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1.
Summary The present study examines the long term trend in sea surface temperatures (SSTs) of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean in the context of global warming for the period 1901–2002 and for a subset period 1971–2002. An attempt has also been made to identify the relationship between SST variations over three different ocean areas, and All-India and homogeneous region summer monsoon rainfall variability, including the role of El-Ni?o/Southern Oscillation (ENSO). Annual sea surface temperatures of the Arabian Sea, Bay of Bengal and Equatorial South India Ocean show a significant warming trend of 0.7 °C, 0.6 °C and 0.5 °C per hundred years, respectively, and a relatively accelerated warming of 0.16 °C, 0.14 °C and 0.14 °C per decade during the 1971–2002 period. There is a positive and statistically significant relationship between SSTs over the Arabian Sea from the preceding November to the current February, and Indian monsoon rainfall during the period 1901–2002. The correlation coefficient increases from October and peaks in December, decreasing from February to September. This significant relationship is also found in the recent period 1971–2002, whereas, during 1901–70, the relationship is not significant. On the seasonal scale, Arabian Sea winter SSTs are positively and significantly correlated with Indian monsoon rainfall, while spring SSTs have no significant positive relationship. Nino3 spring SSTs have a negative significant relationship with Indian monsoon rainfall and it is postulated that there is a combined effect of Nino3 and Arabian Sea SSTs on Indian monsoon. If the Nino3 SST effect is removed, the spring SSTs over the Arabian Sea also have a significant relationship with monsoon rainfall. Similarly, the Bay of Bengal and Equatorial South Indian Ocean spring SSTs are significantly and positively correlated with Indian monsoon rainfall after removing the Nino3 effect, and correlation values are more pronounced than for the Arabian Sea. Authors’ address: Dr. D. R. Kothawale, A. A. Munot, H. P. Borgaonkar, Climatology and Hydrometeorology divisions, Indian Institute of Tropical Meteorology, Pune 411008, India.  相似文献   

2.
张永生  吴国雄 《气象学报》1998,56(5):513-528
该工作将亚洲季风区作为一个复杂的海-陆-气耦合系统,来深入考察季风区海-气、陆-气相互作用的基本事实和物理过程,探讨它们在决定亚洲季风爆发及北半球行星尺度大气环流的季节突变的物理机理。本文是系列文章的第一篇,着重研究亚洲夏季风爆发的区域性和阶段性特征,以及过渡季节热带、副热带地区海-气、陆-气相互作用的基本事实,初步分析了它们之间的联系。研究表明,热带季风对流于4月底到5月初越过赤道进入北半球,首先出现在孟加拉湾东部-中南半岛西南部地区,然后于5月中旬和6月上旬末分别出现在南海和印度半岛地区,呈阶段性爆发的特征。季风对流在孟加拉湾东部-中南半岛西南部地区爆发阶段,在大气环流变化和对流活动中心位置出现区别于南海季风和印度季风爆发的特征。通过对地表感热通量和海表潜热通量的分析,表明热带海洋上海表感热通量甚小于海表潜热通量,南海季风爆发时期印度洋上海表潜热通量显著增大,印度季风爆发后海表潜热通量的高值中心在孟加拉湾和阿拉伯海上建立起来。印度洋上低层增强的过赤道气流引起的强烈的海-气相互作用导致海表水汽的大量蒸发,并通过其输送作用,为季风对流的爆发提供了充足的水汽来源。过渡季节在副热带地区(沿27.5～37.5°N纬带上), 青藏高原和西太平洋上地(海)表感热通量和潜热通量均有迅速的季节变化性, 但趋势相反。当青藏高原上地表感热通量和潜热通量呈阶段性的显著加大, 西太平洋上海表感热通量和潜热通量迅速减小。这种大陆和海洋对大气加热的显著的季节化的差异, 影响着大气环流的季节转变。  相似文献   

