Inhomogeneous trends in the onset date of extreme hot days in China over the last five decades

Using a homogenized daily maximum temperature(T_(max))dataset across China,this study characterized the spatiotemporal variation of the onset date of extreme hot days in a year(i.e.,FirstEHD)during 1960-2018.Inhomogeneous trends of FirstEHD over China during 1960-2018 can be found,with the advanced trend of FirstEHD over most parts in China,while a number of stations in North-Central China(NC)show the delayed trend of FirstEHD.Moreover,there exist interdecadal changes of FirstEHD trend,with a remarkable difference in the trend magnitude before and after the 1990s over South China(SC),and the sign of trend can even reverse from negative to positive after the 1990s in Xinjiang(XJ)and Yangtze River Basin(YR),and from positive to negative in NC.The overall trends of FirstEHD over NC,YR,and XJ during 1960-2018 are dominated by the trends before the 1990s,while they are dominated by the sharp advance after the 1990s over SC.It is further found that the trend of FirstEHD can generally be explained by the long-term trend in T_(max) over most parts of China,but the contribution from T_(max) variabilities is also non-negligible and can even account for more than 75% of the overall trend over NC.The possible factors responsible for the decadal changes in FirstEHD trends are also discussed.

Spatio-temporal analysis of the melt onset dates over Arctic sea ice from 1979 to 2017

The melt onset dates(MOD)over Arctic sea ice plays an important role in the seasonal cycle of sea ice surface properties,which impacts Arctic surface solar radiation absorbed by the ice-ocean system.Monitoring interannual variations in MOD is valuable for understanding climate change.In this study,we investigated the spatio-temporal variability of MOD over Arctic sea ice and 14 Arctic sub-regions in the period of 1979 to 2017 from passive microwave satellite data.A set of mathematical and statistical methods,including the Sen’s slope and Mann-Kendall mutation tests,were used to comprehensively assess the variation trend and abrupt points of MOD during the past 39 years for different Arctic sub-regions.Additionally,the correlation between Arctic Oscillation(AO)and MOD was analyzed.The results indicate that:(1)all Arctic sub-regions show a trend toward earlier MOD except the Bering Sea and St.Lawrence Gulf.The East Siberian Sea exhibits a significantly earlier trend,with the highest rate of-9.45 d/decade;(2)the temporal variability and statistical significance of MOD trend exhibit large interannual differences with different time windows for most regions in the Arctic;(3)during the past 39 years,the MOD changed abruptly in different years for different sub-regions;(4)the seasonal AO has more influence on MOD than monthly AO.The findings in this study can improve our knowledge of MOD changes and are beneficial for further Arctic climate change study.

EAST ASIA SUMMER MONSOON ONSET DATE CALCULATED FROM OBSERVED, REANALYZED AND COMBINED DAILY RAINFALL

In this paper, the East Asia summer monsoon onset date lines in East China are calculated by the definition similar to the traditional one, with the ECMWF reanalyzed 850 hPa daily wind and observed, reana-lyzed and combined daily rainfall during 1980 ~ 1993. To make the onset date line as close as possible to the previous work, the earliest onset date limits have to be applied for the regions with different latitude and the daily mean datasets have to be smoothed by space before calculation, therefore their space-resolution is reduced to about 3 longitude ×1 latitude. The results show that the multiyear mean summer monsoon onset date lines are quite similar to each other. Compared with the one from the reanalysis, the 14-year average onset date line form combination is obviously improved in the southern Sichuan Basin and the correlation between observed and combined onset date is also slightly higher over the Huaihe valley and Northeast China. Since daily rainfall combination also improved the long term daily mean and standard deviation through the pentad CMAP, if no better daily dataset is available, such a kind of daily rainfall combination can be used to get reasonable result in the Indian monsoon region without sufficient observatories or over the North Pacific without any ground obser-vation at all in future study.

ONSET AND RETREAT DATES OF THE SOUTH CHINA SEA SUMMER MONSOON AND THEIR RELATIONSHIPS WITH THE MONSOON INTENSITY IN THE CONTEXT OF CLIMATE WARMING

Global gridded daily mean data from the NCEP/NCAR Reanalysis(1948-2012) are used to obtain the onset date,retreat date and duration time series of the South China Sea summer monsoon(SCSSM) for the past 65 years.The summer monsoon onset(retreat) date is defined as the time when the mean zonal wind at 850 hPa shifts steadily from easterly(westerly) to westerly(easterly) and the pseudo-equivalent potential temperature at the same level remains steady at greater than 335 K(less than 335 K) in the South China Sea area[110-120°E(10-20°N)].The clockwise vortex of the equatorial Indian Ocean region,together with the cross-equatorial flow and the subtropical high,plays a decisive role in the burst of the SCSSM.The onset date of the SCSSM is closely related to its intensity.With late(early) onset of the summer monsoon,its intensity is relatively strong(weak),and the zonal wind undergoes an early(late) abrupt change in the upper troposphere.Climate warming significantly affects the onset and retreat dates of the SCSSM and its intensity.With climate warming,the number of early-onset(-retreat) years of the SCSSM is clearly greater(less),and the SCSSM is clearly weakened.

