Showing papers on "Monsoon published in 1984"

Fluctuations in All-India summer monsoon rainfall during 1871–1978

Abstract: Analysis of the All-India summer monsoon (June to September) rainfall for the period 1871 to 1978 has been made in order to understand the interannual and long-term variability of the monsoon. On a country level, India receives 85.31 cm mean monsoon rainfall which is 78%; of the annual rainfall. The coefficient of variation of monsoon rainfall at the country level is 9.5%;. The highest and lowest rainfall country level were observed in the years 1961 and 1877 respectively, the range being 41 cm about 48%; of the long term average. There are 13/9 years of large-scale deficit/excess in the 108-yr period. There is a continuous rise in the 10-yr mean rainfall from 1899 to 1953. There are four major climatic rainfall periods in the series. Correlogram and spectrum analysis showed significant 14-yr and 2.8-yr cycles respectively in 108-yr series; however detailed examination indicated that these cycles have developed during the last 30 yr of the data period.

The Monsoon of East Asia and its Global Associations—A Survey

Abstract: Observations concerning the summer and winter monsoons of East Asia and their global associations are reviewed. The seasonal mean structure, transient variation, including intraseasonal to interannual, and synoptic to planetary scale fluctuations are discussed separately for the two monsoon components. Similarities and differences between the East Asian monsoon and that of India are also surveyed. We also present a description of the current status of monsoon related observational and theoretical research and highlight important scientific problems. The importance of understanding the long-term anomalies of the monsoon is stressed, and an attempt is made to put the East Asian monsoon in a global perspective with a view towards identifying with the problems of long-range weather forecasting or short-term climate prediction, in general. Finally, sonic future directions of research are suggested.

Ocean–atmosphere coupling over monsoon regions

Abstract: In monsoon regions, the seasonal migration of the intertropical convergence zone (ITCZ) is manifested as a seasonal reversal of winds. Most of the summer monsoon rainfall over India occurs owing to synoptic and large-scale convection associated with the continental ITCZ (Fig. 1). We have investigated the interaction between these large-scale convective systems and the ocean over which they are generated1â3, concentrating on the relationship between organized convection over the Indian Ocean and sea surface temperature (SST). We report here that on a monthly basis the degree of cloudiness correlates well with SST for the relatively colder oceans, but when SST is maintained above 28 °C it ceases to be an important factor in determining the variability of cloudiness. Over the major regions of convection east of 70°E, which are warm year after year, the observed cloudiness cannot be correlated with variations in SST.

The Large-Scale Circulation and Heat Sources over the Tibetan Plateau and Surrounding Areas during the Early Summer of 1979. Part II: Heat and Moisture Budgets

Abstract: The large-scale heat and moisture budgets over the Tibetan Plateau and surrounding area during a 40-day period from late May to early July 1979 are studied using the FGGE Level II-b data. During this period the general circulation over East Asia underwent a distinct seasonal change characterizing the onset of the summer monsoon circulation. The analyses of the horizontal distributions of the vertically integrated heat source and moisture sink reveal the major heat source regions and their different degrees of association with precipitation. The 40-day mean distributions show intense heat sources of 150–300 W m−2 with moisture sinks of nearly equal magnitude over the Assam–Bengal region and in a broad belt extending over the China Plain along the Mei-yu front. The heat source of ∼100–150 W m−21 over the eastern Tibetan Plateau is accompanied by a moisture sink with a magnitude about half as large. The heat sources over the western Plateau and the Takla Makan Desert are not accompanied by appreciab...

Plateau monsoons of the northern hemisphere: a comparison between North America and Tibet

Abstract: Detailed analyses are presented of the temperature and pressure fields of the planetary boundary layer (PBL) and their seasonal variability over the western United States and over the Plateau of Tibet (Qinghai-Xizang Plateau). Over the United States these analyses rely on 850 mb data, augmented by surface data. Over Tibet the 600 mb surface adequately describes the PBL. The effects of a “plateau monsoon” appear prominently during winter and summer over both plateaus. Together with continental monsoon effects they help to shape prominent circulation features, such as the low-level jet stream (LLJ) over Texas and Oklahoma. The complex, seasonal characteristics of precipitation regimes over the North American continent can be explained, to a large extent, by considering these monsoonal changes in the PBL, especially over the mountains.