3.
Summary The air-sea interaction processes over the tropical Indian Ocean region are studied using sea surface temperature data from the Advanced Very High Resolution Radiometer sensor onboard the NOAA series of satellites. The columnar water-vapour content, low-level atmospheric humidity, precipitation, wind speed, and back radiation from the Special Sensor Microwave Imager on board the U.S. Defense Meteorological Satellite Program are all examined for two contrasting monsoon years, namely 1987 (deficit rainfall) and 1988 (excess rainfall). From these parameters the longwave radiative net flux at the sea surface and the ocean-air moisture flux are derived for further analysis of the air-sea interaction in the Arabian Sea, the Bay of Bengal, the south China Sea and the southern Indian Ocean. An analysis of ten-day and monthly mean evaporation rates over the Arabian Sea and Bay of Bengal shows that the evaporation was higher in these areas during the low rainfall year (1987) indicating little or no influence of this parameter on the ensuing monsoon activity over the Indian subcontinent. On the other hand, the evaporation in the southern Indian Ocean was higher during July and September 1988 when compared with the same months of 1987. The evaporation rate over the south Indian Ocean and the low-level cross-equatorial moisture flux seem to play a major role on the ensuing monsoon activity over India while the evaporation over the Arabian Sea is less important. Since we have only analysed one deficit/ excess monsoon cycle the results presented here are of preliminary nature. Received November 5, 1997 Revised March 20, 1998  相似文献   

4.
This paper presents the results of a diagnostic study of a typical case of very heavy rainfall during the South Asian summer monsoon when a mesoscale low in a desert climate merged with a diffused tropical depression. The former low was located over Pakistan‘s desert region and the latter depression originated over the Bay of Bengal. Surface and NCEP reanalysis data supported by satellite and radar images were incorporated in the diagnosis. The relationship between the heavy precipitation process and large-scale circulations such as monsoon trough, subtropical high, westerly jet, low level jet and water vapor transport were investigated to further understand the mechanism of this peculiar interaction. It was found that: (1) the mesoscale low developed as a result of cold air advection aloft from northern latitudes and strong convection over the region of humidity convergence on 24 July 2003 over the lndian Rajistan area. (2) On the same day, a low that formed over the Bay of Bengal was transformed into a monsoon depression and moved westward to the mesoscale low which existed over southwest India and the adjoining southeastern parts of Pakistan. (3) Initially, the mesoscale low received moisture supply from both the Bay of Bengal as well as the Arabian Sea, whereas the Bay of Bengal maintained the continuous supply of moisture to the monsoon depression. (4) After the depression crossed central India, the Bay‘s moisture supply was cut off and the Arabian Sea became the only source of moisture to both the closely located systems. On 27 July, both of the systems merged together and the merger resulted in a heavy downpour in the Karachi metropolitan and in its surroundings. (5) With the intensification as well as the southeastward extensionof the subtropical high and the shift of the monsoon trough axis from southwest-west to northeast-east,the monsoon depression moved southwestward. In this situation, there existed a very favourable condition for a merger of the two systems in the presence of cross-latitude influence. (6) A number of convective cloud clusters were developed and organized in the mesoscale low. Probably, interactions existed among the multi-scale systems.  相似文献   

