Showing papers on "Monsoon published in 1978"

Some Features of a Long Homogeneous Series of Indian Summer Monsoon Rainfall

Abstract: A long rainfall series for the contiguous Indian region for the summer monsoon season (June-September), when more than 75% of the annual rainfall occurs over large parts of the country, has been constructed by considering the rainfall data of a very large number of raingages since 1841 to present. The series from 1866–1970 has been found to be homogeneous. The statistical properties of this homogeneous time series have been investigated. The average monsoon rainfall of India is 88.75 cm with a standard deviation of 7.64 cm. Fisher's statistics g1, g2 and the chi-square statistic indicate that the that series is normally distributed. The years 1877, 1899, 1918, 1920, 1951 and 1965 were very bad monsoon years when the rainfall was below the 5th percentile of the distribution. The increase of 4.6% in the 30-year average from 1901–30 to 1931–60 is significant at the 5% level. The mean for the period 1931–60 is also significantly higher than the overall mean for the period 1866–1970 at the 5% level. D...

Influence of the Arabian Sea on the Indian summer monsoon

Abstract: Based on aerological observations over the Arabian Sea and adjoining north Indian Ocean, taken on board USSR research vessels during MONEX-1973, patterns of vergence of air and water vapour fluxes are studied. The influence of the Arabian Sea on the fluctuations in intensity of the summer monsoon over the west coast of India is shown. It is inferred that evaporation exceeds precipitation over the east Arabian Sea near the west coast of India even during active monsoon. A zonal cell with the ascending limb over the east Arabian Sea and the descending branch over the west Arabian Sea is proposed. DOI: 10.1111/j.2153-3490.1978.tb00825.x

On foreshadowing southwest monsoon rainfall over India with mid-tropospheric circulation anomaly of April

Abstract: In recent years there have been attempts to relate the distribution of rainfall of Indian southwest monsoon with antecedent upper air factors near the scene of activity for the purpose of long range forecasting" Several upper air factors arc critically examined in this context to make a judicious selection. It is found that the total monsoon rainfall over the country is significantly correlated with the latitudinal position of the sub-tropical ridge on the mean circulation chart of April at 500 mb level. A regression equation is developed with a sample data for a period of 21 years (1950-1970). On testing the regression equation for an earlier eleven-year period (1939-1949) and a later five-year period (1971-1975), it is observed that the average deviation between computed and actual southwest monsoon seasonal rainfall distribution turns out to be 6 per cent, which is quite encouraging. Years, when the sub-tropical ridge at 500 mb in April is poleward of the normal position are followed by good monsoon rainfall distribution while years when the ridge is equatorward of the normal position are followed by poor southwest monsoon rainfall. The study specially brings out that it may be possible with this technique to obtain an indication, in early May, about the probability of occurrence of a highly deficient southwest monsoon season. Such an advance indication of a probable extreme sub-normal rainfall year would be of vital economic significance in the execution of such weather dependent projects as water resources planning, power distribution adjustment, alternative food production programmes etc.

Geochemical Implications of Summer Monsoonal Rainwater Composition over India

Abstract: Rainwater was sampled from ten coastal and inland stations over India during the entire monsoon period of 1975. Major sea salt constituents were analytically determined. Sea salt deposition values based on the sodium content are presented for each station. It is estimated that during a monsoon period some 4 000 000 tons of sea salt is likely to be transported into the country. With the exception of calcium, the overall seasonal proportions of sea salt constituents in coastal rainwater indicate either minor or no differences as compared with the composition of seawater. A high excess of calcium has been found in all rainwater samples. Average excess at the coast is found to be 400–1000% over that expected on the basis of the sodium content. The calcium is uniformly distributed over the country with average concentration of 1.1 mg 1−1 for the present network of sampling stations. It is proposed that the excess calcium is predominantly of desert origin and is mostly from Arabia and/or Iran. Excesses...