Interaction of low-level flow with the Western Ghat mountains and offshore convection in the summer monsoon

Abstract: Seven-year averaged values of percent frequency of occurrence of highly reflective cloud for the months June, July, and August indicate that offshore convection is a major component of the cloudiness of the southwest monsoon. Principal areas of convection occur off of the western coats of India, Burma, Thailand, and the Philippines. This study concentrates on the area upstream of the Western Ghats Mountains of India. Analysis of a special boundary layer mission flown during the WMO/ICSU Summer Monsoon Experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection. Data obtained on this mission allow a large-scale momentum budget computation for the subcloud layer, which shows pressure deceleration to be significant. The budget, dominated by advection, predicts an increase of average wind speed which is observed. The pressure decelera...

Onsets of The Asian Summer Monsoon 1979–82

Abstract: Analyses of moisture and mean tropospheric enthalpy distributions based on FGGE data are described and interpreted for the months of May and June 1979. Their detailed spatial characteristics are inferred using 15-day means for the region 0–150°E, 40°S–40°N, and the detailed temporal evolutions from time series at daily intervals for two regions-the Arabian Sea and the larger area 0°–150°E, 22.5°5–41.25°N. Vertical motion fields are calculated using O'Brien's method and are used to determine the distributions of moisture convergence and heating. The onset is found to consist of two main phases: 1) a moisture buildup over the Arabian Sea during which synoptic- and mesoscale transient disturbances develop; the relationship of this buildup with planetary wave activity is discussed. This is followed by 2) a rapid intensification of the Arabian Sea winds and a substantial increase in latent heat release, essentially a large-scale feedback process. The fully established monsoonal flow is interpreted in ...

Interaction of the Monsoon and Pacific Trade Wind System at Interannual Time Scales. Part III: A Partial Anatomy of the Southern Oscillation

Abstract: Studies of surface wind fields, sea surface temperature (SST), precipitation and sea level pressure in the tropical band extending ±30° of the equator from Africa to South America led to the following conclusions. The Southern Oscillation, El Niflo, and climatic variations in the Monsoon System are all part of one global scale phenomenon. The new information is that this phenomenon appears in the sea level pressure field to have a strong propagating component that appears first in the northern Indian Ocean and moves eastwardinto the eastern Pacific. Similar propagation of information was found in the surface wind field and equatorial precipitation regimes. These same conditions were amply demonstrated during the 1982-83 event and so it may be concluded that the evolution of that event bears many similarities to those in the historical record studies referred to in this paper. In the surface wind field of the equatorial wave guide, the large-scale signal appears to take the form of a forced Kelvin...

The southern oscillation and its relation to the monsoon rainfall

Abstract: The interannual fluctuations in the Southern Oscillation indices (Wright, 1975) and their relations to the Indian monsoon (June-September) rainfall have been examined for the period of 106 years from 1875 to 1980. The monsoon rainfall is significantly (99.9 per cent level) correlated with the Southern Oscillation indices for the seasons: MJJ (0.59), ASO (0.67), NDJ (0.53), and FMA of the following year (0.38). The fluctuations in the Southern Oscillation index for the ASO season appear strongly related to the nearly simultaneous monsoon rainfall of India. This implies that the large positive (negative) value of the Southern Oscillation index, signifying strengthening (weakening) of the Walker circulation coincides with large excess (deficient) monsoon rainfall over India. The coherence spectrum reveals that the Southern Oscillation index and the monsoon rainfall are highly correlated in the period range of about 2–2.5 years and 4–6 years. The first of these periods corresponds with the Quasi-Biennial Oscillation and the latter agrees with the features of the Southern Oscillation, suggesting a strong link between Indian monsoon rainfall and these two phenomena. The striking feature of the composites of the Southern Oscillation index averaged for all the drought years and for all the flood years is the simultaneous occurrence of low (high) Southern Oscillation index and droughts (floods) in India. However, this association has limited use in long-range prediction. A preliminary study suggests that a nearly simultaneous occurrence of major climatic anomalies of the tropics, such as droughts in India and El Nino off the coast of Peru, are linked to the Southern Oscillation, indicating some kind of time dependent zonal east-west circulation, i.e. Walker circulation.

