Showing papers on "Monsoon published in 1973"

Physical Oceanography of the Indian Ocean

Abstract: During the preparation of the Atlas on the physical oceanography of the Indian Ocean (WYRTKI, 1971) a comprehensive analysis of its oceanographic conditions had to be undertaken to construct meaningful maps, sections and diagrams, and to interpret the structure and circulation of this ocean, which is in so many ways different from the other oceans. On a large scale 3 distinct circulation systems can be delineated. These are: I. the seasonally changing monsoon gyre II. the south hemispheric subtropical anticyclonic gyre III. the Antarctic waters with the Circumpolar Current

An Equatorial Jet in the Indian Ocean

Abstract: At the surface of the Indian Ocean along the equator a narrow, jet-like current flows eastward at high speed during both transition periods between the two monsoons. The formation of the jet is accompanied by thermocline uplifting at the western origin of the jet and by sinking at its eastern terminus. This demonstrates that a time-variable current can have profound effects in changing the mass structure in the ocean.

Break-Monsoon Over India

Abstract: Interruption of monsoon rainfall by prolonged spells of sparse rainfall (break-monsoon) during the mid-monsoon months of July and August over the plains of northern India (20°–29°N, 75°–85°E) has been investigated. When a tropical low-pressure system moves from the plains in a northerly direction toward the submontane region of the Himalaya, the monsoon trough of low pressure also moves from the plains to that region. This deprives the plains of northern India of significant ascending motion and normal rainfall and brings about the break-monsoon. During the break-monsoon period, descending motion takes place from the stratosphere to the lower troposphere over the plains, while ascending motion occurs in the regions to the north and south of the plains. As the descending motion persists over the plains, the break-monsoon intensifies and the weather turns mainly dry. A vertical circulation model for the break-monsoon period is proposed. The break-monsoon ceases and normal monsoon conditions are re-...

Some Aspects of Indian Monsoon Depressions and the Associated Rainfall

Abstract: This study is based on all the monsoon depressions that moved westward across India between Calcutta, Allahabad, and Delhi on the right and Gopalpur, Nagpur, and Ahmadabad on the left during July and August for the period 1891–1960. Statistical distribution, of 24-hr motion and of the intensity of the depression, the relation between 24-hr motion and concurrent 24-hr rainfall, and the relation between the intensity of the depression and subsequent 24-hr rainfall are examined. In addition, the average rainfall per depression day and its standard deviation, the contribution of depression rainfall to the total rainfall, and the efficiency of the depression as a rain giver are computed. Mean patterns of 24-hr rainfall within 500 km of the center of the depression along longitudes 87°E, 80°E, and 75°E are obtained, and the main points of difference between them are discussed. In the quadrants to the right of the depression track, the rainfall field is flat. In the quadrants to the left, however, large...