Fluctuations of monsoon activity

Abstract: In order to study the evolution of flow patterns associated with 'strong' and 'break' monsoon we have studied six cases each of such epochs during the period 1965 to 1973. As we are interested in the departure of the flow patterns from the normal, we have subtracted long term climatological pentad means from the daily data and constructed anomaly wind, height and thickness charts at standard levels. The several contrasting features during the epochs as well as prior to the epochs. A few days prior to the beginning of the strong monsoon epoch (usually in association with a depression), an eastwest anomaly trough is seen extending from SE Asia to peninsular India in the lower and, middle tropospheres. The trough over peninsular India shifts northwards with the advance of the epoch. Westward movement of a cyclonic anomaly from across Southeast Asia IS also seen. In the upper troposphere a warm anomaly ridge isseen to the northwest of India. Prior to the beginning of breaks, however, an anomaly ridge is seen extending from peninsular India to Malaysia in the lower and middle tropospheres. This feature also shows some northward shift with the advance of the epoch. In the upper troposphere, an anomaly trough is seen to the northwest of India.

Large scale rainfall over India during the summer monsoon and its relation to the lower and upper tropospheric vorticity

Abstract: The daily variation of the lower and upper tropospheric wind field for the monsoon seasons of 1972 and 1973 is investigated, It is shown that the changes in the large-scale rainfall are directly related to those in the large-scale vorticity above the frictional boundary layer on a day-to-day basis throughout the two seasons, large cyclonic vorticity being associated with active spells and anticyclonic vorticity with weak monsoon spells. In the upper troposphere, the regional average of the anticyclonic vorticity is generally found to increase in active periods, decrease slightly in breaks and spectacularly towards the end of the break and increase again during revival. The maximum value of the kinetic energy of the easterlies during a season is found to occur during the break or weak monsoon spell for the seasons studied. The major difference in the upper tropospheric circulation of the two seasons is in the meridional component which is southerly over large regions over India in 1972 and northerly over the entire region on most days in the season of 1973. The contribution of the meridional advection of vorticity in determining the upper level divergence is found to be significant and its variations are found to be correlated to those of the lower-level convergence.

Some studies of the growth of monsoon disturbances

Abstract: The structure of the monsoon depression and the observed flow features prior to and at the time of monsoon depression formation (composite of 15 depressions) are examined. The composite monsoon depression (transient eddy) has a scale of 25° longitude and extends up to 300 mb and has the greast intensity at 700 mb. It shows north-north-east to south-south-west tilt in the lower levels indicating that it may draw upon zonal kinetic energy for its growth. The disturbance has lower temperatures to its west and tilts westwards with height indicating that eddy available potential energy is not converted from zonal available potential energy by large scale advection. There appears to be a reduction of vertical shear at the time of formation of monsoon depressions and this possibly aids cumulus convection.

Primary Productivity of the Bay of Bengal during August-September 1976

Abstract: Primary productivity was measured at 29 stations in the Bay of Bengal during the southwest monsoon of 1976 together with related parameters like phytoplankton counts, chlorophyll a, pheophytin, particulate organic carbon, inorganic phosphate and nitrate.

The southwest monsoon over India and its teleconnections with the middle and upper tropospheric flow patterns over the Southern Hemisphere

Abstract: The paper contains an analysis of the daily synoptic charts of July 1972 between 50° N and the South Pole and between 50° E and 160° E with reference to the activity of the southwest monsoon (summer monsoon) over India. The month commenced with a spell of normally active monsoon for 8 days and ended with a spell of large-scale break in the monsoon for 9 days with two transitional phases in between. There was nearly simultaneous development in the middle-latitude westerlies in the middle and upper troposphere in both the hemispheres especially during the break period. Blocking ridges over and near the Tasman Sea extended far towards the southsoutheast during the break conditions. The pronounced troughs and ridges observed in the meridional profiles along 35° N and 35° S, especially along the latter during break conditions, have been discussed and certain tentative explanations have been put forward regarding the developments in the Southern Hemisphere. DOI: 10.1111/j.2153-3490.1978.tb00826.x

Input by Indian rivers into the world oceans

Abstract: The chemical, sediment and total load carried by the major river basins in India—Ganges, Brahmaputra, Indus (Jhelum), Godavari, Krishna, Narmada, Tapti, Mahnadi and Cauvery have been calculated, based partly on new set of data and partly on existing data. There is a significant amount of chemical load transported by all the Indian rivers, and for global mass transfer calculation, these cannot and should not be ignored. The chemical mass transfer during the monsoon is not surprisingly small, as would be expected for excess discharge and dilution controlled run-off. The sediment mass transfer from non-Himalyan rivers, all within the same range of magnitude, accounts for less than a tenth of that of the Ganges but during the monsoon, except for Cauvery, all the Indian rivers carry a sediment load of greater than 1000 ppm. The total mass transfer from the Indian subcontinent accounts for 6·5 per cent of the global transfer.