The Sensitivity of Monsoon Climates to Orbital Parameter Changes for 9 000 Years BP : Experiments with the NCAR General Circulation Model

Abstract: The July and January climates of 9 000 years ago are simulated with a high resolution general circulation model. The incoming solar radiation is specified from the orbital parameters for 9 000 years ago, and the effect of the remnant North American ice sheet is taken into account (sea-surface temperatures are not changed from modern values). The change in radiation causes temperatures to be higher in July and lower in January over large areas of the continents; the increased temperature contrast between land and ocean results in intensified monsoon circulation and increased precipitation over tropical lands. These findings agree with paleoclimatic evidence and with the results of experiments done using a low resolution model.

Plankton of the Red Sea and the Arabian Gulf

Abstract: Though presenting some similarities, the Red Sea and the Arabian Gulf differ much in their configuration, hydrography and plankton populations. Both the recruitment of pelagic organisms into the Red Sea from the Gulf of Aden and their subsequent northward diffusion within the Red Sea basin, and the fluctuations in biomass and primary productivity are governed by the circulation pattern, which is itself dependent on the periodical Monsoon wind systems. The species diversity of Red Sea plankton is reduced relative to the Indian Ocean, but much higher than that of the Arabian Gulf. An indigenous assemblage of dinoflagellates, tintinnids, copepods and chaetognath species, however, appears to be well adapted to the conditions of this sea. Their wide distributions do not depend on the seasonal inflow. As a rule both the primary productivity and the zooplankton biomass are higher during the NE monsoon. The southern Red Sea is more productive than the northern with the two zones being separated by a low-productivity discontinuity zone at about 20° to 25°N, which is the zone of wind convergence in summer. More than 95% of the zooplankton biomass occurs in the upper 1000m and several endemic species have been reported. The Arabian Gulf is a vast, relatively shallow lagoon connected to the Gulf of Oman through the narrow strait of Hormuz. Circulation is anti-clockwise. Along the Iranian coast salinity rises from 36.6% near the strait to 40.6% in the NE Gulf and even higher along the Arabian coast. The Indian Ocean bathypelagic species as well as the dinoflagellate “Schattenarten” are completely missing from the Gulf. The species diversity is much poorer than that of either the Gulf of Oman or the Red Sea. An indigenous community, however, has developed which at first sight appears to be uniformly distributed especially along the Iranian coast, which is dominated by copepods and myodocopid ostracods. Some horizontal zonation is observable. Both nutrient and biomass concentrations are higher in mid-basin than in coastal waters. The Shatt-el-Arab to Kuwait area where estuarine components are not uncommon, and cladocerans replace ostracods in importance, is more productive and more diversified than the Trucial coast.

Interannual and long-term variability of Indian summer monsoon rainfall

Abstract: Analysis of summer monsoon (June to September) rainfall series of 29 subdivisions based on a fixed number of raingauges (306 stations) has been made for the 108-year period 1871–1978 for interannual and long-term variability of the rainfall. Statistical tests show that the rainfall series of 29 sub-divisions are homogeneous, Gaussian-distributed and do not contain any persistence. The highest and the lowest normal rainfall of 284 and 26 cm are observed over coastal Karnataka and west Rajasthan sub-divisions respectively. The interannual variability (range) varies over different sub-divisions, the lowest being 55 and the highest 231% of the normal rainfall, for south Assam and Saurashtra and Kutch sub-divisions respectively. High spatial coherency is observed between neighbouring sub-divisions; northeast region and northern west and peninsular Indian sub-divisions show oppositic correlation tendency. Significant change in mean rainfall of six sub-divisions is noticed. Correlogram and spectrum analysis show the presence of 14-year and QBO cycles in a few sub-divisional rainfall series.

