夏季的季风环流

早在五十年代,陶诗言和陈隆勋(1957)就已指出,印度西南季风的爆发和我国长江流域梅雨的开始有密切关系。随后,我国不少气象工作者指出,孟加拉湾是我国夏季重要的水汽源地之一,而西南季风则起了水汽输送者的作用。所以,有关季风的研究对于解决我国夏季旱涝的予报有重大的意义。从大气环流来看,季风是夏季热带和付热带大气环流中重要的一员,季风云团提供了大量的凝结潜热,季风又是夏季热带地区角动量输送     

Relationships between the global general circulation and the indian summer monsoon

The relationships between the global general circulation and the Indian monsoon during active and break phases are investigated with the help of FGGE IIIb data.It was found that the ultralong wave component positive and negative height anomalies over Tibet are associated with active and break monsoon phases respectively. This ultralong wave component has significant effect even upto 22oN over the Indian region which is the monsoon trough region. During a monsoon break, the general circulation was found to be more turbulent in the sense that more waves become energised.It was observed that during a break, blocking prevails over the Siberian region and cold air advection takes place toward Indian region from Siberian region depressing the temperatures over the Indian region by about 1oC. During the break, the Indian region gets connected with higher latitudes by the south winds blowing from polar Soviet re-gions to the Indian region. From active to break phase the zonal component weakens by about 25% from Indian ocean area right upto Alaskan region, along the east coast of Asia.     

Relationships between Regional Indian Summer Monsoon Rainfall and Eurasian Snow Cover

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Long-Term variability of the indian summer monsoon and related parameters

The long-term variability of the Indian summer monsoon rain-fall and related regional and global param-eters are studied. The cubic spline is used as a digital filter to smooth the high frequency signals in the time series of the various parameters. The length of the data series varies from 95 to 115 years during the period 1871-1985. The parameters studied within the monsoon system are: (a) monsoon rainfall of the country as a whole; (b) number of break-monsoon days during July and August; (c) number of storms/ depressions in Bay of Bengal and Arabian Sea during summer monsoon season; and (d) dates of onset of summer monsoon over South Kerala Coast. The parameters studied outside the monsoon system are: (a) the Wright’s Southern Oscillation Index (June-July-August); (b) the January mean Northern Hemi-spheric surface air temperature anomaly; and (c) the East-equatorial Pacific sea surface temperature anomaly.In order to examine the variability under various degrees of the smoothing, the series are filtered with splines of 50% variance reduction frequency of one cycle per 10, 20 and 30 years. It is observed that the smoothed time series of the parameters within the monsoon system comprise a common slowly varying com-ponent in an episodic manner distinctly showing the excess and deficient rainfall epochs. The change of intercorrelations between the time series with increasing degree of smoothing throws some light on the time scales of the dominant interactions. The relation between Southern Oscillation and East equatorial Pacific sea surface temperature and the Indian summer monsoon seems to be dominant on the interannual scale. The low frequency variations are found to have significantly contributed to the instability of the correlations of monsoon rainfall with parameters outside the monsoon system.     

Large-Scale features of the indian summer monsoon rainfall and their association with some oceanic and atmospheric variables

The summer monsoon rainfall totals for 31 meteorological subdivisions of India for the years 1901-1980 are analysed. The analysis reveals that four leading eigenvectors (EVs) are significant and account for 65 % of the total variance.The spatial pattern of the first EV exhibits in phase fluctuations over almost the whole India. The large coefficients of this vector can be considered as representative of the conditions of large-scale flood and drought over the country. The second pattern reveals the fluctuations mostly over the North Indian region (north of 20o latitude) probably in association with the Western Disturbances. The third pattern indicates fluctuations over the North-West and the North-East India in opposite phase and the fourth pattern exhibits the characteristic features of fluctuations associated with ‘break’. The spectral analysis of the coefficients of these EVs revealed quasi-periodicities of 2-5 years.On the basis of examination of the elements of these EVs the country has been divided into seven homogeneous regions. Rainfall indices of these regions and of the four EVs have been examined for seek-ing for association with some oceanic and atmospheric variables. The association is significant for the coefficients of the first EV and for the rainfall indices of central and South India. Among all the variables examined, Darwin pressure tendencies have the highest association and appear to be of special significance in prediction of the monsoon rainfall.     

