Decadal change in the boreal summer intraseasonal oscillation

A decadal change in activity of the boreal summer intraseasonal oscillation (BSISO) was identified at a broad scale. The change was more prominent during August–October in the boreal summer. The BSISO activity during 1999–2008 (P2) was significantly greater than that during 1984–1998 (P1). Compared to P1, convection in the BSISO was enhanced and the phase speed of northward-propagating convection was reduced in P2. Under background conditions, warm sea surface temperature (SST) anomalies in P2 were apparent over the tropical Indian Ocean and the western tropical Pacific. The former supplied favorable conditions for the active convection of the BSISO, whereas the latter led to a strengthened Walker circulation through enhanced convection. This induced descending anomalies over the tropical Indian Ocean. Thermal convection tends to be suppressed by descending anomalies, whereas once an active BSISO signal enters the Indian Ocean, convection is enhanced through convective instability by positive SST anomalies. After P2, the BSISO activity was weakened during 2009–2014 (P3). Compared to P2, convective activity in the BSISO tended to be inactive over the southern tropical Indian Ocean in P3. The phase speed of the northward-propagating convection was accelerated. Under background conditions during P3, warmer SST anomalies over the maritime continent enhance convection, which strengthened the local Hadley circulation between the western tropical Pacific and the southern tropical Indian Ocean. Hence, the convection in the BSISO over the southern tropical Indian Ocean was suppressed. The decadal change in BSISO activity correlates with the variability in seasonal mean SST over the tropical Asian monsoon region, which suggests that it is possible to predict the decadal change.

     

A possible new mechanism for northward propagation of boreal summer intraseasonal oscillations based on TRMM and MERRA reanalysis

Boreal summer intraseasonal oscillations (BSISOs) manifest in the active and break spells and act as the primary building block of the Indian summer monsoon. Although recent research has evolved a basic framework for understanding the scale selection and northward propagation of the BSISO, the role of different hydrometeors in modulating these processes remains poorly explored. In this study, TRMM-2A12 retrievals and Modern Era Retrospective-analysis for Research and Applications reanalysis data are examined to establish relationship between cloud hydrometeors and other atmospheric dynamical parameters with the northward propagation of the BSISOs. The study reveals that the cloud liquid water leads the deep convection during the northward propagation of BSISOs in the lower troposphere, while the cloud ice slightly lags the convection. This distribution indicates the occurrence of a possible mechanism of the lower level moistening through the large scale moisture advection in lower atmosphere and boundary layer (PBL) convergence, followed by triggering of the deep convection. The analyses of moisture advection and the dynamical fields with respect to the convection center show that low level moistening is a manifestation of the barotropic vorticity and PBL convergence of moisture anomaly north of the convection center. A new internal dynamical-thermodynamical mechanism is unraveled to understand the reason behind the middle tropospheric heating maximum and its role on the northward propagation. It is shown that the enhanced moisture perturbation in lower levels together with the heat transport by the sub-grid scale eddies within the PBL induces lower level instability required to precondition the lower atmosphere for triggering the deep convection. Vigorous upward motion inside the deep convection uplifts the liquid hydrometeors to upper levels and the formation of precipitable ice leads to the heating maxima in the middle troposphere. To check the robustness of the proposed hypothesis, similar analysis is performed for the weak northward propagating BSISO cases.     

The East Asia-western North Pacific boreal summer intraseasonal oscillation simulated in GAMIL 1.1.1

We evaluate the performance of GAMIL1.1.1 in a 27-year forced simulation of the summer intraseasonal oscillation (ISO) over East Asia (EA)-western North Pacific (WNP). The assessment is based on two measures: climatological ISO (CISO) and transient ISO (TISO). CISO is the ISO component that is phase-locked to the annual cycle and describes seasonal march. TISO is the ISO component that varies year by year. The model reasonably captures many observed features of the ISO, including the stepwise northward advance of the rain belt of CISO, the dominant periodicities of TISO in both the South China Sea-Philippine Sea (SCS-PS) and the Yangtze River Basin (YRB), the northward propagation of 30--50-day TISO and the westward propagation of the 12--25-day TISO mode over the SCS-PS, and the zonal propagating features of three major TISO modes over the YRB. However, the model has notable deficiencies. These include the early onset of the South China Sea monsoon associated with CISO, too fast northward propagation of CISO from 20^oN to 40^oN and the absence of the CISO signal south of 10^oN, the deficient eastward propagation of the 30--50-day TISO mode and the absence of a southward propagation in the YRB TISO modes. The authors found that the deficiencies in the ISO simulation are closely related to the model's biases in the mean states, suggesting that the improvement of the model mean state is crucial for realistic simulation of the intraseasonal variation.     

