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1.
We analyze the dynamical features and responsible factors of the low-frequency intraseasonal time scales which influenced the nature of onset, intensity and duration of active/break phases and withdrawal of the monsoon during the anomalous Indian summer monsoon of 2002 — the most severe drought recorded in recent times. During that season, persistent warm sea surface temperature anomalies over the equatorial Indian Ocean played a significant role in modulating the strength of the monsoon Hadley circulation. This in turn affected the onset and intense break spells especially the long break during the peak monsoon month of July. Strong low-frequency intraseasonal modulations with significant impact on the onset and active/break phases occurred in 2002 which were manifested as a good association between low-frequency intraseasonal oscillations and the onset and active/break spells. Further, SST anomalies over the equatorial Indo-Pacific region on low-frequency intraseasonal time scales were found to affect the equatorial eastward and thereby off-equatorial northward propagations of enhanced convection over the Indian region. These propagations in turn modulated the active/break cycle deciding the consequent severity of the 2002 drought.  相似文献   

2.
The relative impacts of the ENSO and Indian Ocean dipole (IOD) events on Indian summer (June–September) monsoon rainfall at sub-regional scales have been examined in this study. GISST datasets from 1958 to 1998, along with Willmott and Matsuura gridded rainfall data, all India summer monsoon rainfall data, and homogeneous and sub-regional Indian rainfall datasets were used. The spatial distribution of partial correlations between the IOD and summer rainfall over India indicates a significant impact on rainfall along the monsoon trough regions, parts of the southwest coastal regions of India, and also over Pakistan, Afghanistan, and Iran. ENSO events have a wider impact, although opposite in nature over the monsoon trough region to that of IOD events. The ENSO (IOD) index is negatively (positively) correlated (significant at the 95% confidence level from a two-tailed Student t-test) with summer monsoon rainfall over seven (four) of the eight homogeneous rainfall zones of India. During summer, ENSO events also cause drought over northern Sri Lanka, whereas the IOD events cause surplus rainfall in its south. On monthly scales, the ENSO and IOD events have significant impacts on many parts of India. In general, the magnitude of ENSO-related correlations is greater than those related to the IOD. The monthly-stratified IOD variability during each of the months from July to September has a significant impact on Indian summer monsoon rainfall variability over different parts of India, confirming that strong IOD events indeed affect the Indian summer monsoon.
Karumuri AshokEmail:
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3.
The impact of different land-surface parameterisation schemes for the simulation of monsoon circulation during a normal monsoon year over India has been analysed. For this purpose, three land-surface parameterisation schemes, the NoaH, the Multi-layer soil model and the Pleim-Xiu were tested using the latest version of the regional model (MM5) of the Pennsylvania State University (PSU)/National Center for Atmospheric Research (NCAR) over the Indian summer monsoon region. With respect to different land-surface parameterisation schemes, latent and sensible heat fluxes and rainfall were estimated over the Indian region. The sensitivity of some monsoon features, such as Somali jet, tropical easterly jet and mean sea level pressure, is discussed. Although some features of the Indian summer monsoon, such as wind and mean sea level pressure, were fairly well-simulated by all three schemes, many differences were seen in the simulation of the typical characteristics of the Indian summer monsoon. It was noticed from the results that the features of the Indian summer monsoon, such as strength of the low-level westerly jet, the cross-equatorial flow and the tropical easterly jet were better simulated by NoaH compared with verification analysis than other land-surface schemes. It was also observed that the distribution of precipitation over India during the peak period of monsoon (July) was better represented with the use of the NoaH scheme than by other schemes.
U. C. MohantyEmail:
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4.
With an objective to understand the influence of surface marine meteorological parameters in relation to the extreme monsoon activity over the Indian sub-continent leading to flood/drought, a detailed analysis of the sea level pressure over the Southern Hemisphere and various surface meteorological parameters over the Indian seas is carried out. The present study using the long term data sets (Southern Hemispheric Sea Level Pressure Analysis; Comprehensive Ocean Atmospheric Data Set over the Indian Seas; Surface Station Climatology Data) clearly indicates that the sea surface temperature changes over the south eastern Pacific (El Ninõ/La Niña) have only a moderate impact (not exceeding 50% reliability) on the Indian summer monsoon activity. On the other hand, the sea level pressure anomaly (SOI) over Australia and the south Pacific has a reasonably high degree of significance (more than 70%) with the monsoon activity over India. However, these two parameters (SLP and SST) do not show any significant variability over the Indian seas in relation to the summer monsoon activity. Over the Indian seas, the parameters which are mainly associated with the convective activity such as cloud cover, relative humidity and the surface wind were found to have a strong association with the extreme monsoon activity (flood/drought) and thus the net oceanic heat loss over the Indian seas provides a strong positive feed-back for the monsoon activity over India.  相似文献   

