Periodicity in intraseasonal variation of summer monsoon rainfall over Orissa, India in relation to synoptic disturbances

Summary The summer monsoon rainfall over Orissa, a state of eastern India, shows characteristic intraseasonal and interannual variability, due to interaction of basic westerly flow with orography and the synoptic scale monsoon disturbances including low-pressure systems and cyclonic circulations extending upto mid-tropospheric level (LPSC). These systems normally develop over the north Bay of Bengal and move west-northwestwards along the monsoon trough. The essence of this study is to find out the main features of the intraseasonal variability of daily monsoon rainfall over Orissa in relation to synoptic systems like LPSC and its implication on the interannual variation of rainfall. For this purpose, the actual and mean daily rainfall data of 31 uniformly distributed stations, six homogeneous regions and Orissa as a whole during monsoon season (June–September) over a period of 20 years (1980–1999) are subjected to auto-correlation and power spectrum analyses. The actual and average daily scores of significant EOFs and actual daily occurrence along with daily probability of occurrence of the LPSC influencing rainfall over Orissa during the same period are also subjected to auto-correlation and power spectrum analyses. The intraseasonal variation of monsoon rainfall over Orissa and different homogeneous regions is dominated by the synoptic mode (3–9 days) of variation due to the similar mode of variation in the occurrence of LPSC influencing the rainfall. The seasonal rainfall and hence the interannual variation depends on the intraseasonal variation of rainfall modulated with the synoptic mode of variation in the occurrence of the LPSC. The occurrence of LPSC over the northwest (NW) Bay/NW and adjoining northeast (NE) Bay and its subsequent movement and persistence over Orissa and east Madhya Pradesh & Chhattisgarh in synoptic mode (3–6 days) alongwith absence of similar mode in the occurrence of the LPSC over NE Bay, Gangetic West Bengal (GWB) in the north and west central (WC) Bay to the south leads to excess rainfall over different homogeneous regions and Orissa as a whole. The reverse is the case in deficient years over Orissa and all homogeneous regions except southwest Orissa. The occurrence of the LPSC over GWB in synoptic mode (about 5 days) alongwith absence of synoptic mode in the occurrence of the LPSC over NW Bay leads to deficient rainfall year over southwest Orissa. Correspondence: U. C. Mohanty, Centre for Atmospheric Sciences, Indian Institute of Technology, Delhi Hauz Khas, New Delhi 110016, India     

Impact of MJO on the intraseasonal variation of summer monsoon rainfall over India

The summer monsoon rainfall over India exhibits strong intraseasonal variability. Earlier studies have identified Madden Julian Oscillation (MJO) as one of the most influencing factors of the intraseasonal variability of the monsoon rainfall. In this study, using India Meteorological Department (IMD) high resolution daily gridded rainfall data and Wheeler?CHendon MJO indices, the intra-seasonal variation of daily rainfall distribution over India associated with various Phases of eastward propagating MJO life cycle was examined to understand the mechanism linking the MJO to the intraseasonal variability. During MJO Phases of 1 and 2, formation of MJO associated positive convective anomaly over the equatorial Indian Ocean activated the oceanic tropical convergence zone (OTCZ) and the resultant changes in the monsoon circulation caused break monsoon type rainfall distribution. Associated with this, negative convective anomalies over monsoon trough zone region extended eastwards to date line indicating weaker than normal northern hemisphere inter tropical convergence zone (ITCZ). The positive convective anomalies over OTCZ and negative convective anomalies over ITCZ formed a dipole like pattern. Subsequently, as the MJO propagated eastwards to west equatorial Pacific through the maritime continent, a gradual northward shift of the OTCZ was observed and negative convective anomalies started appearing over equatorial Indian Ocean. During Phase 4, while the eastwards propagating MJO linked positive convective anomalies activated the eastern part of the ITCZ, the northward propagating OTCZ merged with monsoon trough (western part of the ITCZ) and induced positive convective anomalies over the region. During Phases 5 and 6, the dipole pattern in convective anomalies was reversed compared to that during Phases 1 and 2. This resulted active monsoon type rainfall distribution over India. During the subsequent Phases (7 and 8), the convective and lower tropospheric anomaly patterns were very similar to that during Phase 1 and 2 except for above normal convective anomalies over equatorial Indian Ocean. A general decrease in the rainfall was also observed over most parts of the country. The associated dry conditions extended up to northwest Pacific. Thus the impact of the MJO on the monsoon was not limited to the Indian region. The impact was rather felt over larger spatial scale extending up to Pacific. This study also revealed that the onset of break and active events over India and the duration of these events are strongly related to the Phase and strength of the MJO. The break events were relatively better associated with the strong MJO Phases than the active events. About 83% of the break events were found to be set in during the Phases 7, 8, 1 and 2 of MJO with maximum during Phase 1 (40%). On the other hand, about 70% of the active events were set in during the MJO Phases of 3 to 6 with maximum during Phase 4 (21%). The results of this study indicate an opportunity for using the real time information and skillful prediction of MJO Phases for the prediction of break and active conditions which are very crucial for agriculture decisions.     

