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1.
The summer monsoon rainfall over Orissa occurs mostly due to low pressure systems (LPS) developing over the Bay of Bengal and moving along the monsoon trough. A study is hence undertaken to find out characteristic features of the relationship between LPS over different regions and rain-fall over Orissa during the summer monsoon season (June-September). For this purpose, rainfall and rainy days over 31 selected stations in Orissa and LPS days over Orissa and adjoining land and sea regions during different monsoon months and the season as a whole over a period of 20 years (1980-1999) are analysed. The principal objective of this study is to find out the role of LPS on spatial and temporal variability of summer monsoon rainfall over Orissa. The rainfall has been significantly less than normal over most parts of Orissa except the eastern side of Eastern Ghats during July and hence during the season as a whole due to a significantly less number of LPS days over northwest Bay in July over the period of 1980-1999. The seasonal rainfall shows higher interannual variation (increase in coefficient of variation by about 5%) during 1980-1999 than that during 1901-1990 over most parts of Orissa except northeast Orissa. Most parts of Orissa, especially the region extending from central part of coastal Orissa to western Orissa (central zone) and western side of the Eastern Ghats get more seasonal monsoon rainfall with the development and persistence of LPS over northwest Bay and their subsequent movement and persistence over Orissa. The north Orissa adjoining central zone also gets more seasonal rainfall with development and persistence of LPS over northwest Bay. While the seasonal rainfall over the western side of the Eastern Ghats is adversely affected due to increase in LPS days over west central Bay, Jharkhand and Bangladesh, that over the eastern side of the Eastern Ghats is adversely affected due to increase in LPS days over all the regions to the north of Orissa. There are significant decreasing trends in rainfall and number of rainy days over some parts of southwest Orissa during June and decreasing trends in rainy days over some parts of north interior Orissa and central part of coastal Orissa during July over the period of 1980-1999  相似文献   

2.
Spatial variations in the relationship between topography and mean annual and seasonal rainfalls in southwestern Saudi Arabia are examined using Kruskal–Wallis one-way analysis of variance. The topographic factors include physiographic features (topography), altitude, slope, proximity to a ridge or crest of mountains, and proximity to the Red Sea. There is a statistically significant effect of topographic factors on the mean annual rainfall, but the results are more significant for the mean seasonal rainfall. The largest amount of mean rainfall in the study area occurs during spring, when rainfall displays a significant relationship with topographic factors, in which more orographic rainfall patterns are associated with higher altitude, greater proximity to a ridge and steeper slopes. Higher altitudes do not necessarily receive more rain; some low altitude locations (i.e., the southern part of the study area) receive more rain during summer and fall because they are located on the windward side of the Asir Mountains and are exposed to the moist air masses brought by the southwest monsoonal system in summer and by the northwest air flow from the Mediterranean and Red Sea in the fall in addition to local convective rainfall patterns. Rainfall in winter increases in the foothills of the Asir Mountains above the coastal plain of the Red Sea. The steep western slopes (windward side) receive more rainfall than the gentle eastern slopes (leeward side): this may be due to the Asir Mountains forcing moist air masses carried by the westerly and northwest winds to rise and cool before they descend and warm on the leeward side.  相似文献   

3.
Meteorological drought during the southwest monsoon season and for the northeast monsoon season over five meteorological subdivisions of India for the period 1901–2015 has been examined using district and all India standardized precipitation index (SPI). Whenever all India southwest monsoon rainfall was less than ?10% or below normal, for those years all India SPI was found as ?1 or less. Composite analysis of SPI for the below normal years, viz., less than ?15% and ?20% of normal rainfall years indicate that during those years more than 30% of country’s area was under drought condition, whenever all India southwest monsoon rainfall was –15% or less than normal. Trend analysis of monthly SPI for the monsoon months identified the districts experiencing significant increase in drought occurrences. Significant positive correlation has been found with the meteorological drought over most of the districts of central, northern and peninsular India, while negative correlation was seen over the districts of eastern India with NINO 3.4 SST. For the first time, meteorological drought analysis over districts and its association with equatorial pacific SST and probability analysis has been done for the northeast monsoon over the affected regions of south peninsular India. Temporal correlation of all India southwest monsoon SPI and south peninsular India northeast monsoon SPI has been done with the global SST to identify the teleconnection of drought in India with global parameters.  相似文献   