CHARACTERISTICS OF MIDDLE EAST JET STREAM DURING SEASONAL TRANSITION AND ITS RELATION WITH INDIAN SUMMER MONSOON ONSET

By using the NCEP/NCAR pentad reanalysis data from 1968 to 2009, the variation characteristics of Middle East jet stream(MEJS) and its thermal mechanism during seasonal transition are studied. Results show that the intensity and south-north location of MEJS center exhibit obvious seasonal variation characteristics. When MEJS is strong, it is at 27.5°N from the 67 th pentad to the 24 th pentad the following year; when MEJS is weak, it is at 45°N from the 38 th pentad to the 44 th pentad. The first Empirical Orthogonal Function(EOF) mode of 200-hPa zonal wind field shows that MEJS is mainly over Egypt and Saudi Arabia in winter and over the eastern Black Sea and the eastern Aral Sea in summer. MEJS intensity markedly weakens in summer in comparison with that in winter. The 26th-31 st pentad is the spring-summer transition of MEJS, and the 54th-61 st pentad the autumn-winter transition. During the two seasonal transitions, the temporal variations of the 500-200 hPa south-north temperature difference(SNTD) well match with 200-hPa zonal wind velocity, indicating that the former leads to the latter following the principle of thermal wind. A case analysis shows that there is a close relation between the onset date of Indian summer monsoon and the transition date of MEJS seasonal transition. When the outbreak date of Indian summer monsoon is earlier than normal, MEJS moves northward earlier because the larger SNTD between 500-200 hPa moves northward earlier, with the westerly jet in the lower troposphere over 40°-90°E appearing earlier than normal, and vice versa.

DETERMINATION OF SOUTH CHINA SEA MONSOON ONSET AND EAST ASIAN SUMMER MONSOON INDEX

Results of the definition of South China Sea summer monsoon onset date and East Asian summermonsoon index in recent years are summarized in this paper. And more questions to be resolved are introducedlater.

东北地区入春日期变化特征及前期信号场

本文利用1961~2020年水平分辨率为0.25°×0.25°的CN05.1日平均气温资料、NCEP/NCAR逐日再分析数据和NOAA逐日海表温度资料,对东北地区入春日期变化特征及前期信号场进行了分析,结果表明:(1)东北地区入春呈由南向北推进的特征,大部地区集中在第26和27候。东北地区入春日期近60年来呈显著提前的特征。(2)入春偏早典型年份,3月份东北地区地表温度和降水均呈现正距平特征,入春偏晚典型年份则相反;500 hPa环流场上,入春偏早典型年份,西西伯利亚平原、贝加尔湖和勘察加半岛存在一个低压槽、高压脊和低压槽的Ω形态环流配置,而入春偏晚典型年份,乌拉尔山地区存在一个高压脊,其东侧为一个强的斜槽,槽底位于东北地区上空,该环流配置从第22候持续到第26候。(3)入春偏早典型年份,前冬北太平洋最明显的关键区海温信号为赤道中东太平洋呈现类La Niña的模态,北大西洋格陵兰岛以南的海域和西太平洋菲律宾以东的海域为明显的海温正距平,入春偏晚典型年份则位相相反,通过相关分析发现,前冬北大西洋和西北太平洋关键区海温均对东北地区入春日期有较好的指示性。