The interdecadal variations of the summer monsoon rainfall over South China

Summary This paper is to promote a further understanding of the interdecadal variations of the summer monsoon rainfall over South China (SCMR). With this focus, we will specifically aim at better understanding possible mechanism responsible for such an interdecadal variation relationship between the SCMR and El Ni?o/Southern Oscillation (ENSO). In many of the previous studies on precipitation, the datasets used are satellite observations or gridded reanalyzed data due to the lack of long-term reliable observations over the marginal seas of the Asian continent. Such an approach could lead to possible errors in the results. In this work, several representative stations with long-term rain-gauge observations are chosen to reduce such uncertainty. The study of the interdecadal variabilities of SCMR indicates that there is a strong linkage between SCMR and ENSO on the interdecadal variations. These results agree well with those from previous studies that the Pacific Decadal Oscillation (PDO) and ENSO are not independent of each other, the interannual and interdecadal variations of tropical Pacific Sea Surface temperatures (SSTs) could affect the interdecadal variations of the SCMR, and the incorporating information on the PDO/ENSO could improve the long-term prediction of the SCMR.     

Experimental 15-day-lead statistical forecast of intraseasonal summer monsoon rainfall over Eastern China

1996年长江中下游、2002年华中南部以及2006年华南沿海地区出现了显著洪涝,并均伴随强季节内降水活动。在30–60天和10–20天这两个时间尺度上,季节内降水异常均与亚洲季风区大气季节内振荡密切相关。本文选取亚洲季风区大气季节内振荡的关键要素作为预报因子,应用贝叶斯小波频段方案对季节内降水进行了预报时效为15天的延伸期预测。该方案对这三年的季节内降水中心的预测水平均较好,预测与观测的季节内降水相关系数均在0.6以上。     

江淮夏季降水异常与西印度洋地区大气环流异常的关系

运用NCEP/NCAR再分析资料和中国气候中心整编的160站月平均降水资料应用经验正交函数、线性相关分析等,分析了江淮地区夏季降水异常特征及其与西印度洋区域大气环流年际异常关系的变化及其可能的机理。结果表明:当500 hPa中纬度低槽活动偏多(少),西太平洋副热带高压偏强(弱),东亚夏季风偏强(弱)时,江淮地区降水偏多(少)。进一步分析还发现西印度洋上空的垂直环流与江淮夏季降水存在较好的关系,但这种年际异常之间的联系受到背景场的影响明显:1979—1993年西印度洋垂直上升运动与江淮夏季降水的变化趋势基本相反,两者线性相关系数为-0.43;1994—2010年两者的变化趋势基本一致,相关系数达0.71。即当西印度洋地区存在环流异常下沉(上升)时,西太平洋副热带高压通常异常减弱东退(增强西伸),副热带季风减弱(增强),有利于雨带偏南(北)。因此,在西太平洋副热带高压和副热带季风年代际偏强(弱)阶段,西印度洋环流与江淮夏季降水呈负(正)相关。     

Origins of intraseasonal rainfall variations over the southern South China Sea in boreal winter

