Showing papers on "Monsoon published in 1981"

Monsoon Climate of the Early Holocene: Climate Experiment with the Earth's Orbital Parameters for 9000 Years Ago

Abstract: Values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for July 9000 years ago was 7 percent greater than at present. When the estimated solar radiation values are used in a low-resulation climate model, the model simulates an intensified continent-scale monsoon circulation. This result agrees with paleoclimatic evidence from Africa, Arabia, and India that monsoon rains were stronger between 10,000 and 5000 years ago than they are today.

Monsoon dynamics: Predictability of monsoons

Abstract: It is shown by numerical simulation that the variability of average pressure and rainfall for July due to short-period flow instabilities occurring in the absence of boundary anomalies can account for most of the observed variability at midlatitudes but not at low latitudes. On the basis of the available evidence it is suggested that a large part of the low-latitude variability is due to boundary anomalies in such quantities as sea-surface temperature, albedo and soil moisture, which, having longer time constants, are more predictable than the flow instabilities. Additional variability due to long-period natural fluctuations would likewise be more predictable. The degree of predictability of monsoons is a matter of considerable social and economic importance. Large agrarian populations exist in monsoon areas, and monsoon rains have a critical influence on food production and human welfare. The long-range prediction of average rainfall could be of immense value for water management and agricultural planning. In this chapter, evidence that mean flow conditions and precipitation patterns at low latitudes are in principle more predictable than those at high latitudes is presented. Among the low-latitude circulations are the African and Asian monsoons and perhaps also the southeasterly monsoon flow east of the North American Cordillera. In particular we wish to show that the natural flow instabilities on synoptic scales account for most of the interannual variability of monthly mean quantities at midlatitudes, but cannot explain the observed variability at low latitudes.

Winter Monsoon Convection in the Vicinity of North Borneo. Part I: Structure and Time Variation of the Clouds and Precipitation

Abstract: Radar and satellite observations in the vicinity of northern Borneo obtained during the International Winter Monsoon Experiment (WMONEX) showed that the convection in that region underwent an extremely regular diurnal cycle. Over the sea to the north of Borneo, the general level of convective activity was increased during monsoon surges and during the passages of westward-propagating near-equatorial disturbances. Convective activity was decreased during monsoon lulls. The diurnal cycle was well-defined, regardless of whether, the general level of convective activity was enhanced or suppressed by synoptic-scale events. The cycle of convection over the sea was especially well documented. It was typically initiated at about midnight when an offshore low-level wind began. Where this wind met the monsoonal northeasterly flow, usually just off the coast, convective cells formed. After midnight, the convection continued to develop and by 0800 LST it had evolved into an organized mesoscale system with a ...

On the Onset Vortex of the Summer Monsoon

Abstract: In this paper we examine the evolution of the low-level flow over the Arabian Sea during the onset of the summer monsoon. A detailed examination of the onset vortex that forms over the Arabian Sea just prior to the commencement of heavy rains over central India is carried out. The unique aspect of this study is the use of data sets from the Global Atmospheric Research Program (GARP) Monsoon Experiment (MONEX) from a variety of observing platforms. These include winds from geostationary satellites, constant level balloons, dropwindsonde aircraft and an enhanced World Weather Watch network. The data sets were analyzed for a 46-day period from 16 May through 30 June 1979. A number of calculations were performed with this analysis. Of major interest is a finding that the kinetic energy of the zonal flow over the central Arabian Sea increases by an order of magnitude one week prior to the commencement of monsoon rain over central India. This study provides a MONEX time-averaged analysis for the low-le...