5.
Summary The atmospheric and oceanic conditions associated with the southwest monsoon during the contrasting monsoon years of 2002 and 2003 over the Arabian Sea have been analyzed in the present study. Early onset of southwesterlies and reduced net heat gain due to low solar radiation were responsible for low sea-surface temperatures (SSTs) over the Arabian Sea during 2002 pre-monsoon (particularly in May). Conversely, light winds and an increased net heat gain set up the pre-monsoon warming in 2003. The development and intensification of deep convection over a large area of the Arabian Sea prior to the onset of the monsoon was observed during 2003, but was absent in 2002. Weak cross equatorial flow and a weak low level jet over the Arabian Sea reduced moisture transport towards the Indian subcontinent in July 2002. This scenario helped to contribute to a prolonged break in monsoon conditions during July. However, no such break in conditions occurred during July 2003. In 2002, the summer monsoon cooling of the Arabian Sea occurred well before July, whereas in 2003 cooling occurred during July. Estimates of wind driven Ekman (horizontal) and vertical transports showed maximum values in the month of June (July) in 2002 (2003). These estimates clearly show the importance of horizontal and vertical advection in the summer cooling of the Arabian Sea. During the southwest monsoon period, the Arabian Sea was warmer in 2003 than in 2002. Late onset of the southwesterlies in June, late cooling of the Arabian Sea in July, and downwelling Rossby wave propagation were responsible for the warm SSTs in 2003. Weak wind stress curl in July dampened the westward propagating sea surface height anomaly signals (Rossby waves) before they reached the western Arabian Sea in 2002, whereas, in 2003 strong wind stress curl enhanced Rossby wave propagation. During the summer monsoon period, subsurface temperatures in the south central Arabian Sea were warmer in 2003 than in 2002, particularly in July and August. Strong Ekman convergence, solar penetration, and downwelling (downward velocities) are responsible for the enhanced subsurface warming in 2003.  相似文献   

6.
Precise specification of the vertical distribution of cloud optical properties is important to reduce the uncertainty in quantifying the radiative impacts of clouds. The new global observations of vertical profiles of clouds from the CloudSat mission provide opportunities to describe cloud structures and to improve parameterization of clouds in the weather and climate prediction models. In this study, four years (2007–2010) of observations of vertical structure of clouds from the CloudSat cloud profiling radar have been used to document the mean vertical structure of clouds associated with the Indian summer monsoon (ISM) and its intra-seasonal variability. Active and break monsoon spells associated with the intra-seasonal variability of ISM have been identified by an objective criterion. For the present analysis, we considered CloudSat derived column integrated cloud liquid and ice water, and vertically profiles of cloud liquid and ice water content. Over the South Asian monsoon region, deep convective clouds with large vertical extent (up to 14 km) and large values of cloud water and ice content are observed over the north Bay of Bengal. Deep clouds with large ice water content are also observed over north Arabian Sea and adjoining northwest India, along the west coast of India and the south equatorial Indian Ocean. The active monsoon spells are characterized by enhanced deep convection over the Bay of Bengal, west coast of India and northeast Arabian Sea and suppressed convection over the equatorial Indian Ocean. Over the Bay of Bengal, cloud liquid water content and ice water content is enhanced by ~90 and ~200 % respectively during the active spells. An interesting feature associated with the active spell is the vertical tilting structure of positive CLWC and CIWC anomalies over the Arabian Sea and the Bay of Bengal, which suggests a pre-conditioning process for the northward propagation of the boreal summer intra-seasonal variability. It is also observed that during the break spells, clouds are not completely suppressed over central India. Instead, clouds with smaller vertical extent (3–5 km) are observed due to the presence of a heat low type of circulation. The present results will be useful for validating the vertical structure of clouds in weather and climate prediction models.  相似文献   

7.
In this study,we aimed to elucidate the critical role of moisture transport affecting monsoon activity in two contrasting summers over the Arabian Sea during the years 1994,a relatively wet year,and 2002,a relatively dry year.A comprehensive diagnostic evaluation and comparisons of the moisture fields were conducted;we focused on the precipitation and evaporation as well as the moisture transport and its divergence or convergence in the atmosphere.Monthly mean reanalysis data were obtained from the National Centers for Environmental Prediction(NCEP-I and-II).A detailed evaluation of the moisture budgets over Pakistan during these two years was made by calculating the latent energy flux at the surface(E P) from the divergence of the total moisture transport.Our results confirm the moisture supply over the Arabian Sea to be the major source of rainfall in Pakistan and neighboring regions.In 1994,Pakistan received more rainfall compared to 2002 during the summer monsoon.Moisture flow deepens and strengthens over Arabian Sea during the peak summer monsoon months of July and August.Our analysis shows that vertically integrated moisture transport flux have a significant role in supplying moisture to the convective centers over Pakistan and neighboring regions from the divergent regions of the Arabian Sea and the Bay of Bengal.Moreover,in 1994,a deeper vertically integrated moisture convergence progression occurred over Pakistan compared to that in 2002.Perhaps that deeper convergence resulted in a more intense moisture depression over Pakistan and also caused more rainfall in 1994 during the summer monsoon.Finally,from the water budget analysis,it has been surmised that the water budget was larger in 1994 than in 2002 during the summer monsoon.  相似文献   

