North Bay Of Bengal Research Articles

An ocean–atmosphere interaction mechanism for the active break cycle of the Asian summer monsoon

A typical active–break cycle of the Asian summer monsoon is taken as beginning with maximum SST (pentad 0) over the north Bay of Bengal when the oceans to its west and east from longitude 40°–160°E, and between latitudes 10° and 25°N (area A) also has maximum SST. During this pentad the recently found “Cold Pool” of the Bay of Bengal (between latitudes 3°N and 10°N) has its minimum SST. An area of convection takes genesis over the Bay of Bengal immediately after pentad 0 in the zone of large SST gradient north of the Cold Pool and it pulls the monsoon Low Level Jetstream (LLJ) through peninsular India. Convection and the LLJ westerlies then spread to the western Pacific Ocean during pentads 1–4 taken as the active phase of the monsoon during which convection and LLJ have grown in a positive feed back process. The cyclonic vorticity to the north of the LLJ axis is hypothesized to act as a flywheel maintaining the convection during the long active phase against the dissipating effect of atmospheric stabilization by each short spell of deep convection. By the end of pentad 4 the SST over area A has cooled and the convection weakens there, when the LLJ turns clockwise over the Arabian Sea and flows close to the equator in the Indian ocean. A band of convection develops at pentad 5 between the equator and latitude 10°S over the Indian ocean and it is nourished by the cyclonic vorticity of the LLJ now near the equator and the moisture supply through it. This is taken as the break monsoon phase lasting for about three to four pentads beginning from pentad 5 of a composite active–break cycle of 40 day duration. With reduced wind and convection over the area A during the break phase, solar radiation and light winds make the SST there warm rapidly and a new active–break cycle begins. SST, convection, LLJ and the net heat flux at the ocean surface have important roles in this new way of looking at the active–break cycle as a coupled ocean–atmosphere phenomenon.

Variability of Convective Activity over the North Indian Ocean and its Associations with Monsoon Rainfall over India

The present study is an attempt to examine the variability of convective activity over the north Indian Ocean (Bay of Bengal and Arabian Sea) on interannual and longer time scale and its association with the rainfall activity over the four different homogeneous regions of India (viz., northeast India, northwest India, central India and south peninsular India) during the monsoon season from June to September (JJAS) for the 26 year period (1979 to 2004). The monthly mean Outgoing Long-wave Radiation (OLR) data obtained from National Oceanic and Atmospheric Administration (NOAA) polar orbiting spacecraft are used in this study and the 26-year period has been divided into two periods of 13 years each with period-i from 1979 to 1991 and period -ii from 1992 to 2004. It is ascertained that the convective activity increases over the Arabian Sea and the Bay of Bengal in the recent period (period -ii; 1992 to 2004) compared to that of the former period (period -i; 1979 to 1991) during JJAS and is associated with a significantly increasing trend (at 95% level) of convective activity over the north Bay of Bengal (NBAY). On a monthly scale, July and August also show increase in convective activity over the Arabian Sea and the Bay of Bengal during the recent period and this is associated with slight changes in the monsoon activity cycle over India. The increase in convective activity particularly over the Arabian Sea during the recent period of June is basically associated with about three days early onset of the monsoon over Delhi and relatively faster progress of the monsoon northward from the southern tip of India. Over the homogeneous regions of India the correlation coefficient (CC) of OLR anomalies over the south Arabian Sea (SARA) is highly significant with the rainfall over central India, south peninsular India and northwest India, and for the north Arabian Sea (NARA), it is significant with northwest India rainfall and south peninsular rainfall. Similarly, the OLR anomalies over the south Bay of Bengal (SBAY) have significant CC with northwest India and south peninsular rainfall, whereas the most active convective region of the NBAY is not significantly correlated with rainfall over India. It is also found that the region over northeastern parts of India and its surroundings has a negative correlation with the OLR anomalies over the NARA and is associated with an anomalous sinking (rising) motion over the northeastern parts of India during the years of increase (decrease) of convective activity over the NARA.