4.
The aim of this study was to investigate temporal variation in seasonal and annual rainfall trend over Ranchi district of Jharkhand, India for the period (1901–2014: 113 years). Mean monthly rainfall data series were used to determine the significance and magnitude of the trend using non-parametric Mann–Kendall and Sen’s slope estimator. The analysis showed a significant decreased in rainfall during annual, winter and southwest monsoon rainfall while increased in pre-monsoon and post-monsoon rainfall over the Ranchi district. A positive trend is detected in pre-monsoon and post-monsoon rainfall data series while annual, winter and southwest monsoon rainfall showed a negative trend. The maximum decrease in rainfall was found for monsoon (? 1.348 mm year?1) and minimum (? 0.098 mm year?1) during winter rainfall. The trend of post-monsoon rainfall was found upward (0.068 mm year?1). The positive and negative trends of annual and seasonal rainfall were found statistically non-significant except monsoon rainfall at 5% level of significance. Rainfall variability pattern was calculated using coefficient of variation CV, %. Post-monsoon rainfall showed the maximum value of CV (70.80%), whereas annual rainfall exhibited the minimum value of CV (17.09%), respectively. In general, high variation of CV was found which showed that the entire region is very vulnerable to droughts and floods.  相似文献   

5.
The Western Ghats plays a pivotal role in determining the hydrological and hydroclimatic regime of Peninsular India. The mountainous catchments of the Ghats are the primary contributors of flow in the rivers that sustains the life and agricultural productivity in the area. Although many studies have been conducted in the past decades to understand long term trends in the meteorological and hydrological variables of major river basins, not much attention have been made to unfold the relationship existing among rainfall and river hydrology of natural drainages on either side of the Western Ghats which host one of the unique biodiversity hotspots across the world. Therefore, an attempt has been made in this paper to examine the short term (last three decades) changes in the rainfall pattern and its influence on the hydrological characteristics of some of the important rivers draining the southern Western Ghats as a case study. The short term, annual and seasonal trends in the rainfall, and its variability and discharge were analyzed using Mann-Kendall test and Sen’s estimator of slope. The study showed a decreasing trend in rainfall in the southwest monsoon while a reverse trend is noticed in northeast monsoon. Correspondingly, the discharge of the west and east flowing rivers also showed a declining trend in the southwest monsoon season. The runoff coefficient also followed the trends in the discharge. The runoff coefficient of the Periyar river showed a decreasing trend, whereas the Cauvery river exhibited an increasing trend. A high-resolution analysis of rainfall data revealed that the number of moderate rainfall events showed a decreasing trend throughout the southern Western Ghats, whereas the high intensity rainfall events showed an opposite trend. The decline in groundwater level in the areas which recorded an increase in high intensity rainfall events and decrease in moderate rainfall events showed that the groundwater recharge process is significantly affected by changes in the rainfall pattern of the area.  相似文献   

6.
我国西南山区降雨侵蚀力时空变化趋势研究   总被引：14，自引：0，他引：14  
降雨是我国西南山区土壤侵蚀的主要动力因素,降雨侵蚀力反映了降雨对土壤侵蚀的潜在能力,研究降雨侵蚀力的时空变化趋势对我国西南山区土壤侵蚀的监测、评估、预报和治理具有重要意义。利用1960—2009年129个气象站逐日降雨量资料,计算出西南山区各气象站逐年降雨侵蚀力。采用趋势系数、气候倾向率和克吕格插值等方法对西南山区降雨侵蚀力50年来的时空变化趋势进行了探讨。结果表明:西南山区降雨侵蚀力空间分布特征与年降水量的空间分布特征一致;西南山区西北部的青藏高原区域降雨侵蚀力年际变化明显,变差系数Cv一般高于0.40;西南山区大部地区降雨侵蚀力呈上升趋势,说明由降雨侵蚀力引起的土壤侵蚀风险在增加,但在成都平原附近降雨侵蚀力在明显下降;降雨侵蚀力变化趋势系数随海拔高度升高而不断增加,在海拔2 500 m以上地区尤为明显,西南山区西北部的高海拔地区海拔高度对降雨侵蚀力增加具有放大效应。  相似文献   