Influences of ENSO on boreal summer intraseasonal oscillation over the western Pacific in decaying summer

Climate Dynamics - Characteristics of boreal summer intraseasonal oscillation (BSISO) over the western Pacific in ENSO decaying summer are revealed in this study. BSISO activity over the western...     

Real-time multivariate indices for the boreal summer intraseasonal oscillation over the Asian summer monsoon region

The boreal summer intraseasonal oscillation (BSISO) of the Asian summer monsoon (ASM) is one of the most prominent sources of short-term climate variability in the global monsoon system. Compared with the related Madden-Julian Oscillation (MJO) it is more complex in nature, with prominent northward propagation and variability extending much further from the equator. In order to facilitate detection, monitoring and prediction of the BSISO we suggest two real-time indices: BSISO1 and BSISO2, based on multivariate empirical orthogonal function (MV-EOF) analysis of daily anomalies of outgoing longwave radiation (OLR) and zonal wind at 850 hPa (U850) in the region 10°S–40°N, 40°–160°E, for the extended boreal summer (May–October) season over the 30-year period 1981–2010. BSISO1 is defined by the first two principal components (PCs) of the MV-EOF analysis, which together represent the canonical northward propagating variability that often occurs in conjunction with the eastward MJO with quasi-oscillating periods of 30–60 days. BSISO2 is defined by the third and fourth PCs, which together mainly capture the northward/northwestward propagating variability with periods of 10–30 days during primarily the pre-monsoon and monsoon-onset season. The BSISO1 circulation cells are more Rossby wave like with a northwest to southeast slope, whereas the circulation associated with BSISO2 is more elongated and front-like with a southwest to northeast slope. BSISO2 is shown to modulate the timing of the onset of Indian and South China Sea monsoons. Together, the two BSISO indices are capable of describing a large fraction of the total intraseasonal variability in the ASM region, and better represent the northward and northwestward propagation than the real-time multivariate MJO (RMM) index of Wheeler and Hendon.     

中高纬大气ISO对夏季鄂海阻高形成和维持的调节作用

运用1979—2015年的逐日NCEP-DOE再分析资料,探讨了夏季鄂霍次克海阻塞高压(简称“鄂海阻高”)与中高纬大气季节内振荡(简称“ISO”)之间的可能联系。研究发现,ISO扰动与夏季鄂海阻高在关键区(130°~160°E,60°~75°N)有最好的耦合关系,且关键区位势高度表现出显著的10~30 d的振荡周期。超前滞后合成分析表明,与鄂海阻高相联系的ISO扰动具有明显的向西传播的特征。波活动通量分析结果显示,波能量在中高纬140°W附近累积,因此西传的ISO扰动可能源自中高纬140°W附近。对位势倾向方程的诊断结果显示,动力作用对关键区位势高度的时间变化起主要作用。进一步运用尺度分析表明,在夏季鄂海阻高发生和维持的过程中,ISO经向风引导的平均涡度平流对位势高度的时间变化起主导作用。     

东亚—西北太平洋地区夏季季节内振荡的年代际变化

本文重点分析对比热带夏季季节内振荡(Boreal Summer Intraseasonal Oscillation, BSISO)1987—1995年(P1),1996—2007年(P2)和2008—2017年(P3)三阶段东亚—西北太平洋地区(East Asian-Western North Pacific, EAWNP)5—9月BSISO年代际变化的季节内差异特征。结果表明,在P1和P3两阶段,5—7月EAWNP BSISO强度几乎相同,但P2中每个月均显著增强,表明5—7月EAWNP BSISO经历了P1—P2增强和P2—P3减弱的年代际变化。8月,EAWNP BSISO强度从P1到P3逐渐增强,P3阶段比P1有显著增强,孟加拉湾和东亚副热带区域的BSISO活动增强。和P1相比,南海地区BSISO活动在P2阶段异常活跃,在5—7月强度增强,并且北传显著。在P2阶段,负位相的太平洋年代际(Interdecadal Pacific Oscillation, IPO)对应的赤道西太平洋和印度洋海温增暖,及Walker环流的增强为5—7月BSISO活动提供了水汽和对流发展的有利条件,而南...     

