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1.
利用1979—2013年NCEP/DOE再分析资料的大气多要素日平均资料、美国NOAA日平均向外长波辐射资料和ERSST月平均海温资料,分析赤道大气季节内振荡(简称MJO)活动对南海夏季风爆发的影响及其与热带海温信号等的协同作用.结果表明,赤道MJO活动与南海夏季风爆发密切联系,MJO的湿位相(即对流活跃位相)处于西太平洋位相时,有利于南海夏季风爆发,而MJO湿位相处于印度洋位相时,则不利于南海夏季风爆发.赤道MJO活动影响南海夏季风爆发的物理过程主要是大气对热源响应的结果,当MJO湿位相处于西太平洋位相时,一方面热带西太平洋对流加强使潜热释放增加,导致处于热源西北侧的南海—西北太平洋地区对流层低层由于Rossby响应产生气旋性环流异常,气旋性环流异常则有利于西太平洋副热带高压的东退,另一方面菲律宾附近热源促进对流层高层南亚高压在中南半岛和南海北部的建立,使南海地区高层为偏东风,从而有利于南海夏季风建立;当湿位相MJO处于印度洋位相时,热带西太平洋对流减弱转为大气冷源,情况基本相反,不利于南海夏季风建立.MJO活动、孟加拉湾气旋性环流与年际尺度海温变化协同作用,共同对南海夏季风爆发迟早产生影响,近35年南海夏季风爆发时间与海温信号不一致的年份,基本上是由于季节转换期间的MJO活动特征及孟加拉湾气旋性环流是否形成而造成,因此三者综合考虑对于提高季风爆发时间预测水平具有重要意义. 相似文献
2.
通过一系列的理想数值试验,研究了亚、非地区热带次尺度的海陆分布和青藏高原大地形在亚洲夏季风形成中的作用.试验结果显示：海陆分布的存在以及海陆分布的几何形状对亚洲夏季风的形成有非常重要的影响.下垫面全是海洋,没有陆地时,无季风现象的存在.当仅有副热带大尺度陆地,而缺乏南亚次尺度陆地和非洲大陆热带陆地时,夏季无明显的越赤道气流,仅在欧亚副热带陆地的东南部有弱的季风,无印度、孟加拉湾和南海夏季风.中南半岛、印度半岛和非洲大陆热带陆地的存在,在夏季引导南半球的东南信风越赤道转向为西南气流,使得南海的北部、中南半岛、孟加拉湾和印度半岛、阿拉伯海上空的低层为强西南气流控制,印度、孟加拉湾和南海夏季风产生.副热带陆地向热带的深入对副热带陆上产生夏季强对流性降水起着至关重要的作用.青藏高原的存在加强了高原东侧的季风,使得季风区向北发展,青藏高原对东亚季风起放大器的作用;减弱了高原西侧的季风,使得季风区向南收缩. 相似文献
3.
Possible mechanism of the effect of convection over Asian-Australian “land bridge” on the East Asian summer monsoon onset 总被引：1，自引：0，他引：1
The Asian-Australian “land bridge” is an area with the most vigorous convection in Asian monsoon region in boreal spring,
where the onset and march of convection are well associated with the onset of East Asian summer monsoon. The convection occurs
over Indo-China Peninsula as early as mid-April, which exerts critical impact on the evolution of monsoon circulation. Before
mid-April there are primarily sensible heatings to the atmosphere over Indo-China Peninsula and Indian Peninsula, so the apparent
heating ratios over them decrease with height. However, after mid-April it changes into latent heating over Indo-China Peninsula
due to the onset of convection, and the apparent heating ratio increases with height in mid-and lower troposphere. The vertical
distribution of heating ratio and its differences between Indo-China Peninsula and Indian Peninsula are the key factors leading
to the splitting of boreal subtropical high belt over the Bay of Bengal. Such mechanism is strongly supported by the fact
that the evolution of the vertical heating ratio gradient above Indo-China Peninsula leads that of 850 hPa vorticity over
the Bay of Bengal. Convections over Indo-China Peninsula and its surrounding areas further increase after the splitting. Since
then, there is a positive feedback lying among the convective heating, the eastward retreat of the subtropical high and the
march of monsoon, which is a possible mechanism of the advance of summer monsoon and convection from Indo-China Peninsula
to South China Sea. 相似文献
4.