Regional Energetics of the 200 mb Summer Circulations

Abstract: Some of the characteristic features of the summer monsoon circulation are investigated using 200 mb wind data operationally determined by the National Meteorological Center during three summers (1970–72). Three-year summer mean 200 mb divergence was large near India, Japan and the Philippines, three of the most actively convective regions during the summer monsoon. A pronounced 200 mb anticyclone lay centered over north India, while the 200 mb velocity potential outflow center was located near the Philippines with a prominent north–south overturning, vertical circulation along 125°E. The intensity and location of the 200 mb anticyclone changed substantially from one monsoon season to the other. Little change was observed in the location of the primary 200 mb velocity potential outflow center. The energy equation was applied to specific limited regions to investigate the effect of boundary fluxes and barotropic processes upon area-averaged eddy kinetic energy during summer. Area-averaged 200 mb zo...

Sub-tropical westerlies in relation to large scale failure of Indian monsoon

Abstract: In the years of large scale failure of southwest monsoon rainfall over India during the decade 1965 to 1974, the mean meridional circulation over India during July and August is of the type occurring in association with 'break monsoon' situations, i.e.. meridional northward motion in the upper troposphere, north of-latitude about 200 N. During such years subtropical westerlies of the upper troposphere seem to protrude more southward into the areas immediately west of India during the monsoon season; this feature persisting on many occasions, right from the preceeding winter, which may explain the preferred northward tracks of the November cyclones of the Bay of Bengal, the occurrence of low 500 mb altitudes over northwest Indian stations during winter and pre-monsoon seasons and the observed greater frequency, and duration of 'break monsoon' spells associated with the large scale monsoon failures as earlier reported by Joseph (1975, 1976).

Interaction Between the West Arabian Sea and the Indian Monsoon

Abstract: Examination of the July 1964 sea surface temperatures in the west Arabian Sea shows that during the weak monsoon over India the sea surface experienced a significant drop in temperature over a larger area compared to a period of strong monsoon. Associated with weak and strong monsoon over India are large-scale pressure changes which occur almost over the entire monsoon region and these changes are not considered to be directly related to the sea surface temperature. The pressure change over India appears to have a controlling influence on the strength of the cross-equatorial flow in the west Arabian Sea.

A numerical index to monitor the Afro-Asian monsoon during the northern summers

Abstract: The identification of the core of the major low-level air current of the Afro-Asian summer monsoon at a topographically-fixed position over eastern Africa has allowed an attempt to be made to monitor the low-level flow and relate it to rainfall downstream over western India. An index of the southerly flow over eastern Africa at a station a few degrees south of the equator for the month of July for a period of 24 years, is compared with the July rainfall of ten stations in the western part of Maharashtra Province of India. It is found that months of high or low wind index correspond well with months of high or low rainfall respectively, especially when two-year overlapping averages are used. An interesting feature of the analysis is that there is a one-year lag between maxima and minima of the wind index over eastern Africa and the corresponding features of the rainfall of western India. A tentative calculation is made to illustrate how the lag might be used for long-range rainfall prediction.

Quasi-biennial oscillation and summer southwest monsoon

Abstract: Stratospheric circulation in the height range of 16 to 60 km in the equatorial region, has been studied using the rocket wind data of Thumba. It is inferred that the quasi-biennial oscillation in the lower and middle stratosphere are out of, phase by six months. The dominant feature in the lower stratosphere is the appearance of the monsoon easterlies during the period June to September. The westerlies or easterlies in the lower stratosphere over the equator extend to about 10°N in the period November to May. In the middle stratosphere, the quasi-biennial oscillation follows a regular alternation of easterlies and westerlies with a downward phase propogation of one km per month, while the quasi-biennial oscillation in the lower stratosphere interacts with the monsoon circulation. The four possible wind phases W, W IE, E and EIW during March to May in the equatorial lower stratosphere are linked with the onset of southwest monsoon near the normal date and normal rains over India. Appearance of the easterlies in the lower stratosphere over Gan Island in May is an indication of the onset of monsoon over Kerala a month later. The Berson westerlies are a poleward extension of the equatorial westerlies, rather than a descent from the upper stratosphere. The out of phase relationship between QBO in the lower and middle, stratospheres suggests an interaction between the QBO and monsoon circulation. The phase of the QBO, over the equator appears to give a prior indication of the onset of the southwest monsoon over Kerala and also the rainfall in the central parts of the country.

