Drought fluctuations based on dendrochronology since 1786 for the Lenglongling Mountains at the northwestern fringe of the East Asian summer monsoon region

The Lenglongling Mountains （LLM） located in northeastern part of the Tibet Plateau, belong to a marginal area of the East Asian summer monsoon （EASM） and are sensitive to monsoon dynamics. Two tree-ring width chronologies developed from six sites of Picea crassifolia in the LLM were employed to study the regional drought variability. Correlation and temporal correlation analyses showed that relationships between the two chronologies and self-calibrated Palmer Drought Severity Index （sc_PDSI） were significant and stable across time, demonstrating the strength of sc_PDSI in modeling drought conditions in this region. Based on the relationships, the mean sc_PDSI was reconstructed for the period from 1786 to 2013. Dry conditions prevailed during 1817-1819, 1829-1831, 1928-1931 and 1999-2001. Relatively wet periods were identified for 1792-1795 and 1954-1956. Spatial correlations with other fourteen precipitation/drought reconstructed series in previous studies revealed that in arid regions of Northwest China, long-term variability of moisture conditions was synchronous before the 1950s at a decadal scale （1791-1954）. In northwestern margin of the EASM, most of all selected reconstructions had better consistency in low-frequency variation, especially during dry periods, indicating similar regional moisture variations and analogous modes of climate forcing on tree growth in the region.

Regional-scale Surface Air Temperature and East Asian Summer Monsoon Changes during the Last Millennium Simulated by the FGOALS-gl Climate System Model

The spatial patterns and regional-scale surface air temperature （SAT） changes during the last millennium,as well as the variability of the East Asian summer monsoon （EASM） were simulated with a low-resolution version of Flexible Global Ocean-Atmosphere-Land-Sea-ice （FGOALS-gl） model.The model was driven by both natural and anthropogenic forcing agents.Major features of the simulated past millennial Northern Hemisphere （NH） mean SAT variations,including the Medieval Climate Anomaly （MCA）,the Little Ice Age （LIA） and the 20th Century Warming （20CW）,were generally consistent with the reconstructions.The simulated MCA showed a global cooling pattern with reference to the 1961-90 mean conditions,indicating the 20CW to be unprecedented over the last millennium in the simulation.The LIA was characterized by pronounced coldness over the continental extratropical NH in both the reconstruction and the simulation.The simulated global mean SAT difference between the MCA and LIA was 0.14°C,with enhanced warming over high-latitude NH continental regions.Consistencies between the simulation and the reconstruction on regional scales were lower than those on hemispheric scales.The major features agreed well between the simulated and reconstructed SAT variations over the Chinese domain,despite some inconsistency in details among different reconstructions.The EASM circulation during the MCA was stronger than that during the LIA The corresponding rainfall anomalies exhibited excessive rainfall in the north but deficient rainfall in the south.Both the zonal and meridional thermal contrast were enhanced during the MCA.This temperature anomaly pattern favored a stronger monsoon circulation.

Seasonal and Intraseasonal Variations of East Asian Summer Monsoon Precipitation Simulated by a Regional Air-Sea Coupled Model

The performance of a regional air-sea coupled model, comprising the Regional Integrated Environment Model System （RIEMS） and the Princeton Ocean Model （POM）, in simulating the seasonal and intraseasonal variations of East Asian summer monsoon （EASM） rainfall was investigated. Through comparisons of the model results among the coupled model, the uncoupled RIEMS, and observations, the impact of air-sea coupling on simulating the EASM was also evaluated. Results showed that the regional air sea coupled climate model performed better in simulating the spatial pattern of the precipitation climatology and produced more realistic variations of the EASM rainfall in terms of its amplitude and principal EOF modes. The coupled model also showed greater skill than the uncoupled RIEMS in reproducing the principal features of climatological intraseasonal oscillation （CISO） of EASM rainfall, including its dominant period, intensity, and northward propagation. Further analysis indicated that the improvements in the simulation of the EASM rainfall climatology and its seasonal variation in the coupled model were due to better simulation of the western North Pacific Subtropical High, while the improvements of CISO simulation were owing to the realistic phase relationship between the intraseasonal convection and the underlying SST resulting from the air-sea coupling.