Environmental characteristics of the Mandovi-Zuari estuarine system in Goa

Abstract: Two rivers, the Mandovi and the Zuari, with their interconnecting canal, form an estuarine system in Goa on the west coast of India. Physical, chemical and biological features of this estuary are adapted to a seasonal rhythm induced by the annual cycle of the monsoon. Heavy precipitation and land runoff from June to September bring about large changes in temperature, salinity, flow pattern, dissolved oxygen and nutrients when the estuary becomes freshwater dominated. The monsoon season (July–September) is followed by a recovery period during the post-monsoon season (October–January) and thereafter a stable period of the pre-monsoon season (February–May) when the estuary becomes marine dominated. During the pre-monsoon (dry) season, the water in the estuarine system remains well mixed and the intrusion of salt water is felt as far as 65 km upstream in both the rivers; but during the monsoon season the rivers become stratified and a salt wedge is formed in each river which extends up to about 10 km upstream in the Mandovi and 12 km in the Zuari. The flow of the estuarine system is regulated by the entry of seawater with the incoming tide through Zuari which reaches Mandovi through the canal. The flow is reversed during the outgoing tide when the estuarine system is flushed. Dilution factors in both the estuaries are similar and vary from 1·2 to 8; highest values occur during the pre-monsoon season. Two shoals/sand bars occur permanently in Mandovi (Aguada Bay) close to a ramp-like inlet to the sea. This inlet poses no navigational problems for about 9 months during the dry season; but for a 3-month period during the monsoon, the waterway becomes hazardous and is closed to boat traffic. Heavy swell and intense wave activity lead to the transfer of sediments into the navigational inlet and the calm season brings the materials back to their original position with practically no overall change in the bathymetry of the bay. The oxygen cycle in the estuarine system is closely related to seasonal changes in temperature and bears an inverse relationship with salinity. In both the estuaries, the sulphate/chlorinity relationship remains uniform and similar to that of the sea except during the monsoon months when the relationship gets disturbed. Changes in phosphorus, nitrogen and silicon are largely regulated by rainfall and land runoff. There is no significant difference in the phytoplankton counts between Mandovi and Zuari which follow a rhythm similar to that of nitrate. Zooplankton biomass is higher in Zuari because of its greater marine influence. No seasonal variation was observed in the density of microflora in the two rivers. Bacterial counts were higher in the lower reaches of the estuarine system and decreased upstream. From its environmental features, the estuarine system can be classified as a tide-dominated coastal plain estuary.

Westward Propagating Predecessors of Monsoon Depressions

Abstract: Sea-level pressure data for the period 1969-1978 are used to investigate the relation between Bay of Bengal lows and depressions and disturbances propagating from the east. Of the 52 lows and depressions studied, 45 were associated with such predecessor disturbances. In 12 cases, the predecessor was associated with a typhoon or named tropical storm in the South China Sea, while the remaining 33 were weaker systems originating over a broad region of land and sea. From examination of time sections over the same period from eastern Thailand to the Burmese coast, 50 westward-moving disturbances with considerable vertical extent were identified, 64% of which developed into lows or depressions on reaching the Bay. In 60% of the 50 instances, the disturbance could be traced to the South China Sea (32% typhoons and 28% weaker circulations). The remaining 40% of the 50 disturbances appeared to originate over land.

Blocking highs over Asia and monsoon droughts over India.

Abstract: Severe summer droughts of the Indian subcontinent have been found to accompany prolonged ‘breaks’ in the south-west monsoon. Data from the years of severe drought suggest that the associated breaks were due to upper tropospheric blocking ridges over East Asia. We present here a model for the evolution of two such blocking ridges: the East Asia (EABR) and the West Asia (WABR). Apparently an intense WABR—the initiator of the monsoon break—precedes the formation of the EABR. This cycle could be a way of predicting years of intense drought or monsoon activity.

Water vapour transport over the Indian Ocean during summer 1975

Abstract: Water vapour transport over the Indian Ocean during the 1975 Indian summer monsoon is investigated. The main data set used in this study is constituted of twice-daily surface assimilated meteorological fields determined on a 2.5 degree long grid from an objective assimilation of surface data over the Indian ocean. Upper air data are also used. Daily evolution of surface water vapour fluxes across different sections are presented. In the Arabian Sea, the onset of the monsoon is characterized by a large increase of the surface water vapour fluxes. The main feature of the fluxes is the existence of an oscillation with a fundamental period of 10 ± 2 days. The analysis of correlation coefficients between fluxes across sections in the Arabian Sea suggests that periodic variations of water vapour fluxes result from downstream forcing. Mean monthly water vapour budgets have been estimated over the Arabian Sea and the Bay of Bengal. The results show that about 70% of the water vapour that crosses the western coast of India comes from the Southern Hemisphere whereas 30% is picked up by evaporation over the Arabian ocean. In the Bay of Bengal, water vapour picked up by evaporation over the ocean seems to be largely used for local weather development. DOI: 10.1111/j.2153-3490.1981.tb01773.x