Changes in the Pattern of Distribution of Southwest Monsoon Rainfall Over India

Abstract: Despite the systematic nature of the monsoon rains over India, large year-to-year variations in the pat- tern of distribution of rainfall during the season occur. The yearly pattern of rainfall distribution during the mon- soon season (May 31-October 2) for each of the years 1901-51 for a network of 105 stations over India is charac- terized by a set of six distribution parameters. A brief description of the spatial distribution of the different patterns is given to indicate the nature of the component patterns. Polynomial trend analyses of the time series of the distribution parameters indicate oscillatory features. Power spectrum analyses reveal certain significant periods corresponding to the sunspot cycle or some higher har- monics with regional preferences. The variation of dis- tribution parameters in the different parts of the country with the different sunspot epochs is demonstrated. Studies of the distribution of surface pressure anomalies, frequency of storms and depressions, and the frequency of "breaks in monsoon" associated with the contrasting sunspot epochs suggest that the monsoon circulation features as well as the characteristics of the rainfall distribution have a periodicity nearing the sunspot cycle. I. INTRODUCTION An average annual rainfall of about 105 cm has been estimated for the plains of India. Even though abundant rain falls over the Indian territory as a whole, the disparity in its distribution over the different parts of the country is so great that some parts suffer from perennial dryness. In other parts, however, so much rain falls that only a small fraction can be utilized. In the worst famine year on record (Le., lS99), the total annual rainfall was 26 percent below normal; in 1917, the year of very heavy rainfall, it was 29 percent above normal. The variability of rainfall (coefficient of variation, CV, which is equated to the standard deviation expressed as percentage of the mean) exceeds 30 percent over large areas of the country and is over 40-50 percent in parts of Saurashtra, Kutch, and Rajasthan. The variability is more than 100 percent at some stations in the interior tracts in these areas, indicating that these are particularly liable to very heavy rainfall in some years and to very scanty rainfall in others. The variability is least (about 15-20 percent) over North- east India and the western coast of the peninsula, indi- cating high reliability of rainfall in these areas. As a general rule, areas having the lowest mean rainfall are also those in which the rainfall is most irregular. Rainfall amount varies with the season of the year. Except for Assam, Jammu and Kashmir, and the south peninsula, more than 75 percent of India's annual rainfall is received during the southwest monsoon season, June through September. The year-to-year variations in month- ly rainfall are much larger than those of the annual rain- fall. Table 1 illustrates the extreme variability of the monthly rainfall in the arid and semiarid subdivisions of the country. The variations are highest during the nonmonsoon months, indicating the highly erratic nature of the rainfall during these months. Even during the monsoon months, the variations are high, the lowest being in the month of July when the monsoon is at its height.

Primary Production in the Indian Ocean I

Abstract: One of the basic aims of the International Indian Ocean Expedition (IIOE) was to accumulate data on the rate of primary production in the region and the environmental phenomena that regulate it. These data could then be used to prepare distribution charts which would show, among other things, areas of seasonal extremes in high and low productivity, and their spatial and temporal variations. A good understanding of these processes in the Indian Ocean is of great importance in the construction of models of seasonal energy transfer and the food pyramid and for their theoretical interpretation. Such models must take into consideration the influences of the Indian Ocean monsoons, intensity of upwelling, circulation pattern, etc., and the effects of these on rate of sedimentation, secondary production and spatial transfer. From the models, we could then estimate the level of fish production that each area could theoretically support and indicate regions of potentially exploitable fisheries.

Changes in the Pattern of Distribution of Southwest Monsoon Rainfall Over India Associated With Sunspots

Abstract: Despite the systematic nature of the monsoon rains over India, large year-to-year variations in the pattern of distribution of rainfall during the season occur. The yearly pattern of rainfall distribution during the monsoon season (May 31–October 2) for each of the years 1901–51 for a network of 105 stations over India is characterized by a set of six distribution parameters. A brief description of the spatial distribution of the different patterns is given to indicate the nature of the component patterns. Polynomial trend analyses of the time series of the distribution parameters indicate oscillatory features. Power spectrum analyses reveal certain significant periods corresponding to the sunspot cycle or some higher harmonics with regional preferences. The variation of distribution parameters in the different parts of the country with the different sunspot epochs is demonstrated. Studies of the distribution of surface pressure anomalies, frequency of storms and depressions, and the frequency of...

Tibetan anticyclone and tropical easterly jet

Abstract: During the summer monsoon the upper tropospheric subtropical anticyclone of Asia is centred over SE Tibet (when it is called the Tibetan anticyclone). Further, the equatorward outflow from this anticyclone gains easterly angular momentum and therefore it appears as an easterly jet stream over SE Asia south of 20N between 150 mb and 100 mb. On finding these current concepts questionable, this study offers an alternative explanation for the migration of the upper tropospheric anticyclone to the Tibetan Plateau and also for the development of the tropical easterly jet. In summer the Bay of Bengal is cold compared to its adjoining continental plains in the north. Therefore in the beginning of summer the lower levels of the anticyclone migrate from their winter position in the Bay of Bengal to the warm plains in the north. As they reach the plains by about June, the upper levels of the anticyclone above 150 mb extend north over the Tibetan Plateau irrespective of whether the Plateau is a warm source or cold source because the upper levels of the subtropical anticyclone have a characteristic poleward slope in all seasons. By about July, when the lower levels of the anticyclone migrate from the plains to still warmer areas in the north over the Plateau, the upper levels which are already over the Plateau continue to remain there throughout the season. The zonal component of the equatorward outflow from the Tibetan anticyclone computed from the law of conservation of angular momentum does not bear any comparison with the observed winds in the upper troposphere over India. On the other hand the winds computed from a thermal gradient show a reasonable agreement with the observed winds indicating thereby that the upper tropospheric high winds are thermally generated. These high winds have been found as a unique phenomenon distinct from a jet stream and therefore it is considered appropriate to call them Tropical Strong Easterlies (TSE) rather than as a tropical easterly jet stream. Some of the characteristic features of the TSE are discussed and they are ascribed to the peculiar temperature distribution in the atmosphere between 200 mb and 60 mb mainly brought about by the vertical motion associated with the summer monsoon.