Impact of Different East Asian Summer Monsoon Circulations on Aerosol-Induced Climatic Effects

The different spatial distributions of aerosol-induced direct radiative forcing and climatic effects in a weak(2003)and a strong(2006)East Asian summer monsoon(EASM)circulation were simulated using a high-resolution regional climate model(RegCM3).Results showed that the atmospheric circulations of summer monsoon have direct relations with transport of aerosols and their climatic effects.Both the top-of-the-atmosphere(TOA)and the surface-negative radiative forcing of aerosols were stronger in weak EASM circulations.The main difference in aerosol-induced negative forcing in two summers varied between 2 and 14 W m 2from the Sichuan Basin to North China,where a maximum in aerosol-induced negative forcing was also noticed in the EASM-dominated areas.The spatial difference in the simulated aerosol optical depth(AOD)in two summers generally showed the similar pictures.Surface cooling effects induced by aerosols were spatially more uniform in weak EASM circulations and cooler by about 1–4.5 C.A preliminary analysis here indicated that a weaker low-level wind speed not conducive to the transport and diffusion of aerosols could make more contributions to the differences in the two circulations.

THE ANALYSIS OF MECHANISM OF IMPACT OF AEROSOLS ON EAST ASIAN SUMMER MONSOON INDEX AND ONSET

RegCM4.3, a high-resolution regional climate model, which includes five kinds of aerosols(dust, sea salt,sulfate, black carbon and organic carbon), is employed to simulate the East Asian summer monsoon(EASM) from 1995 to 2010 and the simulation data are used to study the possible impact of natural and anthropogenic aerosols on EASM.The results show that the regional climate model can well simulate the EASM and the spatial and temporal distribution of aerosols. The EASM index is reduced by about 5% by the natural and anthropogenic aerosols and the monsoon onset time is also delayed by about a pentad except for Southeast China. The aerosols heat the middle atmosphere through absorbing solar radiation and the air column expands in Southeast China and its offshore areas. As a result, the geopotential height decreases and a cyclonic circulation anomaly is generated in the lower atmosphere. Northerly wind located in the west of cyclonic circulation weakens the low-level southerly wind in the EASM region. Negative surface radiative forcing due to aerosols causes downward motion and an indirect meridional circulation is formed with the low-level northerly wind and high-level southerly wind anomaly in the north of 25° N in the monsoon area, which weakens the vertical circulation of EASM. The summer precipitation of the monsoon region is significantly reduced,especially in North and Southwest China where the value of moisture flux divergence increases.

DIAGNOSTIC ANALYSIS OF PERSISTENT DROUGHT/FLOOD EVENTS IN SUMMER OVER THE TWO-LAKE REGION OF CHINA

Based on the daily regional mean rainfall,the Z-index method is used to identify persistent flood and drought events lasting for at least 10 days over a region where Dongting Lake and Poyang Lake sit(referred to as the"two-lake region"hereafter).The National Centers for Environmental Prediction(NCEP)reanalysis data are then utilized to perform a preliminary diagnostic analysis on these events.The results indicate that the composite standardized geopotential height at 500 hPa presents two different meridional wave trains from north to south over the East Asian-Pacific region,i.e.,a"-+-"pattern for the droughts and a"+-+"pattern for the floods,respectively.The developing,maintaining and decaying phases in the drought and flood events are closely related to the intensity and location of a subtropical high and an extra-tropical blocking high.It is shown that the East Asian summer monsoon is strong(weak)with the occurrence of persistent drought(flood)events.Droughts(floods)are accompanied by a weak(strong)tropical convergent system and a strong(weak)subtropical convergent system.Furthermore,the persistent drought(flood)events are associated with a divergence(convergence)of vertically integrated water vapor flux.In the vertical profile of water vapor flux,divergence(convergence)in the mid-and lower-levels and convergence(divergence)in the higher levels are evident in the droughts(floods).Both the divergence in the droughts and the convergence in floods are strongest at 850 hPa.