Rainfall analysis for Indian monsoon region using the merged rain gauge observations and satellite estimates: Evaluation of monsoon rainfall features

Objective analysis of daily rainfall at the resolution of 1° grid for the Indian monsoon region has been carried out merging dense land rainfall observations and INSAT derived precipitation estimates. This daily analysis, being based on high dense rain gauge observations was found to be very realistic and able to reproduce detailed features of Indian summer monsoon. The inter-comparison with the observations suggests that the new analysis could distinctly capture characteristic features of the summer monsoon such as north-south oriented belt of heavy rainfall along the Western Ghats with sharp gradient of rainfall between the west coast heavy rain region and the rain shadow region to the east, pockets of heavy rainfall along the location of monsoon trough/low, over the east central parts of the country, over north-east India, along the foothills of Himalayas and over the north Bay of Bengal. When this product was used to assess the quality of other available standard climate products (CMAP and ECMWF reanalysis) at the gird resolution of 2.5°, it was found that the orographic heavy rainfall along Western Ghats of India was poorly identified by them. However, the GPCC analysis (gauge only) at the resolution of 1° grid closely discerns the new analysis. This suggests that there is a need for a higher resolution analysis with adequate rain gauge observations to retain important aspects of the summer monsoon over India. The case studies illustrated show that the daily analysis is able to capture large-scale as well as mesoscale features of monsoon precipitation systems. This study with data of two seasons (2001 and 2003) has shown sufficiently promising results for operational application, particularly for the validation of NWP models.

Some salient features of the atmosphere observed over the north Bay of Bengal during BOBMEX

This paper describes the near surface characteristics and vertical variations based on the observations made at 17.5‡N and 89‡E from ORV Sagar Kanya in the north Bay of Bengal during the Bay of Bengal Monsoon Experiment (BOBMEX) carried out in July–August 1999. BOBMEX captured both the active and weak phases of convection. SST remained above the convection threshold throughout the BOBMEX. While the response of the SST to atmospheric forcing was clearly observed, the response of the atmosphere to SST changes was not clear. SST decreased during periods of large scale precipitation, and increased during a weak phase of convection. It is shown that the latent heat flux at comparable wind speeds was about 25–50% lower over the Bay during BOBMEX compared to that over the Indian Ocean during other seasons and tropical west Pacific. On the other hand, the largest variations in the surface daily net heat flux are observed over the Bay during BOBMEX. SST predicted using observed surface fluxes showed that 1-D heat balance model works sometime but not always, and horizontal advection is important. The high resolution Vaisala radiosondes launched during BOBMEX could clearly bring out the changes in the vertical structure of the atmosphere between active and weak phases of convection. Convective Available Potential Energy of the surface air decreased by 2–3 kJ kg-1 following convection, and recovered in a time period of one or two days. The mid tropospheric relative humidity and water vapor content, and wind direction show the major changes between the active and weak phases of convection.

Meteorological fields variability over the Indian seas in pre and summer monsoon months during extreme monsoon seasons