On upwelling along the west coast of India

Abstract: The Oceanographic features prevailing along the west coast of India from Ratnagiri to Cape Comorin during the monsoon season of 1962 are presented and discussed. Nineteen hydrographic sections were investigated out of which twelve have been taken up for consideration. It was found that upwelling was noticeable to a marked extent in the region Calicut to Karwar and during this period a strong southward flow existed along the west coast. The topography of the bottom of the convection layer was also studied to have additional information regarding the monsoon features. In this connection the associated vertical migration of the thermocline during this period is shown and the probable causes of upwelling along this coast are discussed.

Breezes along the Tanzanian east coast

Abstract: At the Tanzanian east coast the prevailing winds of the general circulation exhibit seasonal changes in direction in a kind of monsoonal system. The sea and land breezes, which are limited here to diurnal modifications of the general winds, therefore also vary with the seasons. During the northeast monsoon a sea breeze trend prevails most of the time, while in the southeast monsoon season the main trend is from the land. Only during the intermediate periods between the monsoons a diurnal reversal of sea and land breezes develops regularly. The effects of these winds, especially the sea breeze, on the physiological temperature conditions near the coast are of considerable importance. A decrease of the physiological temperature is produced by the advection of cool air, and a further reduction of thermal stress is provided by the improved air ventilation related to the higher wind velocities of the breezes. Since all impressions of physiological temperature are subjective, it is difficult to express these influences quantitatively. However, estimates indicate that the old adage that “the winter in the tropics occurs at night” is no longer valid at the Tanzanian coast when the physiological temperature is considered, because the seasonal differences are larger than the diurnal variations.

Equatorial undercurrent in the western Indian Ocean during the southwest monsoon

Abstract: Measurements along meridional equatorial sections (two sections previously unpublished), which include hydrographic stations and direct current measurements, in the western Indian Ocean during the peak of the southwest monsoon (late August) are discussed. In each section the undercurrent was not present at the equator, but a large westward component was found that extended from the equator as far north as 5°N and had transports amounting to about 40 × 106 m3 sec−1 to the west. When it was observed, the undercurrent was located at 1°S–2°S and had transports as low as 5 × 106 m3 sec−1 to the east; this finding suggested that in the western ocean during the southwest monsoon the strength of the current may be considerably reduced and the axis shifted to the south.

Break in the Indian summer monsoon

Abstract: The changes in circulation patterns over Eurasia during break monsoon condition over India are studied in comparison to the active monsoon condition. Break monsoon condition seems to set in over the India Sub-Continent in association with eastward movement of middle latitude westerly trough at 500 mb, having large amplitude extending into west Pakistan and northern India. Simultaneously the subtropical anticyclonic ridge over Arabia protrudes into central and Peninsular India. The high latitude blocking high over East Siberia retrogrades and the East Asiatic trough deepens and moves eastwards. The west Pacific subtropical ridge recedes eastward from the China continent. During this period the monsoon trough shifts, from its normal position over Gangetic plains, northwards to the foot of the Himalayas and the monsoon westerlies in the lower troposphere extends right upto the rim of the Tibetan Plateau. The sub-tropical ridge line in the upper troposphere shifts northwards during break and lies approximately above the lower monsoon trough. This seems to provide an effective process of removing ascending air in the lower monsoon trough causing exceptionally heavy rainfall over Assam and along the foot of the Himalayas.