Poleward propagation of boreal summer intraseasonal oscillations in a coupled model: role of internal processes

The study compares the simulated poleward migration characteristics of boreal summer intraseasonal oscillations (BSISO) in a suite of coupled ocean?Catmospheric model sensitivity integrations. The sensitivity experiments are designed in such a manner to allow full coupling in specific ocean basins but forced by temporally varying monthly climatological sea surface temperature (SST) adopted from the fully coupled model control runs (ES10). While the local air?Csea interaction is suppressed in the tropical Indian Ocean and allowed in the other oceans in the ESdI run, it is suppressed in the tropical Pacific and allowed in the other oceans in the ESdP run. Our diagnostics show that the basic mean state in precipitation and easterly vertical shear as well as the BSISO properties remain unchanged due to either inclusion or exclusion of local air?Csea interaction. In the presence of realistic easterly vertical shear, the continuous emanation of Rossby waves from the equatorial convection is trapped over the monsoon region that enables the poleward propagation of BSISO anomalies in all the model sensitivity experiments. To explore the internal processes that maintain the tropospheric moisture anomalies ahead of BSISO precipitation anomalies, moisture and moist static energy budgets are performed. In all model experiments, advection of anomalous moisture by climatological winds anchors the moisture anomalies that in turn promote the northward migration of BSISO precipitation. While the results indicate the need for realistic simulation of all aspects of the basic state, our model results need to be taken with caution because in the ECHAM family of coupled models the internal variance at intraseasonal timescales is indeed very high, and therefore local air?Csea interactions may not play a pivotal role.     

The boreal summer intraseasonal oscillation simulated by four Chinese AGCMs participating in the CMIP5 project

The performances of four Chinese AGCMs participating in the Coupled Model Intercomparison Project Phase 5 （CMIP5） in the simulation of the boreal summer intraseasonal oscillation （BSISO） are assessed. The authors focus on the major characteristics of BSISO： the intensity, significant period, and propagation. The results show that the four AGCMs can reproduce boreal summer intraseasonal signals of precipitation; however their limitations are also evident. Compared with the Climate Prediction Center Merged Analysis of Precipitation （CMAP） data, the models underestimate the strength of the intraseasonal oscillation （ISO） over the eastern equatorial Indian Ocean （IO） during the boreal summer （May to October）, but overestimate the intraseasonal variability over the western Pacific （WP）. In the model results, the westward propagation dominates, whereas the eastward propagation dominates in the CMAP data. The northward propagation in these models is tilted southwest-northeast, which is also different from the CMAP result. Thus, there is not a northeast-southwest tilted rain belt revolution off the equator during the BSISO＇s eastward journey in the models. The biases of the BSISO are consistent with the summer mean state, especially the vertical shear. Analysis also shows that there is a positive feedback between the intraseasonal precipitation and the summer mean precipitation. The positive feedback processes may amplify the models＇ biases in the BSISO simulation.     

Influence of three types of boreal summer intraseasonal oscillation on summer precipitation over the Yangtze River Valley

本文研究了三种来源于印度洋的北半球夏季季节内振荡(BSISO)对长江中下游地区降水的影响.结果表明,在所有BSISO类型中水汽辐合对降水异常的贡献最大.在经典型中,伴随着自副热带西太平洋向西北太平洋传播的正(负)对流异常的气旋(反气旋)使得降水在-2(-1至3)侯减少(增加).在向东扩展型中,由于在印度洋的正(负)对流异常激发的遥响应在西北太平洋引起反气旋(气旋),降水在-2至0(1至3)侯增加(减少),在向北偶极子型中,由于伴随着正(负)对流异常的气旋(反气旋)从中国南海移动到西北太平洋,降水在-2至0(1至3)侯减少(增加).     