The Variability of the Interannual Oscillations of the Indian Summer Monsoon Rainfall

A new method of analysis namely, Singular Spectrum Analysis (SSA) is applied to the Indian Summer Monsoon (June-September) Rainfall (ISMR) series. The method is efficient in extracting the statistically significant oscillations with periods 2.8 and 2.3 year from the white noise of the ISMR series. The study shows that 2.8 / 2.3 year cycle captures the variability of the ISMR related to Southern Oscillation / Quasi Biennial Oscillation. The temporal structure of these oscillations show that these are in phase in extreme (excess and drought) monsoon conditions as well as in El Nino Southern Oscillation (ENSO) years. Both these oscillations show minimum variability during the period 1920-1940 and there is an increasing trend in the variability of these oscillations in the recent decades. The study enables to obtain pure signal consisting of reconstructed time series using these two Oscillations, from the original white noise series.     

Some aspects of an early summer monsoon onset over Andaman Sea and further progress towards the Indian mainland

This work addresses the mechanisms that leads to an early onset of monsoon over Andaman Sea but advances further rapidly (slowly) to the Indian mainland resulting in the early (delayed) onset over Kerala. The upper tropospheric temperature, production of kinetic energy (KE) and outgoing long wave radiation (OLR) from the month of May till onset over Kerala are analysed for two delayed onset years (1997, 1995) and two early onset years (2004, 1990). It is observed that the maximum temperature over Tibetan plateau (TP), an increase in the production of KE and strong equatorial convection in early May, is associated with early onset over Andaman Sea. However, when there is a lull in advance of monsoon after the early onset over Andaman Sea, shifting of the warm region south of TP, weak production of KE in the lower troposphere and convective region shifting to Western Pacific resulted in the delayed onset over Kerala in 1997 and 1995. During the early onset years viz. 2004 and 1990, the warm region moving westwards, high production of KE extending to mid troposphere and deep convection moving westwards in the north Indian Ocean (10–15°N) is noticed.     

Pacific decadal oscillation and variability of the Indian summer monsoon rainfall

Recent studies have furnished evidence for interdecadal variability in the tropical Pacific Ocean. The importance of this phenomenon in causing persistent anomalies over different regions of the globe has drawn considerable attention in view of its relevance in climate assessment. Here, we examine multi-source climate records in order to identify possible signatures of this longer time scale variability on the Indian summer monsoon. The findings indicate a coherent inverse relationship between the inter-decadal fluctuations of Pacific Ocean sea surface temperature (SST) and the Indian monsoon rainfall during the last century. A warm (cold) phase of the Pacific interdecadal variability is characterized by a decrease (increase) in the monsoon rainfall and a corresponding increase (decrease) in the surface air temperature over the Indian subcontinent. This interdecadal relationship can also be confirmed from the teleconnection patterns evident from long-period sea level pressure (SLP) dataset. The SLP anomalies over South and Southeast Asia and the equatorial west Pacific are dynamically consistent in showing an out-of-phase pattern with the SLP anomalies over the tropical central-eastern Pacific. The remote influence of the Pacific interdecadal variability on the monsoon is shown to be associated with prominent signals in the tropical and southern Indian Ocean indicative of coherent inter-basin variability on decadal time scales. If indeed, the atmosphere–ocean coupling associated with the Pacific interdecadal variability is independent from that of the interannual El Niño-Southern Oscillation (ENSO), then the climate response should depend on the evolutionary characteristics of both the time scales. It is seen from our analysis that the Indian monsoon is more vulnerable to drought situations, when El Niño events occur during warm phases of the Pacific interdecadal variability. Conversely, wet monsoons are more likely to prevail, when La Niña events coincide during cold phases of the Pacific interdecadal variability.     