亚洲夏季风对江淮梅雨入梅早晚的影响

入梅时间早晚直接影响梅雨期雨量的多寡,其准确预测对农业、交通和旅游业等气象服务具有重要意义。利用中国气象局2017年发布的《梅雨监测业务规定》中的入梅日期资料和NCEP/NCAR再分析资料,研究1981—2020年江淮梅雨入梅早晚的气候特征,分析亚洲夏季风对入梅日早晚的影响。结果表明:(1)江淮梅雨入梅日具有显著的年际变化特征,平均入梅日为6月21日,标准差为11 d,最早和最晚入梅日相差39 d。(2)入梅日与南亚夏季风SASM(South Asian Summer Monsoon)呈显著负相关,与东亚夏季风EASM(East Asian Summer Monsoon)呈正相关。强SASM年,南亚高压偏东,中高纬度高空急流偏南,江淮地区为水汽辐合区,有利于江淮区入梅偏早;强EASM年,西太副高偏北偏强,南风气流旺盛,水汽在华南和东北地区辐合,在江淮地区辐散,不利于梅雨的发生。(3)由于亚洲夏季风具有协同爆发的特点,强SASM-弱EASM协同年,平均入梅日较常年平均偏早4 d,与之相反的协同年入梅日偏晚11.6 d。强SASM-弱EASM年,江淮地区位于高空急流出口右侧,伊朗高压位置偏东,高空受小槽东移影响,低层南亚夏季风发展旺盛,水汽在江淮地区辐合,有利于入梅偏早。弱SASM-强EASM年,中高纬度高空急流轴偏北,江淮地区受脊前偏北气流控制,低层亚洲夏季风辐合区偏东,江淮地区仅为过路水汽通道,不利于江淮区入梅。故亚洲夏季风的强弱与协同发展对江淮梅雨入梅早晚具有一定的指示意义。

东北雨季的划分及其特征

利用1951—2001年东北地区9站旬平均降水量资料划分了东北雨季,确定了雨季开始旬序、结束旬序、持续期和降水量,分析了雨季各特征量之间的关系,探讨了雨季前后大气流场的差异,研究了雨季特征量与热带海温指数的关系。结果表明:(1)东北雨季平均在7月中上旬开始,8月中上旬结束;51 a雨季降水量与9站夏季Z指数密切相关,表明了雨季降水基本反映了该地区夏季降水的特征;(2)一般说来,雨季开始早,则持续期长,降水量大,反之亦然;(3)东北雨季主要受东亚夏季风的控制,季风增强北进,雨季开始,季风减弱南撤,雨季结束;(4)东北雨季特征量有短期气候预测时间尺度上的热带季节海表温度距平前兆信号。

四川盆地夏季降水区域差异及其与季风的联系初探

基于四川盆地逐日气象观测资料、NCEP/NCAR再分析资料,分析了近46年四川盆地夏季降水变化的区域差异及其与东亚夏季风和高原夏季风的联系。结果表明:盆西和盆东夏季降水序列与全国夏季降水的相关分布分别与我国夏季降水第Ⅰ、Ⅱ类雨型分布相类似。使用一元回归方法,分别得到了与东亚夏季风和高原夏季风相关的环流场,通过对两个环流场季风指数高低值年份的合成分析发现,东亚夏季风的影响主要体现在西北气流和东南气流的辐合带在我国东部地区位置变动以及强度变化;高原夏季风对环流场的影响体现在华北到河套地区一带风向的转换。着重分析了1961和1998年夏季与东亚、高原夏季风相关的环流场,发现东亚夏季风与高原夏季风都对四川盆地夏季降水有重要影响,其中盆西夏季降水主要与高原夏季风有关,盆东夏季降水与东亚夏季风和高原夏季风都有关,但以东亚夏季风为主。

东亚夏季风期间水汽输送与西北干旱的关系

利用西北(区)168个测站1961—2000年6～9月月平均降水与温度资料,采用EOF、REOF方法分析了近40年降水异常特征,同时利用同期NCEP/NCAR月平均再分析资料,分析了强(弱)夏季风年西北区水汽通量场的特征及夏季风西北影响区的净水汽通量。结果表明:西北区6～9月降水可分为7个气候异常区;东亚夏季风对我国降水的影响主要位于100°E以东的地区;东亚夏季风西北影响区降水的水汽来源于南风水汽通量;强夏季风年,到达东亚夏季风西北影响区的水汽通量显著增加,该区降水偏多,弱夏季风年则反之。