7.
BOBMEX-Pilot was organised from 23rd October–11th November, 1998 when the seasonal trough had already shifted to south Bay of Bengal. The activity during this period was marked by the development of a monsoon depression from 26th–29th October that weakened over the sea; onset of northeast monsoon along the east coast of India on 29th October; a low pressure area that formed on 2nd November over southwest Bay off Sri Lanka — southTamilnadu coast; and another cyclonic circulation that formed towards the end of the BOBMEX-Pilot period. This paper describes the development of these synoptic systems through synoptic charts and satellite data.  相似文献   

8.
Spatial variability of aridity over northern India (north of 20°N) is studied by examining variations in the arid area. Area with an objectively determined summer monsoon rainfall (June to September total) of less than 500 mm is identified as arid area. The summer monsoon rainfall of 212 rain-gauges from 212 districts of the region for the period 1871–1984 are used in the analysis. An interesting feature of the arid area series is that it shows decreasing trend from beginning of the present century. The summer monsoon rainfall fluctuations over five subjectively divided zones over northern India are examined to understand the association between rainfall and the arid area variations. The rainfall series for northwest India shows a significant increasing trend and that for northeast India a significant decreasing trend from the beginning of this century. Rainfall fluctuations over the remaining zones can be considered intermediate stages of a systematic spatial change in the rainfall pattern. This suggested that the recent decreasing trend in the arid area is due to a westward shift in the monsoon rainfall activities. From correlation analyses it is inferred that perhaps the recent decreasing trend in the arid area and increasing trend in the monsoon rainfall over northwest India are associated with a warming trend of the northern hemisphere.  相似文献   

9.
Orissa is one of the most flood prone states of India. The floods in Orissa mostly occur during monsoon season due to very heavy rainfall caused by synoptic scale monsoon disturbances. Hence a study is undertaken to find out the characteristic features of very heavy rainfall (24 hours rainfall ≥125 mm) over Orissa during summer monsoon season (June–September) by analysing 20 years (1980–1999) daily rainfall data of different stations in Orissa. The principal objective of this study is to find out the role of synoptic scale monsoon disturbances in spatial and temporal variability of very heavy rainfall over Orissa. Most of the very heavy rainfall events occur in July and August. The region, extending from central part of coastal Orissa in the southeast towards Sambalpur district in the northwest, experiences higher frequency and higher intensity of very heavy rainfall with less interannual variability. It is due to the fact that most of the causative synoptic disturbances like low pressure systems (LPS) develop over northwest (NW) Bay of Bengal with minimum interannual variation and the monsoon trough extends in west-northwesterly direction from the centre of the system. The very heavy rainfall occurs more frequently with less interannual variability on the western side of Eastern Ghat during all the months and the season except September. It occurs more frequently with less interannual variability on the eastern side of Eastern Ghat during September. The NW Bay followed by Gangetic West Bengal/Orissa is the most favourable region of LPS to cause very heavy rainfall over different parts of Orissa except eastern side of Eastern Ghat. The NW Bay and west central (WC) Bay are equally favourable regions of LPS to cause very heavy rainfall over eastern side of Eastern Ghat. The frequency of very heavy rainfall does not show any significant trend in recent years over Orissa except some places in north-east Orissa which exhibit significant rising trend in all the monsoon months and the season as a whole.  相似文献   

10.
Using the satellite derived sea surface temperature (SST) data for 1979 (bad monsoon) and 1983 (good monsoon), the SST variability for two contrasting monsoon seasons is studied. The study indicates that large negative anomalies off the Somali and Arabian coasts are associated with good monsoon rainfall over India. The strong monsoonal cooling in these regions can be attributed to strong low level winds and intense upwelling. The reappearance of 27°C isotherm off Somali coast in May/June coincides with the onset of southwest monsoon over India. Further, the influence of zonal anomaly of SST off Somalia Coast (SCZASST) and Central Indian Ocean Zonal Anomaly of SST (CIOZASST) with monsoon rainfall over India is brought out. The former is negatively related to the monsoon rainfall over western and central parts of India, whilst CIOZASST is positively related.  相似文献   