On the Spatio-Temporal Variations of the Tropopause Height over India and Indian Summer Monsoon Activity

The spatio-temporal variation of the tropopause height (TH) over the Indian region (5°N-35°N, 70°E-95°E) has been studied using monthly mean TH data, for 22-year period, 1965 to 1986. The study revealed that the stations south of 20° showed maximum TH in April / May and minimum in September. This variation in TH has been attributed to the corresponding variation of average sea surface temperature (SST) over ± 20° latitudinal belt over Indian Ocean, Arabian Sea and Bay of Bengal. Further the stations north of 20°N showed maximum in June and minimum in October/ November. This maximum in TH has primarily been attributed to the increased insolation and convection. Furthermore it is noticed that the anomaly of TH moved northwards during the period April to July.The interannual variability of the Indian Summer Monsoon Activity (ISMA) has been studied in relation to all India mean TH (at 12 GMT) for six months April through September. The composites of mean TH for good and bad monsoon years showed that     

孟加拉湾西南季风与南海热带季风季节内振荡特征的比较

采用美国国家环境预报中心的向外长波辐射和风场资料及日本气象厅的降水资料,用30-60d滤波后的夏季风指数在孟加拉湾和南海的区域平均值分别代表孟加拉湾西南季风和南海热带季风季节内振荡,对两支季风的季节内振荡特征进行比较分析,发现孟加拉湾西南季风的季节内振荡和南海热带季风的季节内振荡在夏季风期间(5-10月)都有约3次半的波动.夏季风期间,在阿拉伯海-西太平洋纬带上,夏季风的季节内振荡有4次从阿拉伯海的东传和3次从西太平洋的西传,其中7月后东传可直达西太平洋.孟加拉湾和南海在夏季风期间都有4次季节内振荡的经向传播,但孟加拉湾在约15°N以南为季节内振荡从热带东印度洋的北传,在约15°N以北则为副热带季风季节内振荡的南传;而在南海则是4次季节内振荡从热带的北传.在以孟加拉湾西南季风季节内振荡和南海热带季风季节内振荡分别划分的6个位相中,都存在1-3位相和4-6位相中低频对流、环流形势相反的特征,这是由热带东印度洋季节内振荡的东传和北传所致.热带印度洋季节内振荡沿西南-东北向经过约14d传到孟加拉湾,激发了孟加拉湾西南季风季节内振荡的东传,经过约6d到达南海,激发了南海热带季风季节内振荡的北传,经过约25d到达华南,形成热带印度洋季节内振荡向华南的经纬向接力传播(45d).孟加拉湾西南季风季节内振荡所影响的降水主要是在20°N以南的热带雨带随低频对流的东移而东移;而南海热带季风季节内振荡所影响的降水除了这种热带雨带随低频对流的东移外,还有在20°N以北的东亚副热带地区存在雨带随南海低频对流的北移而北移.     

Teleconnections associated with Northern Hemisphere summer monsoon intraseasonal oscillation