Hydrography and plankton as indicators of marine resources

Abstract: Available data on the standing crop of phytoplankton and zooplankton are presented for the Indian region and hydrographical factors such as upwelling, divergence, convergence, currents, nutrients responsible for production of plankton, and distribution of plankton in time and space are dealt with. Role of southwest monsoon and the magnitude of the intense bloom of phytoplankton during this period are pointed out. The significance of the distribution of plankton to other organisms in the food chain including fish is indicated. Attempt is made to correlate the fisheries of the region with the above several factors and point out the potential resources and their location. The possibilities of using some of the factors as indicators of fishery resources are examined. Certain similarities in the hydrological features, production of plankton, its distribution and fisheries occurring here and elsewhere are reviewed. It is also suggested that the high production of plankton which is also rich in oil—a product of the photosynthesis of the diatoms which form the bulk of the synthesizers of the organic matter-is responsible for the rather extensive oil deposits in the past geological ages in the northern Arabian Sea region which we are exploiting now; this is a continuing process. It is also pointed out that the general pattern of circulation of water during the period of heavy bloom of plankton, viz., south-west monsoon in the Arabian Sea, is clockwise which is likely to lead to anticyclonic eddies on a large scale, particularly around Saurashtra coast which would have the effect of taking plankton to the bottom leading to gradual deposition of matter.

Microbiomass and Detritus in the Upper 50 m of the Arabian Sea along the Coast of Africa and India during the NE Monsoon 1964/65

Abstract: This contribution is based on the results of plankton investigations, which were carried out during the German participation in IIOE with RV “Meteor”.

Weather and Equatorial Waves Over Southeast Asia During the Summer Monsoon

Abstract: Analysis of wind, pressure and cloudiness patterns shows that the weather of Southeast Asia during the southwest monsoon relates best to westward-moving equatorial waves. These waves are basically the same as those found over the western Pacific by other investigators although they are often not perceived without special analysis as they move through the vigorous monsoonal circulation. They are characterized by average wavelength of 3100 km, average period of 5½ days or speed of 6.5 m sec−1, a deep cool center from the surface to about the 300-mb level, and a deep band of convergence and cloudiness which feeds into the center from the southwest and south. Relatively good weather with lower amounts of cloudiness and areal precipitation coverage is normally found in the area of synoptic-scale subsidence between waves.

Patterns of distribution of Arabian Sea hydromedusae

Abstract: A comparison between the surface and subsurface waters of the Arabian Sea and those of Bay of Bengal from the hydromedusan fauna point of view shows the following similarities and differences; In both the seas there is a marked difference between surface and subsurface waters as compared to intermediate and deep waters. In both the seas the surface and subsurface waters are under the monsoonal regime with biannual fluctuations of the amount of incoming solar radiation, rate of evaporation, pattern of circulation, convection currents and upwelling. Both seas are very deep. In the Arabian sea, evaporation is greater than what the total influx of fresh water could compensate. In the Bay of Bengal, the influx of fresh water is greater than the evaporation due to the enormous amount of fresh water contributed by the Ganga-Brahmaputra system and by the Irrawaddi.

The Somali current and the southwest monsoon: An ocean atmosphere instability

Abstract: The Somali current and the southwest monsoon are identified as arising from an ocean atmosphere instability which is associated with the low latitude variation of the Coriolis force. Data on the fall of isotherms, surface wind speed, ocean response to frictional force, and upwelling of cold water are discussed.

Rainfall fluctuations—Break-associated synoptic systems

Abstract: Monsoon conditions of two types, i.e., active monsoon and break monsoon over a tropical country (India), are studied in detail. Methods of analysis such as cross-sections, time sections, contour analysis of 700 mb (lower troposphere), 500 mb (middle troposphere) and 200 mb (upper troposphere), and streamline analysis are used in the present study. Consistent results are obtained in distinguishing the break monsoon condition to that of an active or normal monsoon condition.