夏季风北推和强度对我国北方夏季降水影响的年代际特征

利用近50年NCEP/NCAR的候平均风场和比湿场资料,计算了东亚夏季风北界的位置,发现季风北界存在着明显的年代际变化,季风北界偏北的年份集中于1950年代和1960年代,而偏南的年份则集中于1980年代和1990年代,并且各个季风区的季风北界的年代际变化比较一致.利用这些资料定义了'区域夏季风指数',更好地反映东亚夏季风的强弱和北方各地区旱涝之间的关系;用合成分析方法比较了夏季风强度与北方各干旱半干旱区域旱涝变化的关系,说明华北地区最为敏感,区域季风指数与7,8月降水量的关系最好.最后用小波分析方法,讨论了这些区域的夏季风强度和夏季降水量在不同时间尺度上的关系,发现区域季风强度指数和各季风区降水量的关系在15年左右的年代际尺度上关系最好,在3～5年的年际尺度上也有比较好的相关.这些结果对于北方地区的气候趋势预测有一定参考价值.

气溶胶对东亚夏季风指数和爆发的影响及其机理分析

利用高分辨率区域气候模式Reg CM4.3,通过引入沙尘、海盐、硫酸盐、黑碳和有机碳等5种气溶胶,对1995—2010年的东亚夏季风进行数值模拟,研究了自然和人为气溶胶对东亚夏季风的可能影响。结果表明:区域气候模式对东亚夏季风和气溶胶的时空分布都有较好的模拟效果,自然和人为气溶胶造成东亚夏季风指数减小约5%,且除我国东南部地区外,气溶胶使整个季风区的季风爆发时间推迟了1候左右。在我国东南部及近海地区,气溶胶通过吸收太阳辐射对中层大气起到加热作用,气柱受热会出现膨胀,从而造成了低层大气的位势高度下降并激发出气旋式环流距平,气旋式环流距平西侧偏北风能削弱东亚夏季风区低层的偏南气流。气溶胶的加入引起的地表负的辐射强迫造成了空气出现下沉运动并配合低层偏北风和高层偏南风距平,在25°N以北地区形成了间接经向环流距平,从而也削弱了东亚夏季风的垂直环流。气溶胶增加了我国季风区的水汽通量散度值,从而造成了夏季降水的明显减少,其中我国华北和西南地区为2个主要的降水减少区域。

区域气候模式侧边界的处理对东亚夏季风降水模拟的影响

在区域气候模式模拟中，侧边界的作用是引入大尺度强迫场。如何处理好侧边界，即大尺度强迫场和区域气候模式本身之间的关系问题，对于区域气候模式模拟和预报东亚夏季风降水具有重要意义。本文利用美国纽约州立大学Ａｌｂａｎｙ分校的区域气候模式（ＳＵＮＹＡ－ＲｅＣＭ），设计了两种不同的侧边界处理方法，来探讨侧边界对东亚夏季风降水模拟的影响。驱动区域模式的大尺度强迫场来自欧洲中期天气预报中心（ＥＣＭＷＦ）及热带海洋大气研究计划（ＴＯＧＡ）的分析资料场。试验结果表明：（１）当模式的区域较大时，采用较小的侧边界缓冲区会在缓冲区与模式内部的交界处产生不连续；扩大缓冲区并且考虑不同尺度强迫在垂直方向上的不同作用，可以避免这一缺陷。（２）更重要的是采用后一种方案，即减少了区域气候模式在模拟大尺度环流场方面所起的作用，使得模式更多地依赖侧边界来得到更真实的、对东亚夏季风降水起重大影响的一些气流，如副高、西南季风和南海季风，对东亚夏季风降水无论是在大小上还是在雨带位置的演变上都能进行更好的模拟。