Possible mechanism of the effect of convection over Asian-Australian "land bridge" on the East Asian summer monsoon onset 总被引：2，自引：0，他引：2
The Asian-Australian "land bridge" is an area with the most vigorous convection in Asian monsoon region in boreal spring, where the onset and march of convection are well associated with the onset of East Asian summer monsoon. The convection occurs over Indo-China Peninsula as early as mid-April, which exerts critical impact on the evolution of monsoon circulation. Before mid-April there are primarily sensible heatings to the atmosphere over Indo-China Peninsula and Indian Peninsula, so the apparent heating ratios over them decrease with height. However, after mid-April it changes into latent heating over Indo-China Peninsula due to the onset of convection, and the apparent heating ratio increases with height in mid- and lower troposphere. The vertical distribution of heating ratio and its differences between Indo-China Peninsula and Indian Peninsula are the key factors leading to the splitting of boreal subtropical high belt over the Bay of Bengal. Such mechanism is strongly supported by the fact that the evolution of the vertical heating ratio gradient above Indo-China Peninsula leads that of 850 hPa vorticity over the Bay of Bengal. Convections over Indo-China Peninsula and its surrounding areas further increase after the splitting. Since then, there is a positive feedback lying among the convective heating, the eastward retreat of the subtropical high and the march of monsoon, which is a possible mechanism of the advance of summer monsoon and convection from Indo-China Peninsula to South China Sea. 相似文献
5.
ABSTRACTBroad disagreement between modelled and observed trends of Indian summer monsoon (ISM) over the north-central part of the Indian subcontinent (NCI) implies a gap in understanding of the relationship between the forcing factors and monsoonal precipitation. Although the strength of the land–sea thermal gradient (LSG) is believed to dictate monsoon intensity, its state and fate under continuous warming over the Bay of Bengal (BoB) and part of the NCI (23–28°N, 80–95°E) are less explored. Precipitation (1901–2017) and temperature data (1948–2017) at different vertical heights are used to understand the impact of warming in the ISM. In NCI, surface air temperature increased by 0.1–0.2°C decade?1, comparable to the global warming rate. The ISM precipitation prominently weakened and seasonality reduced after 1950, which is caused by a decrease in the LSG at the depth of the troposphere. Warming-induced increase in local convection over the BoB further reduced ISM precipitation over NCI. 相似文献
6.
A Study on Climatological Features of the Asian Summer Monsoon: Dynamics, Energetics and Variability 总被引：1，自引：0，他引：1
A continuing goal in the diagnostic studies of the atmospheric general circulation is to estimate various quantities that cannot be directly observed. Evaluation of all the dynamical terms in the budget equations for kinetic energy, vorticity, heat and moisture provide estimates of kinetic energy and vorticity generation, diabatic heating and source/sinks of moisture. All these are important forcing factors to the climate system. In this paper, diagnostic aspects of the dynamics and energetics of the Asian summer monsoon and its spatial variability in terms of contrasting features of surplus and deficient summer monsoon seasons over India are studied with reanalysis data sets. The daily reanalysis data sets from the National Centre for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) are used for a fifty-two year (1948–1999) period to investigate the large-scale budget of kinetic energy, vorticity, heat and moisture. The primary objectives of the study are to comprehend the climate diagnostics of the Asian summer monsoon and the role of equatorial convection of the summer monsoon activity over India.It is observed that the entrance/exit regions of the Tropical Easterly Jet (TEJ) are characterized by the production/destruction of the kinetic energy, which is essential to maintain outflow/inflow prevailing at the respective location of the TEJ. Both zonal and meridional components contribute to the production of kinetic energy over the monsoon domain, though the significant contribution to the adiabatic generation of kinetic energy originates from the meridional component over the Bay of Bengal in the upper level and over the Somali Coast in the low level. The results indicate that the entire Indian peninsula including the Bay of Bengal is quite unstable during the summer monsoon associated with the production of vorticity within the domain itself and maintain the circulation. The summer monsoon evinces strong convergence of heat and moisture over the monsoon domain. Also, considerable heat energy is generated through the action of the adiabatic process. The combined effect of these processes leads to the formation of a strong diabatic heat source in the region to maintain the monsoon circulation. The interesting aspect noted in this study is that the large-scale budgets of heat and moisture indicate excess magnitudes over the Arabian Sea and the western equatorial Indian Ocean during surplus monsoon. On the other hand, the east equatorial Indian Ocean and the Bay of Bengal region show stronger activity during deficient monsoon. This is reflected in various budget terms considered in this study. 相似文献
7.
