over southeastern China, accounting for 49.7 % of the total variance in the precipitation. Both the El Niño-Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) can impact EOF1. A positive (negative) EOF1 is accompanied by warm (cold) ENSO events and weak (strong) EAWM, and the latters can cause anomalous southerlies (northerlies) along the coast of southeastern China, accompanied by the transportation of water vapor from the Bay of Bengal and the South China Sea favoring a wet (dry) winter over southeastern China. An abrupt transition of the EOF1 is observed around the mid-1980s. Therefore, the data are divided into two subperiods, i.e., 1960–1987 (P1) and 1988–2009 (P2). Significant differences in the large scale atmospheric circulation and sea surface temperature anomalies associated with EOF1 during these two subperiods are observed. EOF1 is closely related to the mid- to high-latitude atmospheric circulation in P1, while its relationship to the tropics obviously increases during P2. The partial regression analysis results show that the interdecadal change of EOF1 is caused by both the interdecadal changes of the EAWM and ENSO around the mid-1980s. In P1, the lower-level anomalous southerlies along the coastal southeastern China accompanied by water vapor transportation that causes above-average precipitation are related to an anti-cyclonic system centered over the mid-latitude western North Pacific associated with EAWM. In P2, the influence of the EAWM is weaker, and the southerly anomaly over the coastal southeastern China is mainly caused by the anticyclone over Philippines, which is related to the ENSO.

The interdecadal change in the relationship between the winter mean surface air temperature (SAT) over East Asia (EA) and the El Nino-Southern Oscillation (ENSO) is investigated using both observational data and a simple general circulation model. The positive phase of the first empirical orthogonal function (EOF) of SAT over EA is characterized by significant warming over midlatitude to high-latitude EA and is linked to the Arctic Oscillation. The second EOF (SAT-EOF2) is represented by significant cooling extending from 55 degrees N to the tropics and abnormal warming over high-latitude EA. Focus is given to SAT-EOF2 which has a close relationship with the La Nina-type sea surface temperature (SST) anomalies. A clear shift in SAT-EOF2 is observed in the mid-1980s. The relationships between SAT-EOF2 and ENSO in two subperiods, i.e., 1957 to 1982 (P1) and 1986 to 2010 (P2), are discussed and compared. Results show that the relationship between SAT-EOF2 and ENSO significantly strengthens after the mid-1980s due to stronger SST and precipitation anomalies in P2 than in P1 associated with ENSO in the tropical western Pacific. In the midlatitudes, the Pacific-North American teleconnection pattern is more closely related to ENSO in P2, whereas in P1, the ENSO-related atmospheric circulation anomalies are more similar to a zonally orientated teleconnection pattern. Numerical experiments suggest that the difference in the ENSO-related circulation anomaly in the midlatitudes is likely related to the difference in the climatological mean flows of these two subperiods.