Intraseasonal oscillation of the southwest monsoon over Sri Lanka and evaluation of its subseasonal forecast skill

斯里兰卡的雨季发生于5-9月间,主要受西南季风的控制.本文发现该地区的西南季风降水存在很强的次季节变率,主导周期为10-35天.降水的季节内变化与西传的异常气旋有关.进一步,利用S2S比较计划中欧洲中心的数值预报模式(ECMWF)提供的回报试验数据,评估了当今动力模式对斯里兰卡西南季风次季节变化的预报技巧.结果显示,对季风指数的预测技巧超过30天,而对降水指数的预测技巧大约两周,且模式的预报技巧具有明显的年际差异.分析表明,能否正确模拟出大尺度环流对热带对流的响应是影响斯里兰卡降水预测的重要因子.     

1991年江淮梅雨与副热带高压的低频振荡

利用NCEP/NCAR再分析资料和观测的台站降水资料研究了1991年梅雨期间江淮洪涝区降水的季节内振荡及其环流特征,着重考察了不同层次副热带高压的低频变化与降水的关系。小波分析表明1991年江淮梅雨期间降水低频振荡的主周期是15~35 d。在低空,15~35 d振荡以异常反气旋(气旋)的形式在中国东南沿海地区交替出现,调控着西太平洋副热带高压进入(退出)南海。低空的这种低频环流型与高空的偶极型涡旋对相耦合,偶极型涡旋对使得南亚高压东伸(西退),形成有利(不利)江淮流域降水的环流形势。江淮流域降水的低频振荡与500hPa北太平洋副热带高压的低频变化及其传播密切相关。当北太平洋西部的位势高度偏高、中部位势高度偏低时,江淮流域降水偏多;反之偏少。这种低频振荡可能起源于北太平洋中部夏威夷群岛附近,然后沿着副热带高压脊线附近向西传播到中国东南沿海。     

Daily modes of South Asian summer monsoon variability in the NCEP climate forecast system

The leading modes of daily variability of the Indian summer monsoon in the climate forecast system (CFS), a coupled general circulation model, of the National Centers for Environmental Predictions (NCEP) are examined. The space?Ctime structures of the daily modes are obtained by applying multi-channel singular spectrum analysis (MSSA) on the daily anomalies of rainfall. Relations of the daily modes to intraseasonal and interannual variability of the monsoon are investigated. The CFS has three intraseasonal oscillations with periods around 106, 57 and 30?days with a combined variance of 7%. The 106-day mode has spatial structure and propagation features similar to the northeastward propagating 45-day mode in the observations except for its longer period. The 57-day mode, despite being in the same time scale as of the observations has poor eastward propagation. The 30-day mode is northwestward propagating and is similar to its observational counterpart. The 106-day mode is specific to the model and should not be mistaken for a new scale of variability in observations. The dominant interannual signal is related to El Ni?o-Southern Oscillation (ENSO), and, unlike in the observations, has maximum variance in the eastern equatorial Indian Ocean. Although the Indian Ocean Dipole (IOD) mode was not obtained as a separate mode in the rainfall, the ENSO signal has good correlations with the dipole variability, which, therefore, indicates the dominance of ENSO in the model. The interannual variability is largely determined by the ENSO signal over the regions where it has maximum variance. The interannual variability of the intraseasonal oscillations is smaller in comparison.     

中国雨季的气候学特征

利用中国740站气候平均逐候降雨量对中国的主雨季进行定义,并对雨季（包括主雨季,春雨和秋雨）的气候学特征进行了讨论。结果表明:全国主雨季最早爆发于华南中部,最晚结束于华西地区。主雨季能持续4到14候不等,雨量占年总降水的30%～60%。主雨季在中国东部为季风雨季,自南向北推进;在西部受西风带影响,北方略早于南方, 且局地性强。中国雨季具有明显的区域性和阶段性特征。中国气候的夏季降水时间序列主要反映了季节循环特征, 但气候季节内振荡（CISO）对东部雨季的持续和推进具有明显的调制作用,其中长江中下游及其以南地区以30～60天周期为主。     

