scholarly journals How Does the Tibetan Plateau Affect the Transition of Indian Monsoon Rainfall?

2007 ◽  
Vol 135 (5) ◽  
pp. 2006-2015 ◽  
Author(s):  
Tomonori Sato ◽  
Fujio Kimura
Keyword(s):  
South Asian ◽  
Summer Monsoon ◽  
Climate Model ◽  
Monsoon Rainfall ◽  
Regional Climate ◽  
Reanalysis Data ◽  
North India ◽  
The Tibetan Plateau ◽  
The South ◽  
Northern India

Abstract The roles of the Tibetan Plateau (TP) upon the transition of precipitation in the south Asian summer monsoon are investigated using a simplified regional climate model. Before the onset of the south Asian monsoon, descending flow in the midtroposphere, which can be considered as a suppressor against precipitation, prevails over northern India as revealed by the NCEP–NCAR reanalysis data. The descending motion gradually weakens and retreats from this region before July, consistent with the northwestward migration of the monsoon rainfall. To examine a hypothesis that the dynamical and thermal effects of TP cause the midtropospheric subsidence and its seasonal variation, a series of numerical experiments are conducted using a simplified regional climate model. The mechanical effect of the TP generates robust descending flow over northern India during winter and spring when the zonal westerly flow is relatively strong, but the effect becomes weaker after April as the westerly flow tends to be weaker. The thermal effect of the TP, contrastingly, enhances the descending flow over north India in the premonsoonal season. The descending flow enhanced by the thermal effect of the TP has a seasonal cycle because the global-scale upper-level westerly changes the energy propagation of the thermal forcing response. The subsidence formed by the mechanical and thermal effects of the TP disappears over northern India after the subtropical westerly shifts north of the plateau, the seasonal change of which is in good agreement with that in the reanalysis data. The retreat of the descending flow can be regarded as the withdrawal of the premonsoon season and the commencement of the south Asian monsoon. After that, the deep convection, indicating the onset of the Indian summer monsoon, is able to develop over north India in relation to the ocean–atmosphere and land–atmosphere interaction processes. Northwest India is known to be the latest region of summer monsoon onset in south Asia. Thus, the thermal and mechanical forcing of the TP has great impact on the transition of the Indian monsoon rainfall by changing the midtropospheric circulation.

Investigating the sensitivity of the onset and withdrawal of the south Asian summer monsoon system to changes in anthropogenic emissions using a climate model

2020 ◽  
Author(s):  
Shiwansha Mishra ◽  
Dilip Ganguly ◽  
Puneet Sharma
Keyword(s):  
South Asian ◽  
Summer Monsoon ◽  
Climate Model ◽  
Asian Summer Monsoon ◽  
Anthropogenic Emissions ◽  
South Asian Summer Monsoon ◽  
The South ◽  
Monsoon System ◽  
Asian Summer ◽  
The Impact

<p>While the monsoon onset is recognized as a rapid, substantial, and sustained increase in rainfall over large parts of south Asia, the withdrawal marks the return to dry conditions. Normally, the south Asian summer monsoon onset occurs around 1<sup>st</sup> June over extreme south of peninsular India, which gradually advances to extreme northwest of India by around 15<sup>th</sup> July. The withdrawal starts from northwest India from around 1st September and from extreme south peninsular India by around 30th September. The determinations of the onset and withdrawal dates of monsoon have great economic significance for this region as they influence many agriculture and water resource management decisions in one of the most highly populated regions of the world. Several studies involving global model simulations have shown that changing aerosol emissions could result in significant changes in the seasonal mean precipitation distribution over India. A few studies also show that presence of absorbing aerosols in the foothills of Himalayas and over the Tibetan plateau could increase the moisture convergence over India thereby causing an advancement and intensification of the monsoon precipitation. However, most of the previous studies, which investigated the impact of anthropogenic emissions on the monsoon, are limited to understanding the impact of various emission changes on the seasonal mean monsoon characteristics. In the present study, we try to understand the sensitivity of the onset and withdrawal period of the south Asian summer monsoon system to changes in anthropogenic emissions using a climate model (CESM1.2). We diagnose the onset and withdrawal of the south Asian monsoon by analyzing the variability in vertically integrated moisture transport (VIMT) over the south Asian region and following the definition of hydrologic onset and withdrawal index (HOWI) defined by Fasullo et al. (2002). We examined the effect of changing emissions anthropogenic aerosol, greenhouse gases and both on the onset and withdrawal of the south Asian summer monsoon system. Our preliminary results suggest that increases in the emissions of aerosols and greenhouse gases from anthropogenic sources from pre-industrial to present day could possibly result in significant delay in the onset and advancement in withdrawal of the south Asian summer monsoon system thereby shortening the length of the monsoon season. More results with greater detail will be presented.</p>

scholarly journals Mid-Holocene monsoons in South and Southeast Asia: dynamically downscaled simulations and the influence of the Green Sahara

2021 ◽  
Author(s):  
Yiling Huo ◽  
William Richard Peltier ◽  
Deepak Chandan
Keyword(s):  
Southeast Asia ◽  
Data Model ◽  
Model Comparison ◽  
Climate Model ◽  
Global Climate Model ◽  
Regional Climate ◽  
Global Model ◽  
The Tibetan Plateau ◽  
The South ◽  
Climate System Model