本文运用NCAR/NCEP再分析数据和APHRO_MA_V1003R1降水数据,对比分析了孟加拉湾西南季风和南海热带季风的气候特征异同以及对降水分布的影响,得到如下结论:(1)孟加拉湾西南季风比南海热带季风爆发更早、强度更强、持续时间更久、向北推进更北.(2)孟加拉湾西南季风建立过程缓慢,主要是索马里越赤道西南气流的逐渐加强和热带印度洋ITCZ(赤道辐合带)的逐渐北移;而南海热带季风建立过程迅速,主要是东亚大槽的一次替换过程伴随西太平洋副热带高压的突然东撤和热带西太平洋ITCZ的突然北跳.(3)孟加拉湾西南风纬向分量较强,季风建立前后主要变化在于偏西风的强度;而南海西南风经向分量较强,季风建立后风向突然逆转,东南风由于副高东撤而迅速被西南风取代.(4)孟加拉湾西南季风撤退较快,而南海季风则撤退较慢.(5)根据季风进程将夏季风期划分为季风发展期(5月)、强盛期(6-8月)和减退期(9-10月).其间对流活跃区的发展和推进、季风槽的位置以及对应降水区域均有明显差异.(6)在夏季风期,孟加拉湾和南海经度上分别存在着由ITCZ北抬引起的、在季风槽对流活跃区上升而在南北两侧下沉的、南北对称分布的季风经向次级环流.由于孟加拉湾和青藏高原强大热源的存在,孟加拉湾上升区南北跨度比南海的更大;孟加拉湾经圈环流更加稳定,而南海经圈环流的南北摆动更明显;孟加拉湾上升中心区比南海的偏北;在季风减退期,由于南海ITCZ撤退较慢,其上升区比孟加拉湾上升区偏北. 相似文献
8.
Teleconnection between the Indian summer monsoon onset and the Meiyu over the Yangtze River Valley 总被引：11，自引：0，他引：11
Based on the Indian and Chinese precipitation data and the NCEP-NCAR reanalysis circulation data, the relationship between the Indian summer monsoon (ISM) onset and the Meiyu over the Yangtze River Valley has been discussed by the methods of correlation analysis and composite analysis. The results show that the date of ISM onset over Kerala in the southwestern coast of the Indian Peninsula is about two weeks earlier than the beginning of the Meiyu over the Yangtze River Valley. After the outbreak of ISM, the teleconnection mode sets up from the western coast of India via the Bay of Bengal (BOB) to the Yangtze River Valley and southern Japan. It is different both in time and space from the telecon- nection mode which is from the northwest of India via the Tibetan Plateau to northern China. The for- mer mode is defined as the "south" teleconnection of the Asian summer monsoon, forming in the pe- riod of ISM onset; while the latter mode is called the "north" teleconnection, mainly occurring in the Asian monsoon culminant period. During the process of the "south" teleconnection’s formation, the Asian monsoon circulation has experienced a series of important changes: ISM onset, the northward movement of the south Asia high (SAH), the onset vortex occurrence, the eastward extension of the stronger tropical westerly belt, and the northeastward jump of the western Pacific subtropical high (WPSH), etc. Consequently, since ISM sets up over Kerala, the whole Asian continent is covered by the upper SAH after about two weeks, while in the mid- and lower troposphere, a strong wind belt forms from the Arabian Sea via the southern India, BOB and the South China Sea (SCS), then along the western flank of WPSH, to the Yangtze River Valley and southern Japan. With the northward moving of the subtropical jet streams, the upper westerly jet stream and the low level jet have been coupled ver- tically over east Asia, while the Yangtze River Valley happens to locate in the ascending motion area between the upper jet stream and the low level jet, i.e. right of the entrance of the upper jet stream and left of the low level jet. Such a structure of the vertical circulation can trigger the Meiyu onset over the Yangtze River Valley. 相似文献
9.