In this study, the possible linkage between summer monsoon rainfall over India and surface meteorological fields (basic fields and heat budget components) over monsoon region (30‡E-120‡E, 30‡S30‡N) during the pre-monsoon month of May and summer monsoon season (June to September) are examined. For this purpose, monthly surface meteorological fields anomaly are analyzed for 42 years (1958-1999) using reanalysis data of NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research). The statistical significance of the anomaly (difference) between the surplus and deficient monsoon years in the surface meteorological fields are also examined by Student’s t-test at 95% confidence level.Significant negative anomalies of mean sea level pressure are observed over India, Arabian Sea and Arabian Peninsular in the pre-monsoon month of May and monsoon season. Significant positive anomalies in the zonal and meridional wind (at 2 m) in the month of May are observed in the west Arabian Sea off Somali coast and for monsoon season it is in the central Arabian Sea that extends up to Somalia. Significant positive anomalies of the surface temperature and air temperature (at 2 m) in the month of May are observed over north India and adjoining Pakistan and Afghanistan region. During monsoon season this region is replaced by significant negative anomalies. In the month of May, significant positive anomalies of cloud amount are observed over Somali coast, north Bay of Bengal and adjoining West Bengal and Bangladesh. During monsoon season, cloud amount shows positive anomalies over NW India and north Arabian Sea.There is overall reduction in the incoming shortwave radiation flux during surplus monsoon years. A higher magnitude of latent heat flux is also found in surplus monsoon years for the month of May as well as the monsoon season. The significant positive anomaly of latent heat flux in May, observed over southwest Arabian Sea, may be considered as an advance indicator of the possible behavior of the subsequent monsoon season. The distribution of net heat flux is predominantly negative over eastern Arabian Sea, Bay of Bengal and Indian Ocean. Anomaly between the two extreme monsoon years in post 1980 (i.e., 1988 and 1987) shows that shortwave flux, latent heat flux and net heat flux indicate reversal in sign, particularly in south Indian Ocean. Variations of the heat budget components over four smaller sectors of Indian seas, namely Arabian Sea, Bay of Bengal and west Indian Ocean and east Indian Ocean show that a small sector of Arabian Sea is most dominant during May and other sectors showing reversal in sign of latent heat flux during monsoon season.

Cloud climatology of the Indian Ocean based on ship observations

Surface cloud data based on synoptic observations made by Voluntary Observing Ships (VOS) during the period 1951-98 were used to prepare the seasonal and annual cloud climatology of the Indian Ocean. The analysis has been carried out by separating the long-term trends, decadal and inter-annual components from the monthly cloud anomaly time series at each 5° × 5° grids.
 
 Maximum zone of total and low cloud cover shifts from equator to northern parts of India during the monsoon season. During the monsoon season (June-September), maximum total cloud cover exceeding 70% and low cloud cover exceeding 50% are observed over north Bay of Bengal. Maximum standard deviation of total and low cloud cover is observed near the equator and in the southern hemisphere. Both total and low cloud cover over Arabian Sea and the equatorial Indian Ocean are observed to decrease during the ENSO events. However, cloud cover over Bay of Bengal is not modulated by the ENSO events. On inter-decadal scale, low cloud cover shifted from a "low regime" to a "high regime" after 1980 which may be associated with the corresponding inter-decadal changes of sea surface temperatures over north Indian Ocean observed during the late 1970s.

Near surface atmospheric characteristics over the North Bay of Bengal during the Indian Summer Monsoon

Observations were made from a ship at 17.5°N & 89°E in the North Bay of Bengal during July–August 1999. The Bay was convectively active during this period with almost one convective event every week. The surface wind speed varied from 3 to 15 m s−1. SST decreased by 0.5° to 1° C following convection. The latent heat flux was about 25–35% lower over the Bay compared to that over the West Pacific at comparable wind speeds. High resolution radiosonde observations have been made for the first time in the North Bay. The values of atmospheric mixed layer height, equivalent potential temperature, convective available potential energy and convection inhibition energy over the North Bay are comparable to those observed over the western Pacific warm pool region. The recovery timescale of the atmospheric variables such as mixed layer height, equivalent potential temperature and convective available potential energy is one to two days.

A broad-scale circulation index for the interannual variability of the Indian summer monsoon