Synoptic Climatology of the Daily 700 mb Summer Monsoon Flow Patterns over India

Abstract: The daily (mean of 0000 and 1200 GMT) 700 mb contour patterns over India are classified in five broad types for each summer monsoon month by using a chart-to-chart correlation method. Certain characteristics of these patterns, such as mutual transitions, persistence, preferred periods of occurrence and interrelationships are studied. Statistical probabilities of two threshold 24 h rainfall amounts (2.5 and 10 mm) being equated or exceeded for each type are computed for 107 stations, more or less uniformly distributed over India. This knowledge of the spatial distribution of precipitation probabilities associated with various circulation types can be used in forecasting probabilities of precipitation over the country if the circulation patterns can he forecast by numerical methods. These probabilities are then compared to the climatological and conditional probabilities of obtaining threshold rainfall amounts on different days of the subsequent 5-day period—given that the threshold rainfall occurr...

Wind shear in a monsoon depression

Abstract: DEPRESSIONS of the Bay of Bengal are an important component of the south-west monsoon circulation. Moving WNW, they account for much of the rainfall in India. Synoptic conditions favouring formation of monsoon depressions, so far described, are: (1) trailing of the axis of the monsoon trough into the head Bay of Bengal; (2) movement of a low pressure system from the east across Burma; and (3) the building downwards of a midtropospheric cyclonic circulation1. Studying the structure of monsoon depressions in their later stages of evolution, using principally upper air data of land stations, earlier workers2 have invoked upper divergence as a crucial factor for development. Gray3 questions the role of upper divergence in tropical cyclogenesis and emphasises that small vertical wind shear is a primary factor in storm development. The mechanism of initial formation over the Bay, however, has remained unexplored for want of adequate data. To gain further insight, a close network of ships and coastal and island stations was organised in the joint Indo–USSR Monsoon-77 Experiment (Fig. 1). Four Soviet research ships in polygon formation recorded for the first time, upperwind observations over the Bay of Bengal. The formation of a depression in the north Bay just to the east of the ships on 19 August and its passage across the polygon the next day was monitored. Sharp decrease in tropospheric vertical wind shear was identified along the coast to start with and subsequently over the north-east of the polygon, before formation of the depression. Marked anticyclonic vorticity first appeared in the upper troposphere and was then totally replaced by cyclonic vorticity of comparable magnitude. This cyclonic vorticity gradually descended to the lower troposphere. The above features, hitherto undocumented, are described here.

Vortices embedded in 'the trough of low pressure off Maharashtra- Goa coasts -during the month of July

Abstract: Two troughs of low pressure are generally noticed over Indian region during the monsoon. One is the monsoon trough Over Gangetic plain persisting throughout the monsoon season. The other is of comparatively less pronounced trough persisting off the west coast of India, being most prominent during the month of July. Vortices embedded in these troughs cause considerable amount of rainfall. While the vortices and low pressure areas associated with the monsoon trough passing over Gangetic plain have been studied extensively, those embedded in the trough off the west coast do not appear to have received enough attention. In this paper the study of latter vortices during the month of July has been conducted. The following conclusions have been arrived at. These vortices are most prominent in the month of July. They have a tendency to form in series in first half of the month of July, (b) They form just south of Goa, (c) They move generally northwards with an approximate speed of 100 km per day. During second half of the month of July some of them are found to move northwestwards, (d) Heavy rainfall associated with them has the peculiarity that coastal stations get more rain than the Ghats, and (e) There may be an association between the formation of the series of vortices, during first half of July and the existence of an upper air cyclonic circulation between 2.1 and 3.6 km above sea level over Gujarat region and neighbourhood.