The boreal summer intraseasonal oscillation (BSISO) has strong convective activity centers in Indian (I), Western North Pacific (WNP), and North American (NA) summer monsoon (SM) regions. The present study attempts to reveal BSISO teleconnection patterns associated with these dominant intraseasonal variability centers. During the active phase of ISM, a zonally elongated band of enhanced convection extends from India via the Bay of Bengal and Philippine Sea to tropical central Pacific with suppressed convection over the eastern Pacific near Mexico. The corresponding extratropical circulation anomalies occur along the waveguides generated by the North African-Asian jet and North Atlantic-North European jet. When the tropical convection strengthens over the WNPSM sector, a distinct great circle-like Rossby wave train emanates from the WNP to the western coast of United States (US) with an eastward shift of enhanced meridional circulation. In the active phase of NASM, large anticyclonic anomalies anchor over the western coast of US and eastern Canada and the global teleconnection pattern is similar to that during a break phase of the ISM. Examination of the evolution of the BSISO teleconnection reveals quasi-stationary patterns with preferred centers of teleconnection located at Europe, Russia, central Asia, East Asia, western US, and eastern US and Canada, respectively. Most centers are embedded in the waveguide along the westerly jet stream, but the centers at Europe and Russia occur to the north of the jet-induced waveguide. Eastward propagation of the ISO teleconnection is evident over the Pacific-North America sector. The rainfall anomalies over the elongated band near the monsoon domain over the Indo-western Pacific sector have an opposite tendency with that over the central and southern China, Mexico and southern US, providing a source of intraseasonal predictability to extratropical regions. The BSISO teleconnection along and to the north of the subtropical jet provides a good indication of the surface sir temperature anomalies in the NH extratropics.     

亚洲热带夏季风指数与中国南方降水的关系

利用1979-2013年夏季全球2.5°×2.5°逐日环流资料和中国气象站点降水观测资料,采用动力学因子(西南风)与热力学因子(Radiation Longwave covting,OLR)相结合定义了标准化的亚洲热带夏季风指数(Tropical Summer Monsoon Index,TSMI).结果 表明,该指数...     

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.     

Propagation and mechanisms of the quasi-biweekly oscillation over the Asian summer monsoon region

The propagation and underlying mechanisms of the boreal summer quasi-biweekly oscillation (QBWO) over the entire Asian monsoon region are investigated, based on ECMWF Interim reanalysis (ERA-Interim) data, GPCP precipitation data, and an atmospheric general circulation model (AGCM). Statistical analyses indicate that the QBWO over the Asian monsoon region derives its main origin from the equatorial western Pacific and moves northwestward to the Bay of Bengal and northern India, and then northward to the Tibetan Plateau (TP) area, with a baroclinic vertical structure. Northward propagation of the QBWO is promoted by three main mechanisms: barotropic vorticity, boundary moisture advection, and surface sensible heating (SSH). It is dominated by the barotropic vorticity effect when the QBWO signals are situated to the south of 20°N. During the propagation taking place farther north toward the TP, the boundary moisture advection and SSH are the leading mechanisms. We use an AGCM to verify the importance of SSH on the northward propagation of the QBWO. Numerical simulations confirm the diagnostic conclusion that the equatorial western Pacific is the source of the QBWO. Importantly, the model can accurately simulate the propagation pathway of the QBWO signals over the Asian monsoon region. Simultaneously, sensitivity experiments demonstrate that the SSH over northern India and the southern slope of the TP greatly contributes to the northward propagation of the QBWO as far as the TP area.     

Relationship between 500 hPa ridge axis positions over the indian and the west Pacific regions and the Indian summer monsoon rainfall

500 hPa ridge positions over the Indian and the West Pacific regions during April are related with the summer monsoon rainfall over India. The ridge position over the Indian region shows better relation with monsoon rainfall than that shown by the ridge over the Pacific region. The multiple correlation of these ridge positions with monsoon rainfall exceeds 0.7. These predictive relationships are better than those shown by other parameters, viz. (1) Northern Hemispheric surface temperature; (2) East-Pacific sea surface tempera-ture; (3) El-Nino events and (4) Tahiti-Darwin pressure difference, and index of southern oscillation, over the 30-year samples analysed.     