During the summer monsoon season over India a range of intraseasonal modulations of the monsoon rains occur due to genesis of weather disturbances over the Bay of Bengal (BOB) and the east Arabian Sea. The amplitudes of the fluctuations in the surface state of the ocean (sea-surface temperature and salinity) and atmosphere are quite large due to these monsoonal modulations on the intraseasonal scale as shown by the data collected during the field programs under Bay of Bengal Monsoon Experiment (BOBMEX) and Arabian Sea Monsoon Experiments (ARMEX). The focus of BOBMEX was to understand the role of ocean-atmospheric processes in organizing convection over the BOB on intra-seasonal scale. ARMEX-I was aimed at understanding the coupled processes in the development of deep convection off the West Coast of India. ARMEX-II was focused on the formation of the mini-warm pool across the southeast Arabian Sea in April-May and its role in the abrupt onset of the monsoon along the Southwest Coast of India and its further progress along the West Coast of India. The paper attempts to integrate the results of the observational studies and brings out an important finding that atmospheric instability is prominently responsible for convective organization whereas the upper ocean parameters regulate the episodes of the intraseasonal oscillations. 相似文献
10.
Neethu Chacko Muthalagu Ravichandran Rokkam R. Rao Sadananda Satheesh Chandra Shenoi 《Ocean Dynamics》2012,62(5):671-681
Sea surface temperature (SST) variability over the Bay of Bengal (BoB) has the potential to trigger deep moist convection
thereby affecting the active-break cycle of the monsoons. Normally, during the summer monsoon season, SST over the BoB is
observed to be greater than 28°C which is a pre-requisite for convection. During June 2009, satellite observations revealed
an anomalous basin-wide cooling and the month is noted for reduced rainfall over the Indian subcontinent. In this study, we
analyze the likely mechanisms of this cooling event using both satellite and moored buoy observations. Observations showed
deepened mixed layer, stronger surface currents, and enhanced heat loss at the surface in the BoB. Mixed layer heat balance
analysis is carried out to resolve the relative importance of various processes involved. We show that the cooling event is
primarily induced by the heat losses at the surface resulting from the strong wind anomalies, and advection and vertical entrainment
playing secondary roles. 相似文献
11.
利用高低层大气环流、OLR（向外长波辐射）、CMAP降水、SST（海表温度）等资料分析了孟加拉湾地区3—5月多年气候平均大气环流及不同要素的演变特征,定义了一个新的孟加拉湾夏季风（BOBSM,下同）爆发指标为孟加拉湾地区（5°N—15°N,90°E—97.5°E）850 hPa和200 hPa纬向风区域平均的变化同时满足U850 > 3 m·s-1和U200 < -5 m·s-1,并持续5天的第一天即作为BOBSM爆发日期.该季风指数有明确的天气学意义,可以反映孟加拉湾低层西南风持续稳定和南亚高压在青藏高原建立早晚的特征.文章进一步分析了BOBSM爆发的年际特征及其前兆海洋信号特征,结果表明：1981—2010年BOBSM爆发的平均日期为5月10日,季风爆发有显著的年际波动,爆发最早在1999年（4月11日）和最晚在1968年（6月1日）,年代际尺度上表现为由爆发偏晚至偏早的变化趋势;BOBSM爆发早（晚）与热带印度洋地区850 hPa的越赤道气流和西风异常加强（减弱）,以及200 hPa青藏高原南亚高压的季节性建立偏早（晚）等密切联系;前期冬季赤道西太平洋的海温冷（暖）变化对BOBSM爆发早（晚）有很好的指示意义,前期冬季海温偏高（低）有利于季风偏早（晚）,其影响的主要途径是通过热源变化激发纬向垂直环流及其热带印度洋和太平洋低层环流异常,进而影响季风爆发早晚. 相似文献
12.
--A large part of the rainfall over India during the summer monsoon season (June-September) is contributed by synoptic scale disturbances such as monsoon depressions. To study the evolution of such disturbances in Atmospheric General Circulation Models (AGCM), the Hadley Centre AGCM (HadAM2b) has been integrated for 15 summer monsoons (1979-1993) and the output was examined for the presence of synoptic scale disturbances such as monsoon depressions, low pressure areas, land lows and land depressions over the Indian summer monsoon region. The atmospheric initial condition for each of these integrations was of 23rd May and observed Sea Surface Temperatures (SST) were described as a boundary condition.¶Although the horizontal resolution of the AGCM used in this study is only 2.5° 2 3.75° lat. long., the model is able to simulate a few monsoon disturbances. The important features of these simulated disturbances are presented. The features of the simulated disturbances are realistic. The morphologies of a well simulated monsoon depression and a simulated low pressure area are presented as examples. The frequency of the simulated monsoon depressions is less than the climatological frequency of the depressions during all four monsoon months. 相似文献
13.