11.
The dynamics and thermodynamics of the surface layer of the Arabian Sea, north of about 10N, are dominated by the monsoon-related annual cycle of air-sea fluxes of momentum and heat. The currents in open-sea regime of this layer can be largely accounted for by Ekman drift and the thermal field is dominated by local heat fluxes. The geostrophic currents in open-sea subsurface regime also show a seasonal cycle and there is some evidence that signatures of this cycle appear as deep as 1000 m. The forcing due to Ekman suction is an important mechanism for the geostrophic currents in the central and western parts of the Sea. Recent studies suggest that the eastern part is strongly influenced by the Rossby waves radiated by the Kelvin waves propagating along the west coast of India. The circulation in the coastal region off Oman is driven mainly by local winds and there is no remotely driven western boundary current. Local wind-driving is also important to the coastal circulation off western India during the southwest monsoon but not during the northeast monsoon when a strong (approximately 7 × 106m3/sec) current moves poleward against weak winds. This current is driven by a pressure gradient which forms along this coast during the northeast monsoon due to either thermohaline-forcing or due to the arrival of Kelvin waves from the Bay of Bengal. The present speculation about flow of bottom water (deeper than about 3500 m) in the Arabian Sea is that it moves northward and upwells into the layer of North Indian Deep Water (approximately 1500–3500m). It is further speculated that the flow in this layer consists of a poleward western boundary current and a weak equatorward flow in the interior. It is not known if there is an annual cycle associated with the deep and the bottom water circulation.  相似文献   

12.
1971-2015年青藏高原东北边坡降水特征及主要影响因子分析   总被引：2，自引：1，他引：1  
利用1971-2015年青藏高原东北边坡20个站的降水观测资料和美国国家环境预报中心（NCEP）再分析资料,分析了青藏高原东北边坡年、季降水量空间分布和变化趋势,并采用相关系数法分析和讨论其所受的影响因素。结果表明：青藏高原东北边坡地区的年、季平均降水量空间分布极为不均,总体上是从南向北递减,东北部最少;青藏高原东北边坡年、夏、秋季平均降水量北部呈上升趋势,南部呈下降趋势;青藏高原东北边坡地区年平均降水量呈下降趋势,气候倾向率为-3.1 mm·（10a）-1,其中春、秋、冬季平均降水量呈上升趋势,夏季平均降水量呈明显下降趋势;青藏高原东北边坡地区年、季降水量的显著周期为2～3 a、4～5 a及10～15 a;南亚季风对青藏高原东北边坡地区降水量影响显著,为明显的正相关,西风指数对高原东北边坡地区降水量有一定影响,相关不是很明显,与其北部降水量呈正相关,南部降水量呈负相关。  相似文献   

13.
南亚季风降水的双极振荡*   总被引：1，自引：2，他引：1  
文章利用气象资料揭示在印度半岛南部和北部,南亚季风降水变化在10年尺度以上呈翘翘板变化形式;利用更长的季风降水资料,即300年的喜马拉雅山达索普冰芯降水记录和印度半岛南部石笋降水记录,发现印度南部和喜马拉雅山季风降水呈双极振荡行为。自1700年以来,喜马拉雅山,即印度北部(或印度半岛南部)季风降水经历了1700~1764年期间的减小(或增加)趋势,1764~1876年期间的增大(或减小)趋势,1876~2000年期间的减小(或增加)趋势。同时,发现印度半岛南部的季风降水同北半球温度变化具有相同的变化特征,而喜马拉雅山季风降水同北半球温度变化具有相反的变化特征。南亚季风降水的这种南北翘翘板变化形式,与跨赤道气流有密切的联系。  相似文献   

14.
A two-dimensional, nonlinear, vertically integrated model was used to simulate depth-mean wind-driven circulation in the upper Ekman layers of the Bay of Bengal and Andaman Sea. The model resolution was one third of a degree in the latitude and longitude directions. Monthly mean wind stress components used to drive the model were obtained from the climatic monthly mean wind data compiled by Hastenrath and Lamb. A steady-state solution was obtained after numerical integration of the model for 15 days. The sensitivity of the model to two types of open boundary conditions, namely, a radiation type and clamped type, was tested. A comparison of simulated results for January with available ship drift data showed that the application of the latter along the open boundary could reproduce all the observed features near the boundary and the interior of the model domain. The model was integrated for 365 days to study the circulation during the southwest and northeast monsoon seasons. The model was successful in simulating the broad features of circulation including gyres and eddies observed during both the seasons, the development of north equatorial current during the northeast monsoon period and eastward moving monsoon drift current up to 90°E during the southwest monsoon season. During the latter season, two anticyclonic gyres were observed in the central and the southern parts of the Bay. A cyclonic type of circulation was prevalent in the central and western parts of the Bay of Bengal during the northeast monsoon months of November and December. The simulated western boundary current along the east coast of India, flows northward and southward during the southwest and northeast monsoon seasons respectively. It is presumed that this western boundary current, simulated during both the seasons, is locally wind-driven.  相似文献   