Dynamics of orographic rainfall

Abstract: A two-dimensional dyn1mical model for orographic rainfall was developed earlier (Sarker 1966, 1967) with particular reference to the Western Ghats. The model assumes a saturated atmosphere with neutral static stability. In the present paper this model has been applied for some more cases in Western Ghats and also for some cases in Khasi and Jaintia hills. With the model the terrain induced vertical velocities have been computed and the same have been used to compute rainfall. The computed rainfall distribution agrees well with the observed rainfall distribution both for Western Ghats and Khasi and Jaintia hills during the active monsoon situations. The peak in rainfall distribution is seen to be purely an orographic effect. Some modifications have been made in the model for its application during the weak monsoon situations. The results for the various c1ses are discussed vis-a-vis the different synoptic situations.

Temporal variations in the northern hemispheric summer circulations

Abstract: Changes in kinetic energy averaged over the tropical (5.0°S-19.6°N), subtropical (24.2°N-37.1 oN) and mid-latitude (41.0°N-48.1°N) belts were examined for the 92-day period from June through August 1970. Some-what systematic fluctuations (10-15 day period) were observed in the eddy and zonal mean kinetic energies of the 200 mb tropical belt. Mid-latitude periodicity was longer at about 20 days. Short period (~5 day) fluctuations were considerably stronger at 700 mb than that at 200 mb, over the tropical belt. Abnormal transitions were observed to begin around 12 July with a simultaneous decrease in 200 mb eddy kinetic energy at all latitudes, 700 mb eddy kinetic energy over the tropical belt, and tropical baroclinic activity as determined from cloudiness; all reached minimum values around 20-24 July. On the other hand, tropical zonal mean kinetic energy increased to maximum values in the same period. During 20-24 July, the tropospheric circulation displayed singularly different characteristics compared to normal, and break monsoon" conditions prevailed over India. The zones of cloudiness and upper divergence maxima over the monsoon region shifted northwards to the Tibetan area, while dry weather and abnormal upper convergent wind inflow was experienced over southeast Asia. A conspicuous and anomalous excitation of convective activity over 'tropical regions in the western North and South Pacific was associated with pronounced upper (lower) tropospheric anticyclonic (cyclonic) circulations. Consequently, 200 mb equatorial easterlies over the western and central Pacific were unusually strong. Concurrently the anticyclone over the Mexican region reached maximum intensity and was displaced to the north of its normal position

Computation of the average precipitation over the western part of peninsular India during the summer monsoon from the continuity equation for atmospheric water vapour

Abstract: Water vapour fluxes computed across different walls of the triangular volume of peninsular India, bounded by Trivandrum, Bombay and Nagpur, were used to compute the net flux convergence on a monthly mean basis for the months June through September for the years 1967–72. The precipitation rates over the region were computed by using the flux convergence values and the equation of continuity for water vapour and were compared with the actual rainfall. The agreement between the computed precipitation and actual rainfall was found to be fairly close. DOI: 10.1111/j.2153-3490.1978.tb00870.x

Monitoring of semi-permanent troughs and ridges in relation to monsoon

Abstract: Two contrasting years (1967 and 1972) have been selected for intensive study in respect of large scale features of northern hemispheric contour field. In the present study, 700 and 300 mb mean monthly contours at different latitude circles at 5 degree interval, were subjected to Fourier analysis and horizontal and vertical tilts of troughs and ridges determined. At and north of 20°N, under quasi-geostrophic assumption, computations have been made to calculate meridional transports of momentum by tilted troughs and ridge at 700 and 300 mb levels and meridional transport of sensible heat at 500 mb level. The following chief results have been obtained. (i) The differences in momentum transport during the two contrasting years were far more marked at 300 mb level than at 700 mb level. (ii) In a year of good monsoon (1967), there was an anomaly of northward transport of momentum across sub-tropical latitudes while in a year of poor monsoon (1972), there was anomaly of southward transport of momentum across the subtropical latitudes, at 300 mb level. (iii) During a year of good monsoon (1967), there was anomaly of southward transport of sensible heat across the sub-tropical latitude of 30°N at 500 mb level, while in a year of poor monsoon (1972), the anomaly was in the form of northward transport. (iv) The anomalies in the transports of momentum and sensible heat during 1967 and 1972 were perceptible about a month before the onset of monsoon over India. If this be true also for other years which have not been studied here, then there is a possibility of foreshadowing large-scale behaviour of the Indian monsoon one month or so ahead of the arrival of the monsoon. (v) In terms of the anomalies in meridional transports or momentum and sensible heat across the sub-tropical latitudes, we present a model of abnormal monsoon in India. If the summer tropical atmosphere is warmer (cooler) than normal, the monsoon rainfall over India is likely to be more (less) than normal. This is shown to be in broad agreement with what is presently known about the theory of monsoon and observations of short-period anomalies in the behaviour of the Indian monsoon.