— The work deals with the computation and analysis of spectral energetics in the frequency domain at 850?hPa and 200?hPa over the tropics (20°S–20°N) and extratropics (20°N–60°N). The data for the winter months, i.e., November, December and January of 1995, 1996 and 1997 are selected for this purpose. The results suggest that much of the low frequency variability of the Northern Hemisphere wintertime general circulation is associated with disturbances which derive their energy from the time-mean flow through barotropic instability. Low frequency fluctuations tend to be larger in horizontal scale and their kinetic energy is largely confined to the upper troposphere. At 850?hPa, strong energy interaction south of 5°N is noticed due to a southward shift of major inflow channel, originating from the Bay of Bengal and entering the ITCZ from the western Arabian Sea. The energy balances in the tropics and the extratropics during winter have different characteristics from those during summer. In contrast to the summer circulation, instead of a downscale decascade as in the case of the extratropics, kinetic energy is transferred in an opposite sense, namely from transients of shorter to those of longer time scales in the tropics during winter. The strong nonlinear energy interactions associated with low frequency waves over the Indian Ocean (5°N–5°S) during winter is the manifestation of the deep convection due to warm water coupled with the crossequatorial low level flow along the ITCZ over this region. Forcing from this region readily excites a large response in terms of nonlinear energy interaction over the extratropical northeast Pacific. 相似文献
14.
利用1979-2009年NCEP第二套大气再分析资料和ERSST海温资料,分析南海夏季风爆发时间的年际和年代际变化特征,考察南海夏季风爆发早晚与南大洋海温之间的联系.主要结果为:(1)南海夏季风爆发时间年际和年代际变化明显,1979-1993年与1994-2009年前后两个阶段爆发时间存在阶段性突变;(2)南海夏季风爆发时间与前期冬季(12-1月)印度洋-南大洋(0-80°E,75°S-50°S)海温、春季(2-3月)太平洋-南大洋(170°E -80°W,75°S-50°S)海温都存在正相关关系,当前期冬、春季南大洋海温偏低(高)时,南海夏季风爆发偏早(晚).南大洋海温信号,无论是年际还是年代际变化,都对南海夏季风爆发具有一定的预测指示作用;(3)南大洋海温异常通过海气相互作用和大气遥相关影响南海夏季风爆发的迟早.当南大洋海温异常偏低(偏高)时,冬季南极涛动偏强(偏弱),同时通过遥相关作用使热带印度洋-西太平洋地区位势高度偏低(偏高)、纬向风加强(减弱),热带大气这种环流异常一直维持到春季4、5月份,位势高度和纬向风异常范围逐步向北扩展并伴随索马里越赤道气流的加强(减弱),从而为南海夏季风爆发偏早(偏晚)提供有利的环流条件.初步分析认为,热带大气环流对南大洋海气相互作用的遥响应与半球际大气质量重新分布引起的南北涛动有关. 相似文献
15.
Orissa State, a meteorological subdivision of India, lies on the east coast of India close to north Bay of Bengal and to the
south of the normal position of the monsoon trough. The monsoon disturbances such as depressions and cyclonic storms mostly
develop to the north of 15° N over the Bay of Bengal and move along the monsoon trough. As Orissa lies in the southwest sector
of such disturbances, it experiences very heavy rainfall due to the interaction of these systems with mesoscale convection
sometimes leading to flood. The orography due to the Eastern Ghat and other hill peaks in Orissa and environs play a significant
role in this interaction. The objective of this study is to develop an objective statistical model to predict the occurrence
and quantity of precipitation during the next 24 hours over specific locations of Orissa, due to monsoon disturbances over
north Bay and adjoining west central Bay of Bengal based on observations to up 0300 UTC of the day. A probability of precipitation
(PoP) model has been developed by applying forward stepwise regression with available surface and upper air meteorological
parameters observed in and around Orissa in association with monsoon disturbances during the summer monsoon season (June-September).