Hydrobiological studies in the gradient zone of the Vellar Estuary: 2. Nutrients

Abstract: Water samples collected over a period of two years from the gradient zone of the Vellar Estuary, Tamil Nadu, India were analysed for PO4-P, NO2-N, NO3-N and SiO3-Si. Reactive phosphorus (PO4-P) showed wide fluctuations and inconsistent seasonal variations. Nitrite and nitrate values were high during monsoon and low during summer. Unlike other nutrients, silicate was high in the surface waters and showed significant negative relationship with salinity. Wide fluctuations in the N:P ratio was discernible

Observed air-sea interface conditions and a monsoon depression during MONEX-79

Abstract: The air-sea interface properties during the early stages of formation of a depression over East Central Arabian Sea during summer MONEX are examined by analyzing the ship data for air and sea temperatures, sea level pressures, sea state numbers and wind fields. Analysis of the data revealed a pronounced increase in sea-air temperature difference (2–4 °C) and this increase in the value is considered, obviously, to be due to a drop in the air temperature. The variations in the sea-air temperature difference and surface pressure are in opposite phase to each other. Some of the plausible mechanisms for the incipient development and movement of the depression are also discussed.

On the relationship between the sea surface temperatures in the equatorial Pacific and the Indian monsoon rainfall

Abstract: There has been a resurgence of studies relating to Sea Surface Temperature (SST) variations in the equatorial Pacific and variations in atmospheric flow patterns over tropical as well as over middle latitudes. In this note, the SST variations over the equatorial eastern Pacific and the variations in the Indian monsoon rainfall (June - September) are re-examined with particular reference to drought and flood years. Our analysis reveals that major drought (flood) years are associated with warmer (cooler) than normal SST values before and after the monsoon season. A possible connection with a feedback mechanism between the SST variation in the equatorial Pacific and the monsoon rainfall over India and vicinity is hypothesized.

The spatial and temporal relationships between the Indian summer monsoon rainfall and the Southern Oscillation.

Abstract: The correlations for the concurrent season (SOI-JJA) show a stable and highly significant relationship, whereas the corelations of the previous season (SOI-MAM) with summer monsoon rainfall are marginally significant for few areas of the country. The space and time relationships between the Southern Oscillation and Indian summer monsoon rainfall have potential applications for the understanding and prediction of the large-scale performance of the monsoon.

Study of the Dynamics of the Intertropical Convergence Zone with a Symmetric Version of the GLAS Climate Model

Abstract: The results of some calculations with a zonally symmetric version of the Goddard Laboratory of Atmospheric Sciences (GLAS) climate model are described. The model was first used to study the nature of symmetric circulation in response to various zonally-averaged latent heating fields based on observations. Three experiments with distribution of Intent beating corresponding to the equinox condition, Northern Hemisphere summer condition and south Asian monsoon condition showed reasonable similarity to the observed distribution of surface easterlies and westerlies and the subtropical westerly jets. In the south Asian monsoon experiment, surface westerlies as well as the upper-level easterly jet in the subtropics of the Northern Hemisphere were found. The strength of the subtropical westerly jet increased with decrease in the vertical eddy viscosity. Additional experiments were carried out in which the model was allowed to determine its own latent beat sources and the results were analyzed to examine ...

Divergent Circulations during the Onset of the 1978–79 Australian Monsoon

Abstract: Large scale numerical analyses of divergence and the divergent component of wind are examined at two levels in the lower and upper troposphere. The synoptic sequence studied includes the onset of the Southern Hemisphere summer monsoon. Comparison with satellite-observed cloudiness leads to the conclusion that the analyzed patterns of divergence contain synoptically realistic meteorological information. Them seems to be virtually no information, however, in the day-to-day changes in magnitude of analyzed divergence in the lower troposphere, and only a weak signal in the upper troposphere. The divergent wind analyses reveal the Intertropical Convergence Zone (ITCZ) to be a readily identifiable feature on individual days, and its location to he both vertically consistent and coincident with the satellite-observed cloud. Two days prior to monsoon onset the analyzed ITCZ moves poleward by 8° latitude. Monsoon convection exists at the intersection of Northern and Southern Hemisphere Hadley cells; it is...