长江中游和下游夏季降水季节内振荡的差异

利用1979~2013年中国站点逐日降水资料和NCEP/NCAR再分析资料,对长江中下游夏季降水的季节内振荡最显著周期进行了分析研究。结果表明长江中游最显著周期为10~30天,长江下游最显著周期为30~60天。为了揭示这种差异产生的物理原因,进一步利用位相合成的方法对这两个区域不同周期的季节内振荡降水、高低空风场和高度场以及垂直结构和水汽等循环过程的演变特征进行分析。在200 hPa环流场上,长江中游的降水主要受到高纬度自西向东传播的波列影响,而长江下游的降水与鄂霍次克海的高度场的变化相关。在风场的垂直涡度和散度的位相结构演变过程中,10~30天的垂直涡度和散度有自北向南的移动,30~60天的垂直涡度和散度在长江以南地区有自南向北的传播。水汽输送的位相发展过程表明,长江中游的水汽分别来自于南海的向北输送和长江以北地区向南的水汽输送;长江下游地区的水汽则主要来自于热带东印度洋经孟加拉湾的向东输送并在南海的北向输送,以及西太平洋水汽向西输送到南海再向长江下游的输送。从高层大尺度环流场和整层积分的水汽通量输送上解释了长江中游10~30天降水的自北向南移动,和长江下游30~60天降水自南向北传播的原因。     

Meridional Propagation of the 30- to 60-day Variability of Precipitation in the East Asian Subtropical Summer Monsoon Region: Monitoring and Prediction

The meridional propagation of the 30- to 60-day intraseasonal variability (ISV) of precipitation in the East Asian subtropical summer monsoon (EASSM) region and its monitoring and prediction are investigated in the current study. Based on a multivariate empirical orthogonal function (MV-EOF) analysis of precipitation and relative vorticity at 700?hPa in East Asia, a bivariate index referred to as the EASSM-ISV index is designed using the two leading MV-EOF modes, with the objective of real-time monitoring of the 30- to 60-day variability of precipitation in the EASSM region. It is found that this index, with its eight phases, can explain the meridional propagation of the 30- to 60-day ISV in precipitation and circulation in the EASSM region. Based on a singular value decomposition technique, a statistical forecast model is developed in which the EASSM-ISV indices from the preceding five pentads are used to predict the indices in five pentads in the future. Meanwhile, the indices are used to predict the meridional propagation of the 30- to 60-day precipitation anomaly in the EASSM region. This model thus provides a useful tool for intraseasonal prediction of precipitation during the rainy season in China.     

THE IMPACT OF PRECEDING ATMOSPHERIC CIRCULATION AND SST VARIATION ON FLOOD SEASON RAINFALL IN YUNNAN

Spatial and temporal distribution characteristics and scale range of two significant areas were obtained by analyzing the relationship among summer rainfall in Yunnan province, height field and SST field (40°S – 40°N, 30 °E – 70°W) across the North Hemisphere at 200 hPa, 500 hPa and 850 hPa for Jan. to May and correlation, and field wave structure. Remote key regions among summer rainfall in Yunnan province, height field and SST field (40°S – 40°N, 30°E – 70°W) across the North Hemisphere at 200 hPa, 500 hPa and 850 hPa were studied through further analyzing of the circulation system and its climate / weather significance. The result shows that the forecast has dependable physical basis when height and SST fields were viewed as predictors and physical models of impacts on rainy season precipitation in Yunnan are preliminarily concluded.     