本文主要探讨冬季南海南部季节内降水变化的来源,数据主要来自于TRMM和NCEP-DOE资料,方法主要有谱分析和回归分析。研究发现南海南部的降水变化在10–20天和30–60天时间尺度上有不同的来源。10–20天时间尺度上,东亚冬季风超前于南海南部降水异常2天。而在30–60天时间尺度上,两者几乎同时出现。此外,热带东南印度洋上的降水异常超前于南海南部一周左右,这表明热带印度洋上的对流引起的环流异常可能对南海南部30–60天时间尺度上降水异常有重要的作用,但对10–20天时间尺度上降水异常的作用并不明显。     

Summer precipitation variability over South America on long and short intraseasonal timescales

A dipole pattern in convection between the South Atlantic convergence zone and the subtropical plains of southeastern South America characterizes summer intraseasonal variability over the region. The dipole pattern presents two main bands of temporal variability, with periods between 10 and 30 days, and 30 and 90 days; each influenced by different large-scale dynamical forcings. The dipole activity on the 30–90-day band is related to an eastward traveling wavenumber-1 structure in both OLR and circulation anomalies in the tropics, similar to that associated with the Madden–Julian oscillation. The dipole is also related to a teleconnection pattern extended along the South Pacific between Australia and South America. Conversely, the dipole activity on the 10–30-day band does not seem to be associated with tropical convection anomalies. The corresponding circulation anomalies exhibit, in the extratropics, the structure of Rossby-like wave trains, although their sources are not completely clear.     

西北太平洋SST季内振荡及其与中国东部夏季降水季内振荡的关系

利用NOAA逐日海表面温度(sea surface temperature,SST)资料、NCEP/NCAR逐日风场和比湿资料以及中国国家气象信息中心提供的逐日降水资料,研究了西北太平洋气候SST的低频周期,进一步分析了夏季西北太平洋SST季节内振荡与中国东部同期降水异常的关系。结果表明:夏季西北太平洋季节内SST异常影响中国东部同期季节内降水最显著的三个区域为:长江中游及华南沿海;江淮流域;华北大部。其影响途径主要是通过西北太平洋季节内海温与850 h Pa环流场之间相互作用,在东亚沿岸自南向北逐渐形成气旋—反气旋—气旋(反气旋—气旋—反气旋)的波列结构,引起东亚沿海局地水汽的辐合辐散,使得中国东部夏季季节内雨带从江淮流域向华北推进(从华北南撤到长江中游及华南沿海地区)。     

Impact of projected SST changes on summer rainfall in southeastern South America

Recent studies have shown that global warming and associated sea-surface temperature (SST) changes may trigger an important rainfall increase in southeastern South America (SESA) during the austral summer (December–January–February, DJF). The goal of this paper is to provide some insight into processes which may link global and SESA changes. For this purpose, a “two-way nesting” system coupling interactively the regional and global versions of the LMDZ4 atmospheric model is used to study the response to prescribed SST changes. The regional model is a variable-grid version of the global model, with a zoom focused over South America. An ensemble of simulations forced by distinct patterns of DJF SST changes has been carried out using a decomposition of full SST changes into their longitudinal and latitudinal components. The full SST changes are based on projections for the end of the twenty-first century from a multi-model ensemble of WCRP/CMIP3. Results confirm the presence of a major rainfall dipole structure, characterized by an increase in SESA and a decrease in the South Atlantic Convergence Zone region. Rainfall changes found in the WCRP/CMIP3 models are largely explained as a response of this dipole structure to the zonally-asymmetric (or longitudinal) component of SST changes. The rainfall response to the zonal-mean (or latitudinal) SST changes (including the global warming signal itself) shows an opposite contribution. The processes explaining the role of zonally-asymmetric SST changes involve remote effects of SST warming over the equatorial Indian and Pacific oceans inducing an atmospheric wave-train extended across the South Pacific into South America.     

