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1
Hurley, John V., and William R. Boos. "A global climatology of monsoon low-pressure systems." Quarterly Journal of the Royal Meteorological Society 141, no. 689 (October 28, 2014): 1049–64. http://dx.doi.org/10.1002/qj.2447.
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Dong, Wenhao, Yi Ming, and V. Ramaswamy. "Projected Changes in South Asian Monsoon Low Pressure Systems." Journal of Climate 33, no. 17 (September 1, 2020): 7275–87. http://dx.doi.org/10.1175/jcli-d-20-0168.1.
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AbstractMonsoon low pressure systems (MLPSs) are among the most important synoptic-scale disturbances of the South Asian summer monsoon. Potential changes in their characteristics in a warmer climate would have broad societal impacts. Yet, the findings from a few existing studies are inconclusive. We use the Geophysical Fluid Dynamics Laboratory (GFDL) coupled climate model CM4.0 to examine the projected changes in the simulated MLPS activity under a future emission scenario. It is shown that CM4.0 can skillfully simulate the number, genesis location, intensity, and lifetime of MLPSs. Global warming gives rise to a significant decrease in MLPS activity. An analysis of several large-scale environmental variables, both dynamic and thermodynamic, suggests that the decrease in MLPS activity can be attributed mainly to a reduction in low-level relative vorticity over the core genesis region. The decreased vorticity is consistent with weaker large-scale ascent, which leads to less vorticity production through the stretching term in the vorticity equation. Assuming a fixed radius of influence, the projected reduction in MLPSs would significantly lower the associated precipitation over north-central India, despite an overall increase in mean precipitation.
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Krishnamurthy, V., and R. S. Ajayamohan. "Composite Structure of Monsoon Low Pressure Systems and Its Relation to Indian Rainfall." Journal of Climate 23, no. 16 (August 15, 2010): 4285–305. http://dx.doi.org/10.1175/2010jcli2953.1.
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Abstract The tropical disturbances formed in the Bay of Bengal and the Arabian Sea and over land points in central India, known as low pressure systems (LPSs), are shown to contribute significantly to the seasonal monsoon rainfall over India. Analyses of daily rainfall over India and statistics of the LPSs for the period of 1901–2003 show that the rainfall pattern when the LPSs are present captures the most dominant daily rainfall pattern that represents the active monsoon phase. The rainfall pattern when the LPSs are absent is similar to the pattern representing the break monsoon phase. The location, number, and duration of the LPSs are found to be closely related to the phases and propagation of the dominant intraseasonal modes of the Indian rainfall. The LPSs are also associated with the strengthening of the monsoon trough and low-level monsoon winds. The number of LPSs and their total duration and the corresponding rainfall during July and August exceed those in June and September. The LPS tracks reach up to northwest India during flood years, whereas they are confined to central India during drought years. However, the contribution of rainfall during the LPSs to the total seasonal rainfall is same during flood or drought years. Although the LPSs seem to play an important role in the monsoon rainfall, they alone may not determine the interannual variability of the seasonal mean monsoon rainfall.
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KUMAR, J. RAJENDRA, and S. K. DASH. "Inter-annual and intra-seasonal variation of some characteristics of monsoon disturbances formed over the Bay." MAUSAM 50, no. 1 (December 17, 2021): 55–62. http://dx.doi.org/10.54302/mausam.v50i1.1804.
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The characteristics of monsoon disturbances during drought and flood years for the period 1971-96 are studied to find out their inter-annual variations. Variations of some of the characteristics of monsoon disturbances formed over Bay during 1979-88, with respect to different monsoon conditions such as strong, weak and break monsoons, are also studied. The results show that monsoon disturbance days are higher during flood years than during drought years. Drought years are associated with higher chances of low pressure areas to intensity into depressions, less westward movement, more horizontal extent, intense pressure departure from normal in comparison with flood years. However, more monsoon disturbances tilt significantly during flood years. The rainfall associated with these disturbances is highly variable and does not depend on the density, horizontal and vertical extent of the individual system. More number of lows intensify into depressions during strong monsoon conditions compared to those of weak monsoon conditions. Lows and depressions during strong monsoons have more westward movement and longer life period. Generally, very few lows form during break monsoon and none of them intensify into depression. Hence, the presence of mid-tropospheric heating during strong and weak monsoons is essential for the formation of depression. Synoptic systems which abate break monsoon condition and re-establish normal monsoon are also discussed.
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Hunt, Kieran M. R., and Andrew G. Turner. "Non-linear intensification of monsoon low-pressure systems by the BSISO." Weather and Climate Dynamics 3, no. 4 (November 18, 2022): 1341–58. http://dx.doi.org/10.5194/wcd-3-1341-2022.
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Abstract. More than half of the rainfall brought to the Indian subcontinent by the summer monsoon is associated with low-pressure systems (LPSs). Yet their relationship with the boreal summer intraseasonal oscillation (BSISO) – the dominant intraseasonal forcing on the monsoon – is only superficially understood. Using reanalysis data, we explore the relationship between the BSISO and LPS intensity, propagation and precipitation, and associated underlying mechanisms. The BSISO has a large impact on mean monsoon vorticity and rainfall as it moves northward – maximising both in phases 2–3 over southern India and phases 5–6 over northern India – but a much weaker relationship with total column water vapour. We present evidence that LPS genesis also preferentially follows these phases of the BSISO. We identify significant relationships between BSISO phase and LPS precipitation and propagation: for example, during BSISO phase 5, LPSs over northern India produce 51 % heavier rainfall and propagate northwestward 20 % more quickly. Using a combination of moisture flux linearisation and quasi-geostrophic theory, we show that these relationships are driven by changes to the underlying dynamics rather than the moisture content or thermodynamic structure of the monsoon. Using the example of LPSs over northern India during BSISO phase 5, we show that the vertical structure of anomalous vorticity can be split into contributions from the BSISO background circulation and the non-linear response of the LPS to anomalous BSISO circulation. Complementary hypotheses emerge about the source of this non-linear vorticity response: non-linear frictional convergence and secondary barotropic growth. We show that both are important. The BSISO imparts greater meridional shear on the background state, supporting LPS intensification. The BSISO background and non-linear LPS response both contribute significantly to anomalous boundary layer convergence, and we show through vortex budget arguments that the former supports additional LPS intensification in boundary layer, while the latter supports faster westward propagation. This work therefore yields important insights into the scale interactions controlling one of the dominant synoptic systems contributing to rainfall during the monsoon.
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Hunt, Kieran M. R., and Jennifer K. Fletcher. "The relationship between Indian monsoon rainfall and low-pressure systems." Climate Dynamics 53, no. 3-4 (March 30, 2019): 1859–71. http://dx.doi.org/10.1007/s00382-019-04744-x.
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JADHAV, S. K., and A. A. MUNOT. "Statistical study of the low pressure systems during summer monsoon season over the Indian region." MAUSAM 55, no. 1 (January 19, 2022): 15–30. http://dx.doi.org/10.54302/mausam.v55i1.853.
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The occurrence of a closed low pressure area due to low, depression or cyclonic storm is termed as Low Pressure System (LPS). LPS plays an important role in the distribution of rainfall during the southwest monsoon season. Lows produce widespread rainfall as compared to depressions and cyclonic storms which yield concentrated rainfall over a smaller area. The distribution of rainfall depends upon the track and life span of LPS. Most of the LPS formed over the Bay of Bengal travel in northwest direction strengthening the rainfall activities over the large parts of the country. In this study, the LPS formed during the monsoon season, June to September for the period 1891-2000 over the region covering India, Pakistan, Bangladesh, Bay of Bengal and Arabian Sea are taken into account. The duration of LPS is also studied in terms of LPS Days during the monsoon season. The statistical analysis of LPS and LPS Days is carried out for the monsoon months and for the monsoon season as a whole, for the period 1891-2000. It is seen that the frequency of LPS during any of the monsoon months does not exceed six but three LPS in a month are more common. Total number of LPS during the monsoon season ranges from 9 to 18. In June and July maximum total duration of LPS lies between 10-14 LPS Days while in August and September, it is between 15-19 LPS Days. August is the month having maximum number of LPS and LPS Days. The frequency analysis shows that monthly and seasonal LPS and LPS Days are normally distributed. It is also observed that even though season's total number of LPS has no significant increase or decrease, the LPS Days have significantly increased during the decades, 1971-80 and 1981-90.