Influence of the Tibetian Plateau on cumulative and diurnal changes of weather and climate in summer

Abstract: An investigation of the dynamic and thermodynamic influences of the Tibetian Plateau on the monsoon circulation in southeastern Asia and China is reported. A five-day primitive equation numerical model was used to simulate the influences of the high mountains in the 0-180 deg E and 25-55 deg N region on a typical July day. The equations predicted horizontal motion, heat, the water vapor mixing ratio, and surface pressure. Consideration was given to solar and longwave radiation and the release of latent heat by deep cumulus clouds. A net warming over the Plateau as opposed to the surrounding areas was found to lead to vertical air ascent and the establishment of strong summer monsoon circulation to the south. It is noted that the grid was too coarse to adequately describe the distribution of precipitation on the steep mountains.

Winter Monsoon Convection in the Vicinity of North Borneo. Part II: Effects on Large-Scale Fields

Abstract: An observational study of the thermodynamic and kinematic structure and properties of monsoon convective systems and their large-scale environment over the southern South China Sea during the field phase (December 1978) of the Winter Monsoon Experiment (WMONEX) has been carried out. The primary observations used are from three Soviet research vessels positioned in a triangular array off the north coast of Borneo during the period 6–28 December. Computations of thermodynamic fields, divergence and vertical motion have been made for the duration of the field phase based on six-hourly rawinsonde releases at the ship sites. Analysis of the data indicates that the degree of convective activity over the southern South China Sea is modulated by long-period synoptic forcing (monsoon surges, easterly waves) and also significantly by diurnal forcing (land-sea breeze circulations). A diurnal cycle of convection persists in the region whether the synoptic-scale forcing is weak or strong. Convection over wate...

Propagation of thermal fronts in the somali current system

Abstract: Surface ship-of-opportunity and satellite remotely sensed sea-surface temperature (SST) data were obtained during the mature phase of the Southwest Monsoon in the north-western Indian Ocean for the years 1976–1979. Large wedge-like areas of upwelled water are observed at 5°N and 10°N after Somali Current spinup, indicative of a two-gyre circulation in the Somali Current system. Several months later the southern separation region is observed to translate poleward (5–75 cm/s) in three of the four years. This translation occurs in several distinct phases over a one-month interval with the southern wedge eventually coalescing with the northern wedge between 8 and 10°N (1976, 1979). (In 1978 the northern wedge translated past Socotra and dissipated in the Arabian Sea). The thermal signature of the “new” northern wedge becomes indistinct over a period of two to three months later.

Monsoon dynamics: The effect of elevation on monsoon rainfall distribution in the central Himalayas

Abstract: In this study, an attempt is made to ascertain up to what elevation in the central Himalayas rainfall increases with height. In the central Himalayas there are about 50 rainfall stations which were installed from 1948 onwards in connection with the Kosi dam project and other flood control projects. The mean monsoon (June to October) rainfall data of these stations have been utilized in this study with a view to obtaining a suitable relationship between rainfall and elevation in this section of the Himalayas. This study has shown: (i) that there exist no linear relationships between elevation and monsoon rainfall; (ii) that the elevation and rainfall parameters can best be related by a polynomial of fourth degree; and (iii) rainfall-elevation profiles show that the zones of maximum rainfall occur near the foothills and at an elevation of 2.0 to 2.4 km. Beyond this elevation, rainfall decreases continuously as elevation increases until the great Himalayan range is reached. Introduction Hill (1881) made a detailed study of the distribution of rainfall in the northwest Himalayas and found that rainfall increases with elevation up to a height of about 1.2 km and thereafter it diminishes as the elevation increases. In the Sierra Nevada mountains in the USA, the rainfall increases up to a height of 1.5 km (Linsley et al . 1949). Rumley (1965) investigated the distribution of rainfall with elevation in the Andes mountains in Ecuador and found two zones of maximum rainfall along the western and eastern slopes at elevations of 1.0 and 1.4 km respectively.