Indo-Pacific remote forcing in summer rainfall variability over the South China Sea

This study investigates summer rainfall variability in the South China Sea (SCS) region and the roles of remote sea surface temperature (SST) forcing in the tropical Indian and Pacific Ocean regions. The SCS summer rainfall displays a positive and negative relationship with simultaneous SST in the equatorial central Pacific (ECP) and the North Indian Ocean (NIO), respectively. Positive ECP SST anomalies induce an anomalous low-level cyclone over the SCS-western North Pacific as a Rossby-wave type response, leading to above-normal precipitation over northern SCS. Negative NIO SST anomalies contribute to anomalous cyclonic winds over the western North Pacific by an anomalous east–west vertical circulation north of the equator, favoring more rainfall over northern SCS. These NIO SST anomalies are closely related to preceding La Niña and El Niño events through the “atmospheric bridge”. Thus, the NIO SST anomalies serve as a medium for an indirect impact of preceding ECP SST anomalies on the SCS summer rainfall variability. The ECP SST influence is identified to be dominant after 1990 and the NIO SST impact is relatively more important during 1980s. These Indo-Pacific SST effects are further investigated by conducting numerical experiments with an atmospheric general circulation model. The consistency between the numerical experiments and the observations enhances the credibility of the Indo-Pacific SST influence on the SCS summer rainfall variability.     

强弱南海夏季风活动及大气季节内振荡

应用ＮＣＥＰ再分析资料和中国降水资料，分析研究了对应南海强、弱夏季风的环流形势及其与之相应的中国东部的降水异常。其结果表明，由强、弱夏季风所引起的中国气候异常是完全不同（甚至反相）的。分析大气季节内振荡（ＩＳＯ）的活动还表明，对应大气强（弱）南海夏季风，南海地区 ８５０ ｈＰａ也有强（弱）大气 ＩＳＯ；而强、弱南海夏季风环流（２００ ｈＰａ和 ８５０ ｈＰａ）主要由异常的大气ＩＳＯ所激发。本研究还揭示了南海地区大气ＩＳＯ的变化往往与江淮地区大气ＩＳＯ的变化反相，例如南海地区的强（弱）大气ＩＳＯ常与江淮流域的弱（强）大气ＩＳＯ相对应。对于大气ＩＳＯ的强度，一般多表现出局地激发特征，经向传播相对较弱。     

Characteristics of summertime daily rainfall variability over South America and the South Atlantic Convergence Zone

Summary ?This paper presents an objective analysis of the structure of daily rainfall variability over the South American/South Atlantic region (15°–60° W and 0°–40° S) during individual austral summer months of November to March. From EOF analysis of satellite derived daily rainfall we find that the leading mode of variability is represented by a highly coherent meridional dipole structure, organised into 2 extensive bands, oriented northwest to southeast across the continent and Atlantic Ocean. We argue that this dipole structure represents variability in the meridional position of the South Atlantic Convergence Zone (SACZ). During early and later summer, in the positive (negative) phase of the dipole, enhanced (suppressed) rainfall over eastern tropical Brazil links with that over the subtropical and extra-tropical Atlantic and is associated with suppressed (enhanced) rainfall over the sub-tropical plains and adjacent Atlantic Ocean. This structure is indicative of interaction between the tropical, subtropical and temperate zones. Composite fields from NCEP reanalysis products (associated with the major positive and negative events) show that in early and late summer the position of the SACZ is associated with variability in: (a) the midlatitude wave structure, (b) the position of the continental low, and (c) the zonal position of the South Atlantic Subtropical High. Harmonic analysis of the 200 hPa geopotential anomaly structure in the midlatitudes indicates that reversals in the rainfall dipole structure are associated primarily with variability in zonal wave 4. There is evidence of a wave train extending throughout the midlatitudes from the western Pacific into the SACZ region. During positive (negative) events the largest anomalous moisture advection occurs within westerlies (easterlies) primarily from Amazonia (the South Atlantic). In both phases a convergent poleward flow results along the leading edge of the low-level trough extending from the tropics into temperate latitudes. High summer events differ from those in early and late summer in that the rainfall dipole is primarily associated with variability in the phase of zonal wave 3, and that tropical-temperate link is not clearly evident in positive events. Received May 31, 2001; revised October 17, 2001; accepted June 13, 2002     