8.
基于1979—2020年逐日的NOAA向外长波辐射资料、NCEP/NCAR再分析风场资料,以及全球CMAP再分析降水资料,探讨了气候态亚洲热带夏季风涌的传播过程及与我国夏季相应的降水联系。分析结果表明,主汛期亚洲热带气候态夏季风季节内振荡(CISO)活动是亚洲夏季风活动的主要特征,随时间北传的亚洲热带夏季风CISO称为亚洲热带夏季风涌,主要有南亚夏季风涌和南海夏季风涌。亚洲热带夏季风涌的传播可分为四个阶段。在亚洲热带夏季风涌的发展阶段,印度洋区域低频气旋与对流活跃,孟加拉湾和南海热带区域被低频东风控制,我国大部分地区无降水发生,降水中心位于两广地区。当进入亚洲热带夏季风涌活跃阶段,孟加拉湾和南海热带地区低频气旋和对流活跃,东亚低频“PJ”波列显著,我国降水中心北移到长江以南的附近区域。亚洲热带夏季风涌减弱阶段,孟加拉湾与南海低频气旋消亡,对流减弱,低频西风加强,日本南部附近为低频反气旋控制,我国长江中下游低频南风活跃,降水中心也北移到长江中下游地区,而华南地区已基本无降水,此阶段的大气低频环流场与亚洲热带夏季风涌发展阶段基本相反。进入亚洲热带夏季风涌间歇阶段时,孟加拉湾和南海热带地区低...  相似文献   

9.
亚洲南部的海陆分布对亚洲夏季风形成的作用   总被引：14，自引：3，他引：14       下载免费PDF全文
本文用数值试验方法，研究南亚印度次大陆及印度支那半岛与周围海洋间的海陆分布在亚洲夏季风环流形成过程中的作用。试验表明，不仅亚洲大陆与周围大洋的海陆分布影响夏季亚洲季风的形成，亚洲南部较小尺度的海陆分布也同样影响季风的形成过程；阿拉伯海、孟加拉湾和南海上空季风的强风中心及相应的越赤道气流可由亚洲南部较小尺度的海陆分布引起。  相似文献   

10.
In this study, the Tropical Rainfall Measurement Mission based Microwave Imager estimates (2A12) have been used to compare and contrast the characteristics of cloud liquid water and ice over the Indian land region and the ocean surrounding it, during the premonsoon (May) and monsoon (June–September) seasons. Based on the spatial homogeneity of rainfall, we have selected five regions for our study (three over ocean, two over land). Comparison across three ocean regions suggests that the cloud liquid water (CLW) over the orographically influenced Arabian Sea (close to the Indian west coast) behaves differently from the CLW over a trapped ocean (Bay of Bengal) or an open ocean (equatorial Indian Ocean). Specifically, the Arabian Sea region shows higher liquid water for a lower range of rainfall, whereas the Bay of Bengal and the equatorial Indian Ocean show higher liquid water for a higher range of rainfall. Apart from geographic differences, we also documented seasonal differences by comparing CLW profiles between monsoon and premonsoon periods, as well as between early and peak phases of the monsoon. We find that the CLW during the lean periods of rainfall (May or June) is higher than during the peak and late monsoon season (July–September) for raining clouds. As active and break phases are important signatures of the monsoon progression, we also analysed the differences in CLW during various phases of the monsoon, namely, active, break, active-to-break and break-to-active transition phases. We find that the cloud liquid water content during the break-to-active transition phase is significantly higher than during the active-to-break transition phase over central India. We speculate that this could be attributed to higher amount of aerosol loading over this region during the break phase. We lend credence to this aerosol-CLW/rain association by comparing the central Indian CLW with that over southeast Asia (where the aerosol loading is significantly smaller) and find that in the latter region, there are no significant differences in CLW during the different phases of the monsoon. While our hypothesis needs to be further investigated with numerical models, the results presented in this study can potentially serve as a good benchmark in evaluating the performance of cloud resolving models over the Indian region.  相似文献   