5.
The northeast monsoon rainfall (NEMR) contributes about 20–40 % of annual rainfall over the North Indian Ocean (NIO). In the present study, the relationship between the NEMR and near-surface atmospheric wind convergence (NSAWC) over the NIO is demonstrated using high-resolution multisatellite data. The rainfall product from the Tropical Rainfall Measuring Mission Multisatellite Precipitation Analysis and near-surface wind product from the Cross-Calibration Multi-Platform available at 0.25° × 0.25° spatial resolution are used for the study. Large-scale NSAWC and divergence maps over the tropical Indian Ocean are generated at monthly scale from the wind product for the period of 1988–2010. A preliminary analysis is carried out for two consecutive anomalous Indian Ocean Dipole (IOD) years 2005 (negative) and 2006 (positive). The distinct spatial patterns of rainfall rate and NSAWC fields over the NIO clearly show the evolution of the anomalous IOD events in the south eastern equatorial Indian Ocean (EEIO). The spatially averaged time-series of pentad NSAWC over the south EEIO box suggests that the variability occurs in phase with rainfall rate during both the northeast monsoon years. Furthermore, the scatter plot between area-averaged pentad rainfall and convergence over the south EEIO box for the period of 1998–2010 shows statistically significant linear correlation which reveals that NSAWC plays a key role in regulating the NEMR.  相似文献   

6.
利用中国逐日站点降水资料、逐日季风监测指数及逐日副热带高压指数、74项环流指数及NCEP/NCAR再分析资料, 分析了2009年秋季至2010年春季的秋冬春西南特大干旱过程中各指数及大气环流异常特征.结果表明: 自2009年10月底东亚冬季风建立以来, 至2010年春季, 东亚冬季风强度持续偏强, 加之西太平洋副热带高压较常年偏西偏南, 西南地区长期受副高控制, 气温持续偏高, 加之冷空气虽然总体偏强, 但主要控制我国北方地区, 造成冷暖空气在西南地区少有交汇, 致使降水偏少, 干旱发生发展. 印缅槽强度较常年偏弱, 来自印度洋、孟加拉湾以及南海的水汽条件不足, 向西南地区输送的来自南海和孟加拉湾两条水汽通道的水汽通量均较常年偏弱很多, 加之西南地区、特别是云南地区自2009年秋季以来, 长期处于下沉运动的正距平区, 造成这段时间西南地区干旱少雨, 旱情持续. 2009年9月El Niño事件全面爆发, 南海-西太平洋地区形成异常反气旋流场, 该反气旋流场较常年偏西偏南, 造成副高位置偏西偏南, 从而使得云贵高原及其周边的印度季风区的降雨量明显偏少;高原地区及南海、菲律宾附近及热带辐合带地区OLR异常对西太平洋副热带高压的变化有一定影响, 进而影响西南地区降水, 其内在机制还有待深入研究.  相似文献   