Detection of monsoon inversion by TIROS-N satellite

Abstract: Colon1 and Ramage2 have investigated the thermal stratification of the summer monsoon air and presented evidence of a well-defined temperature inversion in the lower atmosphere over the Arabian Sea. This inversion is low (base between 900 and 800 mbar) and strong over the western Arabian Sea and weakens and rises (base at ∼700 mbar) towards the coast of India and is not observed east of 70 °E, especially during the active monsoon3. The presence of dry warm continental air from Africa and Arabia above the maritime air is thought to be associated with this inversion. This inversion is very important to the rain producing potential of the monsoon current because once the inversion is destroyed there is a favourable stratification for rapid release of moisture upwards leading to precipitation. Observations of the western Arabian Sea inversion features have previously been reported only from in situ ship radiosonde and aircraft dropsonde measurements. Although the basic accuracy and the vertical resolution of the present-day satellite sensors cannot delineate the small-scale variations of temperature4 such as monsoon inversions we have detected these features from just the TIROS-N derived sea-surface temperatures and the 1,000–850 mbar layer-mean temperatures using a simple differencing procedure. From these temperatures and simultaneous satellite-derived mid-tropospheric water vapour content, we show here the close link between the extent of inversion regions and the convective processes with the Indian monsoon at its different phases.

Circulation and distribution of some hydrographical properties during the late winter in the Bay of Bengal

Abstract: Charts showing dynamic topography, mass distribution, temperature, salinity, dissolved oxygen and inorganic phosphate in the upper layers of the Bay of Bengal for the late winter period were analysed and presented. The near surface circulation has been discussed in relation to thermal structure and water mass characteristics. The analysis revealed that the field of motion in the near surface layers of the Bay of Bengal during the late winter is the result of various factors, such as the winter cooling near the head of the Bay, the influence of northeast monsoon winds, the influence of north equatorial current in the southern regions and the influence of fresh water discharges, especially in the northern Bay, Andaman Sea and along the east coast of India. The integral mean concentration of nutrients in the surface mixed layer is in consonance with the near surface circulation in such a way that its higher (lower) value along with the lower (high) value of dissolved oxygen is associated with less (more) thickness of the surface layer projecting very clearly the effect of divergence of cyclonic gyre present in the near surface circulation. The salinity structure presented shows that the Bay of Bengal exhibits an estuarine type of circulation due to heavy discharge of fresh water all along its land peripheries

The Monsoon over the Indian Ocean during Summer 1975. Part I: Mean Fields

Abstract: An assimillaiion of surface data collected from ship reports, land and islands stations during summer 1975 is presented. The domain of this study is the summer MONEX area (35–100°E, 25°N-25°S) and the period covers May, June, July, August and September 1975. Objective analysis of the data for 304 individual half-days (an analysis every 12 h) was carried out over the domain for the wind, pressure, air temperature, sea surface temperature and water vapor mixing ratio fields. In this first part, the mean monthly fields are presented and climatological features are discussed. Charts of latent heat transfer are presented. The water vapor cross-equatorial flux (40–100°E) for the complete monsoon season (May-September) is 4.7=1010 tons day−1. It is shown that maximum input takes place in the Arabian Sea (75% of the total flux), especially between 45 and 60°E (more than 50% of the total flux). The cross-equatorial flux into the Bay of Bengal is not negligible, being about ⅓ of that over the western India...

Temporal and Spatial Variations of Monthly Rainfall in Java, Indonesia

Abstract: The analysis of the long-term fluctuation of monthly rainfall (1951-1973) revealed that the year-to-year variability of the rainfall in and around Java is large in the "east monsoon" (dry) season and relatively small in the "west monsoon" (rainy) season. The empirical orthogonal function analysis of the anomaly rainfall showed that the quasi-biennial oscillation (QBO) is dominant over the whole of Java with the large variance (29.4% of the total variance), and that a large variability in the "east monsoon" season is mostly explained by this component. The second component (6.2% of the total variance) represents the variation in the "west monsoon" season with a contrastive spatial pattern between the Java Sea side and the Indian Ocean side of the island. It was also confirmed that the first component(the QBO mode) in the anomaly rainfall is closely connected with the QBO in the surface pressure field over Australasia through the eastern south Pacific and the second component is associated with the north- (or south-) ward shift of the winter monsoon circulation system in the Northern Hemisphere. In addition, the relations between these two modes and the "southern oscillation" are briefly discussed.