夏季对流和环流10-25天季内变化-5西太平洋暖池热力状况的关系

主要讨论了西太平洋暖池热力状况年际变化相联系的10－25天季内振荡的特征。在西太平洋暖池处于“暖”夏季和“冷”夏季两种状态时，10－25天季内振荡呈现出明显差异，在所讨论的亚洲－太平洋大部分区域，“暖”夏季时对流活动的10－25天季内变化和“冷”夏季时对流活动的10－25天季内变化表现出显的反相关关系，表明对流活动的年际变化与季内变化的相互作用。当西太平洋暖池上空对流活动的10－25天季内振荡处于最强和最弱阶段时，低层大气表现为Gill型环流响应，即气旋式（反气旋式）环流出现在最强（最弱）对流活动的西北，这种对流和环流关系在西太平洋暖池出于“暖”夏季和“冷”夏季两种状态时均成立。而当西太平洋暖池上空对流活动的10－25天季内振荡处于除此以外其他阶段时，就看不到Gill型环流响应。     

Intraseasonal wind anomalies related to wet and dry spells during the “long” and “short” rainy seasons in Kenya

Summary The largest part of Kenya exhibits two major rainy seasons, the March–May «long rains» and the October–December «short rains», both related to the passage of the ITCZ, but differing in the amount of rainfall recorded and its interannual variability. In order to investigate whether these differences also apply at intraseasonal time-scales, daily rainfall data for the peak month of each rainy season (April and November) were collected for 7 consecutive years (1982–1988). The network comprises 68 stations, from which a classification of the spatial patterns of daily rainfall anomalies has been performed. Wind anomalies corresponding to the various rainfall types and to specific regional rainfall departures were determined using four pilot balloon stations and one radiosonde station. They revealed that there exist significant differences between upper-air circulation anomalies exhibited in the «long» and «short» rainy seasons, especially as far as rain spells in the Eastern Highlands are concerned. In that region, easterly anomalies in the «short rains» period are associated with an increase in rainfall. During the «long rains», enhanced easterlies more generally coincide with an overall drop of convection in the country. In Western Kenya, wet conditions are more systematically associated to westerly wind anomalies.With 12 Figures     

北大西洋多年代际振荡正、负位相期间欧亚夏季副热带波列季节内活动特征及与印度降水的联系

利用美国国家环境预测中心与国家大气研究中心（NCEP/NCAR）逐日再分析资料,针对北大西洋多年代际振荡（AMO）两个不同位相,对逐候200 hPa经向风异常进行EOF分析,发现在AMO正、负位相期间,欧亚副热带波列的季节内活动存在明显差异。利用超前—滞后回归,对比了不同AMO位相下副热带波列及其相联系的印度夏季降水的季节内活动演变特征,分析有关的大气环流,探究波列影响降水的机制。结果表明:在AMO负位相期间,由格陵兰岛以南北大西洋经大不列颠岛、地中海、黑海—里海向南亚北部传播的副热带波列的季节内演变,在印度中部引起下沉,导致中部及西北部季节内降水减少,波列负位相相反;在AMO正位相期间,副热带波列西起冰岛以南北大西洋经丹麦南部、俄罗斯西部、中亚向南亚东北部传播,对应该波列的季节内演变,辐合上升区在印度中部和东西两侧,使得该区域季节内降水增加,波列负位相相反。于是,AMO通过调制夏季欧亚副热带波列的季节内活动,可以对印度夏季降水的季节内变化空间型及演变发挥显著影响。     

Interannual variations of the boreal summer intraseasonal variability predicted by ten atmosphere–ocean coupled models

The reproducibility of boreal summer intraseasonal variability (ISV) and its interannual variation by dynamical models are assessed through diagnosing 21-year retrospective forecasts from ten state-of-the-art ocean–atmosphere coupled prediction models. To facilitate the assessment, we have defined the strength of ISV activity by the standard deviation of 20–90 days filtered precipitation during the boreal summer of each year. The observed climatological ISV activity exhibits its largest values over the western North Pacific and Indian monsoon regions. The notable interannual variation of ISV activity is found primarily over the western North Pacific in observation while most models have the largest variability over the central tropical Pacific and exhibit a wide range of variability in spatial patterns that are different from observation. Although the models have large systematic biases in spatial pattern of dominant variability, the leading EOF modes of the ISV activity in the models are closely linked to the models’ El Nino-Southern Oscillation (ENSO), which is a feature that resembles the observed ISV and ENSO relationship. The ENSO-induced easterly vertical shear anomalies in the western and central tropical Pacific, where the summer mean vertical wind shear is weak, result in ENSO-related changes of ISV activity in both observation and models. It is found that the principal components of the predicted dominant modes of ISV activity fluctuate in a very similar way with observed ones. The model biases in the dominant modes are systematic and related to the external SST forcing. Thus the statistical correction method of this study based on singular value decomposition is capable of removing a large portion of the systematic errors in the predicted spatial patterns. The 21-year-averaged pattern correlation skill increases from 0.25 to 0.65 over the entire Asian monsoon region after applying the bias correction method to the multi-model ensemble mean prediction.     