11.
This study examines the Indian summer monsoon hydroclimate in the National Centers for Environmental Prediction (NCEP)-Department of Energy (DOE) Reanalysis (R2), the Climate Forecast System Reanalysis (CFSR), and the Modern Era Retrospective-Analysis for Research and Applications (MERRA). The three reanalyses show significant differences in the climatology of evaporation, low-level winds, and precipitable water fields over India. For example, the continental evaporation is significantly less in CFSR compared to R2 and MERRA. Likewise the mean boreal summer 925?hPa westerly winds in the northern Indian Ocean are stronger in R2. Similarly the continental precipitable water in R2 is much less while it is higher and comparable in MERRA and CFSR. Despite these climatological differences between the reanalyses, the climatological evaporative sources for rain events over central India show some qualitative similarities. Major differences however appear when interannual variations of the Indian summer monsoon are analyzed. The anomalous oceanic sources of moisture from the adjacent Bay of Bengal and Arabian Sea play a significant role in determining the wet or dry year of the Indian monsoon in CFSR. However in R2 the local evaporative sources from the continental region play a more significant role. We also find that the interannual variability of the evaporative sources in the break spells of the intraseasonal variations of the Indian monsoon is stronger than in the wet spells. We therefore claim that instead of rainfall, evaporative sources may be a more appropriate metric to observe the relationship between the seasonal monsoon strength and intraseasonal activity. These findings are consistent across the reanalyses and provide a basis to improve the predictability of intraseasonal variability of the Indian monsoon. This study also has a bearing on improving weather prediction for tropical cyclones in that we suggest targeting enhanced observations in the Bay of Bengal (where it is drawing the most moisture from) for improved analysis during active spells of the intraseasonal variability of the Indian monsoon. The analysis suggests that the land–atmosphere interactions contribute significant uncertainty to the Indian monsoon in the reanalyses, which is consistent with the fact that most of the global reanalyses do not assimilate any land-surface data because the data are not available. Therefore, the land–atmosphere interaction in the reanalyses is highly dependent on the land-surface model and it’s coupling with the atmospheric model.  相似文献   

12.
Possible causes behind the unusual cooling by summer monsoon clouds over India are investigated. Results suggest that the causes behind the cooling over the Bay of Bengal, India (BBI) and Arabian Sea (AS) within the Indian monsoon region are different. Over the BBI, clouds are tall. A unique upper tropospheric easterly jet stream exists over India during the summer monsoon season, which horizontally spreads the vertically growing deep convective clouds and thereby increases the cloud cover. Hence, more incoming solar radiation is reflected back to space, which leads to cooling. A radiative transfer study employing the Santa Barbara DISORT Atmospheric Radiative Transfer model supports this view. Over the Arabian Sea, clouds are shallow, and hence the upper tropospheric jet cannot affect them. Due to their proximity to the ground, Arabian Sea clouds exert less warming effect, but they exert a considerable cooling effect, which arises because of the high reflectivity of the clouds. Over the Equatorial Indian Ocean (EIO), where the monsoon clouds originate and propagate towards the monsoon trough region, both cooling and warming effects are nearly canceled out. The upper tropospheric jet is located hundreds of kilometers north of the EIO, and hence it does not disturb the deep convective clouds of the EIO. Therefore, they behave similarly to other deep convective clouds in the tropical belt.  相似文献   