A broad-scale circulation index representing the interannual variability of the Indian summer monsoon is proposed and is shown to be well correlated with the interannual variability of precipitation in the Indian monsoon region. Using monthly precipitation analysis based on merging rain-gauge data with satellite estimates of precipitation for the period 1979-96, it is shown that the variability of precipitation on seasonal to interannual time-scales is coherent over a large region covering the Indian continent as well as the north Bay of Bengal and parts of south China. A new index, termed Extended Indian Monsoon Rainfall (EIMR), is defined as the precipitation averaged over the region 70°E–110°E, 10°N–30°N. the EIMR index is expected to represent the convective heating fluctuations associated with the Indian monsoon better than the traditional all India Monsoon Rainfall (IMR) based only on the precipitation over the Indian continent. It is shown that large precipitation over the Bay of Bengal with significant interannual variability cannot be ignored in the definition of Indian summer monsoon and its variability. the June-to-September climatological mean EIMR is found to be larger than that of the IMR even though the former is averaged over a larger area. the dominant mode of interannual variability of the Indian summer monsoon is associated with a dipole between the EIMR region and the north-western Pacific region (110°E–160°E, 10°N–30°N) and a meridional dipole between the EIMR region and the equatorial Indian Ocean (70°E–110°E, 10°S–5°N). It is argued that the interannual variability of the monsoon circulation is primarily driven by gradients of diabatic heating associated with variations of the EIMR, and that the regional monsoon Hadley circulation is a manifestation of this heating. an index of the monsoon Hadley (MH) circulation is defined as the meridional wind-shear anomaly (between 850 hPa and 200 hPa) averaged over the same domain as the EIMR, Using circulation data from two independent reanalysis products, namely the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis and the European Centre for Medium-Range Weather Forecasts reanalysis, it is shown that the MH index is significantly correlated with the EIMR. Also it is shown that both the EIMR and MH indices have a dominant quasi-biennial variability, consistent with previous studies of IMR. Teleconnections of IMR, EIMR and MH indices with summer sea surface temperature (SST) have also been investigated. There are indications that the south equatorial Indian Ocean SST has a strong positive correlation with the EIMR. Also it is noted that the correlation of the monsoon indices with the eastern Pacific SST was weak during the period under consideration primarily due to almost a reverse relationship between monsoon and El Niño and Southern Oscillation during the latest eight years.

A broad‐scale circulation index for the interannual variability of the Indian summer monsoon

AbstractA broad‐scale circulation index representing the interannual variability of the Indian summer monsoon is proposed and is shown to be well correlated with the interannual variability of precipitation in the Indian monsoon region. Using monthly precipitation analysis based on merging rain‐gauge data with satellite estimates of precipitation for the period 1979‐96, it is shown that the variability of precipitation on seasonal to interannual time‐scales is coherent over a large region covering the Indian continent as well as the north Bay of Bengal and parts of south China. A new index, termed Extended Indian Monsoon Rainfall (EIMR), is defined as the precipitation averaged over the region 70°E–110°E, 10°N–30°N. the EIMR index is expected to represent the convective heating fluctuations associated with the Indian monsoon better than the traditional all India Monsoon Rainfall (IMR) based only on the precipitation over the Indian continent. It is shown that large precipitation over the Bay of Bengal with significant interannual variability cannot be ignored in the definition of Indian summer monsoon and its variability. the June‐to‐September climatological mean EIMR is found to be larger than that of the IMR even though the former is averaged over a larger area. the dominant mode of interannual variability of the Indian summer monsoon is associated with a dipole between the EIMR region and the north‐western Pacific region (110°E–160°E, 10°N–30°N) and a meridional dipole between the EIMR region and the equatorial Indian Ocean (70°E–110°E, 10°S–5°N).It is argued that the interannual variability of the monsoon circulation is primarily driven by gradients of diabatic heating associated with variations of the EIMR, and that the regional monsoon Hadley circulation is a manifestation of this heating. an index of the monsoon Hadley (MH) circulation is defined as the meridional wind‐shear anomaly (between 850 hPa and 200 hPa) averaged over the same domain as the EIMR, Using circulation data from two independent reanalysis products, namely the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis and the European Centre for Medium‐Range Weather Forecasts reanalysis, it is shown that the MH index is significantly correlated with the EIMR. Also it is shown that both the EIMR and MH indices have a dominant quasi‐biennial variability, consistent with previous studies of IMR. Teleconnections of IMR, EIMR and MH indices with summer sea surface temperature (SST) have also been investigated. There are indications that the south equatorial Indian Ocean SST has a strong positive correlation with the EIMR. Also it is noted that the correlation of the monsoon indices with the eastern Pacific SST was weak during the period under consideration primarily due to almost a reverse relationship between monsoon and El Niño and Southern Oscillation during the latest eight years.