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。     

南海—热带西北太平洋地区季节内尺度海气变化关系

本文系统地回顾了作者近年来关于南海-热带西北太平洋地区大气和海洋季节内尺度变化关系方面的主要研究成果。文中对10~20天和30~60天两种季节内振荡海气变化关系的不同以及冬、夏季间的差异进行了系统地比较。相比较而言,大气中10~20天振荡所占比例大于30~60天振荡,海表温度30~60天的振荡在南海和西北太平洋副热带地区比10~20天振荡的贡献大,而在低纬度西太平洋地区10~20天振荡与30~60天振荡贡献相近或稍大。在北半球夏季,10~20天低频振荡的分布呈西南—东北走向,由赤道西太平洋地区向西北偏西方向传播,而30~60天低频振荡则以东西向分布为主,表现为由南向北的传播特征。在北半球冬季,10~20天和30~60天两种低频振荡的水平结构类似,均表现为西南—东北走向;同时,南海地区季节内变化信号表现出明显的向南传播的独特特征,并与东亚冬季风的季节内变化密切相关。北半球夏季,南海—菲律宾海地区10~20天低频振荡强度在厄尔尼诺发展年得到加强,而30~60天低频振荡强度则在拉尼娜衰减年得以加强。分析还指出,热带西北太平洋地区夏季热带辐合带附近的季节内变化,尤其是10~20天尺度变化,对季节平均海表温度异常有显著的反馈作用。     

北半球冬季中纬度位势高度场的季节内振荡

利用1979—2012年逐日NCEP/DOE再分析资料,分析北半球中纬度冬季(11月1日—4月30日)对流层位势高度的季节内振荡特征。结果表明:对流层上层位势高度的季节内变化强度较中下层更强,中心主要位于太平洋和大西洋上空;对流层上层位势高度场主要为1~3波的超长波形势,功率谱分析结果表明其时间序列呈现显著的季节内振荡(10~60 d)特征;10~60 d滤波的位势高度异常空间分布与原异常场一致,位势高度季节内振荡随时间主要表现为向西传播的特征,尤其表现在北太平洋上空,而亚欧大陆更为复杂一些;亚洲冬季风对北半球中纬度位势高度的季节内振荡有响应,主要表现为蒙古高压位置和强度的异常,继而对我国冬季气温产生影响。     

Dynamical linkage of tropical and subtropical weather systems to the intraseasonal oscillations of the Indian summer monsoon rainfall. Part II: Simulations in the ENSEMBLES project