13.
利用高分辨率卫星观测资料,从气候态角度分析了亚洲热带夏季风爆发特征。研究表明,亚洲热带夏季风最先在中南半岛西部爆发,随后在整个中南半岛和孟加拉湾东部,然后扩大至孟加拉湾西部和南海。夏季风爆发后,与孟加拉湾和南海相比,中南半岛雨量增强形势不明显。第26—28候（即5月第2候—5月第4候）是亚洲热带夏季风的爆发阶段。整个爆发过程,低层风场的时空演变与对流降水相对应,海表温度场增温较海表风场提早约1候左右;华南地区以锋面降水为主,即副热带季风降水。采用对流降水和海表上空10 m风场分别代表夏季风降水和盛行风向的时空演变特征较常规资料更为准确、精细。  相似文献   

14.
Changes in the frequency of tropical cyclones over the North Indian Ocean   总被引：3，自引：0，他引：3  
Summary  Changes in the frequency of tropical cyclones developing over the Arabian Sea and the Bay of Bengal have been studied utilizing 122 year (1877–1998) data of tropical cyclone frequency. There have been significant increasing trends in the cyclone frequency over the Bay of Bengal during November and May which are main cyclone months. During transitional monsoon months; June and September however, the frequency has decreased. The results have been presented for five months, i.e., May-November which are relevant as far as tropical cyclone frequency over the Arabian Sea and the Bay of Bengal are concerned. The tropical cyclone frequency in the Arabian Sea has not shown any significant trend, probably due to small normal frequency. The frequency time series has been subjected to the spectral analysis to obtain the significant periods. The cyclone frequency over the Bay of Bengal during May has shown a 29 year cycle. A significant 44 year cycle has been found during November. Over the Arabian Sea significant cycles of 13 and 10 years have been observed during May-June and November, respectively. The tropical cyclone frequency in the North Indian Ocean has a prominent El Ni?o-Southern Oscillation (ENSO) scale cycle (2–5 years) during all above five months. The annual cyclone frequency exhibits 29 year and ENSO scale (2–4 years) oscillations. There is a reduction in tropical cyclone activity over the Bay of Bengal in severe cyclone months May and November during warm phases of ENSO. Examination of the frequencies of severe cyclones with maximum sustained winds ≥ 48 knots has revealed that these cyclones have become more frequent in the North Indian Ocean during intense cyclone period of the year. The rate of intensification of tropical disturbances to severe cyclone stage has registered an upward trend. Received June 7, 1999/Revised March 20, 2000  相似文献   

15.
Summary Interannual variations of the summer monsoon onset over the South China Sea (SCS) have been studied using data from over seventeen years (1979–1995) of NMC global analysis and of Outgoing Longwave Radiation (OLR) observed with NOAA polar-orbitting satellites. It was found that the summer monsoon onset in the SCS occurs abruptly with a sudden change of zonal wind direction from easterly to westerly and an exploding development of deep convection in the whole SCS region in the middle of May. Based on the criteria defined in this paper for the SCS summer monsoon onset, the average onset date over the SCS from 1979 to 1995 is around the fourth pentad of May. The airflow and general circulation over the SCS changes dramatically after the onset. The ridge of the subtropical high in the western Pacific in the lower troposphere weakens and retreats eastward from the SCS region with an establishment of westerly winds over the whole region. During the SCS monsoon onset, the most direct impact in the vicinity of the SCS are the equatorial westerlies in the Bay of Bengal through their eastward extension and northward movement. An indirect influence on the SCS onset is also caused by the enhancement of the Somali cross-equatorial flow and the vanishing Arabian High over the sea; the latter may be a signal for the SCS onset. There are quite significant interannual variations in the SCS onset. In the years of a delayed onset, the most profound feature is that the easterly winds stay longer in the SCS than on average. Deep convection activities are suppressed. The direct cause is the abnormal existence of the western Pacific subtropical high over the SCS region. Moreover, compared to the average, the equatorial westerlies in the Bay of Bengal are also weaker in the years of a delayed onset. No significant changes for the cross-equatorial flow at 105 °E are observed for these years. It has also been found that the interannual variations of the SCS onset are closely related with the ENSO events. In the years of a delay, the Walker circulation is weaker, and the sea surface temperature (SST) anomalies in the western Pacific are negative. Received April 14, 1997 Revised July 11, 1997  相似文献   