Large scale aspects of India-China summer monsoon rainfall

This study investigates the dominant modes of variability in monthly and seasonal rainfall over the India-China region mainly through Empirical Orthogonal Function (EOF) analysis. The EOFs have shown that whereas the rain-fall over India varies as one coherent zone, that over China varies in east-west oriented bands. The influence of this banded structure extends well into India.Relationship of rainfall with large scale parameters such as the subtropical ridge over the Indian and the western Pacific regions, Southern Oscillation, the Northern Hemispheric surface air temperature and stratospheric winds have also been investigated. These results show that the rainfall over the area around 40oN, 110oE over China is highly related with rainfall over India. The subtropical ridge over the Indian region is an important predictor over India as well as over the northern China legion.     

Onset of the summer monsoon over the southern Vietnam and its predictability

The summer monsoon onset over southern Vietnam is determined through a new criterion based on both in situ daily rainfall at six selected stations provided by the Institute of Meteorology and Hydrology, Vietnam, and the zonal component of the wind at 1,000 hPa from the National Center for Environmental Prediction/Department of Energy Reanalysis 2. Over the period 1979–2004, the summer monsoon onset mean date is on 12 May, with a standard deviation of 11.6 days. The temporal and spatial structures of the atmospheric conditions prevailing during the onset period are detailed. Clear changes are seen in the zonal wind (strengthened over the Bay of Bengal and changed from negative to positive over South Vietnam) and in convection (deeper), in association with an intensification of the meridional gradients of sea level pressure at 1,000 hPa and of moist static energy at 2 m over Southeast Asia. The predictability of onset dates is then assessed. Cross-validated hindcasts based upon four predictors linked to robust signals in the atmospheric dynamics are then provided. They are highly significant when compared to observations (56% of common variance). Basically, late (early) onsets are preceded in March–April by higher (lower) sea level pressure over the East China Sea, stronger (weaker) southeasterly winds over southern Vietnam, decreasing (increasing) deep convection over the Bay of Bengal, and the reverse situation over Indonesia (120–140°E, 0–10°S).     

The burst of indian summer monsoon as revealed by goes satellite during monex 1979

During FGGE year 1979, low-level air flow over the western Indian Ocean was determined from the analysis of GOES images (5－20 June). The wind pattern shows sudden change in low-level air circulation over western Indian Ocean during the initial burst of summer monsoon. The burst of monsoon is characte-rized by sudden establishment of low-level jet and strong cross-equatorial flow. This abrupt change signals the beginning of southwest monsoon over India and it is associated with the first monsoon rainfall over the southern part of western coast of India. Sudden change in low-level air flow is followed by the burst of monsoon within 3－5 days.     

Some physical aspects of summer monsoon clouds—comparison of cloud model results with observations

The physical characteristics of the summer monsoon clouds were investigated. The results of a simple cloud mod-el were compared with the aircraft cloud physical observations collected during the summer monsoon seasons of 1973,1974,1976 and 1981 in the Deccan Plateau region.The model predicted profiles of cloud liquid water content (LWC) are in agreement with the observed profiles. There is reasonable agreement between the model predicted cloud vertical thickness and observed rainfall.The observed cloud-drop spectra were found to be narrow and the concentration of drops with diameter >20μm is either low or absent on many occasions. In such clouds the rain-formation cannot take place under natural atmos-pheric conditions due to the absence of collision-coalescence process. A comparison of the model predicted and ob-served rainfall suggested that the precipitation efficiency in cumulus clouds of small vertical thickness could be as low as 20 per cent.The clouds forming in the Deccan Plateau region during the summer monsoon are, by and large, cumulus and strato-cumulus type. The vertical thickness of the cumulus clouds is in the range of 1.0-2.0 km. The LWC is found to be more in the region between 1.6-1.9 km A. S. L., which corresponds to the level at almost 3 / 4 th of the total verti-cal thickness of the cloud and thereafter the LWC sharply decreased. Nearly 98 per cent of the tops of the low clouds in the region are below freezing level and the most frequent range of occurrence of these cloud-tops is in the range of 2.0-3.0 km A. S. L.. The dominant physical mechanism of rain-formation in these summer monsoon clouds it the col-lision-coalescence process.