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Shi, Xiaohui, George Kiladis, and Min Wen. "Diagnostics of westward propagating East Asian monsoon low-pressure systems that reach the Indian monsoon region." Climate Dynamics 54, no. 1-2 (November 6, 2019): 987–1005. http://dx.doi.org/10.1007/s00382-019-05041-3.
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Praveen, V., S. Sandeep, and R. S. Ajayamohan. "On the Relationship between Mean Monsoon Precipitation and Low Pressure Systems in Climate Model Simulations." Journal of Climate 28, no. 13 (July 1, 2015): 5305–24. http://dx.doi.org/10.1175/jcli-d-14-00415.1.
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Abstract The north-northwest-propagating low pressure systems (LPS) are an important component of the Indian summer monsoon (ISM). The objective detection and tracking of LPS in reanalysis products and climate model simulations are challenging because of the weak structure of the LPS compared to tropical cyclones. Therefore, the skill of reanalyses and climate models in simulating the monsoon LPS is unknown. A robust method is presented here to objectively identify and track LPS, which mimics the conventional identification and tracking algorithm based on detecting closed isobars on surface pressure charts. The new LPS tracking technique allows a fair comparison between the observed and simulated LPS. The analysis based on the new tracking algorithm shows that the reanalyses from ERA-Interim and MERRA were able to reproduce the observed climatology and interannual variability of the monsoon LPS with a fair degree of accuracy. Further, the newly developed LPS detection and tracking algorithm is also applied to the climate model simulations of phase 5 of the Coupled Model Intercomparison Project (CMIP5). The CMIP5 models show considerable spread in terms of their skill in LPS simulation. About 60% of the observed total summer monsoon precipitation over east-central India is found to be associated with LPS activities, while in model simulations this ratio varies between 5% and 60%. Those models that simulate synoptic activity realistically are found to have better skill in simulating seasonal mean monsoon precipitation. The model-to-model variability in the simulated synoptic activity is found to be linked to the intermodel spread in zonal wind shear over the Indian region, which is further linked to inadequate representation of the tropical easterly jet in climate models. These findings elucidate the mechanisms behind the model simulation of ISM precipitation, synoptic activity, and their interdependence.
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Clark, Spencer K., Yi Ming, and Ángel F. Adames. "Monsoon Low Pressure System–Like Variability in an Idealized Moist Model." Journal of Climate 33, no. 6 (March 15, 2020): 2051–74. http://dx.doi.org/10.1175/jcli-d-19-0289.1.
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AbstractIn this paper, it is shown that westward-propagating monsoon low pressure system–like disturbances in the South Asian monsoon region can be simulated in an idealized moist general circulation model through the addition of a simplified parameterization of land. Land is parameterized as having one-tenth the heat capacity of the surrounding slab ocean, with evaporation limited by a bucket hydrology model. In this model, the prominent topography of the Tibetan Plateau does not appear to be necessary for these storm systems to form or propagate; therefore, focus is placed on the simulation with land but no topography. The properties of the simulated storms are elucidated using regression analysis and compared to results from composites of storms from comprehensive GCMs in prior literature and reanalysis. The storms share a similar vertical profile in anomalous Ertel potential vorticity to those in reanalysis. Propagation, however, does not seem to be strongly dictated by beta drift. Rather, it seems to be more closely consistent with linear moisture vortex instability theory, with the exception of the importance of the vertical advection term in the Ertel potential vorticity budget toward the growth and maintenance of disturbances. The results presented here suggest that a simplified GCM configuration might be able to be used to gain a clearer understanding of the sensitivity of monsoon low pressure systems to changes in the mean state climate.
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LOE, B. R., MEHFOOZ ALI, and D. JOARDAR. "Synoptic systems associated with onset of southwest monsoon over Rajasthan." MAUSAM 56, no. 4 (January 20, 2022): 857–72. http://dx.doi.org/10.54302/mausam.v56i4.1045.
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An attempt has been made to identify the main synoptic features, which are responsible for onset of southwest monsoon over the two met sub divisions east and west Rajasthan for the period 1971-2004 based on modern observational technology and developed forecasting technique in the field of short-range prediction of monsoon surges. The low pressure systems (LPS) originate over Arabian Sea, especially the formation of MTC which indicates a good signature for predicting onset of monsoon, which is the prominent feature for causing onset of summer monsoon over Rajasthan. In addition LPSs form over Bay of Bengal moved in west to northwestwards direction and reach over Rajasthan and its neighbourhood region as a lopar or an upper air cycir, also brings monsoon over Rajasthan.
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MOOLEY, DA, and J. SHUKLA. "Main features of the westward-moving low pressure systems which form over the Indian region during the summer monsoon season and their relation to the monsoon rainfall." MAUSAM 40, no. 2 (April 28, 2022): 35–50. http://dx.doi.org/10.54302/mausam.v40i2.2041.
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The main features of the transitory monsoon low pressure systems (LPS) over the Indian region during the period 1888-1983, in respect of their formation, life, movement, intensity and dissipation, as well as their Inter annual variability are examined by utilizing the Information and data contained In the Indian Daily Weather Reports. The relationships of these features with the monsoon rainfall over India/north India/central India/south India are examined.
The paper discusses a variety of statistics about the low pressure systems, e.g., formation, location, life, intensity, movement and dissipation.
The number of LPS formed is not significantly related to the monsoon rainfall over India except for a weak relationship with central India monsoon rainfall. Number of LPS days during the season is significantly and directly related to the Indian monsoon rainfall (abov.e.5%) and to central Indian monsoon rainfall (above I %) and these relationships generally show good stability. Total westward longitudinal displacement of the LPS during the season and the monsoon rainfall over India and central India are directly and significantly (above 1 %) related and these relationships also generally show good stability.
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MUKHERJEE, AK, and G. NATARAJAN. "Westward moving sea level low pressure systems in the south Bay of Bengal during southwest monsoon." MAUSAM 19, no. 3 (May 5, 2022): 285–88. http://dx.doi.org/10.54302/mausam.v19i3.5325.
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The movement of low pressure systems from the east, across south Bay of Bengal and south Indian Peninsula, during July and August, when the southwest monsoon has established itself over India has been studied for a 20-year period from 1946 to 1965, Only well marked systems, which could be delineated on the sea level synoptic chart, are reported In the present paper, The characteristics of these systems and its relation with other systems breaks in southwest monsoon etc are discussed.
The effect of migratory lows on rainfall has been studied for Madras State, Kerala and coastal Mysore, While Madras gets good rain with scattered heavy to very heavy falls, the rainfall over the West Coast particularly in the northern parts, is comparatively less marked.
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MOHAPATRA, M. "Relative contribution of synoptic systems to monsoon rainfall over Orissa." MAUSAM 58, no. 1 (November 26, 2021): 17–32. http://dx.doi.org/10.54302/mausam.v58i1.1125.