16.
Interannual variations of the monsoon onset over Kerala (MOK) have been studied using data from over 60?years (1948?C2009) of NCEP/NCAR reanalysis and outgoing long-wave radiation. The sea surface temperature fields over the North Indian Ocean associated with the MOK have been examined in association with El Nino and Indian Ocean Dipole (IOD) events which originate in the Pacific and Indian Ocean, respectively. An analysis of the tropical convective maximum showed significant differences in its strength and location during the El Nino, IOD, early, normal, and delayed MOK composites. Further, we also looked into the role of the convective systems formed over the Arabian Sea and Bay of Bengal on MOK. The most significant features during early (delayed) MOK years is the abnormal persistence of westerlies (easterlies) several days prior to MOK and enhanced (suppressed) deep convection over the southeastern Arabian Sea and the southern Bay of Bengal. Moisture builds up over peninsular India several pentads prior to MOK during La Nina, negative IOD, and concurrent La Nina and negative IOD years as compared to the El Nino, positive IOD, and concurrent El Nino and positive IOD years, indicating its significant role on MOK. The monsoon Hadley cell and Walker circulations are weaker (stronger) during a delayed (early) MOK. Further, the sea surface temperature anomalies in the western Pacific are negative (positive) during delayed (early) MOK.  相似文献   

17.
W. May 《Climate Dynamics》2004,22(2-3):183-204
In this study the simulation of the variability and extremes of daily rainfall during the Indian summer monsoon for the present-day and the future climate is investigated. This is done on the basis of a global time-slice experiment (TSL) with the ECHAM4 atmospheric general circulation model (GCM) at a high horizontal resolution of T106. The first time-slice (period: 1970–1999) represents the present-day climate and the second (2060–2089) the future climate. Moreover, observational rainfall data from the Global Precipitation Climatology Project (GPCP, 1997–2002) and rainfall data from the ECMWF re-analysis (ERA, 1958–2001) are considered. ERA reveals serious deficiencies in its representation of the variability and extremes of daily rainfall during the Indian summer monsoon. These are mainly a severe overestimation of the frequency of wet days over the oceans and in the Himalayas, where also the rainfall intensity is overestimated. Further, ERA shows unrealistically heavy rainfall events over the tropical Indian Ocean. The ECHAM4 atmospheric GCM at a horizontal resolution of T106, on the other hand, simulates the variability and extremes of daily rainfall in good agreement with the observations. The only marked deficiencies are an underestimation of the rainfall intensity on the west coast of the Indian peninsula and in Bangladesh, an overestimation over the tropical Indian Ocean, due to an erroneous northwestward extension of the tropical convergence zone, and an overestimation of the frequency of wet days in Tibet. Further, heavy rainfall events are relatively strong in the centre of the Indian peninsula. For the future, TSL predicts large increases in the rainfall intensity over the tropical Indian Ocean as well as in northern Pakistan and northwest India, but decreases in southern Pakistan, in the centre of the Indian peninsula, and over the western part of the Bay of Bengal. The frequency of wet days is markedly increased over the tropical Indian Ocean and decreased over the northern part of the Arabian Sea and in Tibet. The intensity of heavy rainfall events is generally increased in the future, with large increases over the Arabian Sea and the tropical Indian Ocean, in northern Pakistan and northwest India as well as in northeast India, Bangladesh, and Myanmar.  相似文献   