北半球夏季热带季节内振荡影响我国夏季降水的规律和预测方法

利用1979—2018年夏季逐日观测和再分析数据,对北半球夏季热带季节内振荡影响我国夏季降水的规律和预测方法开展了研究。首先,利用非传统滤波即异常相对倾向（Anomalous Relative Tendency,ART）方法获取了气象要素的次季节变化分量,并采用EOF分析方法提取了北半球夏季热带主要季节内振荡信号,结果表明向外长波辐射（Outgoing Longwave Radiation,OLR）异常相对倾向EOF前两个模态共同反映了北半球夏季起源于印度洋并向东和向北传播的、具有30~60 d周期的季节内振荡（Boreal Summer Intraseasonal Oscillation,BSISO）信号。回归分析表明,该季节内振荡信号能够导致当地及其北面地区低层风场和位势高度场异常,影响该地区及其北面地区的水汽辐合辐散,从而能引起我国尤其是我国南方地区季节内旱涝变化,并一定程度上反映了我国异常雨带的向北推进过程。而后,将提取的热带主要季节内振荡信号作为预测因子,将降水异常相对倾向作为先行预板对象,利用多元线性回归方法构建了我国夏季旬降水异常相对倾向的预报模型,将预报的旬降水异常相对倾向加上观测已知的降水近期背景距平,从而得到旬降水距平的预报结果。通过历史回报和交叉检验,评估了该模型对梅雨期我国江淮流域降水（包括2020年梅汛期异常降水）的次季节预测能力。     

Diagnosing the column-integrated moist static energy budget associated with the northward-propagating boreal summer intraseasonal oscillation

Climate Dynamics - Column-integrated moist static energy (MSE) budgets associated with the northward-propagating boreal summer intraseasonal oscillation (BSISO) are diagnosed for the Bay of Bangel...     

北半球夏季大气季节内振荡对中国东南沿海热带气旋暴雨的影响

本文采用美国台风联合警报中心整编的1981—2012年的best-track热带气旋资料、中国大陆743站逐日降水数据、亚太经合组织气候中心的北半球夏季季节内振荡(BSISO)指数和美国国家环境预报中心及大气研究中心的再分析资料,分析了中国东南部地区热带气旋暴雨特征及其与BSISO 8个位相的联系。结果表明:7、8月,中国东南沿海地区的热带气旋暴雨发生的频次最多,多个站点热带气旋暴雨占总暴雨频次的比例达40%以上。7、8月热带气旋暴雨主要发生于BSISO1的第1、2、7、8位相,发生的暴雨频次占总频次的78.4%,主要分布于福建省沿海地区和西南部;BSISO2的第5、6、7位相热带气旋暴雨发生的频次也较多,占总频次的73.6%,主要分布于福建省沿海地区及西南部和广东省西南沿海地区。对发生较多热带气旋暴雨的BSISO1和BSISO2位相背景下的环流合成异常的诊断显示,西北太平洋伸向中国东南沿海地区,大尺度引导气流为显著的异常东风带,有利于热带气旋登陆中国大陆;显著异常的纬向东风切变、气旋性相对涡度和整层水汽的异常辐合,有利于热带气旋登陆过程强度的维持,促使热带气旋暴雨发生在中国大陆。     

Changes in the boreal summer intraseasonal oscillation projected by the CNRM-CM5 model under the RCP 8.5 scenario

Climate Dynamics - The 30–60-day boreal summer intraseasonal oscillation (BSISO) is the predominant intraseasonal variability in the Asian summer monsoon (ASM) region, representing the...     

Seminal role of stratiform clouds in large-scale aggregation of tropical rain in boreal summer monsoon intraseasonal oscillations

Climate Dynamics - Modification of the vertical structure of non-adiabatic heating by significant abundance of the stratiform rain in the tropics has been known to influence the large-scale...     

北半球夏季季节内振荡年代际变化对西北太平洋群发台风突变减少的影响

利用台风最佳路径、bimodal IntraSeasonal Oscillation(bimodal ISO)指数和全球逐日向外长波辐射数据,研究了北半球夏季季节内振荡（Boreal Summer IntraSeasonal Oscillation,BSISO）年代际变化造成1996/1997年后西北太平洋群发台风突变减少的可能机制。分析显示,仅有包含3个及以上个数台风成员的“MTC3”群发出现了突变减少,此类台风更倾向在传播速度较慢、低频对流维持时间较长的BSISO活跃位相内出现,对次季节信号强度的要求相对较低。1996/1997年后,BSISO东传范围减小、周期延长、对流活跃位相日数缩短,导致西北太平洋长时间连续维持对流抑制位相,低频对流在145°E以东海域的强度减弱。当偏西海域有先导台风活动时,它向东南侧激发的罗斯贝波频散波列在（5°—20°N,145°—165°E）海域因没有低频对流耦合而快速消散,导致MTC3群发台风年代际突变减少。