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ABSTRACT. The low/depression over northwest (NW) Bay of Bengal is the largest contributor to seasonal monsoon rainfall over all stations in Orissa and Orissa as a whole. The Low Pressure Systems (LPS) and cyclonic circulation (cycir) extending upto 500 hPa level over NW Bay of Bengal alone contribute about 22% to the seasonal monsoon rainfall through about 12 days. The monsoon trough without any significant embedded systems over Orissa and adjoining regions contributes about 28% to seasonal rainfall through about 55 days. All types of LPS including low, depression and cyclonic storm yield maximum rainfall in their left forward (southwest) sectors. The maximum rainfall belt lies more southward due to a depression compared to that due to a low. The spatial distribution of rainfall due to cycir is less systematic. The interaction due to Eastern Ghat plays a significant role in spatial distribution of rainfall over western and eastern sides of the Eastern Ghat due to monsoon lows and depressions over Orissa and adjoining Bay and land regions. The orographic interaction due to Eastern Ghat with the cycirs over Orissa and adjoining Bay and land regions is significantly less leading to no significant difference in spatial distribution of rainfall over eastern and western sides of the Eastern Ghat.
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Srivastava, Ankur, Suryachandra A. Rao, D. Nagarjuna Rao, Gibies George, and Maheswar Pradhan. "Structure, characteristics, and simulation of monsoon low-pressure systems in CFSv2 coupled model." Journal of Geophysical Research: Oceans 122, no. 8 (August 2017): 6394–415. http://dx.doi.org/10.1002/2016jc012322.
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Sørland, Silje Lund, Asgeir Sorteberg, Changhai Liu, and Roy Rasmussen. "Precipitation response of monsoon low‐pressure systems to an idealized uniform temperature increase." Journal of Geophysical Research: Atmospheres 121, no. 11 (June 9, 2016): 6258–72. http://dx.doi.org/10.1002/2015jd024658.
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Geng, Biao, Kunio Yoneyama, and Ryuichi Shirooka. "Observations of Upper-Tropospheric Influence on a Monsoon Trough over the Western North Pacific." Monthly Weather Review 142, no. 4 (March 27, 2014): 1472–88. http://dx.doi.org/10.1175/mwr-d-13-00233.1.
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Abstract This study examined the synoptic evolution and internal structure of a monsoon trough in association with the deep equatorward intrusion of a midlatitude upper trough in the western North Pacific Ocean in June 2008. The study was based on data from routine synoptic observations and intensive observations conducted on board the research vessel Mirai at 12°N, 135°E. The monsoon trough was first observed to extend southeastward from the center of a tropical depression. It then moved northward, with its eastern edge moving faster and approaching a surface low pressure cell induced by the upper trough. The distinct northward migration caused the monsoon trough to become oriented from the southwest to the northeast. The monsoon trough merged with the surface low pressure cell and extended broadly northeastward. The passage of the monsoon trough over the Mirai was accompanied by lower pressure, higher air and sea surface temperature, and minimal rainfall. The monsoon trough extended upward to nearly 500 hPa and sloped southward with height. It was overlain by northwesterly winds, negative geopotential height and temperature anomalies, and extremely dry air in the upper troposphere. Precipitation systems were weak and scattered near the monsoon trough but were intense and extensive south of the surface monsoon trough, where intense low-level convergence and upper-level divergence caused deep and vigorous upward motion. It appears that the upper trough exerted important impacts on the development of both the monsoon trough and associated precipitation, which are discussed according to the observational results.
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A, Kashyapi, Shripad V K, and Natu J C. "Cyclonic storms and Depressions over the north Indian Ocean during 2019*." MAUSAM 71, no. 3 (August 3, 2021): 357–76. http://dx.doi.org/10.54302/mausam.v71i3.35.
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During 2019, in all 12 intense low pressure systems formed over the Indian Seas. These include; one Super cyclonic storm (KYARR), one extremely severe cyclonic storm (FANI), 4 very Severe Cyclonic Storms (VAYU, HIKAA, MAHA & BULBUL), 2 Cyclonic Storms (PABUK & PAWAN), 3 Deep Depressions and 1 Depression. Out of these 12 systems, 4 systems formed over the Bay of Bengal and 8 over the Arabian Sea. Arabian Sea remained exceptionally active in terms of cyclogenesis this year, especially in the post monsoon season. The season-wise distribution had been one cyclonic storm in winter, one in pre-monsoon season, 2 depressions and 2 very severe cyclonic storms during the monsoon season and 4 cyclonic storms and 3 depressions in Post monsoon season.
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JADHAV, S. K. "Summer monsoon low pressure systems over the Indian region and their relationship with the sub-divisional rainfall." MAUSAM 53, no. 2 (January 18, 2022): 177–86. http://dx.doi.org/10.54302/mausam.v53i2.1633.
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In the present paper performance of the monthly sub-divisional summer monsoon rainfall is studied in association with the position of the Low Pressure System (LPS) over the Indian region. Existence of the LPS over a particular location increases the rainfall activities in certain parts of the country while decreases in some other parts. For this study, the Indian region (5°-35° N and 60° -100° E) is divided into 5° Lat. ´ 5° Long. grids. The duration of LPS is taken in terms of LPS days with respect to the location of LPS in a particular grid. Monthly total number of LPS days in each of the grids are computed during the summer monsoon season, June to September for the period 1891 – 1990. Maximum number of LPS days (more than half of the total) are observed in the latitude belt between 20°-25°N. The percentages of total LPS days in this area are higher in July and August which are peak monsoon months as compared to June and September. When there is a LPS are in the area 20°-25° N and 80°-90° E, there is significant increase in the rainfall activities in the sub-divisions along mean monsoon trough while northeast India and southeast peninsular India experience significant decrease in rainfall in the months of July and August. Owing to the movement of LPS from east to west through central India, most parts of the country, excluding northeast India and south peninsular India get good rainfall activity. Correlation coefficients between monthly LPS days over the different grids and monthly sub-divisional rainfall are computed to study the relationships. The performance of sub-divisional rainfall mostly related with the occurrence of LPS in certain grid- locations. The correlation field maps may give some useful information about rainfall performance due to LPS in a particular grid locations.
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Ko, Ken-Chung, and Po-Sheng Chiu. "ISO-Modulating Effects on the East Asian Summer Monsoon Circulation Patterns Associated with Southern Taiwan’s Monsoon Rainfall." Monthly Weather Review 142, no. 9 (September 2014): 3163–77. http://dx.doi.org/10.1175/mwr-d-13-00327.1.
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In this study, the circulation patterns of the summer monsoon associated with monsoon rainfall in southern Taiwan were analyzed and two types of monsoon patterns were defined. The first type was characterized by a broad low pressure area extending northeastward from a low pressure center near southern China to southern Japan. Strong southwesterly flows were observed over the southern flank of the monsoon trough. The second type of monsoon pattern was characterized by a strong westward-extending anticyclone in the area north of Taiwan (including central China, South Korea, and Japan), and a weaker east–west-elongated monsoon trough south of Taiwan, driving the easterly flow to southern Taiwan. The modulating effect of intraseasonal oscillations (ISOs) on these monsoon flow patterns indicates that as ISOs propagate northwestward toward southern China, they create favorable conditions for developing and strengthening southwesterly flows and convection. However, because of the latitudinal limit of northwestward-propagating ISOs, only the edge of the strongest convection over the southern flank of the ISO cyclonic circulation reaches southern Taiwan during the westerly phase. Thus, although the westerly ISO pattern appears to be stronger than the easterly pattern, it brings less rainfall to southern Taiwan. Through the tightening of pressure gradients, the ISO typically generates anomalous cyclones (anticyclones) that can affect the southwesterly and northeasterly flows near its southern (northern) and northern (southern) rims. Therefore, fluctuations in the low-frequency background flow can exert a notable effect on the monsoon rainfall and associated circulation systems near Taiwan.
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Yoden, Shigeo, Vinay Kumar, Surendra Dhaka, and Matthew Hitchman. "Global monsoon systems and their modulation by the equatorial Quasi-Biennial Oscillation." MAUSAM 74, no. 2 (March 29, 2023): 239–52. http://dx.doi.org/10.54302/mausam.v74i2.5948.