7.
Towards understanding the unusual Indian monsoon in 2009   总被引：1，自引：0，他引：1  
The Indian summer monsoon season of 2009 commenced with a massive deficit in all-India rainfall of 48% of the average rainfall in June. The all-India rainfall in July was close to the normal but that in August was deficit by 27%. In this paper, we first focus on June 2009, elucidating the special features and attempting to identify the factors that could have led to the large deficit in rainfall. In June 2009, the phase of the two important modes, viz., El Niño and Southern Oscillation (ENSO) and the equatorial Indian Ocean Oscillation (EQUINOO) was unfavourable. Also, the eastern equatorial Indian Ocean (EEIO) was warmer than in other years and much warmer than the Bay. In almost all the years, the opposite is true, i.e., the Bay is warmer than EEIO in June. It appears that this SST gradient gave an edge to the tropical convergence zone over the eastern equatorial Indian Ocean, in competition with the organized convection over the Bay. Thus, convection was not sustained for more than three or four days over the Bay and no northward propagations occurred. We suggest that the reversal of the sea surface temperature (SST) gradient between the Bay of Bengal and EEIO, played a critical role in the rainfall deficit over the Bay and hence the Indian region. We also suggest that suppression of convection over EEIO in association with the El Niño led to a positive phase of EQUINOO in July and hence revival of the monsoon despite the El Niño. It appears that the transition to a negative phase of EQUINOO in August and the associated large deficit in monsoon rainfall can also be attributed to the El Niño.  相似文献   

8.
In this study, the possible linkage between summer monsoon rainfall over India and surface meteorological fields (basic fields and heat budget components) over monsoon region (30‡E-120‡E, 30‡S30‡N) during the pre-monsoon month of May and summer monsoon season (June to September) are examined. For this purpose, monthly surface meteorological fields anomaly are analyzed for 42 years (1958-1999) using reanalysis data of NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research). The statistical significance of the anomaly (difference) between the surplus and deficient monsoon years in the surface meteorological fields are also examined by Student’s t-test at 95% confidence level. Significant negative anomalies of mean sea level pressure are observed over India, Arabian Sea and Arabian Peninsular in the pre-monsoon month of May and monsoon season. Significant positive anomalies in the zonal and meridional wind (at 2 m) in the month of May are observed in the west Arabian Sea off Somali coast and for monsoon season it is in the central Arabian Sea that extends up to Somalia. Significant positive anomalies of the surface temperature and air temperature (at 2 m) in the month of May are observed over north India and adjoining Pakistan and Afghanistan region. During monsoon season this region is replaced by significant negative anomalies. In the month of May, significant positive anomalies of cloud amount are observed over Somali coast, north Bay of Bengal and adjoining West Bengal and Bangladesh. During monsoon season, cloud amount shows positive anomalies over NW India and north Arabian Sea. There is overall reduction in the incoming shortwave radiation flux during surplus monsoon years. A higher magnitude of latent heat flux is also found in surplus monsoon years for the month of May as well as the monsoon season. The significant positive anomaly of latent heat flux in May, observed over southwest Arabian Sea, may be considered as an advance indicator of the possible behavior of the subsequent monsoon season. The distribution of net heat flux is predominantly negative over eastern Arabian Sea, Bay of Bengal and Indian Ocean. Anomaly between the two extreme monsoon years in post 1980 (i.e., 1988 and 1987) shows that shortwave flux, latent heat flux and net heat flux indicate reversal in sign, particularly in south Indian Ocean. Variations of the heat budget components over four smaller sectors of Indian seas, namely Arabian Sea, Bay of Bengal and west Indian Ocean and east Indian Ocean show that a small sector of Arabian Sea is most dominant during May and other sectors showing reversal in sign of latent heat flux during monsoon season.  相似文献   

9.
A brief summary of Dr. G. V. Rao's research interests is presented. Many of his earlier studies were in conjunction with the summer Monsoon Experiment of 1979 (MONEX-79). These included: 1) the structure of the Somali jet based on aerial observations; 2) sea-level air trajectories over the equatorial Indian Ocean; 3) structural features of the east African low-level flow; 4) effects of Indian Ocean surface temperature anomaly patterns on the summer monsoon circulations; 5) structures of the monsoon low-level flow over the Arabian Sea; 6) characteristics and momentum-flux budgets of the Arabian Sea convective bands; and 7) evaporation and precipitation over the Arabian Sea during the monsoon seasons. Dr. Rao's research efforts in recent years had focused on case studies of mesocyclones spawned by tropical cyclones (TCs) in Florida using Doppler radar data and a mesoscale numerical model. These included: 1) research on tornadic mesocyclones spawned by TC Earl in 1998; 2) documentation of subtle differences between tornadic and non-tornadic mesocyclones in TC Floyd in 1999; and 3) numerical simulation of the tornadic environment observed in peninsular Florida during TC Earl in 1998. Preliminary findings show that the supercells' cold pools interacted with an existing boundary resulting in increased baroclinicity and horizontal vorticity, and a maximization of the tornado production potential by the updrafts. The model successfully simulated the mesoscale features of the mesocyclones and the tornadic environment observed during TC Earl. A 24 h simulation of accumulated rainfall within the inner domain agreed well with the observed precipitation pattern over the region.  相似文献   