Evaporation and Precipitation over the Arabian Sea During Several Monsoon Seasons

Abstract: Evaporation estimates were made over the Arabian Sea for the southwest monsoon seasons (covering June, July and August) of 1963, 1964, 1973, 1974 and 1977. The monthly mean evaporation was maximum in June but fell by 18% in August. The spatial distribution of evaporation was such that the west central Arabian Sea registered high amounts. A secondary maximum, evidently of Southern Hemisphere origin, appeared in the south central Arabian Sea in August and was obviously associated with one of the several low-level wind maxima in the western Indian Ocean. The satellite-derived precipitation in both the 1973 and 1974 monsoon seasons was confined to the eastern Arabian Sea and was mostly aligned parallel to the west coast of India. The total precipitation during these two seasons amounted to only 40% of the total evaporation. Based on the work of Saha and Bavadekar (1973), it is conjectured that a good part of the remaining 60% of evaporation is transported into India.

Potential instability and precipitation occurrence within an inter-tropical discontinuity environment

Abstract: The degree of potential instability along a transect around the longitude of Lagos running through the Intertropical Discontinuity (ITD) is investigated using an instability indexP which is similar to but not the same as the Showalter Index. Over 200 radiosonde/rawinsonde observations made over Lagos, Niamey and Aoulef in West Africa during the IGY, in particular, 1958, are used to investigate the variation ofP across the ITD. Places north, over and up to 2° of latitude south of the ITD are pre dominantly stable even during the Monsoon (rainy) months considered. The cases of instability found are located farther (i. e. greater than 2°) south of the ITD than the precipitation. Contingency tables give a better credence to a modified form of the index, styledP J , and suggest that the Showalter stability index does not work over the region. Potential instability alone is not a sufficient condition for precipitation, however, and the role of the large scale circulation in arranging a “triggering mechanism” should be considered.

Beach changes on a monsoon coast, Peninsular Malaysia

Abstract: The northeast monsoon (November-February) has a greater impact than the southwest monsoon (May-August) on the east coast of Peninsular Malaysia. Beach changes from the southwest monsoon to the northeast monsoon are abrupt, compared to the gradual recovery of the beach from the northeast monsoon to the southwest monsoon. Although its impact varies yearly, the northeast monsoon leaves behind a beach in which the backshore and the upper foreshore are relatively undisturbed by low wave action in the succeeding monsoon until the onset of the next northeast monsoon. The impact of the northeast monsoon on the beaches is essentially erosional; the foreshore and backshore are cut back, ridges are flattened and the beach profile is smoothed out. In contrast, the beaches are accretional during the southwest monsoon; accretion takes place on the foreshore and decreases in intensity landward. Ridges, bars, berms and cuspate features are more common during the southwest monsoon than during the northeast monsoon. Foreshores with sand less than 0.5 mm diameter tend to build a relatively steeper gradient in the southwest monsoon than in the northeast monsoon; foreshores with sand more than 0.5 mm diameter are unable to build a relatively steeper gradient in the southwest monsoon and in fact have a slightly lower gradient in the southwest monsoon. Beach cusps of the northeast monsoon are found at a higher level on the beach and are spaced 24.38-30.48 metres (80-100 feet); they remain in a degraded form and are succeeded in the southwest monsoon by another series at a lower level on the beach with spacings at 15.24-18.29 metres (50-60 feet). The ridge and runnel topography is more common in the southwest monsoon than in the northeast monsoon. For beaches with such topography in both monsoons, the ridges and runnels are more pronounced and of a higher amplitude in the southwest monsoon than in the northeast monsoon. Under constructive wave action, the ridges migrate upbeach and eventually merge with the upper foreshore or the berms. Where there is an abundant supply of fine sand, the ridge topography is further enhanced by aeolian action. Beach vegetation also helps to maintain the height and configuration of the ridges. The alternation of 'cut' during the northeast monsoon and 'fill' during the southwest monsoon in an area of active ridge formation will eventually produce a successive series of parallel ridges. The monsoons also affect the direction and magnitude of the littoral transport. The nearshore topography acts as a reservoir for material removed from the beaches during the northeast monsoon and a source of migratory forms moving landward and upbeach during the southwest monsoon.