Skeletal extension of Acropora formosa at a fringing reef in the Andaman Sea

Abstract: A growth study of Acropora formosa (Dana) was conducted in situ at a fringing reef in front of Phuket Marine Biological Center, the Andaman Sea, southern Thailand Monthly extensions of branches tagged with wire were measured in addition to sunshine, rainfall, settlement of sediment, turbidity, salinity, and temperature The average extension of coral branches was 8 cm in 344 days Growth was found to be approximately two times faster during the dry northeast monsoon compared with the wet southwest monsoon Factors which can cause the observed pattern of growth are discussed

Rainfall over the southern slopes of the Himalayas and the adjoining plains during ‘breaks’ in the monsoon

Abstract: During ‘breaks’ in the south-west (or summer) monsoon there is a general cessation of rainfall activity over most of the Indian area, but this activity increases over the southern slopes of the Himalayas and its adjoining plains. In this study, rainfall distribution during the ‘break’ monsoon situations has been studied, over the southern slopes of the Sikkim, Nepal and Uttar Pradesh Himalayas (i.e. from longitude 78°E to 89°E) and their adjoining plains. Rainfall distributions during the major ‘break’ situations which occurred during the principal monsoon months of July and August of the period from 1957 to 1969 were studied. This study has shown that, on a ‘break’ day, positive percentage departures of rainfall as high as 100 to 300 per cent have occurred at stations located in the outer Himalayas and the adjoining plains of north Bengal, east and central Nepal, Bihar and east Uttar Pradesh. However, the width of this heavy rain-belt is maximum over the hills and plains of south-eastern Nepal and the adjoining plains of north Bihar. This study has also shown that on a ‘break’ day there is a decrease of rainfall activity over the Himalayas west of longitude 80°E. Decrease of rainfall also takes place along a narrow belt immediately to the south of the great Himalayan range to the east of longitude 80cE outside Sikkim. Over the adjoining Gangetic plains, an increase in rainfall is noticed even to the south of the Ganga river in Bihar and its neighbourhood. Thus, owing to the increase in rainfall activity during ‘break’ monsoon situations in the catchments of Himalayan rivers located in Sikkim and Nepal, a peculiar situation arises which is responsible for causing floods in the downstream plains which experience almost drought conditions.

Short-Term Fluctuations in the Australian Monsoon during Winter Monex

Abstract: Satellite cloud imagery and single station rainfall data are used to define large-scale cloudy and clear phases of the Australian monsoon for the Winter (W) Monex period 20 December 1978 to 31 January 1979. To examine the flow characteristics and possible forcing mechanisms of these phases, large-wale, objective wind and mean sea level pressure analyses have been produced using W Monex IIB data. Composited Row configurations for each phase show marked differences in areal mean divergence and vertical motion over the monsoon region. The other main distinguishing features are the strength of the northeas trades, the strength of the southeast trades over the Indian Ocean, the amplitude of Southern Hemisphere midlatitude upper troughs and the location of the Northern Hemisphere jet maximum. No obvious differences in the wind field are evident over the deep tropics. For each mean phase, diagnosed divergent wind analyses indicate that the ITCZ is farther south and the Southern Hemisphere Hadley cell mo...