Fluxes of heat and momentum over sea surface during the passage of a depression in the north Bay of Bengal

Time variation of surface fluxes of heat, moisture and momentum over a sea station (20°N 89°E) in the north Bay of Bengal has been computed by profile method for the period 18th–25th August 1990 using meteorological data of MONTBLEX-90 from ORVSagarkanya. The fluxes showed synoptic and diurnal variations which are marked during depression (20th–21st August) compared to their variation prior to and after this period. Variations of heat and water vapour fluxes were in phase. Night time fluxes are relatively high compared to day time. Average momentum transfer during depression was two to three times large. Variations in Bowen ratio were relatively large during day time. During depression, it varied between 0·2 in day time and about 0·3 at night and in the undisturbed period between −0·1 and 0·2 during day time and 0·2 and 0·25 at night. The study shows that the assumptionCD=CH=CE of the exchange coefficients normally used in estimating the fluxes by the bulk aerodynamic method is not appropriate becauseCH/CD≈2,CE/CD≈1·5 andCH/CE≈1·4.

Synoptic scale disturbances of the Indian summer monsoon as simulated in a high resolution climate model

Hamburg atmospheric general circulation model ECHAM3 at T106 resolution (1.125' lat.Aon.) has considerable skill in reproducing the observed seasonal reversal of mean sea level pressure, the location of the summer heat low as well as the position of the monsoon trough over the Indian subcontinent. The present-day climate and its seasonal cycle are realistically simulated by the model over this region. The model simulates the structure, intensity, frequency, movement and lifetime of monsoon depressions remarkably well. The number of monsoon depressions/storms simulated by the model in a year ranged from 5 to 12 with an average frequency of 8.4 yr-', not significantly different from the observed climatology. The model also simulates the interannual variability in the formation of depressions over the north Bay of Bengal during the summer monsoon season. In the warmer atmosphere under doubled CO2 conditions, the number of monsoon depressions/cyclonic storms forming in Indian seas in a year ranged from 5 to 11 with an average frequency of 7.6 yr-', not significantly different from those inferred in the control run of the model. However, under doubled CO2 conditions, fewer depressions formed in the month of June. Neither the lowest central pressure nor the maximum wind speed changes appreciably in monsoon depressions identified under simulated enhanced greenhouse conditions. The analysis suggests there will be no significant changes in the number and intensity of monsoon depressions in a warmer atmosphere.

Diagnostic study of a Bay of Bengal monsoon depression, July 1979

Upper winds from the depression field especially in the formation stage are the major constraints, for studying their detailed structure at this stage.
 Monsoon Experiment (MONEX) in 1979 has provided, adequate upper air data from a depression field which formed in the north Bay of Bengal on 7 July 1979. Based on these observations, certain features like thermal structure, distribution of vorticities, divergence, radial and tangential wind, vertical wind shear inflow and out flow and vertical velocities associated with the depression have been computed and the results presented in this paper.
 "Genesis Parameter" designed by Gray (1978) for the development of the storm based on the product of coriolis parameter, vorticity parameter, vertical wind shear parameter, ocean energy parameter, stability parameter and relative humidity parameter have been computed. In the present case, the above genesis parameter is computed to be 46 x 10-8 car K sec-1 cm-3 which is more than the average non-developing depression (26 x 10-8 calo K sec-l cm-3) and less than the developing depression (55 x 10-8 calo K sec-l cm-3) in the Atlantic.

Role of mountains in the development of the Indian summer monsoon circulation and the associated weather