We assess the ability of individual models (single-model ensembles) and the multi-model ensemble (MME) in the European Union-funded ENSEMBLES project to simulate the intraseasonal oscillations (ISOs; specifically in 10–20-day and 30–50-day frequency bands) of the Indian summer monsoon rainfall (ISMR) over the Western Ghats (WG) and the Bay of Bengal (BoB), respectively. This assessment is made on the basis of the dynamical linkages identified from the analysis of observations in a companion study to this work. In general, all models show reasonable skill in simulating the active and break cycles of the 30–50-day ISOs over the Indian summer monsoon region. This skill is closely associated with the proper reproduction of both the northward propagation of the intertropical convergence zone (ITCZ) and the variations of monsoon circulation in this band. However, the models do not manage to correctly simulate the eastward propagation of the 30–50-day ISOs in the western/central tropical Pacific and the eastward extension of the ITCZ in a northwest to southeast tilt. This limitation is closely associated with a limited capacity of models to accurately reproduce the magnitudes of intraseasonal anomalies of both the ITCZ in the Asian tropical summer monsoon regions and trade winds in the tropical Pacific. Poor reproduction of the activity of the western Pacific subtropical high on intraseasonal time scales also amplify this limitation. Conversely, the models make good reproduction of the WG 10–20-day ISOs. This success is closely related to good performance of the models in the representation of the northward propagation of the ITCZ, which is partially promoted by local air–sea interactions in the Indian Ocean in this higher-frequency band. Although the feature of westward propagation is generally represented in the simulated BoB 10–20-day ISOs, the air–sea interactions in the Indian Ocean are spuriously active in the models. This leads to active WG rainfall, which is not present in the observed BoB 10–20-day ISOs. Further analysis indicates that the intraseasonal variability of the ISMR is generally underrepresented in the simulations. Skill of the MME in seasonal ISMR forecasting is strongly dependent on individual model performance. Therefore, in order to improve the model skill with respect to seasonal ISMR forecasting, we suggest it is necessary to better represent the robust dynamical links between the ISOs and the relevant circulation variations, as well as the proportion of intraseasonal variability in the individual models.     

Forecast skill of the tropical intraseasonal oscillation in the NCEP GFS dynamical extended range forecasts

This study examines the forecast performance of tropical intraseasonal oscillation (ISO) in recent dynamical extended range forecast (DERF) experiments conducted with the National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) model. The present study extends earlier work by comparing prediction skill of the northern winter ISO (Madden-Julian Oscillation) between the current and earlier experiments. Prediction skill for the northern summer ISO is also investigated. Since the boreal summer ISO exhibits northward propagation as well as eastward propagation along the equator, forecast skill for both components is computed. For the 5-year period from 1 January, 1998 through 31 December, 2002, 30-day forecasts were made once a day. Compared to the previous DERF experiment, the current model has shown some improvements in forecasting the ISO during winter season so that the skillful forecasts (anomaly correlation>0.6) for upper-level zonal wind anomaly extend from the previous shorter-than 5 days out to 7 days lead-time. A similar level of skill is seen for both northward and eastward propagation components during the summer season as in the winter case. Results also show that forecasts from extreme initial states are more skillful than those from null phases for both seasons, extending the skillful range by 3–6 days. For strong ISO convection phases, the GFS model performs better during the summer season than during the winter season. In summer forecasts, large-scale circulation and convection anomalies exhibit northward propagation during the peak phase. In contrast, the GFS model still has difficulties in sustaining ISO variability during the northern winter as in the previous DERF run. That is, the forecast does not maintain the observed eastward propagating signals associated with large-scale circulation; rather the forecast anomalies appear to be stationary at their initial location and decay with time. The NCEP Coupled Forecast System produces daily operational forecasts and its predication skill of the MJO will be reported in the future.     

春季对流层温度的季内和季节以上分量对南海夏季风爆发的年代际变化的相对影响

南海夏季风爆发时间在1993/1994年出现显著的年代际提早, 探讨了大气要素场的不同时间尺度分量季节演变的年代际变异对南海夏季风爆发时间的年代际变异的相对影响作用。南海夏季风爆发时间的年代际提早与南海季风区对流层经向温度梯度季节性逆转的年代际提早有密切联系。南海季风区5月中对流层经向温度梯度年代际增强主要由季风区北部温度的年代际显著增暖造成。季内分量和季节以上分量对1993年之前南海季风区经向温度梯度逆转及加强时间偏晚的作用同等重要。经向温度梯度距平的季节以上分量主要源于季风区北部温度相应分量的贡献, 而季节内分量则主要由南部相应分量影响所致, 并由25~90 d分量所主导。季节以上分量对1994年之后南海季风区经向温度梯度逆转及加强时间偏早的贡献要大于季节内分量的贡献。经向温度梯度距平的季节以上分量和季内分量对总距平的正贡献都主要来自于季风区北部温度相应分量。两种季内低频分量对温度梯度季内分量的贡献率相当, 10~25 d分量主要由南海北部温度相应分量所主导, 25~90 d分量对总距平的正贡献也源自北部分量。准双周振荡分量对各年代南海夏季风爆发具有明显的触发作用。      