Circulation in the north Arabian Sea at Murray Ridge during S.W. monsoon

Abstract: Circulation in the Arabian Sea north of 20°N has been observed to contain warm and cold core eddies. These have been confirmed from various investigations during the International Indian Ocean Expedition (1962–1967) and subsequent oceanographic surveys. Satellite imageries using records of high resolution infrared radiometer have also delineated these eddies with more clarity. The eddy circulation appears to get intensified and in some areas persists in the SW monsoon (May–September). These months are dominated by upwelling along the Arabian coast and the cold water plumes and wedges extend eastward. Upwelling, comparatively weak, also appears along the Pakistan coast, west of Karachi. In the middle of these two upwelled cold water areas an anticyclonic eddy is found with warm core. Records generally show that this eddy circulation is repeated in the S.W. monsoon. Interaction of permanent seabed topographic features like the Murray Ridge and continental shelf on the eddy field, if any, has been studied. This eddy circulation favoured the vertical and horizontal mixing of nutrient rich water.

Summer cooling in the east central Arabian Sea - a process of dynamic response to the southwest monsoon

Abstract: The cooling of the east central Arabian Sea during summer monsoon season is examined using data sets-of MONEX '79 and MONSOON '77 programmes. These studies have revealed that downward transfer of heat due to the mixing of warm surface and cold sub-surface waters associated with deepening of the current shear zone is the predominant process for the observed cooling in the region. The rate of cooling is greater immediately after the onset of strong monsoon winds and the development of anti-cyc10nic wind stress curl, which give rise to a southerly current off the west coast of India. The downward flux of heat is found to vary in the study area with generally higher values in the western regions where the growth of surface layer is maximum. The cyclone heat potential, during pre-monsoon and monsoon conditions is also studied.

An analysis of trends in the rainfall and droughts occurring in the southwest monsoon and northeast monsoon systems in the southern Peninsular India

Abstract: A detailed climatic classification of Karnataka and Tamil Nadu States of southern Peninsular India which receive rainfall from both southwest and northeast monsoon has been made by using the data of all provincial raingauge stations in these States for the period 1901-75 and a comparison of the same is made with earlier classifications. A comparison of the rainfall normals (1901-1950) with the rainfall amounts recorded at different stations in these States during 1951-1975 indicates that decreases in annual rainfall have occurred in the latter period for many regions in Tamil Nadu which receive rainfall predominently during northeast monsoon, whereas increases have occurred in Malnad area, coastal Karnataka and north Interior Karnataka which receive rainfall mostly during southwest monsoon. Maps showing differences between rainfall during these periods have been presented. Trends of annual rainfall as well as rainfall during principal seasons, viz., southwest monsoon, northeast monsoon and pre-monsoon thunderstorm season of typical stations in these regions have been studied and results discussed. Use of auto-regressive models for study of trends is discussed.

Equatorial Velocity Profiles. Part II: Zonal Component

Abstract: Vertical profiles of horizontal velocity made along 53°E in the western Indian Ocean, during and after he onset of the southwest monsoon in 1976, show features in zonal velocity of relatively small vertical scale. Persistence of the features over the month-long observation period and over 2½ degrees of longitude indicates long temporal and zonal scales. The vertical structure is common to those profiles close to the equator, with no appreciable variation in amplitude or depth. Between 1°30′N and 3°N the phase of the features reverses. There is no evidence of similar features at 5°N. The data suggest meridionally trapped equatorial waves. Similarities between the observed phenomena and the linear theory of equatorial waves are striking but quantitative comparisons lead us to question its naive application here. Equatorial intensification is apparent but it does not scale with the Rossby radius of deformation, indicating that Kelvin waves are not dominant at any vertical scale. The phase change and...

A case study on the influence of summer monsoonal vortex on the thermal structure of upper central Arabian sea during the onset phase of MONEX-79

Abstract: The influence of the summer monsoonal vortex on the characteristics of the near-surface layer and of the thermal structure in the top 200-m water column at two stations in the central Arabian Sea (15°N, 65°N, and 9°N, 68°E) is documented with the aid of time-series measurements made during the onset phase of MONEX-79. The observed cooling in SST of 1.5°C is equally attributed to the heat exchange at the air-sea interface and advection caused by the tonsoonal onset. The mixed layer deph, initially 20 to 30 m, increased to 40 to 50 m as a consequence of wind and buoyancy mixing. An upwelling tendency, confined only to the upper thermocline with the onset of the monsoon, was of the order of 1 m day−1.