区域海气耦合模式模拟的2003年东亚夏季风季节内振荡

评估了一个区域海气耦合模式(由区域环境系统集成模式RIEMS和普林斯顿海洋模式POM组成)对2003年东亚夏季风季节内振荡(ISO)的模拟性能。通过与观测海温驱动单独大气模式结果的比较,分析了海气耦合过程对东亚夏季大气ISO模拟性能的影响。结果表明:耦合模式能够模拟出2003年中国东部地区夏季降水的气候态分布,模拟的中国东部尤其是江淮地区大气ISO活动较单独大气模式更为显著。同时,耦合模式能够较好地再现大气ISO经向上北传的传播特征,模拟的江淮流域降水处于活跃和中断期时西北太平洋地区低频降水和环流异常在强度和空间分布上较单独大气模式都更为合理。相比于单独大气模式,耦合模式对大气ISO模拟的改善,一方面与其对气候态西北太平洋副热带高压和相关对流层底层风场模拟的改善有关,另一方面与其包含海气相互作用,因而对低频降水与海温和水汽辐合位相关系模拟的改善有关。

东亚季风区石笋δ^(18)O解译:基于夏季风与夏季风降雨的思考

近些年,对于东亚季风区石笋δ^(18)O的气候环境指示意义的争论较多,主要在东亚季风区石笋δ^(18)O代表夏季和风强度、夏季风降水还是水汽源变化。基于中国东部华北地区降水与长江中下游地区降水反相变化和长江中下游地区降水与菲律宾海降水反相变化(遥相关),从年际-年代际到千年-轨道尺度对石笋δ^(18)O与夏季风降水、厄尔尼诺-南方涛动(ENSO)的相互关系进行了探讨分析。通过对比石笋δ^(18)O记录与华北和梅雨区降水,发现石笋δ^(18)O偏负对应华北降水增加,梅雨区降水减少;石笋δ^(18)O偏正对应华北降水减少,梅雨区降水增加。这种对应关系不仅存在年际-年代际尺度,而且在千年-轨道尺度同样存在,石笋δ^(18)O不仅反映夏季风强弱变化,同时与中国东部区域降水关系是明确对应的。通过降水的空间相互关系,发现ENSO活动主要通过影响中国东部降水的空间分布格局而作用于石笋δ^(18)O。La Ni a态导致南海及菲律宾海对流加强,西太副高位置偏北,长江中下游地区梅雨期缩短,华北夏季降水增加,东亚季风区石笋δ^(18)O偏负。El Ni o态,南海和菲律宾海对流受到抑制,西太副高位置南移,长江中下游地区梅雨期延长,华北夏季降水减少,东亚季风区石笋δ^(18)O偏正。另外,水汽源分析发现,菲律宾海水汽输送对东亚季风区降水及降水δ^(18)O贡献相对较小。因此,综合分析认为,东亚季风区石笋δ^(18)O主要反映了亚洲夏季风的强弱变化。

积云对流参数化方案对东亚夏季风模拟对比

利用MM5模式对比5种不同积云对流参数化方案,即Kuo方案、Grell方案、Kain-Fritsch方案、Betts-Miller方案和赵思雄修改的Kuo方案对东亚季风区强、弱季风年夏季风的模拟能力。结果表明:5种对流参数化方案对各季节都有一定的模拟能力,各个方案对降雨的预报能力随地区和年际而有所不同,对强季风年grell和bm方案对雨带特征的描述较好,对弱季风年bm和zhao方案对雨带的模拟与实际最为接近。

江淮梅雨与东亚副热带夏季风进程变异的关系

根据中国气象局《梅雨监测业务规定》中的入、出梅标准,结合1960—2016年全国661个常规气象站逐日气象资料,以及NCEP/NCAR月平均再分析资料,分析了江淮梅雨和东亚副热带夏季风进程变异的时空特征,提取季风关键区(32°~34°N,112°~120°E,包含17个站点),并分析了江淮梅雨和季风关键区的联系与成因。结果表明:1960—2016年平均梅雨期为6月8日—7月15日,平均梅雨量为303 mm。比东亚平均梅雨季的开始时间早9 d,比其结束时间晚7 d。梅雨量在近57 a中也呈波动式变化,但整体为上升趋势。入梅越早,出梅越晚,则梅雨期越长,梅雨量越多。副热带夏季风推进到关键区的平均时间为5月19日,其在1970s末和1990s末分别发生了由偏晚向偏早和由偏早向偏晚的突变。夏季风到达关键区偏早时,出梅日偏晚,梅雨量偏多,季风到达偏晚时,出梅日偏早,梅雨量偏少。副热带夏季风推进时间和江淮梅雨量呈全区一致的负相关,负相关区位于湖南、湖北及江西三省临近的两湖地区。东亚副热带夏季风到达关键区时间偏早(晚)年,500 hPa高度场上乌拉尔山—鄂霍茨克海为正(负)距平,阻塞高压增强(减弱);日本海附近为负(正)距平,东亚大槽加深(西退北缩),加强(削弱)了槽后冷空气向南输送且不(有)利于中低纬度副热带高压的北跳,西太平洋副热带高压中心强度增强(减弱),位置偏西(东),其西北侧的西南暖湿气流输送加强(减弱),江淮地区有水汽的辐合(辐散),有(不)利于梅雨量偏多。

