Recent changes in the spatio-temporal characteristics of monsoon intraseasonal oscillations

Theoretical and Applied Climatology - Monsoon intraseasonal oscillations (MISOs) define a significant proportion of intraseasonal variability of the Indian summer monsoon. In the backdrop of...     

Swapping of the Pacific and Atlantic Niño influences on north central India summer monsoon

Climate Dynamics - The highly populated north central India receives 90% of annual rainfall during June to September. The interannual variation of summer monsoon rainfall is less studied...     

The chemical fractionation of atmospheric aerosol as a result of snow crystal formation and growth

The relationships between the physical and chemical properties of mixed-phase clouds were investigated at Storm Peak Laboratory (3220m MSL) located near the continental divide in northwestern Colorado. Interstitial aerosol particles, cloud droplets and snow crystals were concurrently collected when the laboratory was enveloped by a precipitating cloud. All samples were analyzed for trace elements, soluble anions, electrical conductivity and acidity.The results show average trace constituent concentration ratios of cloud water to snow water range from 0.4 to 26. All but six of the 32 elements and ions measured had ratios greater than one. This result suggests a chemical species dependency of in-cloud aerosol particle scavenging processes. Evidence of a decrease of in-cloud aerosol particle scavenging efficiency by snow due to increases in aerosol concentration is also presented.Differences between the chemical composition of cloud water and snow water are manifested most strongly when snow crystals grow by vapor deposition. In-cloud scavenging efficiencies by snow crystals for most aerosol particle chemical species are dependent on the growth of the snow crystals by accretion of cloud droplets. This chemical fractionation of the atmospheric aerosol by snow crystal formation and growth should be most active where narrow, continental cloud droplet size distributions and low liquid water contents are prevalent, enhancing the probability of snow crystal growth by diffusion.     

北印度洋-南海季风区热带海洋极端波候变化特征

利用1979—2016年ERA-Interim有效波高（SWH）和海表风场数据,分析了南海-北印度洋极端海浪场分布和变化.结果表明：南海-北印度洋极端SWH分布和极端风速分布形态以及年际变化趋势高度一致,说明了涌浪为主的北印度洋和风浪为主的南海一样,极端SWH都由局地的极端风速控制;强极端SWH主要分布在阿拉伯海以及南海北部,阿拉伯海北部增长与该区域气旋强度增强有着密切关系,而南海的极端SWH主要受东北季风控制;东非沿岸极端SWH线性增长趋势则与索马里急流的年代际尺度上有逐渐增强的线性趋势有关.北印度洋及南海海域极端SWH距平场的EOF分析结果表明,南海极端SWH与北印度洋表现出反相变化的特征.北印度洋（南海海域）极端SWH多出现在西南季风（东北季风）期间,因为在西南季风（东北季风）期间,极端风速也相对增强.     

我国季风区湿度场的一些统计特征

在计算我国7站(其中东部季风区5个站)10年逐日大气湿度参量的基础上,对大气湿润状况的一些基本统计性质,包括均方差、标准误差、偏度及不同等压面上湿度变化的相关性等,进行了分析。结果表明,这些统计量的地理分布、季节变化及不同高度之间的差异等均与季风气候的特征有着密切的联系。     

热带气旋“碧利斯”与南海季风相互作用的强水汽特征数值研究

强热带风暴碧利斯(0604)登陆后其低压环流较长时间地维持,并与南海季风相互作用,造成湖南省东南部发生历史罕见的特大致洪暴雨。文中应用多种常规、非常规细网格观测资料及NECP再分析资料,结合暴雨中尺度数值预报模式AREM对该暴雨过程的强水汽场特征进行了数值模拟和诊断分析,并设计水汽敏感性试验,进一步揭示造成湘东南特大暴雨的水汽通道和水汽来源。结果表明:碧利斯低压环流南侧的西南气流对湘东南暴雨区起到了主要水汽输送作用,且随着碧利斯逆时针旋转,水汽沿着环流中心东侧的强风速带夹卷到环流北侧,并通过增强的东北风源源不断地输送至湘东南。这南北两支主要水汽通道在湘东南长时间交汇,形成了湘东南暴雨区深厚的湿层和强水汽辐合,对碧利斯低压环流较长时间的维持及对湘东南特大暴雨的形成和发展有重要的作用。     