10.
Detailed analysis of the surface winds over the Indian Ocean derived from ERS-1 scatterometer data during the years 1993 and 1994 has been used to understand and unambiguously identify the onset phase of south-west monsoon. Five day (pentad) averaged wind vectors for the period April to June during both years have been examined to study the exact reversal of wind direction as well as the increase in wind speed over the Arabian Sea in relation to the onset of monsoon over the Indian west coast (Kerala). The related upper level humidity available from other satellites has also been analysed. The results of our analysis clearly show a consistent dramatic reversal in wind direction over the western Arabian Sea three weeks in advance of the onset of monsoon. The wind speed shows a large increase coinciding with the onset of monsoon. These findings together show the dominant role of sea surface winds in establishing the monsoon circulation. The study confirms that the cross equatorial current phenomenon becomes more important after the onset of monsoon.  相似文献   

11.
Monsoon onset over Kerala (India) which occurs every year is a major climatic phenomenon that involves large scale changes in wind, rainfall and sea surface temperature (SST). Over the last 150 years, the date of monsoon onset over Kerala (DMOK) has varied widely, the earliest being 11 May, 1918 and the most delayed being 18 June, 1972. DMOK has a long term (1870–2014) mean of 01 June and standard deviation of 7–8 days. We have studied the inter-annual and decadal time scale variability of DMOK and their relation with SST. We found that SST anomalies of large spatial scale similar to those in El Nino/La Nina are associated with the inter-annual variability in DMOK. Indian Ocean between latitudes \(5^{\circ }\hbox {S}\) and \(20^{\circ }\hbox {N}\) has two episodes of active convection associated with monsoon onset over Kerala (MOK), one around DMOK and the other about six weeks earlier (called pre-monsoon rain peak or bogus monsoon onset) and in between a two week period of suppressed convection occurs over north Indian Ocean. A prominent decadal time scale variability was found in DMOK having large and statistically significant linear correlation with the SST gradient across the equator over Indian and Pacific oceans, the large correlation persisting for several months prior to the MOK. However, no linear trend was seen in DMOK during the long period from 1870 to 2014.  相似文献   

12.
Due to the IO monsoon impact, the tropical IO circulation has significant seasonal variation, especially in the northern IO. However, in mean-state, a relatively closed current loop is established by eastward current along the equator and westward current south of equator, which is regarded as Tropical Gyre in the Indian Ocean. Based on this circulation system, relevant studies were reviewed. Its impact on heat and salt transports and regional climate changes were discussed.  相似文献   

13.
The impact of Southern Oscillation on thecyclogenesis over the Bay of Bengal duringthe summer monsoon has been investigated.The analysis of correlation coefficients(CCs) between the frequency of monsoondepressions and the Southern OscillationIndex (SOI) reveals that more depressionsform during July and August of El Niñoyears. Due to this, the seasonal frequencyof monsoon depressions remains little higherduring El Niño epochs even though thecorrelations for June and September are notsignificant. The CCs for July and August aresignificant at the 99% level.The El Niño-Southern Oscillation (ENSO)is known to affect Indian MonsoonRainfall (IMR) adversely. The enhancedcyclogenesis over the Bay of Bengal duringJuly and August is an impact of ENSO whichneeds to be examined closely. Increasedcyclogenesis over the Bay of Bengal may bereducing the deficiency in IMR duringEl Niño years by producing more rainfallover the eastern parts of India duringJuly and August. Thus there is a considerablespatial variation in the impact of ENSOon the monsoon rainfall over India and El Niñoneed not necessarily imply a monsoonfailure everywhere in India.The area of formation of monsoon depressionsshifts eastward during El Niño years.Warmer sea surface temperature (SST) anomaliesprevail over northwest and adjoiningwestcentral Bay of Bengal during premonsoon andmonsoon seasons of El Niño years.May minus March SOI can provide useful predictionsof monsoon depression frequencyduring July and August.  相似文献   