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Monthly-mean data of ERA-Interim reanalysis, precipitation, outgoing longwave radiation (OLR) and sea surface temperature(SST) are investigated for 40 years (1979-2018) to reveal the modulation of the global monsoon systems by the equatorial quasi-biennial oscillation (QBO), focusing only on the neutral El Niño-Southern Oscillation (ENSO) periods (in total 374 months). First, the climatology of the global monsoon systems is viewed with longitude-latitude plots of the precipitation, its proxies and lower tropospheric circulations for the annual mean and two solstice seasons, together with the composite differences between the two seasons. In addition to seasonal variations of Intertropical Convergence Zones (ITCZs), several regional monsoon systems are well identified with an anti-phase of the annual cycle between the two hemispheres. Precipitation-related quantities (OLR and specific humidity), surface conditions [i.e., mean sea level pressure (MSLP) and SST] and circulation fields related to moist convection systems show fundamental features of the global monsoon systems. After introducing eight QBO phases based on the leading two principal components of the zonal-mean zonal wind variations in the equatorial lower-stratosphere, the statistical significance of the composite difference in the precipitation and tropospheric circulation is evaluated for the opposite QBO phases. The composite differences show significant modulations in some regional monsoon systems, dominated by zonally asymmetric components, with the largest magnitudes for specific QBO-phases corresponding to traditional indices of the equatorial zonal-mean zonal wind at 20 and 50 hPa. Along the equator, significant QBO influence is characterized by the modulation of the Walker circulation over the western Pacific. In middle latitudes during boreal summer, for a specific QBO-phase, statistically significant modulation of low-pressure cyclonic perturbation is obtained over the Northern-Hemisphere western Pacific Ocean associated with statistically significant features of heavier precipitation over the eastern side of the cyclonic perturbation and the opposite lighter precipitation over the western side. During boreal winter, similar significant low-pressure cyclonic perturbations were found over the Northern-Hemisphere eastern Pacific and Atlantic Oceans for specific phases.
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MOHAPATRA, M., and U. C. MOHANTY. "Excess and deficient summer monsoon rainfall over Orissa in relation to low pressure systems." MAUSAM 60, no. 1 (November 27, 2021): 25–38. http://dx.doi.org/10.54302/mausam.v60i1.959.
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A study has been undertaken to find out different characteristics like frequency, intensity, movement, region of occurrence etc. of low pressure systems (LPS) including low, depression and cyclonic storm etc. developing over Orissa and neighbouring sea and land regions during excess and deficient monsoon rainfall months (June – September) over Orissa. The study is based on data of 20 years (1980-1999). The principal objective of this study is to find out the contribution of LPS to extreme monsoon rainfall activity over Orissa.
The number of LPS days rather than frequency of formation of LPS over different regions better explain the excess and deficient rainfall over Orissa. The excess rainfall over Orissa during June is not significantly related with the number of LPS days. Significantly less than normal number of LPS days over northwest (NW) Bay of Bengal and Gangetic West Bengal (GWB) and higher number of LPS days over west central (WC) Bay off north coastal Andhra Pradesh (NCAP) cause deficient rainfall over Orissa during June. While significantly higher than normal number of LPS days over NW Bay and Orissa leads to excess rainfall during July, less than normal number of LPS days over WC Bay off NCAP is associated with excess rainfall during August. The less number of LPS days over Orissa due to less frequent movement of LPS across Orissa from the Bay of Bengal leads to deficient rainfall over Orissa during both July and August. Significantly higher/less than normal number of LPS days over NW Bay leads to excess/deficient rainfall over Orissa during September.
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Sørland, Silje Lund, and Asgeir Sorteberg. "The dynamic and thermodynamic structure of monsoon low-pressure systems during extreme rainfall events." Tellus A: Dynamic Meteorology and Oceanography 67, no. 1 (August 12, 2015): 27039. http://dx.doi.org/10.3402/tellusa.v67.27039.
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MOHAPATRA, M., H. R. BISWAS, and G. K. SAWAISARJE. "Daily summer monsoon rainfall over northeast India due to synoptic scale systems." MAUSAM 59, no. 1 (November 27, 2021): 35–50. http://dx.doi.org/10.54302/mausam.v59i1.1130.
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The summer monsoon rainfall over northeast India mostly depends on the synoptic systems over the region and neighbourhood and the convection due to the interaction of orography with the synoptic and sub-synoptic scale systems. Hence, an attempt is made to analyse the mean daily rainfall distribution over northeast India due to different synoptic systems like Low Pressure Systems (LPS) and cyclonic circulations (cycir) extending upto lower/middle tropospheric levels over different regions. The mean daily rainfall due to monsoon trough over various locations in northeast India is also analysed. For the above purpose, the rainfall data over 50 uniformly distributed stations in northeast India during summer monsoon season (June-September) for a period of 10 years (1991-2000) are considered. The principal objective of the study is to find out the contribution of the different synoptic systems to the spatial variability of monsoon rainfall over northeast India.
The developed synoptic analog maps may be useful to the forecasters for 24 hours rainfall forecast with the knowledge of location, intensity and movement of the synoptic systems. Based on larger data set, the results confirm the earlier findings (Srinivasan et al., 1972) with respect to rainfall due to monsoon trough and LPS. The Low Level Cycir (LLC) also plays significant role on the rainfall variability over northeast India, as the number of LLC days is significantly higher over the region, contrary to the days of occurrence of LPS. The study finds out the regions of excess/deficient rainfall and active/weak monsoon conditions due to different synoptic systems.
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Adames, Ángel F., and Yi Ming. "Moisture and Moist Static Energy Budgets of South Asian Monsoon Low Pressure Systems in GFDL AM4.0." Journal of the Atmospheric Sciences 75, no. 6 (June 1, 2018): 2107–23. http://dx.doi.org/10.1175/jas-d-17-0309.1.
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Abstract The mechanisms that lead to the propagation of anomalous moisture and moist static energy (MSE) in monsoon low and high pressure systems, collectively referred to as synoptic-scale monsoonal disturbances (SMDs), are investigated using daily output fields from GFDL’s atmospheric general circulation model, version 4.0 (AM4.0). On the basis of linear regression analysis of westward-propagating rainfall anomalies of time scales shorter than 15 days, it is found that SMDs are organized into wave trains of three to four individual cyclones and anticyclones. These events amplify over the Bay of Bengal, reach a maximum amplitude over the eastern coast of India, and dissipate as they approach the Arabian Sea. The structure and propagation of the simulated SMDs resemble those documented in observations. It is found that moisture and MSE anomalies exhibit similar horizontal structures in the simulated SMDs, indicating that moisture is the leading contributor to MSE. Propagation of the moisture anomalies is governed by vertical moisture advection, while the MSE anomalies propagate because of horizontal advection of dry static energy by the anomalous winds. By combining the budgets, we interpret the propagation of the moisture anomalies in terms of lifting that is forced by horizontal dry static energy advection, that is, ascent along sloping isentropes. This process moistens the lower free troposphere, producing an environment that is more favorable to deep convection. Ascent driven by radiative heating is of primary importance to the maintenance of the moisture anomalies.
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PUTTANNA, B., and GEETA AGNIHOTRI. "Floods in Karnataka during 2009 : A synoptic study." MAUSAM 63, no. 2 (December 16, 2021): 223–30. http://dx.doi.org/10.54302/mausam.v63i2.1396.