Relationships between intraseasonal rainfall variability of coastal Tanzania and ENSO

Summary The influence of ENSO on intraseasonal variability over the Tanzanian coast during the short (OND) and long (MAM) rainy seasons is examined. In particular, variability in the rainfall onset, peak and end dates as well as dry spells are considered. In general, El Niño appears to be associated with above average rainfall while La Niña is associated with below average rainfall over the northern Tanzanian coast during OND, and to lesser extent MAM. Over the southern coast, the ENSO impacts are less coherent and this region appears to be a transition zone between the opposite signed impacts over equatorial East and southern Africa. The increased north coast rainfall during El Niño years is generally due to a longer than normal rainfall season associated with early onset while reduced rainfall during La Niña years tends to be associated with a late onset, and thus a shorter than average rainfall season. Wet conditions during El Niño years were associated with enhanced convection and low-level easterly anomalies over the equatorial western Indian Ocean implying enhanced advection of moisture from the Indian Ocean while the reverse is true for La Niña years. Hovmöller plots for OLR and zonal wind at 850 hPa and 200 hPa show eastward, westward propagating and stationary features over the Indian Ocean. It was observed that the propagating features were absent during strong El Niño years. Based on the Hovmöller results, it is observed that the convective oscillations over the Tanzanian coast have some of the characteristic features of intraseasonal oscillations occurring elsewhere in the tropics.     

基于南海夏季风季节内振荡的降水延伸预报试验

利用代表南海夏季风季节内振荡特征的850 hPa纬向风EOF分解的前两个主成分,定义南海夏季风季节内振荡指数,并利用美国国家环境预测中心第2代气候预报系统 (NCEP Climate Forecast System Version 2, NCEP/CFSv2) 提供的1982—2009年逐日回算预报场计算了南海夏季风季节内振荡指数的预报值,用于我国南方地区持续性强降水的预报试验。试验结果表明:利用南海夏季风季节内振荡实时监测指数与模式直接预报降水量相结合的统计动力延伸预报方法,能够有效提高季节内降水分量的预报效果。同时,该方法能够避免末端数据损失,修正了对模式预报降水直接进行带通滤波而导致的负相关现象,并起到消除模式系统误差的作用。     

Observational study of the seasonality of the submonthly and intraseasonal signal over the tropics

Summary Atmospheric variability in outgoing long-wave radiation (OLR) and tropospheric relative vorticity (VOR) over the South American region was studied from 1979 to 1996 using the complex Morlet wavelet function. The analyses focus on spatial variation in intraseasonal and submonthly scales. Scalograms were used to measure submonthly intraseasonal oscillations in convection, which were found to be predominant in the tropical regions. However, 7-day and 15-day oscillations were observed at tropical and extratropical latitudes in spring and winter, indicating that transient disturbances play a more prominent role. Regarding VOR, tropical energy intensities were highest in the spring and summer, whereas subtropical and extratropical energy intensities were highest in the autumn and winter. The dynamics of the 25-day and 45-day VOR oscillations indicates a possible correlation with Rossby waves over the eastern tropical Pacific Ocean, mainly during the summer. During winter, the 7-day and 15-day VOR oscillations are more frequent at higher latitudes and are enhanced along storm tracks. It was also observed that convection amplitudes in the regions of maximum intensity change appreciably from year to year and from season to season, showing that the behavior of the submonthly and intraseasonal oscillations is nonperiodic and correlates strongly with El Ni?o/Southern Oscillation years. These results confirm the efficiency of wavelet analysis for time-scale studies of atmospheric variability.