Some special features of the Indian summer monsoon mentioned by Hahn and Manabe (l 975) in their model simulated with mountains are discussed with reference to the model for surface layers given by Banerji (1930 a and b) while takin g into acco unt the effect of moun tain ranges on the a ir flow and configura tion of isobars on the basis of thermo-dynamical and hydrodynarnical consideration and the experience of the author for fiftyyears about the facts of weather.
 It is observed that the lower monsoon circulation and the upper tropospheric circulation developed indepe nden t.Iy, The barriers to the west of the east coa st of Africa are respon sible for the deflection of the south-east trades north to northeastward acro ss the equator under the influence of the heat low (location influenced by barriers) over south Pakistanan d the monsoou trough circulation in the levels up to about 600 mb which is set up due to the influence of the moun tains on the Indian sub-continent.
 The westerly jet is not a part of the monsoon circulation and its shift from the south to the north of Tibet need notnecessarily coincide with the onset of the monsoon. There would not appear any cause and effect relation between the development and strength of eas terly jet and the onset and performance of the monsoon in terms of rainfall.The low level jet is embedded in the monsoon current and is noticed even to the east of Madagascar and in thewestern Indian Ocean and the adjoining parts of eastern coastal area s of Africa (Findlater 1969 a and b) and would not appear to be developing due to upwelling of water off the Somalia coast which has led to its being named Somalia jet (Bunker 1965).
 The seasonal rainfall can be understood on the basis influence of mountains, although proximit of the tracksfollowed by the depressions while moving west to northwestwards from the north Bay of Bengal, increase, the amount of rainfall. Small amounts of rainfall over the northwest of the sub-contonent would appear to be due to absence of barriers over thecoast extending From Karachi in Pakistan to veraval in G ujratt (India)and the presence of low level inversion due to spreading of relatively warmer drier air between about 1 and 3 km from the Baluchistan plateau side over the lower cool moist monsoon air. There is no ITCZ over the monsoon trough area upto about 600mb once the monsoon is established. ' Breaks' in monsoon rain would appear to be associated with the high pressure anomalies over the monsoon trough area and the adjoi ning area to th: south which prevent the southern hemispheric air from being drawn to the west coast of India.

Indian summer monsoon cyclogenesis and its variability

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.

Forecast five days ahead of the development of low pressure areas -and depressions /storms over the northern part of the Bay of Bengal during the southwest monsoon season

During the months of June, July, August and September, low pressure areas form with some regularity over the north Bay of Bengal. Most of these low pressure areas concentrate into depressions. Some of them even intensify into cyclonic storms. During the period from 1950 to 1975, 289 depressions/storms formed in the Bay. In this period 163 out of these 289 depressions/storms occurred during the S.W. monsoon m()nths June to September. The low pressure areas not reaching the stage of depressions have been left out of this account. The lows formed but not developed upto the stage of depressions have been accounted for the months June to September of the years 1971 to 1975. These lows and the depressions/storms were found occurring at regular intervals. A somewhat periodicity was maintained in their appearance. Pressure curves showing daily variations of pressure of Chittagong in the months June to September of the years 1971 to 1975 were made. The 0000 GMT mean sea level pressures were taken for the purpose. These curves were found in waveforms. Each wave has .the length, amplitude and period. The maximum and minimum of the pressures, i.e., wave crest and wave trough or the ridge and trough in the wave were found significant in view of the weather the place experienced during the period of the wave. The period of the wave was observed to be about 5 days. In case of a serious abnormality in any cyclonic circulation, the 'period' of. the particular wave was disturbed. The cyclonic circulations developing into lows of pressure and depressions in the Bay fitted fairly satisfactorily in the wave troughs. The coming wave crest and wave trough in the pressure curve may be worked out.