东亚夏季经向水汽输送的变异及其对极端降水的影响

东亚夏季降水的异常与水汽输送的变异密切相关。基于1958—2016年资料,研究了夏季东亚季风区经向水汽输送的主要变异特征及其对东亚夏季极端降水的影响。经向水汽输送的第一主变异模态表现出中国东部和西北太平洋上的水汽经向输送呈现反向异常,以年际变化为主。当中国东部向北输送的水汽增强(减弱)而西北太平洋向北输送减弱(增强),则中国东部大范围的极端降水量及频次增加(减少)。该模态与西太(西太平洋)副高西伸(东撤)有关,并主要受到热带中东印度洋海温的影响。第二变异模态以年代际变化为主兼有年际变化,表现在1980年后中国东部及邻近海域上空的经向水汽输送减弱,使得环渤海地区和华南沿海的极端降水量及频次减少而长江上、下游和贵州的极端降水量及频次增加。该模态与西太副高的减弱有关,并受到热带西太海温年代际增温的影响。第三变异模态以年际变化为主兼有年代际变化,反映中国长江以北地区和日本南部及附近区域的经向水汽输送的反相变化结构。长江以北水汽输送减弱(增强),可导致华北、东北的极端降水量及频次减少(增加)和长江下游及江南地区的极端降水量及频次的减少(增加)。该模态主要受欧亚大陆上空中高纬度纬向遥相关波列和热带印太(印度洋太平洋)海温异常的影响。

燕山南麓夏季降水对东亚夏季风变化的响应

利用燕山南麓1951-2005年夏季降水资料和NCEP/NCAR高度场、风场资料,采用趋势分析、小波变换、突变检验等方法,对燕山南麓夏季降水规律和东亚夏季风变化进行了分析。结果发现:(1)燕山南麓夏季降水从1980年以来呈减少趋势,在1996年发生了突变,近年减少更加显著。线性减少趋势显著,平均每10 a减少29mm。(2)燕山南麓夏季降水存在10 a的变化周期,东亚夏季风存在11 a的变化周期,二者变化周期基本一致。降水的10 a周期显著,东亚夏季风11 a周期不太显著。(3)强夏季风年,蒙古地区低气压显著降低,蒙古上空低槽活动频繁,极涡明显变弱,燕山南麓夏季降水偏多。弱夏季风年,蒙古地区低气压明显减弱,蒙古上空低槽活动较少,极涡显著加深,燕山南麓夏季降水偏少。(4)燕山南麓夏季降水减少是对东亚夏季风减弱和对应的中高度大气纬环流变化的响应。

Effects of the Indo-China Peninsula Heat Fluxes on the 1998 South China Sea Summer Monsoon

Sensitivity experiments to investigate the role and importance of theIndo-China Peninsula (ICP) heat fluxes (sensible and latent heat fluxes) on the South China Seasummer monsoon (SCSSM) onset and its subsequent evolution have been carried out using a regionalclimate model. Two sensitivity experiments have been conducted: cutting off the sensible heat flux(noted as IChfx) and latent heat flux (noted as ICqfx). Results show that by cutting off theunderlying heat flux of the ICP, the precipitation decreases and low-level geopotential heightincreases over the local regions as well as the remote regions of the East China Sea, southernJapan, and western North Pacific. The westerly (easterly) zonal winds at 850 hPa (200 hPa) aregenerally weakened in May and June, especially before and during the onset of the SCSSM towards theend of May. It is worth noting that although the observed sensible heat flux is only about 1/3 ofthe latent heat flux, the differences between the control run and each of the two sensitivityexperiments are comparable.