Abstract. Proxy records suggest that the Northern Hemisphere during the mid-Holocene (MH), to be assumed herein to correspond to 6,000 years ago, was generally warmer than today during summer and colder in the winter due to the enhanced seasonal contrast in the amount of solar radiation reaching the top of the atmosphere. The complex orography of both India and Southeast Asia (SEA), which includes the Himalayas and the Tibetan Plateau (TP) in the north and the Western Ghats mountains along the west coast of India in the south, renders the regional climate complex and the simulation of the intensity and spatial variability of the MH summer monsoon technically challenging. In order to more accurately capture important regional features of the monsoon system in these regions, we have completed a series of regional climate simulations using a coupled modeling system consisting of the University of Toronto version of the Coupled Climate System Model version 4 (UofT-CCSM4), the Weather Research and Forecasting (WRF) regional climate model and the 3D Coastal and Regional Ocean Community model (CROCO) to dynamically downscale MH global simulations constructed using UofT-CCSM4. In the global model, we have taken care to incorporate Green Sahara (GS) boundary conditions in order to compare with standard MH simulations and to capture interactions between the GS and the monsoon circulations in India and SEA. In both the global and the regional models, the response of the South Asia (SA) and SEA monsoons to MH orbital forcing is intensified and accompanies lower surface temperature which is likely related to the increased reflectance of shortwave flux at high levels from the greater cloud cover. Comparison of simulated and reconstructed climates suggest that the dynamically downscaled simulations produce significantly more realistic anomalies in the Asian monsoon than the global climate model, although they both continue to underestimate the inferred changes in precipitation based upon reconstructions using climate proxy information. Monsoon precipitation over SA and SEA is also greatly influenced by the inclusion of a GS, with a large increase in particular being predicted over northern SA and SEA, and a lengthening of the monsoon season. Data-model comparison with downscaled simulations outperform those with the coarser global model, highlighting the crucial role of downscaling in paleo data-model comparison.

scholarly journals Reexamining the barrier effect of Tibetan Plateau on the South Asian summer monsoon

2013 ◽  
Vol 9 (4) ◽  
pp. 5019-5036
Author(s):  
G.-S. Chen ◽  
Z. Liu ◽  
J. E. Kutzbach
Keyword(s):  
Tibetan Plateau ◽  
South Asian ◽  
Summer Monsoon ◽  
Asian Monsoon ◽  
Asian Summer Monsoon ◽  
Monsoon Circulation ◽  
The Tibetan Plateau ◽  
South Asian Summer Monsoon ◽  
The South ◽  
Monsoon System

Abstract. The Tibetan Plateau has been conventionally treated as an elevated heat source driving the Asian monsoon system, especially for the South Asian monsoon. Numerous model simulations with general circulation models (GCMs) support this hypothesis with the finding that the Asian monsoon system is weak or absent with all elevated topographies removed. A recent model simulation shows that the South Asian summer monsoon circulation is little affected with only the Himalayas (no Tibetan Plateau) kept as a barrier, leading to a hypothesis of the barrier "blocking" mechanism of the Tibetan Plateau. In this paper, a new series of experiments are designed to reexamine this barrier effect. We find that with the barrier, the large-scale summer monsoon circulation over South Asia is simulated in general agreement with the full Tibetan Plateau, which is consistent with the previous finding. However there remains significant differences in both wind field and precipitation field elsewhere, suggesting a role of the full Tibetan Plateau as well. Moreover, the proposed barrier "blocking" mechanism is not found in our experiments. The energy of the low-level air and the convection is lower/weaker over the Indian subcontinent in the full Tibetan Plateau experiment than that in the no-Tibetan Plateau experiment or the barrier only experiment, which is opposite to the barrier "blocking" hypothesis. Instead, there is a similar candle-like latent heating in the middle troposphere along the south edge of the Tibetan Plateau in both the full Tibetan Plateau and the barrier experiments, whereas this "candle heating" disappears in the no-Tibetan Plateau experiment. We propose that this candle heating is the key to understand the mechanisms of the Tibetan Plateau on the South Asian monsoon. Future studies are needed to check the source of the "candle heating" and its effect on the Asian monsoon.

scholarly journals On understanding the role of South Asian High on the Onset and Withdrawal of the Indian Monsoon

2021 ◽  
Author(s):  
Nishtha Agrawal ◽  
Vivek Pandey
Keyword(s):  
South Asian ◽  
Large Scale ◽  
Climate Model ◽  
Indian Monsoon ◽  
Regional Climate ◽  
Reanalysis Data ◽  
Hadley Cell ◽  
South Asian High ◽  
Annual Variability

Abstract The Indian monsoon is always considered to be a large-scale process that has a profound impact on the agriculture and economic conditions of India. The present study addresses the role of South Asian High (SAH) and subtropical westerly jet (STJ) on the onset and withdrawal of Indian monsoon. For this purpose, we have utilized the output of the Regional Climate Model (RegCM v4.6) and reanalysis ERA5 pressure level data for 24 years (1982–2005) of study. We begin our analysis with the evaluation of Tibetan Plateau (TP) heating and its connection with different atmospheric factors during the seasonal transition of monsoon is perused. We have further tried to decipher the link between SAH and inter-annual variability of monsoon. Our analysis shows the efficiency of the model in simulating inter-annual variability of monsoon onset and withdrawal features. The days of onset and withdrawal simulated by the model have a similar mid-latitude connection as that obtained from the reanalysis data. The vertical structure of the Hadley cell and the horizontal position of SAH have been produced realistically during the transition of the monsoon. We found that the change in meridional position of STJ has a significant impact on phase-shifting of arrival and departure of monsoon. This repositioning of STJ in a meridional direction is strongly correlated to the upper-level high developed over the eastern periphery of the western pacific which is an important component of monsoon flow over the Bay of Bengal. Thus the zonal position of SAH is observed to have direct implications on the onset and withdrawal dates of India.

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