Incursion of the Pacific Ocean Water into the Indian Ocean

Abstract: Using the data collected during the International Indian Ocean Expedition, maps showing the distribution of depth, acceleration potential, salinity and oxyty were prepared for the northeast monsoon for the four potential thermosteric anomaly surfaces: 160, 120, 80 and 60 cl/t. Zonal components of current aong 84°E were computed from the geopotential dynamic heights. From such an analysis, it became clear that low-salinity water from the Pacific intrudes into the western Indian Ocean through the Banda and Timor seas in the upper layers above 100 cl/t surface, while the North Indian Ocean Water penetrates towards the Eastern Archipelago below 100 cl/t surface. The South Equatorial Countercurrent and the Tropical Countercurrent are well depicted on the vertical section of zonal components as well as on the distribution of acceleration potential.

Indian summer monsoon cyclogenesis and its variability

Abstract: MONSOON disturbances forming over the north Bay of Bengal and moving towards the WNW across northern India are the main rain producers of the Indian summer monsoon. There are some (monsoon) seasons when these disturbances fail to develop in adequate number and some that do form move towards the northwest or north—these are periods of drought. Understanding how monsoon disturbances develop and why they sometimes do not is an interesting scientific and an important economic problem. Studies so far on monsoon cyclogenesis1,2 have shown that the monsoon atmosphere is not baroclinically unstable. Shukla1 obtained barotropically unstable upper tropospheric modes and emphasised the role of the conditional instability of the second kind (CISK) but failed to get a preferred scale of maximum growth for lower tropospheric disturbances. R.N.K. et al.2 found slowly growing barotropically unstable modes in the lower troposphere corresponding to monsoon disturbances. Mak3 studied the baroclinic instability of monsoon zonal flow with a superposed meridional component, but the meridional winds required for instability were large compared to observed winds. Lorenz4 studied the barotropic instability of Rossby waves and found that shorter waves can be barotropically unstable. V.S. et al.5 showed that the superimposition of a long stationary wave, of the scale of wave number two, on monsoon zonal flow yielded fast growing upper tropospheric modes and lower tropospheric modes of scale 2,500 km, a doubling time of 5 days and slow westwards movement. These correspond well to the observed monsoon disturbances. We report here a combined barotropic–baroclinic stability analysis of monsoon zonal flow with a superimposed long stationary wave (global monsoon) using a two-level quasi-geostrophic model. We show that the phase of the global monsoon wave is a factor in deciding whether a particular year is good for monsoons or for drought.

Spatial and temporal variation of the wind stress off the Somali Coast

Abstract: Wind observations from ships along the sea-lane off the Somali coast are used for estimating the seasonal increase in wind stress during the southwest monsoon. The stress has a relatively large maximum (∼4–6 dyn cm−2) in the region between 8°N and 12°N during June, July, and August. Maximum values of the zonal wind stress curl occur about 6°N–8°N. During this monsoon period a large-scale eddy (∼600 km in diameter) or eddies occur in the Somali basin. Wind stress averages (1922–1972) for the north-western Indian Ocean are also given. Yearly southwest monsoon averages for the region subjected to the strong monsoon winds show that stress components may vary from year to year by nearly a factor of 2.