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Karnataka a State in south peninsular India receives 73% of its annual rainfall during southwestmonsoon season. Because of the complex physiographical features, the rainfall pattern over the State shows large spatialvariation from 50 to 350 cms. The coefficient of interannual variation of the monsoon rainfall is about 15% over coastalKarnataka (CK) and between 20-30% over interior Karnataka. The precipitation over this State is mainly dominated bysemi permanent systems like off shore trough running along the west coast, low pressure systems forming over the Bay ofBengal during the monsoon season. A well marked low pressure area during 28 September - 3 October 2009 over the Bayof Bengal caused widespread damage to life and property in north Karnataka. In this study, an attempt has been made toexamine the synoptic features of this system that caused heavy rainfall over many districts of the State leading towidespread destruction.
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MOOLEY, DA, and J. SHUKLA. "Index of activity of the monsoon trough over India." MAUSAM 40, no. 3 (April 28, 2022): 19–30. http://dx.doi.org/10.54302/mausam.v40i3.2109.
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Measures for the daily, monthly and seasonal activity of the Indian monsoon trough are developed; and their variability and relationships with Indian rainfall during the summer monsoon season are examined.
The daily monsoon troug activity index is the anomaly of the daily pressure within the trough from the long-period mean. The frequency of the normalized daily monsoon trough activity index < -1.0 is higher in years of good Indian monsoon than that in years of deficient Indian monsoon, at a level of significance of above 0: 1%. For days in September, the daily monsoon trough activity index is appreciably higher (i.e., trough appreciably weaker) in years of deficient Indian monsoon than that in years of good Indian monsoon.
The monthly measures of the monsoon trough activity, derived from the daily monsoon trough activity index show significant relationship with monthly rainfall for all the monsoon months except June. The relationship for September is highly significant and stable.
Amongst the seasonal measures of the monsoon trough activity, the seasonal frequency of days with active monsoon trough is significantly (at I %) and stably related to Indian monsoon rainfall.
Low pressure systems are found to add largely to the activity of the monsoon trough.
The seasonal frequency of days with active monsoon trough is related directly and significantly (at 1 %) to April 500 mb ridge location over India, inversely and significantly (at 1 %) to the eastern equatorial Pacific SST anomaly tendency from MAM to JJA. The April 500 mb ridge thus appears to be a good indicator of the monsoon trough activity over India during the monsoon season.
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Mohapatra, M., and U. C. Mohanty. "Spatio-temporal variability of summer monsoon rainfall over Orissa in relation to low pressure systems." Journal of Earth System Science 115, no. 2 (April 2006): 203–18. http://dx.doi.org/10.1007/bf02702034.
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MAZUMDAR, A. B. "Southwest monsoon rainfall in India : Part II - Principal components in temporal domain." MAUSAM 49, no. 3 (December 17, 2021): 301–8. http://dx.doi.org/10.54302/mausam.v49i3.3635.
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An attempt has been made towards objective identification of phases of the southwest monsoon by principal component analysis (PCA) in temporal domain (T-mode). The method utilizes the relationship of weekly rainfall activities with principal components (PCs) of southwest monsoon. Based on the relationships, subgroup of weeks with similar spatial patterns have been identified. Synoptic features of these subgroups have been brought out with the help of synoptic charts. The first four significant PCs are associated with four kinds of active phases of the southwest monsoon when the low pressure systems have typical characteristics corresponding to each PC. Thus, the study suggests a method of interpretation of PCs with the help of synoptic charts by objective identification of phases of southwest monsoon.
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Deoras, Akshay, Kieran M. R. Hunt, and Andrew G. Turner. "Comparison of the Prediction of Indian Monsoon Low Pressure Systems by Subseasonal-to-Seasonal Prediction Models." Weather and Forecasting 36, no. 3 (June 2021): 859–77. http://dx.doi.org/10.1175/waf-d-20-0081.1.
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AbstractThis study analyzes the prediction of Indian monsoon low pressure systems (LPSs) on an extended time scale of 15 days by models of the Subseasonal-to-Seasonal (S2S) prediction project. Using a feature-tracking algorithm, LPSs are identified in 11 S2S models during a common reforecast period of June–September 1999–2010, and then compared with 290 and 281 LPSs tracked in ERA-Interim and MERRA-2 reanalysis datasets. The results show that all S2S models underestimate the frequency of LPSs. They are able to represent transits, genesis, and lysis of LPSs; however, large biases are observed in the Australian Bureau of Meteorology, China Meteorological Administration (CMA), and Hydrometeorological Centre of Russia (HMCR) models. The CMA model exhibits large LPS track position error and the intensity of LPSs is overestimated (underestimated) by most models when verified against ERA-Interim (MERRA-2). The European Centre for Medium-Range Weather Forecasts and Met Office models have the best ensemble spread–error relationship for the track position and intensity, whereas the HMCR model has the worst. Most S2S models are underdispersive—more so for the intensity than the position. We find the influence of errors in the LPS simulation on the pattern of total precipitation biases in all S2S models. In most models, precipitation biases increase with forecast lead time over most of the monsoon core zone. These results demonstrate the potential for S2S models at simulating LPSs, thereby giving the possibility of improved disaster preparedness and water resources planning.
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Hatsuzuka, Daisuke, Tetsuzo Yasunari, and Hatsuki Fujinami. "Characteristics of Low Pressure Systems Associated with Intraseasonal Oscillation of Rainfall over Bangladesh during Boreal Summer." Monthly Weather Review 142, no. 12 (December 1, 2014): 4758–74. http://dx.doi.org/10.1175/mwr-d-13-00307.1.
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Abstract Characteristics of low pressure systems (LPSs) responsible for submonthly-scale (7–25 days) intraseasonal oscillation (ISO) in rainfall over Bangladesh and their impact on the amplitude of active peaks are investigated for 29 summer monsoon seasons. Extreme and moderate active peaks are obtained based on the amplitude of 7–25-day-filtered rainfall series. By detecting the LPSs that formed over the Indian monsoon region, it was found that about 59% (62%) of extreme (moderate) active peaks of rainfall are related to LPSs. These LPSs have horizontal scale of about 600 km and vertical scale of about 9 km. For the extreme active peak, the locations of the LPS centers are clustered significantly over and around Bangladesh, accompanied by the maximum convergence in the southeast sector of the LPSs. After their formation, they tend to remain almost stationary over and around Bangladesh. In contrast, for the moderate active peak, the LPS centers are located over the Ganges Plain around 85°E, and the maximum convergence of the LPSs occurs around their centers. This difference in the convergence fields is closely associated with the geographical features to the north and east of Bangladesh and the horizontal scale of the LPSs. These features suggest that the amplitude of the active peaks in the submonthly-scale ISO is modulated by small differences in the locations of the LPS centers. These findings suggest that improved predictions of both genesis location and the tracks of the LPSs are crucial to forecasting seasonal rainfall over Bangladesh.
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MOHAPATRA, M. "Sub-divisional summer monsoon rainfall over India in relation to low pressure systems over the Bay of Bengal and adjoining land regions during 1982-1999." MAUSAM 59, no. 3 (November 27, 2021): 327–38. http://dx.doi.org/10.54302/mausam.v59i3.1264.
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A study is undertaken to find out characteristic features of relationship of the low pressure system (LPS) over the Bay of Bengal and adjoining land regions with the rainfall over different meteorological sub-divisions of India during summer monsoon season (June-September). For this purpose, rainfall over 35 meteorological sub-divisions in India and LPS days over west central (WC) Bay, northwest (NW) Bay, northeast (NE) Bay, Bangladesh (BDS), Gangetic West Bengal (GWB), Orissa, north coastal Andhra Pradesh (NCAP), east Madhya Pradesh and Chattisgarh (EMPC) and Jharkhand (JKD) during different monsoon months and the season as a whole over a period of 18 years (1982-1999) are analysed. There is large month to month variation in the impact of the LPS on the sub-divisional monsoon rainfall over India. However, the results presented in the study including developed correlation maps may be helpful to predict 24 hours rainfall based on the location of the LPS and associated monsoon trough.