Role of intra-seasonal oscillations on monsoon floods and droughts over India

Aim of this diagnostic study is to investigate the impact of intra-seasonal oscillations in terms of number, duration and intensity on rainfall during June through September, 1979–2006. Analysis of wavelet spectra for winds at 850 hPa field for monsoon period reveals number and duration of oscillations, which exercise profound influence on monsoon rainfall. Results indicate that four to six oscillations appear in normal rainfall or flood cases, while two or three oscillations are identified in the years of drought episodes. Though total duration of above oscillations is varied from 25 to 85 days, the duration is short (20 to 35 days) obviously in the years of less number of oscillations and also the number of oscillations are directly related to the monsoon rainfall. The coefficient of correlation between them is 0.56, which is significant at 1% level. To examine the strength of intra-seasonal oscillations in terms of different indices on seasonal rainfall is investigated. The Madden and Julian Oscillation Index shows an inverse relationship with rainfall, where as a direct relationship is noticed between Monsoon Shear Index and rainfall for the study period. Both results are significant at 5% level. To consolidate the above statistical relationships, seasonal circulation changes in the contrasting years of monsoon rainfall have been examined; present study reveals that anomaly negative outgoing longwave radiation is noticed over most of Arabian Sea, Indian sub-continent and the Bay of Bengal during June through September in flood year (1988). But opposite convective activity is true in drought year (2002). Similarly the spatial U-850 hPa field distribution showed much stronger monsoon winds in 1988, while zonal circulation was very weak in 2002. Such differences are observed in the anomaly zonal wind field at 200 hPa also. Over the monsoon region U-850 hPa field is almost a mirror image of U-200 hPa distribution of wind field. Finally annual cycles of U-850 and U-200 hPa fields reflect striking difference at 200 hPa level during the summer monsoon period in flood and drought years.     

东亚夏季风与北太平洋SSTA关系的年代际变化特征及其机制研究

利用NCEP／NCAR提供的40年再分析资料和英国气象局提供的海温资料以及中国气象局整编的160站降水资料，分析了东亚夏季风以及我国华北夏季降水的年代际变化特征及其与北太平洋SSTA的关系，提出了东亚存季风在70年代中期发生显著变化，1976年前东亚夏季风偏强，受其影响华北地区夏季降水偏多，1976年以后，东亚转为弱夏季风阶段，华北地区进入少雨期，研究表明，1976年前北太平洋SSTA对大气作用显著，北太平洋海温异常对大圆波列会产生一种年际尺度的“刺激”作用叠加在年代际背景上，加强或减弱波列强度，造成强夏季风段结北夏季降水偏多气候态下的年际变化，1976年后，北太平洋地区海气温差小，海温对大气加热作用不明显，因此北太平洋海温异常通过大圆波列与东亚夏季风的联系也变得淡漠，对我国华北地区夏季降水的影响不再显著。     

华北夏季大气水汽输送特征及其与夏季旱涝的关系

本文利用ECMWF再分析资料ERA40和中国160站的降水资料,分析了我国华北地区1958年-2002年夏季的大气水汽含量和水汽输送的基本气候特征,研究了华北夏季旱涝年的大气水汽含量和水汽输送异常情况,最后利用线性回归的方法探讨了该地区大气水汽含量和水汽输送的变化趋势.结果表明:华北地区夏季降水的大气水汽来源主要有3支:来自孟加拉湾的水汽、来自我国南海和西太平洋的水汽以及中高纬西风带的水汽输送.华北地区对流层低层以经向水汽输送通量为主,到了中高层则以纬向输送通量为主;与华北地区夏季旱涝密切相关的异常水汽输送主要是南海和西太平洋以及西风带水汽输送异常,华北地区南边界水汽输入异常和东边界水汽输出异常是造成华北夏季旱涝年水汽收支异常的主要原因;近半个世纪以来,伴随着华北干旱化的加剧,华北地区南边界输入的大气水汽呈现显著的减少趋势.     

Prediction and monitoring of monsoon intraseasonal oscillations over Indian monsoon region in an ensemble prediction system using CFSv2

An ensemble prediction system (EPS) is devised for the extended range prediction (ERP) of monsoon intraseasonal oscillations (MISO) of Indian summer monsoon (ISM) using National Centers for Environmental Prediction Climate Forecast System model version 2 at T126 horizontal resolution. The EPS is formulated by generating 11 member ensembles through the perturbation of atmospheric initial conditions. The hindcast experiments were conducted at every 5-day interval for 45 days lead time starting from 16th May to 28th September during 2001–2012. The general simulation of ISM characteristics and the ERP skill of the proposed EPS at pentad mean scale are evaluated in the present study. Though the EPS underestimates both the mean and variability of ISM rainfall, it simulates the northward propagation of MISO reasonably well. It is found that the signal-to-noise ratio of the forecasted rainfall becomes unity by about 18 days. The potential predictability error of the forecasted rainfall saturates by about 25 days. Though useful deterministic forecasts could be generated up to 2nd pentad lead, significant correlations are found even up to 4th pentad lead. The skill in predicting large-scale MISO, which is assessed by comparing the predicted and observed MISO indices, is found to be ~17 days. It is noted that the prediction skill of actual rainfall is closely related to the prediction of large-scale MISO amplitude as well as the initial conditions related to the different phases of MISO. An analysis of categorical prediction skills reveals that break is more skillfully predicted, followed by active and then normal. The categorical probability skill scores suggest that useful probabilistic forecasts could be generated even up to 4th pentad lead.     