The PoP forecast has been converted into the deterministic occurrence/non-occurrence of precipitation forecast using the critical
value of PoP. The parameters selected through stepwise regression have been considered to develop quantitative precipitation
forecast (QPF) model using multiple discriminant analysis (MDA) for categorical prediction of precipitation in different ranges
such as 0.1–10, 11–25, 26–50, 51–100 and >100 mm if the occurrence of precipitation is predicted by PoP model. All the above
models have been developed based on data of summer monsoon seasons of 1980–1994, and data during 1995–1998 have been used
for testing the skill of the models. Considering six representative stations for six homogeneous regions in Orissa, the PoP
model performs very well with percentages of correct forecast for occurrence/non-occurrence of precipitation being about 96%
and 88%, respectively for developmental and independent data. The skill of the QPF model, though relatively less, is reasonable
for lower ranges of precipitation. The skill of the model is limited for higher ranges of precipitation.
accepted September 2006 相似文献
16.
Nisha Kurian Matthieu Lengaigne Gopalakrishna Venkata Vissa Jerome Vialard Stephane Pous Anne-Charlotte Peter Fabien Durand Shweta Naik 《Ocean Dynamics》2013,63(4):329-346
Active and break phases of the Indian summer monsoon are associated with sea surface temperature (SST) fluctuations at 30–90 days timescale in the Arabian Sea and Bay of Bengal. Mechanisms responsible for basin-scale intraseasonal SST variations have previously been discussed, but the maxima of SST variability are actually located in three specific offshore regions: the South-Eastern Arabian Sea (SEAS), the Southern Tip of India (STI) and the North-Western Bay of Bengal (NWBoB). In the present study, we use an eddy-permitting 0.25° regional ocean model to investigate mechanisms of this offshore intraseasonal SST variability. Modelled climatological mixed layer and upper thermocline depth are in very good agreement with estimates from three repeated expendable bathythermograph transects perpendicular to the Indian Coast. The model intraseasonal forcing and SST variability agree well with observed estimates, although modelled intraseasonal offshore SST amplitude is undere-stimated by 20–30 %. Our analysis reveals that surface heat flux variations drive a large part of the intraseasonal SST variations along the Indian coastline while oceanic processes have contrasted contributions depending of the region considered. In the SEAS, this contribution is very small because intraseasonal wind variations are essentially cross-shore, and thus not associated with significant upwelling intraseasonal fluctuations. In the STI, vertical advection associated with Ekman pumping contributes to ～30 % of the SST fluctuations. In the NWBoB, vertical mixing diminishes the SST variations driven by the atmospheric heat flux perturbations by 40 %. Simple slab ocean model integrations show that the amplitude of these intraseasonal SST signals is not very sensitive to the heat flux dataset used, but more sensitive to mixed layer depth. 相似文献
17.
The effects of asymmetric potential vorticity forcing on the instability of South Asia High and Indian summer monsoon onset 总被引：1，自引：0，他引：1
Based on the theory of potential vorticity(PV),the unstable development of the South Asia High(SAH)due to diabatic heating and its impacts on the Indian Summer Monsoon(ISM)onset are studied via a case diagnosis of 1998.The Indian Summer Monsoon onset in 1998 is related to the rapidly strengthening and northward moving of a tropical cyclone originally located in the south of Arabian Sea.It is demonstrated that the rapid enhancement of the cyclone is a consequence of a baroclinic development characterized by the phase-lock of high PV systems in the upper and lower troposphere.Both the intensification of the SAH and the development of the zonal asymmetric PV forcing are forced by the rapidly increasing latent heat released from the heavy rainfall in East Asia and South East Asia after the onsets of the Bay of Bengal(BOB)monsoon and the South China Sea(SCS)monsoon.High PV moves southwards along the intensified northerlies on the eastern side of the SAH and travels westwards on its south side,which can reach its northwest.Such a series of high PV eddies are transported to the west of the SAH continuously,which is the main source of PV anomalies in the upper troposphere over the Arabian Sea from late spring to early summer.A cyclonic curvature on the southwest of the SAH associated with increasing divergence,which forms a strong upper tropospheric pumping,is generated by the anomalous positive PV over the Arabian Sea on 355 K.The cyclone in the lower troposphere moves northwards from low latitudes of the Arabian Sea,and the upper-layer high PV extends downwards and southwards.Baroclinic development thus occurs and the tropical low-pressure system develops into an explosive vortex of the ISM,which leads to the onset of the ISM.In addition,evolution of subtropical anticyclone over the Arabian Peninsula is another important factor contributing to the onset of the ISM.Before the onset,the surface sensible heating on the Arabian Peninsula is very strong.Consequently the subtropical anticyclone which dominated the Arabian Sea in spring retreats westwards to the Arabian Peninsula and intensifies rapidly.The zonal asymmetric PV forcing develops gradually with high PV eddies moving southwards along northerlies on the eastern side of the anticyclone,and a high PV trough is formed in the middle troposphere over the Arabian Sea,which is favorable to the explosive barotropic development of the tropical cyclone into the vortex.Results from this study demonstrate that the ISM onset,which is different from the BOB and the SCS monsoon onset,is a special dynamical as well as thermodynamic process occurring under the condition of fully coupling of the upper,middle,and lower tropospheric circulations. 相似文献
18.