The frequent development and persistence of LPS over NW Bay are favourable for higher seasonal monsoon rainfall over east central India. The development and persistence of LPS over WC Bay adversely affect the seasonal rainfall over this region. On the other hand, the frequent development and persistence of LPS over WC Bay and its subsequent westward movement across NCAP are favourable for higher seasonal rainfall over the peninsular region excluding west coast. The seasonal rainfall over northwest India decreases with increase in LPS days over EMPC. The seasonal rainfall over west central India, northeast India and west coast are not significantly related with the number of LPS days over the regions under consideration.
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Roy, Partha, and T. Narayana Rao. "Precipitation Characteristics of Cyclonic Disturbances over the South Asia Region as Revealed by TRMM and GPM." Journal of Climate 35, no. 15 (August 1, 2022): 4943–57. http://dx.doi.org/10.1175/jcli-d-21-0774.1.
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Abstract The relative contributions of cyclonic disturbances (CDs; i.e., low pressure systems, depressions, and cyclonic storms) and non-CDs to annual and seasonal rainfall are studied using 22 years of TRMM and GPM measurements during the passage of 866 CDs in the South Asia region (SAR). The changes in stratiform and convective precipitation within the cyclonic storm and in different CDs are also examined. The rainfall in the wettest regions of the SAR, the west coasts of India and Myanmar, and the slopes of the Himalayas is of non-CD origin, while CD rainfall peaks in the eastern parts of the monsoon trough and the northern Bay of Bengal (BOB). The CD rain fraction (RF) of annual and seasonal rainfall exhibits large spatial variation in the range of 4%–55%. The land–ocean dichotomy exhibited by CD RF is not uniform across India. Large CD RF is confined to the coast in some regions due to topographical barriers, but extends to 800–1000 km inland from the coast in the monsoon trough region. Low pressure systems contribute more to annual rain than depressions and cyclonic storms in the monsoon trough and the northern BOB (∼40%), particularly during the monsoon, mainly due to their frequent occurrence. The stratiform RF and occurrence are higher in CDs than in non-CDs, with the greatest contribution in central India (>80%), whereas the non-CDs are characterized by having higher convective RFs. The stratiform rain occurrence increases with intensification of CDs over both land and ocean, indicating its importance in the intensification of CDs and organizing large-scale systems.
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Farukh, Murad A., Md A. Islam, and Hiroshi Hayasaka. "Wildland Fires in the Subtropical Hill Forests of Southeastern Bangladesh." Atmosphere 14, no. 1 (January 1, 2023): 97. http://dx.doi.org/10.3390/atmos14010097.
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The first ever comprehensive study on wildland fires in Bangladesh is carried out to develop a fire prevention and prediction method. The major causes of huge wildland fires (88%) in the subtropical Chittagong Hill forest (43% of total) of southern Bangladesh are reported as shifting cultivation, grazing and unauthorized settlement. We used satellite hotspot (HS) data from 2003 to 2021 (a total of 54,669 HSs) to clarify the spatio-temporal structure of wildland fires. Fire weather conditions were analyzed using various weather data and synoptic-scale weather maps at different air levels. Fires concentrated from March through April or a transitional period from the dry season, caused by the Asian winter monsoon, to the wet season, due to the Asian summer monsoon. Fire occurrence depended on dry conditions and pre-monsoon showers and their timing. The difference in 925 hPa heights of high and low pressure systems may be attributed to the different types of the Asian winter and summer monsoons. The average maximum air temperature and incident shortwave solar energy in April were the highest and strongest, contributing fire-prone weather conditions. Based on the analysis, a fire prevention and prediction method will be developed, and this report may also facilitate the establishment of future CO2 reduction measures for Bangladesh.
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Hatsuzuka, Daisuke, and Hatsuki Fujinami. "Effects of the South Asian Monsoon Intraseasonal Modes on Genesis of Low Pressure Systems over Bangladesh." Journal of Climate 30, no. 7 (April 2017): 2481–99. http://dx.doi.org/10.1175/jcli-d-16-0360.1.
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The quasi-biweekly oscillation (QBW) is a dominant intraseasonal mode in summer rainfall over Bangladesh. Active phases of the QBW are often accompanied by low pressure systems (LPSs) such as vortex-type lows. This study investigated the effects of two intraseasonal modes, the QBW and the boreal summer intraseasonal oscillation (BSISO), on the genesis of LPSs over Bangladesh during 29 summer monsoon seasons. Daily lag composites of convection and low-level atmospheric circulation were constructed for active-phase cases with LPSs (LPS case) and without LPSs (non-LPS case) based on rainfall in the QBW over Bangladesh. In the QBW mode, a westward propagation of an anticyclonic anomaly from the western Pacific to the Bay of Bengal (BoB) is common in both cases. However, the anticyclonic center in the LPS case is located slightly to the east of that in the non-LPS case, which results in stronger cyclonic vorticity over and around Bangladesh. In contrast, the BSISO mode shows an opposite phase between the two cases: a cyclonic (anticyclonic) anomaly propagating northward from the equator to the BoB in the LPS case (non-LPS case). In the LPS case, the cyclonic anomaly in the BSISO mode enhances the westerly (easterly) flow over the BoB (Bangladesh) in the active phase, resulting in the enhancement of cyclonic vorticity over the northern BoB and Bangladesh, in cooperation with the QBW mode. These results suggest that both the QBW and BSISO modes have significant influence on the environmental conditions for LPS genesis over Bangladesh.
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Dong, Wenhao, Yanluan Lin, Jonathon S. Wright, Yuanyu Xie, Fanghua Xu, Wenqing Xu, and Yan Wang. "Indian Monsoon Low-Pressure Systems Feed Up-and-Over Moisture Transport to the Southwestern Tibetan Plateau." Journal of Geophysical Research: Atmospheres 122, no. 22 (November 20, 2017): 12,140–12,151. http://dx.doi.org/10.1002/2017jd027296.
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Ditchek, Sarah D., William R. Boos, Suzana J. Camargo, and Michael K. Tippett. "A Genesis Index for Monsoon Disturbances." Journal of Climate 29, no. 14 (June 30, 2016): 5189–203. http://dx.doi.org/10.1175/jcli-d-15-0704.1.
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Abstract Synoptic-scale monsoon disturbances produce the majority of continental rainfall in the monsoon regions of South Asia and Australia, yet there is little understanding of the conditions that foster development of these low pressure systems. Here a genesis index is used to associate monsoon disturbance genesis in a global domain with monthly mean, climatological environmental variables. This monsoon disturbance genesis index (MDGI) is based on four objectively selected variables: total column water vapor, low-level absolute vorticity, an approximate measure of convective available potential energy, and midtropospheric relative humidity. A Poisson regression is used to estimate the index coefficients. Unlike existing tropical cyclone genesis indices, the MDGI is defined over both land and ocean, consistent with the fact that monsoon disturbance genesis can occur over land. The index coefficients change little from their global values when estimated separately for the Asian–Australian monsoon region or the Indian monsoon region, suggesting that the conditions favorable for monsoon disturbance genesis, and perhaps the dynamics of genesis itself, are common across multiple monsoon regions. Vertical wind shear is found to be a useful predictor in some regional subdomains; although previous studies suggested that baroclinicity may foster monsoon disturbance genesis, here genesis frequency is shown to be reduced in regions of strong climatological vertical shear. The coefficients of the MDGI suggest that monsoon disturbance genesis is fostered by humid, convectively unstable environments that are rich in vorticity. Similarities with indices used to describe the distribution of tropical cyclone genesis are discussed.