中国北方地区冬季雨雪年度变化与大气气溶胶分布特征研究

近年来,随着大气观测技术的快速发展,为冬季大雪年际变化研究提供了一些新的观测事实,增加了新的认识。认为大气环流对降雪的年际变化的影响只是重要方面之一。新的观测事实启示研究者,冬雪的年际变化和差异还可能与其他影响气候变化有更为复杂的因素——大气气溶胶特征有关。基于1980-2008年中国气象台站降水量资料和1980-2005年北方地区大气气溶胶光学厚度（AOD）资料,研究中国北方地区大范围多雨雪以及少雨雪年度变化与大气气溶胶分布特征的关系。结果表明：1980-2008年,中国北方地区典型的多雨雪年为1980、1984、1989、1993、1998、2003年和2006年;少雨雪年为1982-1983、1985-1988、1997、2001年和2005年。根据1980-2005年华北逐年冬半年雨雪总量与北方地区同期AOD相关分析,揭示中国北方地区雨雪年度变化与气溶胶光学厚度（AOD_550m）的年度变化存在正相关,相关系数达到0．001的显著性水平。同期资料AOD。。。分析表明,中国北方地区冬季多雨雪与少雨年大气气溶胶光学厚度差异显著,多雨雪年大气气溶胶光学厚度显著偏厚,中国华北北部、东北南部地区AOD正距平的平均值达到1×10。-5×10^-3,冬季北方地区少雨雪年与此差异显著,AOD为显著负距平,其平均值为-5×10^-3。     

基于GOSAT卫星遥感的我国中部地区大气CH4时空分布

利用2013—2016年GOSAT上被动红外探测器(TANSO)官方反演的大气CH_4柱浓度,采用普通克里金插值(Ordinary Kriging)方法对GOSAT卫星数据产品进行插值预处理,并利用ArcGIS地理信息系统空间分析软件提取各省份CH_4平均浓度,分析中部地区CH_4浓度的时空分布。结果表明,由GOSAT反演的中部地区大气CH_4年均浓度由2013年的1 827.0×10~(-9)增长到2016年的1 857.9×10~(-9),其平均绝对增长率为10.3×10~(-9)/a。中部地区大气CH_4年均浓度略低于长三角地区,高于京津冀和东三省地区。中部地区大气CH_4呈现较强的季节变化特征,江西、湖南、湖北峰值出现在9月,安徽、河南、山西峰值则出现在8月,中部六省去长期趋势后的月均值均略低于长三角地区,高于京津冀和东三省地区。我国中部地区CH_4浓度高值区主要分布在江西、湖南及周边区域,低值区则集中在河南以北及山西地区。     

中国中东部冬季霾日的形成与东亚冬季风和大气湿度的关系

利用1961-2013年中国地面台站长期观测资料和同期NCEP/NCAR再分析资料,以华北、江淮和华南为研究区,分析了中国中东部冬季霾日的形成与东亚冬季风以及大气湿度的关系。结果表明:(1)冬季霾日与东亚冬季风强度成显著的负相关。首先,东亚冬季风强度的减弱使得地面风速减小,进而导致冬季霾日增多。其中,华北7-8 m/s最大风速日数和江淮6-8 m/s最大风速日数的减少,及华南≤2 m/s最大风速日数的增多对各区冬季霾日的增多作用较大。其次,东亚冬季风减弱引起冬季气温的持续升高,易导致冬季霾日的增多,这在华北地区较之在江淮和华南更为明显。(2)由于气候变暖,冬季气温升高,使得近地面相对湿度减小。在江淮和华南地区,冬季霾日的增多与近地面相对湿度的减小显著相关,而在华北地区这种相关较弱。(3)冬季气温升高也有利于大气层结稳定度的增强,3个区域冬季霾日的增多均与大气层结稳定度的增强显著相关,特别是与对流层中低层(850-500 hPa)大气饱和度的降低显著相关。(4)冬季霾日数变化与区域水汽输送关系密切。其中,华北地区的冬季霾日数与水汽总收入成显著正相关,江淮地区与纬向水汽收入成显著正相关,与经向水汽收入成显著负相关,华南地区与经向水汽收入成显著负相关。     