Vertical structure of the planetary boundary layer in the west Indian Ocean during the Indian summer monsoon as revealed by ISMEX data

Abstract: During the Indo-Soviet Monsoon Experiment (ISMEX) .973, closely spaced aerological measurements were obtained in the west Indian Ocean and the Arabian Sea. Based on these observations a study of the thermodynamical and kinematic structure of the atmospheric planetary boundary layer in this region is made. Under undisturbed weather conditions the planetary boundary layer is mainly divided into two sub-layers-the neutral stratified mixed layer and the cloud layer. In the region north of 10°N latitude over the Arabian Sea, a strong temperature inversion tops the boundary layer and shows a downward slope from the coast of India towards the west, thus reducing the planetary boundary layer in the western region to a mixed layer and a temperature inversion layer. In the extreme eastern region, strong convective mixing weakens the layered structure and leads to the destruction of the inversion. A gradual increase of wind speed with height is observed in the sub-cloud layer in this region. A sharp maximum frequently occurs below 1200 m in the wind profiles in the region north of 7°N latitude over the Arabian Sea. The wind speed maximum occurs usually within the inversion layer in the western Arabian Sea and within the cloud layer in the central Arabian Sea, and is often associated with large values of wind shear at low levels.

Low‐level air flow circulation over the Arabian Sea during the summer monsoon as deduced from satellite‐tracked superpressure balloons. Part II‐Analysis of the flow field

Abstract: Trajectories of superpressure balloon flights in the tropical boundary layer over the Indian Ocean during the 1975 summer monsoon are analysed in relation to conventional meteorological observations, mainly ship reports. An assimilation of these randomly distributed observations is performed to compute mean gridded fields of wind and pressure during four periods defined according to monsoon activity over the Indian sub-continent. Mean cloud cover fields over a 2.5° × 2.5° square are also determined from satellite images. The main features of the low-level monsoon circulation are deduced from the two sets of data; these are in conflict with earlier conclusions deduced from the interpretation of local conditions. The differences in meteorological fields between the various phases of the activity of the monsoon over India are studied and discussed.

Trends and periodicities in annual rainfall in monsoon areas over the northern hemisphere

Abstract: Trends and periodicities in annual rainfalls in monsoon areas over the northern hemisphere have been examined. Stations selected for Southeast Asia are Colaba (Bombay), Trivandrum and Colombo; for West Africa they are Accra, Warri, Tower Hill (Freetown) and Dakar; and for Central America, Quito and Colon*. The data were mostly collected from World Weather Records. For finding out long term trends a 25-point ultra low pass sine terminated filter was used to cut off all periods below 12 years. In Southeast Asia, Colaba showed a rise between 1910 and 1955 and then a fall. Trivandrum does not show any significant variation whereas the trend at Colombo is oscillatory. In West Africa amongst the places to the south of Sahara, Accra and Dakar do not show any trend, a general decrease is noticed at Tower Hill and Warri shows an increase. In Central America Colon shows an oscillatory trends whereas Quito does not show any significant variation. There does not appear to be current decrease in the rainfall over African monsoon countries and over India as has been reported by Winstanley (1973). As regards periodicities, analysis were confied to only 11-year and 22-year cycles. Besides being of physical significance these two cycles have long enough period to cause imbalance in drought prone areas. These periods seem to be present everywhere but they do not show and systematic correlations with sunspot numbers. The amplitudes of the oscillation do not exhibit any systematic dependence either on latitude or on longitude. Large differences were noticed even at neighbouring places. Rainfall in these areas because of its strong dependence on local geography, is modulated strongly by other factors to show a poor correlation with sunspot number.

Aspects of aridity and drought in the monsoon climate of Sri Lanka

Abstract: Typical of the monsoonal rainfall conditions over Sri Lanka are the remarkable inter-annual and seasonal deficits of water, varying considerably in temporal and spatial aspects. Therefore, both the phenomena of aridity and drought are of great significance from the point of the general monsoon climate of Sri Lanka. This paper investigates the annual and monthly aridity by using the 'aridity index' of De Martonne Lauer (1926/1952) showing the increasing extent of aridity from the S. W. quarter to the N.W./N as well as S.E. of Sri Lanka. On the other hand, drought has been studied by the 'drought index' of Gibbs and Maher (1967) underlining a considerable interannual rainfall variability, being expressed by the incidence of heavily drought-stricken years as well as by the occurrence of completety drought-free years. It is pointed out that drought can be called a typical, however irregular and spatially limited phenomenon in the climate of Sri Lanka. All observations are based on 30-year rainfall data (1931-1960) of monthly and annual figures.