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RANALKAR, Manish, RK Giri, and Laxmi Pathak. "The deluge of peninsular India during Northeast Monsoon 2015: Observational aspects of thermodynamical, dynamical and microphysical features." MAUSAM 73, no. 4 (September 30, 2022): 909–24. http://dx.doi.org/10.54302/mausam.v73i4.5853.
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During the period 1st November 2015 to 4th December 2015, Southern India especially the city of Chennai was battered by heavy rainfall owing to the passage of a Deep Depression during 8-11 November 2015 and low-pressure areas during 13-18 November 2015, 19-24 Nov. 2015 and 29 November 2015 - 4 December 2015. The deluge due to heavy rainfall associated with these systems resulted in enormous loss of life and property. A wave perturbation in deep zonal flow moved from east to west in November 2015. The observational, synoptic, dynamical and microphysical aspects of these systems have been analysed using in-situ surface observations, satellite observations and reanalysed datasets.
The results show the existence of a dominant trough in the easterlies off the southeast coast of peninsular India at 850 hPa and anomalous anticyclonic circulation over central parts of the country at 500 hPa extending up to the upper troposphere were conducive to the northward movement of these low-pressure systems and copious rainfall over NMR. The surface observational features of the deluge were explored using the Automatic Weather Stations (AWS), High Wind Speed Recording (HWSR) systems, Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) satellites. The microphysical aspects of these low-pressure systems are also analysed using TRMM and ERA-Interim datasets.
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Zhao, Ping, Song Yang, Huijun Wang, and Qiang Zhang. "Interdecadal Relationships between the Asian–Pacific Oscillation and Summer Climate Anomalies over Asia, North Pacific, and North America during a Recent 100 Years." Journal of Climate 24, no. 18 (September 15, 2011): 4793–99. http://dx.doi.org/10.1175/jcli-d-11-00054.1.
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Abstract Summertime relationships between the Asian–Pacific Oscillation (APO) and climate anomalies over Asia, the North Pacific, and North America are examined on an interdecadal time scale. The values of APO were low from the 1880s to the mid-1910s and high from the 1920s to the 1940s. When the APO was higher, tropospheric temperatures were higher over Asia and lower over the Pacific and North America. From the low-APO decades to the high-APO decades, both upper-tropospheric highs and lower-tropospheric low pressure systems strengthened over South Asia and weakened over North America. As a result, anomalous southerly–southwesterly flow prevailed over the Asian monsoon region, meaning stronger moisture transport over Asia. On the contrary, the weakened upper-tropospheric high and lower-tropospheric low over North America caused anomalous sinking motion over the region. As a result, rainfall generally enhanced over the Asian monsoon regions and decreased over North America.
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Stone, Tim. "Last glacial cycle hydrological change at Lake Tyrrell, southeast Australia." Quaternary Research 66, no. 1 (July 2006): 176–81. http://dx.doi.org/10.1016/j.yqres.2006.03.007.
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AbstractLake Tyrrell is the largest playa in the Murray Basin of southeast Australia. Optical dating of transverse dune (lunette) sediments extends the lake's radiocarbon chronology to the last interglacial period. The highest lake level was attained 131,000 ± 10,000 yr ago, forming Lake Chillingollah, a megalake that persisted until around 77,000 ± 4000 yr ago. Pedogenesis of its sandy lunette continued until buried by a silty clay lunette deflated from the lake floor 27,000 ± 2000 yr ago. The dated soil-stratigraphic units correlate with the upper Tyrrell Beds and contain evidence that humans visited the lakeshore before 27,000 yr ago. The Lake Chillingollah megalake was synchronous with very high lake levels in monsoon-dominated Australia, yet it was not influenced by tropical monsoon systems. It was filled instead by increased winter rainfall from westerly low-pressure fronts. Greater effective precipitation across Australia is evident, the result of a weakened subtropical high-pressure zone.
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Deoras, Akshay, Kieran M. R. Hunt, and Andrew G. Turner. "The four regional varieties of South Asian monsoon low‐pressure systems and their modulation by tropical intraseasonal variability." Weather 76, no. 6 (May 30, 2021): 194–200. http://dx.doi.org/10.1002/wea.3997.
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Al-Qadi, Tagreed Ahmed, Khamis Daham Muslih, and Alyaa Gatea Shiltagh. "Analysis of Correlation and Coupling between El Niño-Southern Oscillation and Dust Storms in Iraq from 1971 to 2016." Iraqi Geological Journal 54, no. 1E (May 31, 2021): 103–13. http://dx.doi.org/10.46717/igj.54.1e.9ms-2021-05-30.
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This study's main objective is to examine the impact of the El Niño-Southern Oscillation on dust storms in Iraq and trying to find a reasonable synoptic explanation of this relationship. In order to achieve our goal, various data collections related to dust storm days from eight stations in Iraq and the El Niño-Southern Oscillation data over the period 1971–2016 were analyzed. The results showed a negative relationship between dust storms and the El Niño-Southern Oscillation. This correlation means high dust storms are associated with La Niña, while the low dust storms occur during the El Niño event. The coupling between the tropical pacific sector and dust storms in Iraq is perhaps related to the El Niño-Southern Oscillation variations and how they can contribute to the change of the main pressure system values that mainly participate in triggering dust storms and their intensity. The subtropical high-pressure and the Indian monsoon low-pressure systems have been recognized as the responsible pressure systems for dust storms formation.
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Raia, Adma, and Iracema Fonsecade Albuquerque Cavalcanti. "The Life Cycle of the South American Monsoon System." Journal of Climate 21, no. 23 (December 1, 2008): 6227–46. http://dx.doi.org/10.1175/2008jcli2249.1.
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Abstract The South American monsoon system (SAMS) life cycle plays an important role in the distribution and duration of the rainy season mainly over southwestern Amazonia, and the central west and southeast Brazil regions, affecting the economy through impacts on the agriculture and hydrology sectors. In this study a new criterion is applied to identify the monsoon onset and demise that was not used before in the SAMS region. This criterion is based on the atmospheric humidity flux over an area recognized as the monsoon core because of zonal wind reversal and changes in humidity from the transition seasons to summer and winter. Areas in Brazil that have a monsoon regime are identified, and several features associated with the life cycle are discussed. The climatological onset and demise are identified as the end of October and the end of March, respectively, and an interannual variability is found in the times of onset/demise. The main observed features in the two phases are discussed, such as the role of the South Atlantic subtropical high displacement, the northwestern moisture flux east of Andes and from the Atlantic Ocean, the zonal wind intensity and direction over central South America, the vertical motion over the continent, and the expansion/reduction of the Bolivian high circulation with associated high-level divergence. The frontal systems contribute to the pressure decrease, wind direction changes, and soil moisture increase previous to the onset. Low-frequency troughs with intraseasonal variability establish conditions favorable to the monsoon onset, and low-frequency ridges are related to late onset.
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Meera, M., E. Suhas, and S. Sandeep. "Downstream and In Situ: Two Perspectives on the Initiation of Monsoon Low‐Pressure Systems Over the Bay of Bengal." Geophysical Research Letters 46, no. 21 (November 3, 2019): 12303–10. http://dx.doi.org/10.1029/2019gl084555.
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De Vleeschouwer, D., A. C. da Silva, F. Boulvain, M. Crucifix, and Ph Claeys. "Precessional and half-precessional climate forcing of Mid-Devonian monsoon-like dynamics." Climate of the Past Discussions 7, no. 3 (May 2, 2011): 1427–55. http://dx.doi.org/10.5194/cpd-7-1427-2011.