气候变暖背景下青海三江源区季节冻土冻融特征研究

青海三江源区是全球气候变化的敏感区和生态环境脆弱区,目前正面临着冻土退化的问题。本研究基于三江源区18个国家气象站1961—2021年气象观测资料,对气候变暖前后季节冻土冻融特征进行对比分析。结果表明:三江源区年平均气温为-0.34℃,呈东高西低分布,总体以0.38℃·(10a)^(-1)的速率上升,并在1997年发生突变,突变后气温显著升高。平均年最大季节冻结深度为142.5 cm,自西北向东南减小,总体以2.4 cm·(10a)^(-1)速率退化,与变暖前相比减少了11 cm。平均地表冻结初日为10月24日,以1.0 d·(10a)^(-1)速率推迟,平均地表冻结终日为5月18日,以3.3 d·(10a)^(-1)速率提前,与变暖前相比,地表冻结终日提前了12 d,地表冻结初日推迟了14 d。季节冻土平均冻结时间为133.9 d,呈西高东低分布,总体以1.9 d·(10a)^(-1)速率减少,与变暖前相比减少了8.8 d。年最大冻结深度及冻结时间分别在2004年和2002年发生突变,相比气温均有一定滞后。这说明,季节冻土在受气温变化影响同时,还受地形、人类活动等其他因素影响。该研究揭示了三江源区季节冻土冻结作用弱化的现象,研究成果可为应对气候变化、工程建设等提供参考。

青藏高原冬季积雪与华南前汛期降水年际变率的联系

利用1979—2018年青藏高原(简称高原,下同)卫星积雪数据集、华南地区261站逐日降水及ERA5再分析资料,探讨了高原冬季积雪与华南前汛期降水的联系。结果表明:1)高原西部积雪与华南前汛期降水的正相关关系最为稳定,其主要影响前汛期的锋面降水,对夏季风降水的影响较小;2)华南前汛期在高原西部积雪偏多年比偏少年偏早20 d,使得前汛期降雨日数偏多,持续时间偏长,总降水量偏多,而降水强度受积雪的影响较小;3)高原积雪偏多年,积雪的冷却作用形成了低层异常反气旋环流,而东亚沿岸为“+-+”的位势高度异常,中纬度“西高东低”的环流配置有利于中高纬冷空气南侵,使得华南上空温度偏低,同时偏强偏南的西太平洋副热带高压加强了低纬地区偏南气流和水汽输送。3—4月锋面在华南北部南北摆动,4月初偏北干冷空气南侵和偏南暖湿气流的持续北推使得锋面加强,触发了前汛期的较早建立;积雪偏少年冷空气和偏南暖湿气流均较弱,华南北部锋面在4月初中断,4月中下旬华南北部锋面在偏北弱冷空气和偏南暖湿气流的共同作用下重新建立,从而华南前汛期开始偏晚。

Influence of Climate on Sugarcane Yield in Côte d’Ivoire: Case of the Ferkessédougou Region

This study aims to understand the current climatic trends and explain the possible losses of agricultural yields. To achieve this objective, this work characterized the evolution of extreme temperature indices in the sugar complexes of Ferké 1 and Ferké 2, two stations located in the northern part of C?te d'Ivoire. The onset and cessation dates of the rainy season and the length of the rainy season were investigated. The agricultural and climatic data were obtained from each sugar complex. The period of study ranges from 2002 to 2019 in Ferké 1 and Ferké 2. The results show significant upward trends in extreme temperature indices. The analysis of sugarcane yield associated with the different climatic parameters shows no significant results in general. However, on the Ferkessédougou sugar complexes, the results highlight that maximum and minimum temperatures could be the variables that influence most yield production. The maximum temperature with coefficients of 1.60 and 0.77 at Ferké 1 and Ferké 2 respectively seems to contribute to an increase in yield while the minimum temperature with coefficients of -0.98 and -0.22 at Ferké 1 and Ferké 2 respectively could lead to a loss in yield. The results obtained with the Single Linear Regression (SLR) and the Multiple Linear Regression (MLR) models also highlight the strong influence of minimum and maximum temperatures.

广东汛期开始日期的年际和年代际变化研究

用小波分析和相关统计等方法 ,对近 5 0多年来广东开汛的年际和年代际变化及与副高、PDO指数的关系进行了研究 ,同时也研究了它与后期天气、前期 5 0 0hPa环流场和海温场的关系。结果表明 ,近 5 0年来广东开汛呈偏早的趋势 ,具有明显的 2～ 7年左右的年际和 34年左右的年代际周期振荡 ;开汛的年代际振荡与副高、PDO指数是一致的 ,并与它们存在反向的关系 ,且突变的时间也均发生在 2 0世纪 70年代中后期 ;广东开汛与后期天气的相关分析表明 ,它与广东 4月降水的相关最好 ,与 5 ,6月的相关则差 ;与广东降水集中期初日只有在 1986年以后至今才具有显著的正相关 ,而与降水集中期次数却在 1977年之前就具有显著的负相关 ;广东开汛的早晚与前冬 5 0 0hPa高度场存在几个显著的相关中心 ,与前秋北太平洋西风漂流区的海温异常也有明显的相关。