Measurements of atmospheric hydroperoxides at a rural site in central Japan

Measurements of hydroperoxides (H₂O₂ and MHP) at ground level were made from 2012 to 2015 in Imizu City, Toyama Prefecture in central Japan. H₂O₂ and MHP concentrations ranged from 0.01 to 3.5 ppb and from below the level of detection (< 0.01 ppb) to 1.4 ppb, respectively. The concentrations of H₂O₂ and MHP were high in the summer and low in the winter. The H₂O₂ concentration was at its maximum in July and August, whereas the concentration of O₃ in the daytime was highest in May and June. The ratio of [H₂O₂]/[SO₂] presented clear seasonal variations. Many cases showed the condition of [H₂O₂] < [SO₂], called oxidant limitation especially in the cold months. Hydroperoxide concentrations in the rainwater were also high in the summer. The concentrations of MHP were much lower than those of H₂O₂ in the rain water. High concentrations of H₂O₂ (> 2.5 ppb) were detected in the summer during the inflow of air pollution. The concentrations of H₂O₂ were significantly high in July and August of 2013. The H₂O₂ was well correlated with the O₃ in July and August whereas there was no correlation between O₃ and H₂O₂ in May and June. There was a negative correlation between NO_X and H₂O₂.     

Simulated impacts of afforestation in East China monsoon region as modulated by ocean variability

Using the National Center for Atmospheric Research Community Climate System Model Version 3.5, this paper examines the climatic effects of afforestation in the East China monsoon region with a focus on land–atmosphere interactions and the modulating influence of ocean variability. In response to afforestation, the local surface air temperature significantly decreases in summer and increases in winter. The summer cooling is attributed to enhanced evapotranspiration from increased tree cover. During winter, afforestation induces greater roughness and weaker winds over the adjacent coastal ocean, leading to diminished latent heat flux and increased sea-surface temperature (SST). The enhanced SST supports greater atmospheric water vapor, which is accompanied by anomalous wind, and transported into the East China monsoon region. The increase in atmospheric water vapor favors more cloud cover and precipitation, especially in the eastern afforestation region. Furthermore, the increase in atmospheric water vapor and cloud cover produce a greenhouse effect, raising the wintertime surface air temperature. By comparing simulations in which ocean temperature are either fixed or variable, we demonstrate that a significant hydrologic response in East China to afforestation only occurs if ocean temperatures are allowed to vary and the oceanic source of moisture to the continent is enhanced.     

Dynamics of the boreal summer African monsoon in the NSIPP1 atmospheric model

A nine-member ensemble of simulations with a state-of-the-art atmospheric model forced only by the observed record of sea surface temperature (SST) over 1930–2000 is shown to capture the dominant patterns of variability of boreal summer African rainfall. One pattern represents variability along the Gulf of Guinea, between the equator and 10°N. It connects rainfall over Africa to the Atlantic marine Intertropical Convergence Zone, is controlled by local, i.e., eastern equatorial Atlantic, SSTs, and is interannual in time scale. The other represents variability in the semi-arid Sahel, between 10°N and 20°N. It is a continental pattern, capturing the essence of the African summer monsoon, while at the same time displaying high sensitivity to SSTs in the global tropics. A land–atmosphere feedback associated with this pattern translates precipitation anomalies into coherent surface temperature and evaporation anomalies, as highlighted by a simulation where soil moisture is held fixed to climatology. As a consequence of such feedback, it is shown that the recent positive trend in surface temperature is consistent with the ocean-forced negative trend in precipitation, without the need to invoke the direct effect of the observed increase in anthropogenic greenhouse gases. We advance plausible mechanisms by which the balance between land–ocean temperature contrast and moisture availability that defines the monsoon could have been altered in recent decades, resulting in persistent drought. This discussion also serves to illustrate ways in which the monsoon may be perturbed, or may already have been perturbed, by anthropogenic climate change.     

东亚和南亚季风协同作用对西南地区夏季降水的影响

为探究东亚夏季风(EASM,East?Asian?summer?monsoon)和南亚夏季风(SASM,South?Asian?summer?monsoon)相互作用及其强弱变化对西南地区夏季降水的影响,利用1979—2019年西南地区161站逐日降水观测资料和ERA-5提供的1979—2019年全球再分析资料,通过对...