15.
Remotely sensed data on ocean colour of waters surrounding Sri Lanka received from the Coastal Zone Colour Scanner (CZCS) are processed and analyzed. Raw data of 1 km resolution on relatively cloud free days during 1978–1986 are processed to produce sea surface chlorophyll maps within latitudes 4.5N-11N and longitudes 78E-85E, a region in the Indian Ocean surrounding Sri Lanka. The processed data include about 110 single day maps and composite averages for each month and season. The months of July, August and September are omitted in the calculation of averages due to insufficient data. The waters in the Gulf of Mannar and Palk Bay areas show high chlorophyll-a concentrations throughout the year. However, these high values may represent other suspended particles and dissolved organic matter besides chlorophyll-a as this region is shallow (< 100 m). Regions with high chlorophyll concentrations (> 0.5 mg m-3) along the coast and western ocean region can be seen in the months of October and November, after the southwest monsoon period. As high surface chlorophyll concentrations may indicate high productivity, these regions need extensive measurements of primary production and also continuous monitoring of fish catches, during and after the southwest monsoon. Studies of particle composition in shallow water areas, in particular waters in Palk Bay and Gulf of Mannar, should be carried out in order to elucidate the effect of non-phytogenic.  相似文献   

16.
Recent analysis of a sediment core in the eastern Arabian Sea revealed a negative pulse of about 1% in the δ18O value of the planktonic Foraminifera around the last glacial maximum (LGM). This pulse has been attributed to (i) increased runoff into the Bay of Bengal from the east-flowing south Indian rivers due to enhancement of the northeast winter monsoon, and (ii) an increase in Arabian sea-surface temperature caused by the weakening of the southwest monsoon at the LGM. We show that the speculation on which the latter hypothesis is based, is not supported by observational data and cannot fully account for the observed magnitude of the spike. With a view to assessing the validity of the first mechanism, we have modelled the mixed layer of the Bay of Bengal as a well-mixed box. The model calculations show that to account for the pulse requires a change of about 10% in either the annual rate of river input or its isotopic composition. For the northeast monsoon to account for the pulse it would mean that the rainfall should have increased by a factor of five to ten during the LGM. No evidence for such an increase is indicated in the available palaeoclimatic data. We explain the freshwater spike by invoking increased discharge of glacial meltwater from the Tibetan plateau into the Bay of Bengal. We show that the proxy climate data from the Indo-Tibetan region that has become available recently provides substantial evidence for the occurrence of a warming event around the LGM, which supports our mechanism.  相似文献   

17.
Having recognized that it is the tropospheric temperature (TT) gradient rather than the land–ocean surface temperature gradient that drives the Indian monsoon, a new mechanism of El Niño/Southern Oscillation (ENSO) monsoon teleconnection has been unveiled in which the ENSO influences the Indian monsoon by modifying the TT gradient over the region. Here we show that equatorial Pacific coralline oxygen isotopes reflect TT gradient variability over the Indian monsoon region and are strongly correlated to monsoon precipitation as well as to the length of the rainy season. Using these relationships we have been able to reconstruct past Indian monsoon rainfall variability of the first half of the 20th century in agreement with the instrumental record. Additionally, an older coral oxygen isotope record has been used to reconstruct seasonally resolved summer monsoon rainfall variability of the latter half of the 17th century, indicating that the average annual rainfall during this period was similar to that during the 20th century. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