インドにおける夏のモンスーンの変動に関する解析

Temporal fluctuations appearing in the summer monsoon over India are investigated by using the data of 1962. By the method of spectrum analysis, it is revealed that two major periodicities exist, at least, in the temporal fluctuation of the monsoon. One is the oscillation around 5 days period and the other is around 15 days pericd.The oscillation around 5 days period appears mainly in the range from the north Bay of Bengal through the monsoon trough region in northern India. The structure of the disturbance which causes this periodicity is examined by the method of cross spectrum analysis. The results show that the disturbance is a westward-moving one and its longitudinal wavelength is about 30°. This disturbance seems to represent the so-called monsoon low. The vertical structure of these monsoon lows indicates that their cyclonic circulation is prevailing in the lower troposphere and the axis of the trough slightly tilts westward. Moreover, the monsoon low is accompanied with a distinct warm core in the upper troposphere. In the lower levels, the amplitude of the temperature is mall and the disturbance is neither warmnor cold cored. It is also shown that the monsoon low has a steering level at the height around 500mb level.The oscillation around 15 days period is revealed to be in connection with the active/weak cycle of monsoon. The intense wind fluctuations associated with this cycle appear both in the upper and the lower troposphere, being in phase with each other. At the stage of active monsoon, it is revealed that the area with cyclonic circulation is formed over the Bay of Bengal. This cyclonic circulation is accompanied with cold temperature anomalies in the lower troposphere and warm anomalies in the upper levels. Besides, it is also shown that the depth of the moist layer over the Bay increases at this stage. These situations strongly suggest that the active monsoon condition over India is characterized by the enhanced convective activity over the Bay. Nearly opposite situations occur at the stage of weak monsoon and the anticyclonic circulation is formed over the Bay. The latitudinal shift of monsoon trough can be explained by superposing these two circulations on the distribution of the mean flow.The transition between the active and the weak stage is also investigated by applying the time-composite technique to the time series of surface pressure anomalies. At the stage of active monsoon, a large low pressure area is formed over the Bay of Bengal and the high pressure anomalies appear at the weak stage. By examining the time sequences of this transition, it is shown that a pair of high- and low pressure anomalies rotate clockwise over the wide area including the Bay of Bengal, Tibetan Plateau, whole Indian subcontinent and Indo-China. As for the nature of this rotation, it is suggested that the north-south standing oscillation between Tibetan Plateau and the Bay of Bengal together with the east-west one between Indo-China and the Indian continent can cause the clockwise rotation mentioned above. It is also discussed that these oscillations seem of reflect the temporal variations of the intensity of the mean meridional- and the mean zonal circulation cells respectively.

Case studies of westerly waves in the development of monsoon depressions

Development of monsoon depressions forming on 3 and 12 August 1972 in the north Bay of Bengal was found to be linked with changes in mid latitude upper tropospheric flow patterns over China and adjoining areas. Development took place in association with upper tropospheric cut off lows which formed over China, retrograded and induced perturbations in the easterlies over the head Bay of Bengal.

Storm-induced sea-level changes at Saugor Island situated in north Bay of Bengal

The sea level at Saugor Island is not significantly affected by freshet discharges of the river Hooghly although there are significant changes at a station about 70 miles upstream (Minist. Irrg. Pwr. 1952, 1953). Daily departures of the observed tidal heights at Saugor Island from the values predicted by the tide predicting machine were obtained for selected periods when cyclonic storms affected the north Bay of Bengal and were located 150-250 miles away from Saugor Island. The. monthly mean departure dm was subtracted from the daily departures and the resultant values of (8-dm ) were found to vary between I and 3 ft. On the assumption of (i) a mean .wind distribution found as the average of a large number of cyclonic storms in the Bay of Bengal, (ii) negligible' static' pressure effect and (iii) a wind stress coefficient of 0.0024 (Sverdrup), the wind pile-up at Saugor Island caused by the wind systems associated with the cyclonic storms, was computed. The depths of the ocean needed for the computation were taken from the Admiralty charts. The computed values of the pile up were found to agree fairly well with the values of (8-dm), the mean percentage deviation being 10 per cent for the six instances studied. This suggests that wind pile-up accounts for nearly 90 per cent of the departure of actual tidal heights from the predicted values. Probably the remaining 10 per cent comes from the mass-transport effects associated with the water currents generated by the wind systems.

Air-Masses and fronts in the Monsoon Depression in India

From a synoptic and aerological studt of the data of radiosonde acents at Vizagapatam, Cuttack, Culcutta and Akyab during a depression in the Bay of Bengal in the beginning of July 1945, it is observed that different airmasses took part in both in the formation stage of the depression and during its subsequent intensification into a storm and movement westnorth westwards.
 The study supports the mechanism of the formation of depression in the north bay of Bengal during the monsoon season suggested by Desai in an earlier paper.