D. R. Pattanaik 《Pure and Applied Geophysics》2007,164(8-9):1527-1545
The present study is an attempt to examine the variability of convective activity over the north Indian Ocean (Bay of Bengal
and Arabian Sea) on interannual and longer time scale and its association with the rainfall activity over the four different
homogeneous regions of India (viz., northeast India, northwest India, central India and south peninsular India) during the
monsoon season from June to September (JJAS) for the 26 year period (1979 to 2004). The monthly mean Outgoing Long-wave Radiation
(OLR) data obtained from National Oceanic and Atmospheric Administration (NOAA) polar orbiting spacecraft are used in this
study and the 26-year period has been divided into two periods of 13 years each with period-i from 1979 to 1991 and period
-ii from 1992 to 2004. It is ascertained that the convective activity increases over the Arabian Sea and the Bay of Bengal
in the recent period (period -ii; 1992 to 2004) compared to that of the former period (period -i; 1979 to 1991) during JJAS
and is associated with a significantly increasing trend (at 95% level) of convective activity over the north Bay of Bengal
(NBAY). On a monthly scale, July and August also show increase in convective activity over the Arabian Sea and the Bay of
Bengal during the recent period and this is associated with slight changes in the monsoon activity cycle over India. The increase
in convective activity particularly over the Arabian Sea during the recent period of June is basically associated with about
three days early onset of the monsoon over Delhi and relatively faster progress of the monsoon northward from the southern
tip of India. Over the homogeneous regions of India the correlation coefficient (CC) of OLR anomalies over the south Arabian
Sea (SARA) is highly significant with the rainfall over central India, south peninsular India and northwest India, and for
the north Arabian Sea (NARA), it is significant with northwest India rainfall and south peninsular rainfall. Similarly, the
OLR anomalies over the south Bay of Bengal (SBAY) have significant CC with northwest India and south peninsular rainfall,
whereas the most active convective region of the NBAY is not significantly correlated with rainfall over India. It is also
found that the region over northeastern parts of India and its surroundings has a negative correlation with the OLR anomalies
over the NARA and is associated with an anomalous sinking (rising) motion over the northeastern parts of India during the
years of increase (decrease) of convective activity over the NARA. 相似文献
19.
G.J. Bhagavathiammal S. Sathishkumar S. Sridharan Manohar Lal S. Gurubaran K.U. Nair 《Journal of Atmospheric and Solar》2010,72(16):1204-1211
Spectral analysis of Tirunelveli (8.7°N, 77.8°E) MF radar winds for the year 2007 indicate the presence of long-period Kelvin waves with periods ～23 and ～16 days in the low-latitude mesosphere during Indian summer monsoon months. The dominant presence of these slow-phase speed waves at mesospheric altitudes motivated us to investigate their origin and vertical propagation characteristics. Space-time Fourier analysis of NCEP winds and OLR show the presence of these periodicities with zonal wavenumber 1 indicating that tropical convection is the potential source for these waves and westward phase of stratospheric QBO winds might have favoured these waves to reach the mesosphere. 相似文献
20.
A kinetic energy budget over the Indian region is computed for the period 4–9 July 1973, when a twin monsoon depression-one in the Bay of Bengal and another in the Arabian sea were the dominant synoptic features. The generation term caused by the cross-contour flow is a dominant source to the kinetic energy. The dissipation term is computed as a residual and is a major sink for the kinetic energy. The horizontal flux divergence is also a sink term but is much smaller in magnitude than other major source and sink terms. From the results it may be inferred that the generation term is the most important for the maintenance of monsoon disturbances. 相似文献