The role of synoptic systems in the interannual variability of Sahel rainfall

Summary Components of the June-September climate over the Sahel are investigated in simulations with the GCM of the NASA/Goddard Institute for Space Studies, forced by SST observed during 1987 and 1988. The study analyzes the role of the synoptic patterns in determining precipitation differences between the two seasons, with special attention given to African wave disturbances (AWD). Emphasis is placed on deducing the characteristics of individual systems which may be missed by spectral and/or composite analyses. Results are derived from time-longitude cross-sections and spatial distributions of daily and weekly averages of key climatological variables. Despite the overall rainier season, rainless AWD are more prevalent in the simulations corresponding to June–September 1988 forcing than for 1987. Daily precipitation is shown to be highly correlated with mid-tropospheric vorticity, near surface convergence and 200 mb divergence. August daily rainfall was some-what better correlated with implied large scale vertical motion for 1988 forcing, emphasizing the dominance of broad circulation influences during that summer. While significant rainfall variability is attributed to AWD, quasistationary mechanisms cannot be ignored. In these simulations, upper tropospheric divergence modulated by changes in the Tropical Easterly Jet serves to both intensify the rainfall triggered by AWD and to sustain broader rainfall patterns during events of massive uplift.With 16 Figures     

Moisture transport and intraseasonal variability in the South America monsoon system

This paper examines moisture transport on intraseasonal timescales over the continent and over the South Atlantic convergence zone (SACZ) during the South America (SA) summer monsoon. Combined Empirical Orthogonal Function analysis (EOFc) of Global Precipitation Climatology Project pentad precipitation, specific humidity, air temperature, zonal and meridional winds at 850?hPa (NCEP/NCAR reanalysis) are performed to identify the large-scale variability of the South America monsoon system and the SACZ. The first EOFc was used as a large-scale index for the South American monsoon (LISAM), whereas the second EOFc characterized the SACZ. LISAM (SACZ) index showed spectral variance on 30?C90 (15?C20) days and were both band filtered (10?C100?days). Intraseasonal wet anomalies were defined when LISAM and SACZ anomalies were above the 75th percentile of their respective distribution. LISAM and SACZ wet events were examined independently of each other and when they occur simultaneously. LISAM wet events were observed with the amplification of wave activity in the Northern Hemisphere and the enhancement of northwesterly cross-equatorial moisture transport over tropical continental SA. Enhanced SACZ was observed with moisture transport from the extratropics of the Southern Hemisphere. Simultaneous LISAM and SACZ wet events are associated with cross-equatorial moisture transport along with moisture transport from Subtropical Southwestern Atlantic.     

Prediction of early summer rainfall over South China by a physical-empirical model

In early summer (May–June, MJ) the strongest rainfall belt of the northern hemisphere occurs over the East Asian (EA) subtropical front. During this period the South China (SC) rainfall reaches its annual peak and represents the maximum rainfall variability over EA. Hence we establish an SC rainfall index, which is the MJ mean precipitation averaged over 72 stations over SC (south of 28°N and east of 110°E) and represents superbly the leading empirical orthogonal function mode of MJ precipitation variability over EA. In order to predict SC rainfall, we established a physical-empirical model. Analysis of 34-year observations (1979–2012) reveals three physically consequential predictors. A plentiful SC rainfall is preceded in the previous winter by (a) a dipole sea surface temperature (SST) tendency in the Indo-Pacific warm pool, (b) a tripolar SST tendency in North Atlantic Ocean, and (c) a warming tendency in northern Asia. These precursors foreshadow enhanced Philippine Sea subtropical High and Okhotsk High in early summer, which are controlling factors for enhanced subtropical frontal rainfall. The physical empirical model built on these predictors achieves a cross-validated forecast correlation skill of 0.75 for 1979–2012. Surprisingly, this skill is substantially higher than four-dynamical models’ ensemble prediction for 1979–2010 period (0.15). The results here suggest that the low prediction skill of current dynamical models is largely due to models’ deficiency and the dynamical prediction has large room to improve.     

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。