南海夏季风活动的年际和年代际特征

利用ＮＣＥＰ风场资料和候平均向外长波辐射（ＯＬＲ）资料分析了南海区域低层风场与对流活动的关系，在此基础上，采用南海中南部的纬向风平均值来定义南海夏季风的爆发，确定了长序列（１９４９～１９９８）的南海夏季风爆发日期和强度指数，并研究南海夏季风活动的年际和年代际变化特征。结果表明：南海夏季风爆发日期和强度指数呈显著的反相关；５０年来的气候趋势是，爆发日期逐渐偏晚，强度指数逐渐减弱。二者都存在着明显的年际和年代际变化，它们在不同阶段上的波动是各种时间尺度振荡叠加的结果，而年代际尺度具有非常重要的作用。东印度洋海温异常在南海夏季风爆发前后，均与南海夏季风强度指数呈显著的反相关。东太平洋海温异常在南海夏季风爆发之前，与强度指数反相关，而爆发之后，与强度指数正相关。这体现了南海夏季风活动与ＥＮＳＯ事件的密切关系。

40a南海夏季风建立日期的确定

对 1 958～ 1 997年 NCEP/NCAR4～ 6月逐候资料的分析表明 ,将 ( 1 0～ 2 0°N,1 1 0～ 1 2 0°E)区域内面积平均的 850 h Pa层上稳定地有θse≥ 335K且纬向风由东风稳定地转变为西风的时刻为南海夏季风爆发时间具有较好的指示意义。所谓稳定是指从该时刻起 ,这一状况必须持续 3候且其后间断不超过 2候 ,或持续 2候后间断 1候但立刻又回到间断前状态。

基于日均温度的华山松径向生长敏感温度研究

树干木质部形成周期与温度密切相关,存在影响形成层活动的敏感温度,但是尚未研究尝试从树木年轮中探索影响径向生长的敏感温度。华山主峰的华山松径向生长对气候变化较敏感,且华山西峰的气象站记录了1953年以来气象资料,这为探索树木径向生长与温度的关系提供了宝贵的材料。以华山主峰的华山松年轮资料和日值温度资料为基础,通过分析历年日平均温度的变化过程和年轮宽度、早材宽度、晚材宽度、最小密度以及最大密度的相关性,尝试探索华山松径向生长与日均温度变化的关系。结果表明春季一定温度的初日时间对华山松径向生长有较大的影响,其中3℃和8℃初日时间和持续时间对年轮宽度的促进作用最明显,而3℃对早材宽度影响较大,8℃初日时间对晚材的影响较大;高于11℃的温度会对华山松的径向生长造成限制,其中以11℃的作用最明显;而温度的终日时间对年轮特征影响较小。说明3℃是早材形成的敏感温度,8℃是晚材形成的敏感温度,温度高于11℃会对华山松的径向生长构成胁迫。这证明华山松年轮特征中包含了较多的气候信息,形成层活动和木质部的生长存在阈值温度,通过分析不同温度的生长期与年轮特征的关系可以找到影响华山松径向生长的敏感温度。这些敏感的温度是通过什么生理过程影响木质部的形成尚无法得知,但是这为解释树木生长对全球温度升高的响应以及利用华山松年轮重建历史气候提供了重要依据。

华南前汛期起止日期的确定及降水年际变化特征分析

利用国家气候中心提供的华南地区74测站1957—2001年的逐日降水资料和NCEP/NCAR再分析资料,对华南前汛期雨季开始期与结束期的定义标准作了改进;研究了华南前汛期降水雨量、雨日的年际变化特征以及变化趋势的空间分布特征;用小波分析法分析华南前汛期降水的周期分布特征。分析结果发现:45年来前汛期开始期总体上呈现偏早趋势,而结束期具有偏晚的趋势,两者的年际变化十分显著;45年来华南全区总雨量、雨日呈正趋势变化,且两者的空间分布特征非常相似。华南前汛期内雨量、雨日总体上呈现增长趋势并且具有明显的年际变化特征,存在准2年、3～5年、6～8年的振荡周期。

华南前汛期降水开始和结束日期确定方法综述

对目前有关前汛期开始和结束日期的研究工作进行了总结。提出应参照梅雨期和季风降水等有关日期划分问题的方法,使用比月平均资料更高精度的逐日(候)资料,选择能够表征华南前汛期降水特征的适宜区域和代表站点,综合考虑降水、环流及其它物理要素在前汛期开始和结束前、后的演变特征,从而客观、准确地划定各年前汛期的起始和结束日期,以用于华南前汛期问题的研究。