14.
In this study Tropospheric Biennial Oscillation (TBO) and south Asian summer monsoon rainfall are examined in the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFSv2) hindcast. High correlation between the observations and model TBO index suggests that the model is able to capture most of the TBO years. Spatial patterns of rainfall anomalies associated with positive TBO over the south Asian region are better represented in the model as in the observations. However, the model predicted rainfall anomaly patterns associated with negative TBO years are improper and magnitudes are underestimated compared to the observations. It is noted that positive (negative) TBO is associated with La Niña (El Niño) like Sea surface temperature (SST) anomalies in the model. This leads to the fact that model TBO is El Niño-Southern Oscillation (ENSO) driven, while in the observations Indian Ocean Dipole (IOD) also plays a role in the negative TBO phase. Detailed analysis suggests that the negative TBO rainfall anomaly pattern in the model is highly influenced by improper teleconnections allied to IOD. Unlike in the observations, rainfall anomalies over the south Asian region are anti-correlated with IOD index in CFSv2. Further, summer monsoon rainfall over south Asian region is highly correlated with IOD western pole than eastern pole in CFSv2 in contrast to the observations. Altogether, the present study highlights the importance of improving Indian Ocean SST teleconnections to south Asian summer rainfall in the model by enhancing the predictability of TBO. This in turn would improve monsoon rainfall prediction skill of the model.  相似文献   

15.
The second campaign of the Arabian Sea Monsoon Experiment (ARMEX-II) was conducted in two phases viz., March–April and May–June 2003. In the present work, the buoy and ocean research vessel data collected during the second phase of ARMEX-II have been analysed to bring out the characteristic features of monsoon onset. The results have shown that the thermodynamical features such as build up of lower tropospheric instability and increased height of zero degree isotherm occurred about a week before the monsoon onset over Kerala and adjoining southeast Arabian Sea. There was a sharp fall in the temperature difference between 850 and 500 hPa, and the height of zero degree isotherm about 2–3 days before the monsoon onset. The flux of sensible heat was positive (sea to air) over south Arabian Sea during the onset phase. Over the Bay of Bengal higher negative (air to sea) values of sensible flux prevailed before the monsoon onset which became less negative with the advance of monsoon over that region. The pre-onset period was characterized by large sea surface temperature (SST) gradient over the Arabian Sea with rapid decrease towards north of the warm pool region. The buoy observations have shown that SST remained close to 30.5°C in the warm pool region during the pre-onset period in 2003 but only 2–3 degrees away (north of this region) SSTs were as low as 28.5–29°C. An interesting aspect of sea level pressure (SLP) variability over the Indian seas during the onset phase of summer monsoon 2003 was undoubtedly, the highest SLP in the warm pool region inspite of very high SSTs.  相似文献   

16.
Research efforts focused on assessing the potential for changes in tropical cyclone activity in the greenhouse-warmed climate have progressed since the IPCC assessment in 1996. Vulnerability to tropical cyclones becoming more pronounced due to the fastest population growth in tropical coastal regions makes it practically important to explore possible changes in tropical cyclone activity due to global warming. This paper investigates the tropical cyclone activity over whole globe and also individually over six different ocean basins. The parameters like storm frequency, storm duration, maximum intensity attained and location of formation of storm have been examined over the past 30-year period from 1977 to 2006. Of all, the north Atlantic Ocean shows a significant increasing trend in storm frequency and storm days, especially for intense cyclones. Lifetime of intense tropical cyclones over south Indian Ocean has been increased. The intense cyclonic activity over north Atlantic, south-west Pacific, north and south Indian Ocean has been increased in recent 15 years as compared to previous 15 years, whereas in the east and west-north Pacific it is decreased, instead weak cyclone activity has been increased there. Examination of maximum intensity shows that cyclones are becoming more and more intense over the south Indian Ocean with the highest rate. The study of the change in the cyclogenesis events in the recent 15 years shows more increase in the north Atlantic. The Arabian Sea experiences increase in the cyclogenesis in general, whereas Bay of Bengal witnesses decrease in these events. Shrinking of cyclogenesis region occurs in the east-north Pacific and south-west Pacific, whereas expansion occurs in west-north Pacific. The change in cyclogenesis events and their spatial distribution in association with the meteorological parameters like sea surface temperature (SST), vertical wind shear has been studied for Indian Ocean. The increase in SST and decrease in wind shear correspond to increase in the cyclogenesis events and vice versa for north Indian Ocean; however, for south Indian Ocean, it is not one to one.  相似文献   