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Abstract. A Devonian magnetic susceptibility (MS) record obtained on limestones ranging from the Uppermost-Eifelian to the Lower-Givetian and located on the southern border of the Dinant Synclinorium in Belgium, was selected for time-series analysis. In these carbonate ramp and platform deposits, spectral analyses highlight persistent high-frequency cycles in both the MS-signal and the microfacies curve, reflecting environmental and climate changes. These meter-scale variations in the MS-signal are interpreted as changes in the flux of magnetic minerals towards the marine system, most likely controlled by monsoon rainfall-intensity. By combining chrono- and biostratigraphic information with theoretical knowledge of sedimentation rates in different depositional environments, these cycles are interpreted as astronomically driven (precession-dominated). It is hypothesized that during precession maxima the trans-equatorial pressure gradient reaches a maximum and intensifies monsoonal circulation. The consequent increased moisture transport towards the continent leads to enhanced precipitation and runoff, which in turn leads to an increased flux of detrital material (including magnetic minerals responsible for the MS-signal) towards the marine system. Moreover, this unique high-resolution climate signal reveals half-precessional cycles. These cycles suggest the important response of intense monsoonal systems to periodic changes in the strength of low-latitude (equatorial) insolation.
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Levine, Richard C., and Gill M. Martin. "On the climate model simulation of Indian monsoon low pressure systems and the effect of remote disturbances and systematic biases." Climate Dynamics 50, no. 11-12 (September 16, 2017): 4721–43. http://dx.doi.org/10.1007/s00382-017-3900-x.
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Adames, Ángel F., and Yi Ming. "Interactions between Water Vapor and Potential Vorticity in Synoptic-Scale Monsoonal Disturbances: Moisture Vortex Instability." Journal of the Atmospheric Sciences 75, no. 6 (June 1, 2018): 2083–106. http://dx.doi.org/10.1175/jas-d-17-0310.1.
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AbstractSouth Asian monsoon low pressure systems, referred to as synoptic-scale monsoonal disturbances (SMDs), are convectively coupled cyclonic disturbances that are responsible for up to half of the total monsoon rainfall. In spite of their importance, the mechanisms that lead to the growth of these systems have remained elusive. It has long been thought that SMDs grow because of a variant of baroclinic instability that includes the effects of convection. Recent work, however, has shown that this framework is inconsistent with the observed structure and dynamics of SMDs. Here, we present an alternative framework that may explain the growth of SMDs and may also be applicable to other modes of tropical variability. Moisture is prognostic and is coupled to precipitation through a simplified Betts–Miller scheme. Interactions between moisture and potential vorticity (PV) in the presence of a moist static energy gradient can be understood in terms of a “gross” PV (qG) equation. The qG summarizes the dynamics of SMDs and reveals the relative role that moist and dry dynamics play in these disturbances, which is largely determined by the gross moist stability. Linear solutions to the coupled PV and moisture equations reveal Rossby-like modes that grow because of a moisture vortex instability. Meridional temperature and moisture advection to the west of the PV maximum moisten and destabilize the column, which results in enhanced convection and SMD intensification through vortex stretching. This instability occurs only if the moistening is in the direction of propagation of the SMD and is strongest at the synoptic scale.
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METRI, S. M., and KHUSHVIR SINGH. "Study of rainfall features over Goa state during southwest monsoon season." MAUSAM 61, no. 2 (November 27, 2021): 155–62. http://dx.doi.org/10.54302/mausam.v61i2.796.
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In this paper the rainfall features at different raingauge stations of Goa state have been studied for the period of 30 years. The statistical parameters such as mean monthly rainfall, Standard Deviation and Coefficient of Variation have been computed for each raingauge station of Goa. Some heavy rainfall events during the period have also been studied. The study shows the significant rising trend of rainfall towards the eastern parts of Goa. Goa experiences an average rainfall of about 330 cm annually and around 90% of annual rainfall occurs during southwest monsoon season i.e. (June to September). Studies revealed that most of heavy rainfall events caused due to active off-shore trough and low pressure systems formed over southeast Arabian Sea. It has also come out from the study that the orography of Goa plays an important role in rainfall distribution. Valpoi receives maximum rainfall due to its orographic effect.
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Fang, Chenwei, Jim M. Haywood, Ju Liang, Ben T. Johnson, Ying Chen, and Bin Zhu. "Impacts of reducing scattering and absorbing aerosols on the temporal extent and intensity of South Asian summer monsoon and East Asian summer monsoon." Atmospheric Chemistry and Physics 23, no. 14 (July 26, 2023): 8341–68. http://dx.doi.org/10.5194/acp-23-8341-2023.
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Abstract. The vast majority of reductions in aerosol emissions are projected to take place in the near future; however, associated impacts on the large-scale circulation over the populated Asian monsoon region remain uncertain. Using the state-of-the-art UK Earth System Model version 1 (UKESM1), this study examines the response of the South Asian summer monsoon and East Asian summer monsoon (SASM and EASM, respectively) to idealized reductions in anthropogenic emissions of carbonaceous aerosols and SO2. The analysis focuses on changes in the monsoon temporal extent and intensity of precipitation following decreases in either scattering (SCT) or absorbing (ABS) aerosols or decreases in both. For SCT, the combination of the early transition of land–sea thermal contrast and sea level pressure gradient during the pre-monsoon season, together with the late transition in the post-monsoon season associated with the tropospheric warming, advances the monsoon onset but delays its withdrawal, which leads to an extension of the summer rainy season across South Asia and East Asia. The northward shift of the upper-tropospheric Asian jet forced by the SCT reduction causes the anomalous convergence of tropospheric moisture and low-level ascent over northern India and eastern China. The intensification of the South Asian high (SAH) due to the warming over land also contributes to the dynamic instability over Asia. These changes enhance the rainy season of these regions in boreal summer. Reductions in absorbing aerosol act in the opposite sense, making the Asia's rainy season shorter and weaker due to the opposite impacts on land–sea contrast, Asian jet displacement and SAH intensity. With reductions in both SCT and ABS aerosol together the monsoon systems intensify, as the overall impact is dominated by aerosol scattering effects and results in the strengthening of monsoon precipitation and 850 hPa circulation. Although aerosol scattering and absorption play quite different roles in the radiation budget, their effects on the monsoon precipitation seem to add almost linearly. Specifically, the patterns of monsoon-related large-scale responses from reducing both SCT and ABS together are similar to the linear summation of separate effect of reducing SCT or ABS alone; this is despite the inherent nonlinearity of the atmospheric systems. The opposing adjustments of Asian rainy season forced by the ABS and SCT aerosol emission reductions suggest that emission controls that target factors like emissions of black carbon that warm the climate would have a different response to those that target overall aerosol emissions.
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Ko, Ken-Chung, and Jyun-Hong Liu. "Quasi-Periodic Behavior of the Pacific–Japan Pattern Affecting Propagation Routes of Summertime Wave Patterns and the Associated Tropical Cyclone Tracks over the Western North Pacific." Monthly Weather Review 144, no. 1 (January 1, 2016): 393–408. http://dx.doi.org/10.1175/mwr-d-15-0080.1.
Full textAbstract:
Abstract This study introduces a modified Pacific–Japan (PJ) index that exhibits a substantial periodicity of 5–16 days in the East Asian summer monsoon region. The quasi-periodic fluctuations of the PJ index can indicate changes in the large-scale circulation systems. In the PJ high phase, the wave pattern propagates northwestward from the western North Pacific tropics to an area near northern Luzon and is then forced to move westward because of a stationary, anomalous high pressure system over southern Japan. The tropical cyclones (TCs) associated with the anomalous low pressure systems tend to follow a straight-moving propagation route through the northern South China Sea. The anomalous cyclonic flow causes heavy rainfall in eastern Taiwan. However, in the PJ low phase, the wave pattern and TCs follow a recurving propagation route toward higher latitudes. The circulation pattern typically brings heavy rainfall to northern Taiwan in the PJ low phase. Therefore, wave patterns under the influence of the quasi-periodic fluctuations of the PJ pattern affect rainfall because of the changing propagation routes of the wave patterns, as well as the TC tracks.