18.
The characteristics of moisture transport over the Asian summer monsoon region and its relationship with summer precipitation in China are examined by a variety of statistical methods using the NCEP/NC AR reanalysis data for 1948-2005.The results show that:1) The zonal-mean moisture transport in the Asian monsoon region is unique because of monsoon activities.The Asian summer monsoon region is a dominant moisture sink during summer.Both the Indian and East Asian monsoon areas have their convergence cente...  相似文献   

19.
利用NCEP/NCAR （National Centers for Environmental Prediction/National Center for Atmospheric Research）2001-2010年再分析资料,检验了全球气候系统模式CESM中大气模块CAM （Community Atmosphere Model）对亚洲夏季风和大气热源的模拟能力。结果显示,模式可以再现亚洲夏季风和大气热源的主要特征。通过敏感试验探讨人为气溶胶影响亚洲夏季风的机理,分析、讨论了气溶胶引起的非均匀加热的变化对辐散风和无辐散风强度的影响,在机理上解释了亚洲夏季风减弱的原因。结果表明,人为气溶胶浓度的升高使东亚夏季风强度在中国东南地区、中南半岛北部和印度半岛北部减弱。而中国东南部季风的减弱促使中国内陆降水减少,沿海降水增多。进一步分析人为气溶胶浓度升高的作用发现,其改变了大气热源的分布,造成阿拉伯海、孟加拉湾和中国南海大气热源增强,中国东部地区和中南半岛大气热源减弱,其中气溶胶通过影响凝结潜热来改变大气热源,主要是对对流过程的影响。此外,大气热源分布的变化改变了季风区的热力结构,使中国东南地区、中南半岛北部的加热减弱,从而减少了全位能的产生,使得全位能向辐散风的转换减小,辐散风减弱;同时,中国东南部、中南半岛北部季风由于辐散风向无辐散风转换的减弱,无辐散风减弱,最终导致了夏季风强度的减弱。而且,人为气溶胶对亚洲夏季风的影响主要通过大气热力和动力过程的响应产生作用。  相似文献   

20.
The reproducibility and future changes of the onset of the Asian summer monsoon were analyzed based on the simulations and projections under the Representative Concentration Pathways(RCP) scenario in which anthropogenic emissions continue to rise throughout the 21 st century(i.e. RCP8.5) by all realizations from four Chinese models that participated in the Coupled Model Intercomparison Project Phase 5(CMIP5). Delayed onset of the monsoon over the Arabian Sea was evident in all simulations for present-day climate, which was associated with a too weak simulation of the low-level Somali jet in May.A consistent advanced onset of the monsoon was found only over the Arabian Sea in the projections, where the advanced onset of the monsoon was accompanied by an increase of rainfall and an anomalous anticyclone over the northern Indian Ocean. In all the models except FGOALS-g2, the enhanced low-level Somali jet transported more water vapor to the Arabian Sea, whereas in FGOALS-g2 the enhanced rainfall was determined more by the increased wind convergence. Furthermore,and again in all models except FGOALS-g2, the equatorial SST warming, with maximum increase over the eastern Pacific,enhanced convection in the central West Pacific and reduced convection over the eastern Indian Ocean and Maritime Continent region, which drove the anomalous anticyclonic circulation over the western Indian Ocean. In contrast, in FGOALS-g2, there was minimal(near-zero) warming of projected SST in the central equatorial Pacific, with decreased convection in the central West Pacific and enhanced convection over the Maritime Continent. The broader-scale differences among the models across the Pacific were related to both the differences in the projected SST pattern and in the present-day simulations.  相似文献   

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