17.
Although previous literature have considered Southern Oscillation Index (SOI), Indian Dipole, and SST as the major teleconnection patterns to explain the variability of summer monsoon rainfall over India. South Asia low pressure and Indian Ocean high are the centers of action that dominates atmospheric circulations in Indian continent. This paper examines the possible impact of South Asian low pressure distribution on the variability of summer monsoon rainfall of India using centers of action approach. Our analysis demonstrates that the explanation of summer monsoon rainfall variability over Central India is improved significantly if the SOI is replaced by South Asian low heat. This contribution also explains the physical mechanisms to establish the relationships between the South Asian low heat and regional climate by examining composite maps of large-scale circulation fields using NCEP/NCAR Reanalysis data.  相似文献   

18.
During the field cruises of the Indian Ocean Experiment (INDOEX) extensive measurements on the atmospheric chemical and aerosol composition are undertaken to study the long-range transport of air pollution from south and southeast Asia towards the Indian Ocean during the dry monsoon season in 1998 and 1999. The present paper discusses the temporal and spatial variations in aerosols and aerosol forcing during the winter monsoon season (January-March) for INDOEX first field phase (FFP) in 1998 and INDOEX intensive field phase (IFP) in 1999. An interactive chemistry/aerosol model (LMDZ.3.3) is used to investigate the variation in the spatial distribution of tropospheric sulphate aerosols during 1998 and 1999. The model results depict major enhancement in the sulphate aerosol concentrations, radiative forcing (RF) and optical depth over the Indian subcontinent and adjoining marine areas between INDOEX-FFP and IFP. A significant increase in transport of sulphate aerosols from the continents to the Indian Ocean region has also been simulated during the winter monsoon in 1999. The mean RF over INDOEX-FFP in 1998 is found to be ?1.2 Wm–2 while it increased to ?1.85 Wm–2 during INDOEX-IFP in 1999. Model results reveal a mean sulphate aerosol optical depth (AOD) of 0.08 and 0.14 over Indian subcontinent during 1998 and 1999, respectively. The model results suggest that elevated AOD downwind of source regions in India can significantly affect the regional air quality and adjoining marine environments.  相似文献   

19.
A high resolution model, using the Miami Isopycnic Coordinate Ocean Model (MICOM), has been implemented for the first time to study the seasonal circulation and coastal upwelling off the southwest Indian coast during 1974. This model is part of a model and data assimilation system capable of describing the ocean circulation and variability in the Indian Ocean and its predictability in response to the monsoon system. Along the southwest coast of India the dominant coastal current is the reversing West Indian Coastal Current which is well simulated and described, in addition to the weaker undercurrent of the opposite direction. Upwelling of cold water, 4‡C lower than offshore temperatures appear in April. The upwelling intensifies with the southwest monsoon and is simulated in accordance within situ observations. Upwelling appears to be strongest off Cochin and Quilon, and the upwelling of cold water is seen together with a decrease in salinity in the model simulation.  相似文献   

20.
Analysis of monthly momentum transport of zonal waves at 850 hPa for the period 1979 to 1993, between ‡S and ‡N for January to April, using zonal (u) and meridional (v) components of wind taken from the ECMWF reanalysis field, shows a positive correlation (.1% level of significance) between the Indian summer monsoon rainfall (June through September) and the momentum transport of wave zero TM(0) over latitudinal belt between 25‡S and 5‡N (LB) during March. Northward (Southward) TM(0) observed in March over LB subsequently leads to a good (drought) monsoon season over India which is found to be true even when the year is marked with the El-Nino event. Similarly a strong westerly zone in the Indian Ocean during March, indicates a good monsoon season for the country, even if the year is marked with El-Nino. The study thus suggests two predictors, TM(0) over LB and the strength of westerly zone in the Indian Ocean during March.  相似文献   

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