Influence of Tropical Disturbances on Monthly Monsoon Rainfall of India

Abstract: An attempt has been made to examine the influence of tropical disturbances (i.e., depressions/cyclonic storms) on the rainfall of individual monsoon months and the season as a whole considering the year record of rainfall and these disturbances over the Indian land area north of 15°N excluding the hilly areas of the country. Individual monsoon months from June to September exhibit a relationship between rainfall and the corresponding frequency of tropical disturbances, but the monsoon season taken as a whole does not show any significant relationship.

On the northward advance of the Itcz and the onset of the southwest monsoon rains over the southeast Bay of Bengal

Abstract: The pentad rainfall data of six stations in the Andaman-Nicobar group of islands for the period 1953-1978 have been analysed to study the northward advance of the ITCZ and the onset of the summer monsoon rainfall over the southeast Bay of Bengal. It is found that at the island stations south of 10°N the summer monsoon rains set in on the average towards the end of April which is a month in advance of the onset of the monsoon over the south Kerala coast of peninsular India. At the island stations to the north of 10°N the onset occurs about the first week of May. These dates are ahead of those shown in the existing diagrams giving the average onset dates of the summer monsoon over the southeast Bay of Bengal. Examination of the frequency and latitude of formation of cyclonic disturbances in the Bay of Bengal and the progressive seasonal variations of the tropospheric winds over the aerological station of Port Blair during April-May lend support to the earlier onset dates.

Measurements of CO and CH4 in the troposphere over Saudi Arabia, India, and the Arabian Sea during the 1979 international summer monsoon experiment (MONEX)

Abstract: During the 1979 Summer MONEX, 150 air samples collected over Saudi Arabia, India, and the Arabian Sea were analyzed for CO and CH4. Near Dhahran and over the Ganges Valley there were high concentrations of CO, around 300 ppbv, in the boundary layer. Out over the Saudi Arabian desert there was no sharp increase in the boundary layer. It is suggested that these high concentrations originate from pollution sources. Low values of CO, down to 80 ppbv, are found over the Arabian Sea as the monsoon progresses, and these may originate from the Southern Hemisphere. Methane over Saudi Arabia (1.59 ppmv) is a little higher than that over the Arabian Sea (1.54 ppmv) probably because the latter region is influenced by air from the Southern Hemisphere.

The onset of the summer monsoon during the FGGE 1979 experiment off the East African coast: A comparison of wind data collected by different means

Abstract: During FGGE 1979, from March to July, an extensive oceanographic experiment with ships and moored stations was carried out in the Somali Current. The development of the monsoon winds off Somalia during the time of that experiment is described in a comparative analysis of standard ship wind observations, moored buoy wind measurements, low-level cloud winds, and winds from land stations. The onset 1979 is found to be of the multiple type, with northward winds off Somalia beginning around May 5 but dying down into early June; the real onset of sustained high winds starts around June 10. Cloud level wind observation numbers off Somalia decrease drastically with the monsoon onset because of lack of clouds over the quickly developing cold upwelling areas. An intercomparison of cloud level and ship winds for the period May 16 to July 6 at five offshore points shows good agreement in directions but reduction of ship wind speeds against cloud level winds off northern Somalia after the onset, which may be explained by the increased vertical wind shear due to high air stability over the upwelled water and by geostrophic shear due to the strong gradients of sea surface temperature. A comparison of 3-day averages of buoy winds measured at 3-m height 30 km offshore, but still inland from the ship lane, with ship winds for the period March 3 to June 10 showed good agreement in directions but lower buoy wind speeds, which could partly be due to sensor height difference and partly due to horizontal wind shear towards the coast. Coastal stations and wind buoys near the coast are found not to be good indicators of the monsoon onset further out in the open ocean.

A Case Study of a Fluctuation of the Somali Jet During the Indian Summer Monsoon

Abstract: A fluctuation in the intensity of the Somali low-level jet flowing along the East African coast during the Indian summer monsoon is studied. It is shown that this variation of the intensity of cross-equatorial flow in western Indian Ocean results from the interaction between extratropical perturbations moving eastward at midlatitudes of the Southern Hemisphere and southeast trade winds of the monsoon circulation over the southern equatorial Indian Ocean.