南海夏季风爆发日期与海温的多尺度关系及最优子集回归预测

采用小波变换,Lanczos时间滤波器,相关分析研究了南海夏季风爆发日期的多尺度特征及与海温场的关系,并研制了南海夏季风爆发日期的短期气候预测新方法——多尺度最优子集回归预测方法。结果表明,1958—2008年南海夏季风爆发的平均日期为5月18日,标准差约10d,具有弱的偏早爆发趋势,存在15.4a的年代际变化和5.3a的年际变化周期。在年际变化尺度上,南海夏季风爆发日期与前期冬季(12—2月)海温场有4个显著的相关区,分别为热带南印度洋,菲律宾以东的热带西北太平洋,赤道中东太平洋,热带东太平洋。在年代际尺度上,南海夏季风爆发日期与前一年(前一年3月至当年2月)海温场的显著相关除有与年际尺度一致的热带西北太平洋海区外,还存在5个显著不同的相关区,即西南印度洋,东南印度洋,北太平洋,热带东南太平洋和东南太平洋。1999—2008年的独立样本检验表明,多尺度最优子集回归预测方法的最大预测误差为8.5d,预测值与实际值相差在5d以内的占50%,且对爆发日期的异常迟早具有较好的预测能力,它明显好于最优子集回归预测与单,多变量的线性回归预测。多尺度最优子集回归预测可以为南海夏季风爆发日期的短期气候预测业务预报提供重要参考。

华南前汛期开始日期异常与大气环流和海温变化的关系

利用1961—2012年美国国家环境预报中心/大气研究中心(NCEP/NCAR)的再分析资料、NOAA海温资料,CMAP降水资料和华南261个测站降水观测资料,首先分析华南前汛期开始日期(以下简称华南开汛)异常的气候特征,然后采用相关分析、合成分析的方法研究华南开汛异常与3—4月大气环流以及海温变化的关系。结果表明,近52 a来华南开汛具有显著的年际变化特征,但变化趋势不明显。开汛最早出现在1983年3月1日,最晚出现在1963年6月1日,1961—2012年华南平均开汛日期是4月6日。华南开汛主要出现在3—4月,占92.3%。华南开汛与3—4月华南降水相关最显著,开汛偏早(晚),对应华南3—4月降水偏多(少)。华南开汛偏早年,在3—4月,对流层高层副热带西风急流偏强,中层西太平洋副热带高压偏强偏西、低层南支槽偏强,华南上空西南气流偏强;华南开汛偏晚年则相反。华南开汛与3—4月中国南海及周边地区海温显著相关,海温偏低(高)对应华南开汛偏晚(早)。华南开汛偏晚年的海温和大气环流异常比早年显著。

近52年广东开汛期特征及与前汛期降水的关系分析

利用广东省86个气象台站近52年(1962—2013年)逐日降水量资料,统计了逐年各台站及全省开汛期(rain season onset date/RSOD),并对广东省开汛特征及其与前汛期降水的关系进行了分析。分析结果表明:(1)广东省开汛期多集中在3月下旬至4月中旬,最早和最迟开汛期相差94 d;(2)广东省东南大部、北部大部较中部大部和西南部开汛早,雷州半岛开汛最迟;(3)广东省开汛可分为突发型开汛和渐进型开汛两种类型;(4)广东省开汛期年(代)际变化特征明显,存在15～16 a的年代际振荡周期,1986年为年代振荡周期变化的转折点,前后分别存在8 a和5～6 a的年代振荡周期;(5)各台站开汛期与3月下旬及4月降水相关性最好,与5、6月及前汛期降水的相关性差;(6)广东开汛异常偏早(晚)年,其前冬500 hPa高度场存在明显差异。

东北冷涡降水集中期的客观识别研究

基于区域关键天气过程客观化识别和监测的需求及东北雨季应包括冷涡降水的事实,采用东北三省及内蒙古东四盟共147站逐日降水量资料,通过对东北区域多年平均5点平滑处理的逐日降水量序列的综合分析及对历年逐日滑动平均雨量的对比试验,确定了判别东北冷涡降水集中期开始日期的阈值及持续时间,进而研制了东北冷涡降水集中期开始日期的客观识别方法。基于该方法的客观识别,得到1981-2010年气候平均态的东北冷涡降水集中期的开始日期为每年的5月26日。同时,定义盛夏降水集中期开始日的前一日为冷涡降水集中期的结束日期,发现冷涡降水集中期的结束日期为6月25日。在此基础上,采用NCEP/NCAR逐日再分析的风场、位势高度场资料,通过对东北冷涡降水集中期前、中和后期各层大气环流场及各系统的逐日变化特征的对比分析,验证了该客观识别方法的合理性。