18.
Man-made coastal structures directly affect sediment balance and sediment dynamics on the surrounding beaches. The Colombo Harbor Expansion Project has created about 5-km-long breakwater nearly perpendicular to the beach. The present study is focused on quantitatively and qualitatively analyzing the effect of the Colombo Harbor Expansion Project on economically important beaches in and around Colombo city area. In this study, the authors measured monthly variations of beach width, beach profile and the mean grain-size of the sediments at mean sea level for complete annual monsoon cycle. Data were analyzed to establish site-specific erosion vulnerability. Monitoring results show that cumulative beach erosion has increased after the construction of the breakwater (rate = 0.7 m/year from May 2000 to April 2011 and rate = 28.2 m/year from April 2011 to June 2012). In addition, the cumulative and site-specific sand accretion and erosion patterns have a clear relationship with the monsoon seasonality. Beaches were narrower during the stormy southwestern monsoon, whereas beaches were wider during fair weather of northeast monsoon and inter-monsoon periods. In contrast, the constructed breakwater obstructs natural longshore sediment dynamics. For example, a significant amount of sediments from the Kelani-Ganga River were buried in the Colombo Harbor due to alteration of prominent longshore sediments transportation on the western coast of Sri Lanka. Therefore, this study shows enhancement of coastal erosion in the studied southern beaches due to a lack of sediment deposition.  相似文献   

19.
Particulate fluxes of aluminium, iron, magnesium and titanium were measured using six time-series sediment traps deployed in the eastern, central and western Arabian Sea. Annual Al fluxes at shallow and deep trap depths were 0.47 and 0.46 g m-2 in the western Arabian Sea, and 0.33 and 0.47 g m-2 in the eastern Arabian Sea. There is a difference of about 0.9–1.8 g m-2y-1 in the lithogenic fluxes determined analytically (residue remaining after leaching out all biogenic particles) and estimated from the Al fluxes in the western Arabian Sea. This arises due to higher fluxes of Mg (as dolomite) in the western Arabian Sea (6–11 times higher than the eastern Arabian Sea). The estimated dolomite fluxes at the western Arabian Sea site range from 0.9 to 1.35gm-2y-1. Fe fluxes in the Arabian Sea were less than that of the reported atmospheric fluxes without any evidence for the presence of labile fraction/excess of Fe in the settling particles. More than 75% of Al, Fe, Ti and Mg fluxes occurred during the southwest (SW) monsoon in the western Arabian Sea. In the eastern Arabian Sea, peak Al, Fe, Mg and Ti fluxes were recorded during both the northeast (NE) and SW monsoons. During the SW monsoon, there exists a time lag of around one month between the increases in lithogenic and dolomite fluxes. Total lithogenic fluxes increase when the southern branch of dust bearing northwesterlies is dragged by the SW monsoon winds to the trap locations. However, the dolomite fluxes increase only when the northern branch of the northwesterlies (which carries a huge amount of dolomite accounting 60% of the total dust load) is dragged, from further north, by SW monsoon winds. The potential for the use of Mg/Fe ratio as a paleo-monsoonal proxy is examined.  相似文献   

20.
纵向岭谷区冬、夏水热条件空间分布研究   总被引：20，自引：0，他引：20  
曹杰  何大明  姚平 《地球科学进展》2005,20(11):1176-1182
利用纵向岭谷区内76个测站降水、温度资料和大气环流资料,研究该区冬、夏两季降水的空间分布规律。结果表明：纵向岭谷区多年平均冬季降水空间上沿河流呈纵向分布;但纵向岭谷独特地形对冬季降水变化的空间分布影响不明显;冬季气候平均温度大致呈东西向带状分布,由低纬到高纬温度逐渐递减;在区域上,纵向岭谷独特地形的“通道—阻隔”作用对冬季温度空间分布的影响不明显;但在怒江和澜沧江流域,这种影响则较为明显。纵向岭谷区多年平均夏季降水空间分布主要由纵向岭谷的“阻隔”效应,以及夏季从孟加拉湾来的气流和从南海来的气流在相应迎风坡面辐合,形成两支较强的上升气流所致;而纵向剖面大气环流的变化则较为均匀,显示了纵向岭谷的“通道”效应。纵向岭谷的这种“通道—阻隔”效应使得西南季风和东南季风在区内交汇,并使区内夏季降水空间上及夏季降水的变化沿河流呈纵向分布。地形对怒江、澜沧江流域夏季温度空间分布的“通道—阻隔”作用较明显;对夏季温度变化的“通道—阻隔”作用则在纵向岭谷西北部地区最明显,但其余地区地形的作用则相对较弱。  相似文献   

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